Power System Technology

Chapter 4

Power System Technology

CONTENTS

INTRODUCTION *.. **. . . . . . *.. .*. . . . . . . . . . . . . . . . . . . *.** *.*.*OVERVIEW OF POWER SYSTEM EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .COORDINATED OPERATIONS AND PLANNING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Performance Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Operating and Planning Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Implementing Coordinated Operations and Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

INCREASING TRANSMISSION CAPABILITYLimits to Transfer Capability . . . . . . . . . . . . . . . .Constraints on Individual Components . . . . . . . .System Operating Constraints . . . . . . . . . . . . . . . .System Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . .Prospects for Increasing Capability . . . . . . . . . . .Increasing the Capacity of an Existing Line . . .Controlling Real Power Flows . . .Stability Response . . . . . . . . . . . . . .Future Trends . . . . . . . . . . . . . . . . . .High-Power Semiconductors . . . . .Improving Computer CapabilitiesSuperconductivity . . . . . . . . . . . . . . .

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Boxes

Three Common Reliability Standards . . . . . .Real and Reactive Power . . . . . . . . . . . . . . . . . .Example of Transmission Constraints . . . . . .

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4-1. A Simple Electric System . . . . . . . . . . . . . . . . . .4-2. Transmission Line . . . . . . . . . . . . . . . . ... . . .4-3. Weekly Load Curve . . . . . . . . . . . . . . . . . . . . . . .4-4. Transmission System Limitations . . . . . . . . . .4-5. Western Interconnected System . . . . . . . . . . . .

TablesTable4-1. ANSI Standard Voltage Limits . . . . . . . . . . . . .4-2. Operation and Planning Functions . . . . . . . . . .4-3. Typical Generator Response Rates . . . . . . . . . .

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4-4. Key Physical Characteristics of Generating Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-5. Key Transmission Planning Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-6. Transmission Capability Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~... . . . . . . .4-7. Technologies To Increase Transfer Capability . . . . . . . . . . +... . . . . . . . . . . . . . . . . . . .

96101102109109109115

Chapter 4

Power System Technology

INTRODUCTIONAn electric power system is a vast, complex

machine composed of a large number of generators,transmission lines, distribution systems, and substa-tions. Although less physically obvious than genera-tors and transmission lines, systems to coordinatethe operation and planning are also vital componentswithout which power systems could not function.Coordination systems include monitoring and com-munication equipment, devices that actually controlgenerators and transmission lines, and engineeringmodels and expertise which specify how to operategenerators and transmission lines as well as planequipment additions.

As discussed in chapter 1, the demands on powersystems are changing. Electricity generation isbecoming increasingly competitive, with new formsof ownership and control, and some new technolo-gies are being introduced. Both generators andpurchasers of power are seeking greater access totransmission. Utilities are pursuing new methods toreduce costs by improving operating efficiency.

Improving operating efficiency, integrating com-petitive power supplies, and wheeling power requiremodifications of uncertain technical feasibility andeconomic merit. Can a power system operate withmany separately owned generating companies andpower purchasers demanding access? Is there suffi-cient transfer capacity to accommodate the changes?What will be the impact on reliability and effi-ciency?

The physical principles of electricity greatlycomplicate answering the technical questions. Be-cause power flows at nearly the speed of light withvirtually no storage of electricity in a system, thesupply of power must balance customer demand atevery instant. Also, the flow of power on individualtransmission lines is difficult to control. Conse-quently, the performance of all components is highlyinterrelated. For example, a generator failure in-stantly alters the power flows on transmission lines,perhaps beyond safe physical limits, while requiringthat other generators immediately increase output tomeet demand. Due to the highly integrated nature of

power systems, answering the technical questionsrequires examining the overall power system and therole played by each of the major components.

Chapter 4 examines current and future powersystem technology, and the opportunities to increasetransfer capabilities and operating efficiencies ofexisting systems. The first section discusses thefundamentals of power systems—the equipmentthat composes the system and their performancerequirements. The second section examines thefunctions involved in coordinating all the individualgenerators and transmission components into anintegrated bulk power system. The last sectionexamines the physics of power transfers, and oppor-tunities to increase transfer capability.

OVERVIEW OF POWERSYSTEM EQUIPMENT

An electric power system is comprised of themajor pieces of equipment we commonly associatewith the utility industry. This equipment includesgenerating units that produce electricity, trans-mission lines that transport electricity over longdistances, distribution lines that deliver the electric-ity to customers, and substations that connect thepieces to each other. The bulk power system includesthe generating plants, transmission lines, and theirassociated equipment. Energy control centers co-ordinate the operation of the bulk power systemcomponents from moment to moment and preparefor the near future. A wide variety of other planningand engineering systems coordinate operating andcapacity expansion plans for the longer term. Figure4-1 shows a simple electric system with two powerplants and three distribution systems connected by atransmission network of four transmission lines.

The U.S. power industry uses a variety of fuelsand technologies to generate electricity. Nuclearfission, fossil fuels, and falling water are all com-monly used to drive electric generators whichconvert mechanical energy into electricity. Conven-tional generators typically produce 60 cycle/second(Hertz or Hz) alternating-current (AC) electricitywith voltages between 12 and 30 kilovolts (kV). The60 Hz power is generated in three time-varying

- 9 1 -

92 ● Electric Power Wheeling and Dealing: Technological Considerationsfor Increasing Competition

.

Power plant/subs tat I n

Figure 4-l-A Simple Electric System

Energy Interconnectioncontrol center

I Power PI ant/substation

Interconnection I I

Distribution system

SOURCE: Offices of Technology Assessment, 1989.

sinusoidal patterns, called phases. Generating unitshave automatic voltage regulators which control theunit’s voltage output and speed governors whichadjust power output and frequency in response todemand and changing system conditions.

A wide and growing variety of unconventionalgeneration technologies have been developed, too.These include cogeneration, conversion of solarenergy to electricity, wind-driven generators, andunconventional fuels such as waste material. Themix of fuels and technologies changes from year toyear as new units are built and old units are retired.]

A generation substation connects generators totransmission lines. To minimize losses over longdistances, transmission lines require high voltages,typically between 69 and 765 kV. Power transform-

ers at the substations raise the voltage to these highlevels for efficient transmission. Substations alsohouse a variety of equipment for monitoring andcommunication and for controlling and protectingboth the transmission and generation facilities. Apower plant consists of one or more generating unitson a site together with a substation.

Transmission lines carry electric energy from thepower plants to the distribution systems. Mosttransmission in the United States consists of over-head AC lines operated at 69 kV or above. Often,lower voltage transmission lines operating at be-tween 23 and 138 kV are termed sub transmission,although the distinction depends on the characteris-tics of the individual utility system and is notuniformly applied.

ISCC ch. 6 for a brc~down of the mix of generating CaPaC@.

Chapter 4--Power System Technology ● 93

Power actually flows along bundled strands ofwire called conductors. Conductors are bare metalcables, typically aluminum strands around a steelcore. AC transmission lines typically have threeindividual or paired conductors to carry three-phasepower. There are some segments of direct currenttransmission and underground cables for specialapplications, although these are less common thanoverhead AC lines. Figure 4-2 shows a typicaltransmission line, consisting of a right-of-way,towers to support the conductors, the conductorsthemselves attached to the towers by insulators, andadditional shield wires to protect the conductorsfrom lightning.

The width of the right of way and the tower designare determined by the voltage of the line and the

Figure 4-2-Transmission Line

Photo credit: Cassaza, Schultz & Associates, Inc

need for air insulation to prevent electricity fromflashing over (i.e., arcing between a conductor andthe ground or the tower). Towers may be made ofwood, concrete, steel, or aluminum depending on thenumber and weight of conductors, the terrain, andthe distance between towers. In addition to theweight of the conductors, the towers must be able tosupport any ice which forms on the lines and theforce of wind. Typically, high-voltage lines havenumerous heavy conductors, requiring use of metaltowers for strength.

In addition to the conductors and towers them-selves, transmission systems have monitoring, con-trol, and protective devices much like those found inpower plant substations. Transmission substationshouse this equipment together with devices used toregulate voltage and power flow on the lines.

An interconnected group of individual trans-mission lines comprises a transmission system. Atransmission line connected at both ends to othertransmission lines is part of the grid or network.Transmission lines connected to the grid at only oneend, with the other end connected either to agenerating plant or customer loads, are called radialor feeder lines. The transmission system shown infigure 4-1 allows each distribution system to receivepower from either of the power plants. Even if onenetwork line is disconnected, each distributionsystem can still receive power from both generators.

Some very large electric consumers, such asmajor industrial plants take their power directlyfrom the transmission system, typically at subtrans-mission voltage levels between 23 and 138 kV. Asubstation containing metering, protective, and switch-ing apparatus connect these large customers to atransmission line. Most customers, however, receivetheir electricity from a distribution system.

Distribution systems operate at lower voltagesthan the transmission system, typically under 35 kV,to transport smaller amounts of electricity relativelyshort distances. Power transformers reduce thehigh-voltage electricity from the transmission sys-tem to the lower distribution system level. Thepower transformers are housed together with controland protection devices in distribution substations.

The distribution system is divided into primaryand secondary systems. The primary distribution

94 ● Electric Power Wheeling and Dealing: Technological Considerations for Increasing Competition

system operates at between 2.4 and 35 kV. Somemoderately large customers, typically industrial orlarge commercial, take their electric service from theprimary system directly. Most customers receivetheir electricity from the secondary distributionsystem at voltages between 110 and 600 volts. Theprimary distribution system delivers power to distri-bution transformers which reduce voltage to thesecondary system voltage levels. Secondary dis-tribution systems typically serve groups of cus-tomers in neighborhoods. Unlike transmission lines,secondary distribution systems typically carry singlephase rather than three phase power. Primarydistribution may be either single or three phase.

Protective apparatus in the distribution systeminclude circuit breakers in distribution substationsthat open automatically when a protective relaydetects a fault (or short circuit) and fuses on thesecondary systems that open when overloads occur.Many of the circuit breakers and switches indistribution circuits are manually operated devices,so restoring service after outages occur is usuallydone manually.

Nearly all electric utilities in the United States areconnected to neighboring utilities through one ormore transmission links, or tie lines. Each utility isresponsible for providing the power used by itscustomers without taking power from neighborsunless alternate arrangements have specifically beenmade. Coordinated operation of interconnected sys-tems is implemented through the institution ofcontrol areas. A control area is a geographic regionwith an energy control center (ECC) responsible foroperating the power system within that area. Thecontrol area is defined electrically by telemeteringequipment on all transmission paths into and out ofthe area. One or more utilities may makeup a controlarea. The control area in figure 4-1 is interconnectedto two neighboring control areas through transmis-sion lines.

Energy control centers employ a variety ofequipment and procedures: monitoring and commu-nication equipment called telemetry to constantlyinform the center of generator output and systemconditions; computer-based analytical and dataprocessing tools which together with engineeringexpertise specify how to operate generators andtransmission lines; and governors, switches, and

other devices which actually control generators andtransmission lines. The control center equipmentand procedures are typically organized into threesomewhat overlapping systems which are some-times integrated in a full energy management system(EMS). They are the automatic generation control(AGC) system which coordinates the power outputof generators; the supervisory control and dataacquisition (SCADA) system which coordinatestransmission line equipment and generator voltages;and advanced applications, such as analytical sys-tems to monitor and evaluate system security andperformance, and plan operations.

COORDINATED OPERATIONSAND PLANNING

The electric power system is a complex entitycomprised of many interacting electrical and me-chanical parts. Utilities may have a dozen or moregenerating units and transmission lines, and hun-dreds of distribution systems serving hundreds ofthousands of customers each with a variety of energyusing devices. Coordinating the operation and plan-ning of a utility’s equipment to meet the demand forelectricity is the responsibility of operating andplanning systems. The integrated operation andplanning of modem power systems represents dec-ades of evolution and development.

Performance Standards

One underlying goal in planning and operating apower system is to provide electricity that meetscustomer requirements safely and reliably. Thisentails:

. providing electricity with the correct voltageand frequency to operate consuming equip-ment; and

. providing that power with an acceptable levelof outages or service interruptions.

In practice voltage, frequency, and reliability maybeviewed as constraints or standards which must bemet.

Frequency Standards

A relatively constant frequency of 60 Hz is takenfor granted in the design of customers’ equipmentsuch as motors, clocks, and electronics. Actualfrequencies in U.S. power systems rarely deviate


beyond 59.9 and 60.1 Hz, well within the toleranceof consumers’ electronic equipment and motors.2

Some clocks work by actually counting thenumber of cycles (i.e., every 60 cycles is 1 second).Keeping correct time requires that total deviationfrom 60 Hz over time is small and balanced. Forexample, when frequency fluctuates, clocks slowdown or speed up accordingly. Later, the frequencymust be adjusted to correct the time. The totaldeviation, called time error, is very small andinsignificant.

Power system equipment itself is more sensitiveto frequency deviations than consumer equipment.In particular, the control systems of modem powersystems are designed to be extremely sensitive tofrequency deviations. To function, the control sys-tems actually require very slight deviations and mustclosely monitor time error. (The slight frequencydeviations are essentially used for communicationbetween generators and the control system, asdiscussed later.) For this reason, standards forfrequency are set by the utilities in designing theircontrols, and frequency fluctuations have virtuallyno consumer impact. Standards for frequency andtime error are set by the Operating Committee of theNorth American Electric Reliability Council.

Besides maintaining a relatively constant systemfrequency around 60 Hz, another frequency re-quirement is to avoid nonsystem frequencies. Someelectrical equipment creates nonsystem frequenciesbesides the normal 60 Hz power, which maypropagate through a transmission or distributionsystem and damage other equipment.3

For example, harmonic frequencies, i.e., integralmultiples of 60 Hz such as 120 or 180 Hz,superimposed on the desired frequency may causecommunication equipment malfunctions. Standardsfor nonsystem frequencies have not been uniformlyestablished, partly because severe problems havebeen limited.

Voltage Standards

Unlike frequency, whichlocations in a power systempoint to point. The voltages

is the same at allvoltage varies fromthroughout a power

system depend on the voltage output of individualgenerators and voltage control devices and the flowsof power through the transmission system.

Some voltage standards for power delivered tocustomers are widely accepted and published by theAmerican National Standards Institute (ANSI) (seetable 4-1).A These standards are given both fornormal, sustained conditions and for emergencyconditions lasting a few hours. The less stringentshorter term standards allow system operators great-er freedom in responding to emergencies. Theselection of voltage standards for delivered elec-tricity reflects an implicit balance between the costof maintaining the standard and consumers’ bene-fits.

The ANSI voltage standards have been developedbecause many types of customer equipment requirecertain minimum standards to function properly. Forexample, with excessively low voltage, electricmotors function poorly and may overheat and lightsdim. Overly high voltages, on the other hand,shorten the lives of lamps substantially and increasemotor power which may damage attached equip-ment.5

Not all equipment has narrow voltage tolerances,however. Electric resistance space and water heaterswork well over a wider range of voltages and areinsensitive to fluctuations, for example.

The ANSI voltage standards do not apply forshort-term voltage fluctuations lasting a few secondsor less. Switching transmission lines and generatorson or off and turning on major appliances may createvoltage spikes or drops. Voltage fluctuations maydamage computers and other electronic equipmentor cause lights to flicker. However, standards forshort-term fluctuations are far less uniformly estab-lished than those for longer term voltage variations.

ISra~Wd ~~~&ook of Electrica/Engi~ering, Donald Fink (cd.) (New York, Ny: McGraw ~iill~ 1978)! PP. 20~6.3Ha~ook of Modern Electronics ~ Electrical Engineering, ch~]es Be]ove (cd. ) ( New York, NY: John Wiley & Sons, 1986), pp. 2318-2319.

E.F. Fuchs, D.J. Roesler, and F.S. Alashhab, Sensitivity ofEleclriculApphances to Harmonics and Fractional Harmonics of the Power System’s Voltuge,Parts 1 and 2, IEEE Transactions of Power Deliver, vol. PWRD-2, No. 2, April 1987.

qAmeri~~ Na[ional st~d~ds Institute, Naziomz/ Electrical Saj?ty Code—1987 Edition, ANSI ~ ‘19~7.SD. Fink, op. C1l., foo~otC 2.


Table 4-1-ANSI Standard Voltage Limits

Voltage limits (in percent of nominal)

Normal operating Emergencyconditions conditions

Nominal voltage Minimum Maximum Minimum Maximum

120-600 . . . . . . . . . . . . . . . 95 105 91.7 105.8600-34,500 . . . . . . . . . . , . . 97.5 105 95 105.8SOURCE: Power Technologies, Inc., “Technical Background and Conwterations m Proposed Irwreasad Wheeling, Transmission Access,

and Non-Utility Generation, ” contractor repon prepared for the Offrca of Technology Assessment, March 1986, pp. 1 14.

Some industrial customers have installed their ownprotective gear to guard equipment against out ofrange voltages. With the proliferation of computersand other sensitive electronics, an increasing num-ber of customers are purchasing protective deviceswhich filter out voltage fluctuations.

Reliability Standards

Reliability is a measure of the ongoing ability ofa power system to avoid outages and continue tosupply electricity with the appropriate frequencyand voltage to customers. In contrast to the standardsfor voltage and frequency, reliability goals reflectcustomer preferences for the trade-off betweenelectricity prices and outages rather than the actualdesign and operating requirements of customerequipment.

Establishing objective, quantitatively derived stand-ards that accurately reflect the value of servicereliability has proven difficult. Ideally, standardsshould balance the customers’ value placed onreliability with the costs of providing it. However,determining the value of reliability to customers hasproven challenging because of the wide variance incustomers’ costs from an outage. Customer outagecosts depend on a host of diverse factors including:

the magnitude of the outage (the total amountof energy or power not supplied);how often outages occur, and the duration of theoutage;how prepared the customer is;the type of customer (e.g., industrial or residen-

tial); and. the time of day, day of week, and season of the

outage.

Determining the cost to a utility of providingincreased reliability is similarly challenging. Bulksystem outages occur when generation and transmis-sion are insufficient to meet total customer demandat any instant. However, neither loads nor theavailability of generation and transmission can beforecast with great accuracy. In particular, relativelyinfrequent and unpredictable events (e.g., a lightningstrike on a transmission line or sudden equipmentfailure) may suddenly reduce the availability of acritical generator or transmission line. Also, ana-lyzing the joint reliability impacts of transmissionand generation is challenging. Thorough examinat-ion of the complex interactions between individualpower system components under the nearly endlessarray of possible conditions is analytically intracta-ble. As a result, it is difficult to calculate theimproved reliability resulting from adding newgeneration or transmission equipments

Further, even with high bulk system reliability, alarge number of outages may occur. In fact, bulksystem failures account for a relatively small portionof customer service outages, around 20 percent byone estimate.7 The remainder is caused by dis-tribution system problems, often the result of stormdamage to distribution lines.

In lieu of more quantitatively derived and definedstandards for reliability, engineering planners as-

6For ~ comprehensive disc~sion of tie cha,tlenges of cvalua(ing reliability impacts focusing on the chalknges of integrating generation andtransmission effects, see Public Service Electric & Gas Co., Composite System Reliability Evaluation: Phase 1, Scoping Study, Electric Power ResearchInstitute, EPRI EL-529(I, Deccmbcr 1987.

IJ.S. Departmcntof Energy, ‘‘The National Electric Reliability Study: Exccuiivc Summary,” DOE/EP-0003, April 1981, as cited in: Power SysfemReliability Evahuuion, Institute of Elcctricat and Electronics Engineers, 1982, p. 42,

Chapter 4----Power System Technology . 97

sume a variety of rules of thumb or de factostandards. Three of the most common reliability-related goals are:

● loss of load probability (LOLP) of 1 day in 10years,

● first (or second) contingency security, and● reserve margins of 15 to 20 percent. (See box

4-A.)

These reliability standards specify the amount ofcapacity to be installed (e.g., reserve margins andLOLP), and how that capacity must be operated(contingency security). Thus, they play a central rolein determining the constraints and capabilities ofmodem power system operations and planning.

While these indices are all commonly used, thereare no uniform definitions of how they are calcu-lated, in part due to the variety of conditionsimportant to different utilities. For example, incalculating LOLP or reserve margins, a utilityrelying heavily on power imports may include theimpact of transmission while other utilities do not.The choice of standard to attain is a matter ofexperience and engineering judgment as well assystem specific characteristics and is not uniformacross the country.8

Many parts of the country currently have higherbulk system reliability than that prescribed by thestandards. The exceeded standards result from thecurrent surplus capacity planned and built to meethigh-load forecasts of the past two decades that didnot materialize.

Whether current standards could be strengthenedor weakened to the benefit of customers is specula-tive. Certainly, some customers may benefit fromreduced reliability if accompanied by reduced elec-tric prices, just as others may prefer higher reliabilityeven with higher prices.

Operating and Planning Requirements

Given the performance standards for deliveringelectricity, there are three general functions forcoordinated operating and planning, They reflect thedynamic and complex nature of power systems andtheir customers. The functions are:

1. following changing loads;

2. maintaining supply reliability; and

3. coordinating transactions of power.

In practice, operating and planning systems seekto perform these functions at minimum cost. Thisrequires a tremendous amount of information, com-puting power, and communication capability, aswell as extensive coordination within and among thevarious organizations involved.

Following Load

At each moment the supply of power must equalthe demand of consumers. However, demandchanges continuously and occasionally unpredict-ably. Some load patterns tend to repeat approxi-mately with the time of day, day of week, and withthe season. Figure 4-3 shows a weekly pattern ofdemand for a U.S. utility. The vagaries of weather,economic conditions, consumer behavior, and in thelonger term, technological change all impede theability to forecast accurately. The continual andsometimes unpredictable changes in demand requirecoordination systems to be able to follow loads frommoment to moment (called regulation) and fromhour to hour (called ramping) as well as plansupplies flexibly for the longer term.

A fundamental constraint in following loadsinvolves ensuring that generating units and trans-mission equipment operate within design tolerances.For example, power system equipment used forgeneration, transmission, and distribution has cer-tain voltage requirements, much like consumerequipment. Standards for power system voltages areset by system engineers based on site-specificequipment design and operating requirements. Somepower system equipment requires voltages withinfairly narrow tolerances to operate properly. Otherpower system equipment particularly some transform-ers, are designed to function over a wide range ofvoltages. Similar constraints apply to the ability oftransmission equipment to accommodate powerflows. The capabilities and constraints of powersystem equipment are discussed in greater detaillater in this chapter.

%= ch. 6 for a discussion of standards in different regions.


Box 4-A—Three Common Reliability Standards

Loss of Load Probability (LOLP)LOLP is a measure of the long-term expectation that a utility will be unable to meet customer demand. Many

utilities prescribe a standard LOLP of 1 day in 10 years. This means that given the uncertain failure of generationand transmission equipment and variations in customer demands, engineering analyses predict that there will be abulk system outage for 1 day in a l0-year period.1 The LOLP specifies neither the duration of the outage (e.g.,whether the outage lasts several seconds or an hour) nor the magnitude (whether the outage affects one distributionsubstation or the entire system). Different utilities use different definitions of LOLP; there is no single consistentmethod of calculating it.

Contingency Security CriteriaAfter a major system component such as a generator or transmission line has failed, the redistribution of power

flow on the remaining system will not automatically meet customer load. Even if sufficient generation is available,voltages and thermal loadings on the transmission system may fall outside acceptable limits, or the system may beunstable resulting in cascading failures. Utilities typically specify a reliability criterion of first (or second)contingency security, meaning that sufficient reserves of transmission and generation are immediately availablesuch that the power system will continue to operate even if the one (or two) most critical components fail.2 Thisis called the principle of n-1 (or n-2) operation, and applies at all times, even when some elements are already outof service. For example, if three lines are out of service, the system’s operation must be adjusted so that it will beable to stand the loss of a fourth line, Usually the critical components are the largest generators or transmission lines,or some component at a critical location in the network. In general, contingency studies assume that no more thanone or two major failures will occur at a time (multiple failures are improbable unless there is some common cause).Contingency studies rely upon engineering judgment to decide which types of failures are reasonable or credible,since the large number of components makes enumerating all possible failure modes impractical.

Capacity or Reserve MarginThe reserve margin is the oldest and most traditional measure of reliability. Reserve margin is the difference

between generating capacity and peak load expressed as a percentage of peak load. Similarly, capacity margin isthe difference between capacity and peak load expressed as a percentage of capacity (rather than peak load ).3 Thus,for example, a system with a peak load of 4,000 MW, and installed capacity of 5000 MW has margins of: ReserveMargin = (5,000-4,000)+ 4,000= 0.25, or 25%; and Capacity Margin = (5,000-4,000)+ 5,000= 0.20, or 20%. Thenumerator for both measures is the same, but the denominator for is capacity margin is smaller, so the capacitymargin is always smaller than reserve margins by a few percentage points. Capacity margin is the measure used bythe North American Electric Reliability Council, although in practice most utilities refer to their reserve margins.Typically reserve margins of about 15 to 20 percent have been considered sufficient to allow for maintenance andunscheduled outages (corresponding to capacity margins of approximately 13 to 17 percent).4 However, the amountof reserve margin required depends on system specific factors such as the number and size of generating units andtheir performance characteristics. For example, a system with a few large units will require higher reserves than asystem with many small units.5

IReca]] Ihat he bulk pwer SySLem klties ge~tlm and transmission but excludes distribution. T?uts, outages are expected to be mOre common man ~

LOLP sIandard indicales.

2N(fi Am~cm E[=~c Rellablll~ Cmll (NERC), Tr~sjer cq~;~ip Rejereflce f30cmcn/ (Princeton, NJ: 1980), pp. 6-7.

3Re=we m~gms ad capacity mag~ ~ ~~film]ly relal~ by [he equati~: capacity Margin = Rescme Mmgin + (1 + Reserve Margin).

4A. Kauf~ SMI K. Nelwn, Library of Congress, Congressional Research Serwm, “DO We Rcidly Need All Those Electric Plants?” August 1982.

5N~ ~acm Electric Reliability Council (NERC), Relmbthiy Concepr.s (Princemn, NJ: Fcbm~ 1985), p. 16.

Maintaining Supply Reliability any piece of equipment may fail, either on its own ordue to external influences (e.g., lightning strikes).

The cost and performance of generation and Preparing for continued operation after equipmenttransmission equipment is variable and uncertain, as failure is called maintaining security. As defined byare customer loads. From one moment to the next, the North American Electric Reliability Council

Chapter Power System Technology ● 9 9

Figure 4-3-Weekly Load Curve

Thousand megawatts

12

10

8

6

4

2

0

I

Inter mediate

Base

Sunday Monday Tuesday Wednesday Thursday Friday Saturday

SOURCE: Power Technologies, kc., “Technical Background and Consdarabons m ProDosed Increased Wheeling, Transmissmn Accessand Non UtJIIty Ganaratim,” contractor reporl prepared for the OMce of T~no@y Assessment, fich 1988, pp. 2-3.

(NERC), “security is the ability of the bulk powerelectric system to withstand sudden disturbancessuch as electric short circuits or unanticipated loss ofsystem components. ”9

Ensuring sufficient availability of supplies iscalled maintaining adequacy. Again according toNERC, “adequacy is the ability of the bulk powerelectric system to supply the aggregate electricpower and energy requirements of the consumers atall times, taking into account scheduled and un-scheduled outages of system components."10 Inaddition to unexpected failure, virtually all equip-ment requires some maintenance, and has operatinglimitations that reduce availability. In the longerterm, the cost and availability of fuels is uncertain,resulting in uncertain operating costs for generating

units. In the longer term still, construction cost andschedules for new equipment are often uncertain, asis the demand for power. Maintaining adequacyinvolves addressing these consties.

Coordinating Transactions

Nearly all utility systems are

tints and uncertain-

interconnected withother systems, allowing for a variety of transactions.Transactions may take a variety of forms, including:short-and long-term purchases and sales with neigh-boring systems; purchases from suppliers within autility’s service area (e.g., an independent powerproducer); operation of jointly owned power plants;and wheeling of power. *l.

gNO~ Amc.i~~ Electric Re]i~bi]i[y co~cil (~R(’), Re/~bill~ Concepu (~inceton, NJ: NERC, February 1985), p. 8.

l“lbid.I IFor a comprehensive di~ussion of tie [y~s of in~~ti]ity u~~[ions u~, xc: EIwrw [formation &irninistration, ~nterlttiflty Bdk pOWer

Transuctiom, DOE/EIA-0418, Ocmtwr 1983.


Except where contrary arrangements are specifi-cally made, it is the responsibility of each utility toprovide the power used by its customers withoutabsorbing power from its neighbors or sendingunwanted power to them. Through NERC, NorthAmerican utilities have set standards for controllinginadvertent interchange.12

One requirement for inadvertent transactions isbased on “area control error” (ACE), a measure ofdifference between the actual and scheduled inter-change at any moment which also accounts forpower frequency deviations. NERC guidelines spec-

ify both that ACE must be zero at least once in each10-minute period and must not average beyond aspecified level for any period. Controlling ACE andinadvertent power transfers requires careful sched-uling and control of transactions between the entitiesas well as monitoring and recording the transactionsfor billing or other compensation.

Implementing CoordinatedOperations and Planning

Performing the functions of following load, main-taining reliability, and coordinating transactions

12N~h ~encm Electric Reliability Council (NERC), Operating Manuul (Princeton, NJ: ~c. 1, 1987)

lsFor a more tW~lca] descrlpl]on, see ex~ple: Institute of Electrical and Electronics Engineers (IEEE) Commitlcc Report, “lhcription ~dBibliography of Major Economy-Security Functions, Parts 1,11, and Ill.” /EEE Transactions on Power Appararus and Systems, vol. PAS-1(M), No, 1,JamlMy 1981, pp. 211-235.


involves executing severaland planning procedures. 13

coordinated operating

The procedures each focus on different timehorizons and different aspects of the power system(see table 4-2). Some procedures such as coordinat-ing the energy output of generating units to balancedemand are performed continuously. Others, such asplanning generation additions, are performed far lessoften. The time horizon each procedure is concernedwith also varies widely. For example, controllingenergy input to a generator focuses on a time horizonof under a minute, while long-term planning mayhave a 20-year or more horizon reflecting the longconstruction time and useful life of generation andtransmission equipment. Each time horizon beyonda few seconds requires forecasts of customer de-mand and performance of system equipment.

Governor Control of Generatorsfor Load Following

At every moment, the power generated mustbalance the amount demanded to maintain the 60 Hzfrequency required by both customers and the powersystem. Frequency fluctuations result from an im-balance between the supply and demand for powerin a system. In any instant, if the total demand forpower exceeds total supply (e.g., when a generator

fails, or as demand increases through the course ofa day), the rotation of all generators slows down,causing the power frequency to decrease. A similarprocess occurs in reverse when generation exceedsloads, with the governors reducing the energy inputto generators to maintain frequency.

Controlling frequency involves balancing thesupply and demand for power. Speed governors onmost generating units constantly monitor frequencyand regulate those units’ power output to helpbalance demand and restore the frequency. Theconstant change in a unit’s power output slightlyincreases maintenance requirements, and slightlydecreases operating efficiency.

Power output from a generator does not changeinstantaneously. The rate at which a generator’spower output can increase or decrease, called theramp rate or response rate, depends on the type ofgenerator. That is, the usefulness of a particulargenerator in regulating frequency varies from unit tounit. Large steam generating units such as nuclearpower plants and large coal units generally changeoutput levels slowly, while gas turbines and hydrounits are very responsive. Table 4-3 shows typicalresponse rates for different types of generators. Theresponse rate is expressed in percent of rated

Table 4-2-Operation and Planning Functions

Function Purpose Procedures involved

Following loadFrequency regulation

Cycling

Maintaining reliabilityMaintaining security

Maintaining adequacy

Coordinating transactions

Following moment-to-momentload fluctuations

Following daily, weekly,and seasonal cycles(within equipmentvoltage, power limits)

Preparing for unplannedequipment failure

Acquiring adequatesupply resources

Purchasing, selling, and

Governor controlAutomatic generation control (AGC)

and economic dispatch

AGC/economic dispatchUnit commitmentVoltage control

Unit commitment (forspinning and ready reserves

Security dispatchVoltage control

Unit commitmentMaintenance schedulingPlanning capacity expansion

AGC/economic dispatchwheeling power in Unit commitmentinterconnected systems

SOURCE Adapted from F, Moboshwi, Southern Cahforma Edison, letter 10 Off- of Technology Assessment, May 13, 19S6.

102 • Electric Power Wheeling and Dealing: Technological Considerations for Increasing Competition

Table 4-3--Typical Generator Response Ratesa

Unit type and size Response rate

Steam units (all fuels)10-50 MW . . . . . . . . . . . . . . 4.8% per minute60-199 MW . . . . . . . . . . . . . 3.8% per minute200 and over . . . . . . . . . . . . 2.80/. per minute

Hydroelectric10-59 MW . . . . . . . . . . . . . . 1-6% per secondabove 60 MW . . . . . . . . . . . 4-6% per second

Combustion TurbineAll Sizes . . . . . . . . . . . . . . . . 55% per minute

‘see “power Plant Response,” Institute of Electrical and El~”cs EngineersVbrking Oroup on Power Piant Response to lad Changea IEEE Paper TP 65-71,1965; and Surwy of Cyclical load Caps4iKties of Fosdt Fkd Ganaratlng Units(PabAlto, CA: Electric Power Research Institute, 1979) EPRI-975.

SOURCE: Power Technologies, Inc., “Teohnolo@oal Considerations in ProposedScenarios for Increasing Competition in the Electric Utility Industry,”contractor report prepared for the Oft&a of Technology Assessent, March1988, pp. 2-16.

capacity per minute or per second at which outputcan change.

The ramp rate of a generator reflects the maximumrate at which the unit’s power output can change.Given this upper limit, each generator’s governor isset to specify how rapidly and to what extent the unitactually will respond to frequency changes. Some,but not all units need to be on governor control torespond quickly. In fact, some units do not respondat all, but instead are set to produce a fixed poweroutput. The amount of generation under governorcontrol required to follow load depends on expectedchanges in load and on the ramp rates of theavailable controlled generators. System engineeringanalyses determine the amount of generation re-quired to be under governor control. Setting thegenerator governors is a function with a slightlylonger time horizon, discussed next.

Economic Dispatch and Automatic GenerationControl for Load Following, Reliability,and Coordinating Transfers

Coordinated operation based on the incrementalcosts of generation, called economic dispatch, is onekey to minimizing cost.14

The incremental production cost of a generatingunit is the additional cost per kilowatthour (kWh) of

generating an additional quantity of energy or thecost reduction per kWh due to generating a lesserquantity of energy. Incremental production costsdepend on the cost of fuel and the efficiency withwhich the unit converts the fuel to electricity, andany other operation costs that vary with the level ofpower output. In economic dispatch, units with thelowest incremental costs are used as much aspossible to meet customer demand. Typically, eco-nomic dispatch is entirely recomputed every 5 to 10minutes.

Automatic computer control of generator outputis used to implement the dispatch of generators in acontrol area. Automatic generation control (AGC)systems calculate what increase or decrease in eachgenerating unit’s output is required to maintain thebalance between supply and demand in the leastcostly way. Based on AGC calculations, generatorgovernors are reset to affect the change. An AGCsystem constantly monitors the power system fre-quency to determine whether increased or decreasedoutput is required. The AGC system typically resetsgenerator governors every 5 to 10 seconds based onan approximation of economic dispatch.

When governors balance supply with loads, highincremental-cost units such as combustion turbinesmay be used because they are able to change poweroutput rapidly. The AGC systems set the governorson power plants so that power output from low-operating cost generators increases to displaceoutput from more expensive generators that wereused to control frequency.

To perform its job, an economic dispatch andAGC system needs cost and performance informa-tion about each of the power system’s operablegenerating units. For example, the system mustknow the range of power each generator can produce(called the control range) and the ramp rate.Typically, the efficiency with which fuel is con-verted to electricity, and hence the incremental cost,depends on whether the plant is being operated atfull or part capacity. Control ranges, efficiency, andincremental costs vary widely with the type ofgenerator, and sometimes on contractual requirementsfor purchasing power.

I+jcc B.F. wollcn~rg ad AOJ. wood, Pwer ce~era~~~, OperurWn ad c~~r~~ (NCW York, NY: Jo~ Wiley & Sons, Inc., 1983); or A.S. ~k,ModerrI Power System Control and Operarwn (Boston, MA: Kluwcr Academic Publishers, 1988).

Chapter Power System Technology ● 103

Minimizing total cost often involves one utilitypurchasing electricity produced by another. Inter-utility transactions are sometimes very highly auto-mated, as in the case of the Pennsylvania-NewJersey-Maryland interconnected system, which isone large control area with 11 member utilitiesacting as a tight power pool. In other cases, theprocess is less automated, carried out throughbrokerage systems or by system operators usingtelephones or small computer networks to exchangesale and purchase information.

AGC systems control both the planned andinadvertent power exchange between control areas.Interutility transactions are implemented by increas-ing the generation of the selling utility and decreas-ing that of the buying utility. When inadvertent,excessive, or insufficient exchanges occur, AGCsystems adjust the governors on generating units toincrease or decrease power output, correcting theamount of power transferred. To properly controlinterutility exchanges, AGC systems must haveinformation on the planned schedule of powertransfers and must constantly meter the actual powerflows for comparison.

In calculating which generating units to operateand at what level, the dispatch system must alsoconsider the effects of the transmission system.Dispatch systems commonly incorporate two princi-pal effects. First, losses on the transmission linesmay be significant in systems with widespreadgeneration and loads. When this is the case, thedispatch systems must consider the incremental costof transmission losses in addition to the incrementaloperating cost of generation. Accurately calculatingincremental transmission losses is difficult and timeconsuming. however. Losses increase dispropor-tionately with increases in power transfers anddepend on the often indirect path of power flows.Both features result in computational difficulty fordetermining actual losses. However, some considera-tion of transmission losses is required. Typically, anapproximate mathematical model of the losses isused.

The second transmission consideration relates toadequacy and reliability. The capacity to transferpower while remaining within voltage and load flowlimits is a constraint on economic dispatch. Whensufficient transmission is not available to deliver

power from the lowest cost generators to loads, othergenerators must be operated. This is called operatingoff-economy. The dispatch system then needs toknow not only the capacity of the transmissionsystem and the current use but also the amount ofcapacity required for the transfer and the effect onsystem voltages. Again, due to the computationaldifficulties of calculating power flows, the dispatchsystem relies on another portion of the energymanagement system to determine the security (e.g.,ability to withstand equipment failure) of the dis-patch scheme chosen and override the economicdispatch if needed. That is the function of securityconstrained dispatch, discussed later.

Voltage Control for Load Following

The job performed by governors and AGC fo-cuses on meeting frequency requirements economi-cally as loads change. However, changing genera-tion dispatch may also change voltages across thesystem. Voltages must be kept within design toler-ances for a power system to provide acceptableservice to customers. Maintaining voltage involvesbalancing the supply and demand of power, althoughin this case, it involves balancing reactive power(called VARs) rather than real power (see box 4-B).An imbalance in the supply and demand of VARscauses voltage to rise or drop across the powersystem. Understanding the pattern of voltages andreactive power flows is a complicated problemarising from the physics of electric systems.

Power system equipment creates the primarydemand for VARs. Long, heavily loaded transmis-sion lines typically consume VARs, as do powertransformers and motors. One effect of reactivepower flows is that the use of distant low-costgenerating units may not be possible if sufficientVAR supplies are not available despite otherwiseadequate transmission line capability.

Maintaining voltages to within the standardsrequired by system equipment is the function ofVAR control. Voltages at various locations aretelemetered to the energy control center fromvarious points in a power system and checked toensure they fall within the acceptable range. Whenvoltages begin to deviate from the acceptable range,both automatic and remotely controlled actions aretaken using a variety of reactive power controldevices. Supervisory control and data acquisition


Box 4-B—Real and Reactive Power

Power is the product of voltage (electrical potential or pressure) times current (the number and velocity ofelectrons flowing). In an AC system, both voltage and current vary sinusoidally over time with a frequency of 60cycles per second (60 Hertz). However, the current and voltage are not necessarily in phase with each other. Thatis, the current may reach its maximum slightly before or after the voltage does in each cycle. Active, or real powerresults from current and voltage in phase with each other. Measured in watts, it is the power delivered to a load tobe transformed into heat, light, or physical motion. Reactive power results from that portion of current and voltagewhich are not in phase. Measured in V’s (for Volt-Amps Reactive), it can be thought of as the flow of power stored(but not consumed) by electric and magnetic fields around circuit components.

That current and voltage may be out of phase results from a phenomenon called reactance. ’ When a voltagecauses a current to begin flowing through a wire, a magnetic field forms around the wire opposing and delaying thechange in current. When the voltage is reduced, the collapsing magnetic field again opposes and delays the reductionin current. The magnetic field may also induce or retard a current in nearby wires (e.g., other conductors in atransmission line). The overall effect of the forming and collapsing magnetic fields in delaying changes in currentrelative to voltage creates inductive reactance, or inductance. The larger the current, the larger the inductive effect.

Similarly, different voltages between circuit components (e.g., between conductors in a transmission line orbetween a conductor and the ground) create electric fields. These forming and collapsing electric fields result incapacitive reactance (or capacitance), in which current changes are advanced relative to voltage changes. The largerthe voltage, the huger the electric field and the capacitive effect.

Capacitance and inductance exist in any piece of electrical equipment. When capacitance and inductance arebalanced in a transmission line the voltage and current are in phase with each other. Then there is no net flow ofreactive power. When the inductive effect is greater than capacitive effect the current lags the voltage at any pointon the transmission line and the line is said to consume reactive power. Similarly, when the capacitive effect isgreater, the voltage lags the current and the line is said to produce reactive power.

A transmission line’s operating voltage is determined by the line design, and the capacitive effect is constant.However, different real power flows on a line result from different currents, with fixed voltages. Thus, the inductiveeffect, due to magnetic fields caused by current flowing, increases as the amount of power flow increases. For thisreason, low real power flows on a line may result in a high flow of produced reactive power. High active power flowson a line may result in a high flow of consumed reactive power.

if&c~ma is me ~~ of tie ive~~ to fl~~ &t~ining how much cufienl will flOW for a given vo]lage. The Otherpart of impedance is Cdkd ~f?~l~klme,wh]ch caurs as the flowing electrons collide with atoms of memf in the conductors. Resistance is a form of electrical friction which cremes hea[.

SOURCE: office of Technology Assessmem, 1989.

systems combine telemetry of voltage to the control VAR output and off-economy dispatch are commoncenter and remote control of VAR supplies.

VARs may be supplied or consumed by genera-tors either automatically or under the control ofsystem operators. Whereas real power output of agenerator is controlled by governors controlling theenergy input, reactive power is regulated by adjust-ing magnetic fields within the generators. As withreal power, reactive power output from a generatoris limited. Limits to reactive power output are due topossible overheating within the generator resultingfrom high levels of output. Control of generator

modes of voltage control on the bulk power system.

Other automatic and manual voltage controldevices include capacitors, shunt reactors, variabletransformers, and static VAR supplies. These de-vices may be installed at various locations in thetransmission, subtransmission, and distribution sys-tems. Voltage problems resulting from VAR flowsare one major cause of transmission limitations.Also, improved VAR control may help reduceoperating costs by reducing VAR flows. ’s

t~sclcnti~c Systems, [~., Optimization of Reactive VAR Sources in System Planning, EpRl EL-3729. Novem~r 19~.

Chapter G-Power System Technology ● 105

As a result, the use of these devices is likely toincrease over time. VAR-related transmission lim-its, and some approaches to reducing them arediscussed in the final section of this chapter,

Security Constrained Dispatch for Reliability

The combined control of real and VAR generationoutput and other VAR sources may not result insecure performance. Maintaining system reliabilityis the job of security constrained dispatch ofgeneration.

The objective of security constrained dispatch isto prevent the possibility of “cascading outages” inwhich the failure of one or two generators ortransmission lines results in the overloading andfailure of other equipment. A key to securityconstrained dispatch is scheduling generation in a“defensive” mode so that the power system willhave enough supplies ready to continue operatingwithin emergency standards for frequency, voltage,and transmission line loadings should contingenciesoccur. In a sense, security constrained dispatchaccounts for reliability constraints on transfer capac-ity.

An important parameter of the defensive operat-ing practice is that transmission capability must beheld in reserve for the possible occurrence of a majorfailure in the system. Generating units are similarlyheld in reserve. Idle generating units and transmis-sion lines with below capacity power flows maymistakenly seem to be surplus, when in fact they areessential for reliability. This difference betweenappearance and reality must be carefully noted inchanges to the power system.

The analytical methods used are based on loadflow calculations of real and reactive power flows inthe power system. Control center operators typicallyexamine a series of contingency cases to determinethe most severe contingency and the resulting powertransfer limit. When that limit is lower than presenttransfers, the economic dispatch is recalculated toreduce the transfer to acceptable levels and imple-mented by the AGC and Supervisory Control andData Acquisition (SCADA) systems. By dispatchinggenerating units “off-economy,” security constraintsresult in higher operating costs.

Power flow and contingency analyses needed forsecurity constrained dispatch are time-consumingand computationally difficult. Complex systemswith many generating sources, transmission com-ponents, and loads have complicated flow patterns,resulting in a large number of contingencies to beexamined. Because of the computational difficul-ties, security constrained dispatch often relies onplanning and analysis to determine transfer capabili-ties and constraints. The result is an approximationof actual security constraints. Utilities are increas-ingly developing automatic energy managementsystems by combining the data acquisition capabili-ties of SCADA systems with load flow and otheranalytical tools needed to evaluate security in realtime. 16

Unit Commitment for Load Following,Reliability, and Coordinating Transactions

Generating units typically need to warmup beforeoperating (unlike transmission lines). To be readyfor operation, generators must not only be warmedup, but must also be rotating in synchronism with the60 Hz of the power system. This requires utilities toestablish a unit commitment plan. Unit commitmentplans seek to ensure a sufficient supply of generationfor immediate operation in case of contingenciessuch as failure of a generating unit or transmissionline. Also, the plan ensures that sufficient generationunder governor control is available for regulatingfrequency in response to changing loads. Suchgeneration which is synchronized and ready to serveadditional demand is called spinning reserves.

Unit commitment plans also specify which unitswill be warmed up and cooled down to follow thecycle of loads over the course of a day, week, orseason. Utilities calculate unit commitment sched-ules which minimize the total expected costs ofoperation and spinning reserves required to maintainreliability and meet expected changes in demand.Often, utilities also schedule power purchases fromother utilities. New unit commitment plans aretypically established each day or after major plantoutages or unexpected load changes.

Unit commitment planning requires a vast amountof information. Virtually all the information aboutgeneration and transmission operating cost and

16’’scAD~Ms Mmket still Vibrant, ” Elecrricul WOr&i, vol. 201, No. 10, November 1987, p. 36.

106 . Electric Power Wheeling and Dealing: Technological Considerations for Increasing Competition

availability required by the dispatch and securitysystems is also needed to develop the best unitcommitment schedule. In addition, the time and costto warm up generating units and the availability ofpersonnel to operate generating units must beconsidered. These factors vary depending on thetype of generating unit. Unit commitment schedulesare typically developed using computers to performthe numerous calculations for identifying the mini-mum expected total costs.

Scheduling Unit Maintenance for Reliability andCoordinating Transactions

Scheduling maintenance is similar to unit com-mitment, although with a somewhat longer timeframe. The objective is to schedule needed genera-tion and transmission equipment maintenance tomeet reliability goals and minimize the expectedcost of operation. Maintenance scheduling requiresinformation about each piece of equipment’s needfor maintenance, and the expected customer de-mand. Maintenance schedules may be establishedannually or following unexpected equipment out-ages.

System Emergency Operations and RestoringPower for Reliability Emergencies

System emergencies occur when there simply isnot enough capacity available either within theutility or through neighboring systems to meet load.When voltages and frequencies deviate too much asa result, relays and circuit breakers may isolateoverloaded generators and transmission componentsfrom the system, exacerbating the imbalance be-tween supply and demand. Emergency operationsinvolve avoiding cascading outages by reducing thepower delivered to consumers. In the extreme, thisrequires disconnecting customers from the system.Plans for load shedding must be coordinated with theautomatic isolation of generating units that occursunder abnormal frequency and voltage conditions.Restoring power also requires coordination of thesystem components and the devices used to isolatethe loads. Following system failures, restorationrequires that some generating units be capable ofstarting on their own, called “black-start capabil-ity, ” Not all generators have this capability, typi-cally taking their starting power from the system.

Planning Generation and Transmission Capacity

System planning has the long-term focus ofadding adequate generation and transmission capac-ity to meet changing demands reliably and at lowcost. Planning begins with forecasting both thechanging patterns of demands on the system, as wellas the costs of alternate fuels and resources. Basedon these forecasts, planners implement generationexpansion plans to meet those changing conditions.Plans for new transmission facilities must reflectboth the changes in demands and in generation.Planning new generation and transmission facilitiesis typically a utility responsibility, often performedwith considerable regulatory oversight. Also, be-cause of the interconnected nature of utilities, plansare also usually coordinated with power pool andNERC region review to assure reliability.

Uncertainty in forecasting presents acute prob-lems for planning, in which time horizons of 2 to 30years reflect the construction and operating lives ofnew generation and transmission facilities. In recentdecades, forecasts of long-term load growth haveoften been highly inaccurate. In addition to uncer-tainty over long-term trends, load forecasting iscomplicated by the effects of unpredictable (butinevitable) variations in weather and economiccycles from year to year. Similarly, the significantuncertainty and swings in fuel prices make operatingcosts highly uncertain. The result of this uncertaintyis a mix of facilities which may not ideally meetexisting conditions (e.g., with surplus or deficitgenerating capacity). At any time, the existing mixof generation and transmission capacity reflectsprevious expectations of fuel prices, constructionschedules, and customer demand which may bequite different from actual outcomes.

Generation expansion planners have many supplytechnologies to choose from, with a wide range ofcost and performance characteristics. Typically,generating units with relatively low operating costs(e.g., nuclear, coal, hydroelectric) have been rela-tively expensive to build and have had long con-struction periods. Generating units that are relativelyquick and inexpensive to build (e.g., gas-or oil-firedcombustion turbines) have had relatively high oper-ating costs. Because of uncertain fuel price andavailability, planners often seek a diverse mix ofgenerating technologies.


The growing interest in conservation and loadmanagement technologies, together called demandside management (DSM), has added further optionsto expansion planners. DSM has an even widervariety of performance and cost characteristics.Increasingly, system planners must also considernonutility generation in transmission and generationplanning. Numerous computer-based analytic toolshave been developed to aid planners in evaluatingthe financial and economic impacts of differentcapacity expansion plans under a variety of demandand economic scenarios.

Choice of generation types is often broken intobase-load, intermediate, and peaking reflecting thetime-varying patterns of demand (figure 4-3). Plantschosen for base-load operation typically have rela-tively high construction costs justified by lowoperating costs. Because of the limited hours they’reexpected to operate, generating units with lowcapital cost are chosen for peaking duty, even thoughthey may have higher fuel and operating costs.

The varied operating and cost characteristics ofdifferent generation technologies give each ad-vantages and disadvantages for use in a powersystem. System planning must ensure adequatecontrollable generation for regulating both fre-quency (by controlling the output of active power)and voltage (by controlling the output of reactivepower). The costs and ability to operate as spinningreserves and to warm up or cool down under unitcommitment plans are also critical. Table 4-4summarizes some of the key characteristics consid-ered in planning of both existing and prospectivegeneration facilities.

Transmission system expansion must be adequateto accommodate generating unit additions as well asthe changing patterns of loads. Siting of powerplants is integrally related to transmission require-ments, and costs and capabilities need to be consid-ered together. Depending on the location of a newgenerating unit relative to the existing transmissionsystem, new transmission may or may not berequired. Transmission additions may also beneeded to increase transfer capability to neighboringutilities. The appropriate level of interutility transfercapability depends on the opportunities such as

exchanging reserve generating capacity and obtain-ing lower-cost energy and capacity .17

A variety of engineering-analytical tools are usedto determine the type of transmission additionsneeded, and the overall impact on the existingsystem. These tools help planners examine suchfactors as:

. the effects on real and reactive power flows,

. the resulting transmission losses,● the need for voltage and reactive power control

devices on the system, and. the transient stability and contingency security

of and the impacts on system reliability.

Table 4-5 summarizes some of the key character-istics of both existing and prospective transmissionfacilities which are considered in planning,

INCREASING TRANSMISSIONCAPABILITY

Transmission systems have a variety of uses suchas:

● delivering power from a utility’s supplies to itscustomers,

. providing for interutility exchanges of econ-omy energy, firm capacity and shared reservecapacity,

. integrating nonutility generation, and

. wheeling power.

In integrated power systems, performing thesefunctions involves moving power from a largenumber of generators to a large number of loadsalong a network of transmission lines. At times,transmission constraints occur which limit the abil-ity to move power from one location to another. Thissection describes the constraints on a system’stransmission capability and some of the technolo-gies available to ease those constraints,

Limits to Transfer Capability

Basic physical principles largely determine thetransmission capability of a power system. A fewfundamental factors underlie the physical limita-tions to transfer capacity of power systems. Thelimitations may be due to either the abilities of

17 For ~ dlwu~~ion of ~jmnlng intemtility ~mission SYs[cms, s~ Pub]ic Semice E]cc~ic & G~ CO., An ApprO~h for Determining Tran@erCapubiliry Objectives, Electric Power Research Institute, EPRI EL-3425, March 1984.


individual components, or to the requirements andchallenges of operating the overall system. Indi-vidual transmission line components have specificvoltage requirements and limited current-carrying orthermal capability, either of which may constraintheir use. System-related constraints involve thecomplex interactions between individual generators,transmission circuits and their control system, andthe needs for maintaining adequate reliability. Table4-6 summarizes the constraints on power transfers.

Physical laws alone do not dictate the absoluteamount of transfer possible. Rather, they indicate atrade-off between level of transfers and reliability.There is no simple power network equivalent of thetelephone company’s busy signal. For example,higher transfers decrease transmission reserves,increasing the possibility of an outage occurring. Asa result, transfer capability depends on both physicalcharacteristics and the reliability standards andprocedures used. (Recall that reliability criteria aresomewhat subjective and not set on a quantitativelyderived balance between the utility’s costs ofproviding reliability and the consumers’ benefits ofuninterrupted service.)

Determining transfer capability and opportunitiesfor improvements can be a challenging matter ofbalancing economics, reliability, engineering, andpolicy, Developing meaningful estimates of trans-mission capability requires considerable engineeringexpertise, data, and analytic tools. This challengearises because capability is not merely the rating ofa single line or a few lines. Rather, transmissioncapability is a function of the strength of the systemas a whole, including not only the transmission linesbut the generating units as well. For example,spinning reserves of generation located near loadsmay reduce the amount of transmission capacityfrom distant generators which must be held inreserve to maintain reliability.

Transmission capability also varies over time,further complicating any assessment of the ade-quacy, limitations or opportunities for expandingcapabilities. It varies as switching operations occurand as demand, generation, and transmission pat-terns change. Fluctuating patterns of demand, chang-ing availability of generators and transmission lines,even weather, all affect capability.

In some cases, there may be a single bindingconstraint that would produce a large increase incapability if it could be relieved. More often thereare multiple constraints on a single transmissionline, or constraints on many lines at the same time sothat reliving a single constraint would make prac-tically no difference. For example, in the PJMsystem, west to east transfers are limited by voltage-related factors 85 percent of the time and thermallimits for the remainder.l8

Although this section discusses physical con-straints, there are also institutional constraints onpower transmission. Even with sufficient physicalcapability, some economically advantageous trans-fers may not take place. For example, lack ofregulatory approval, lack of intercompany agree-ments or contractual basis, or simply lack ofknowledge of economic opportunities may all provereal and significant impediments to full use oftransmission capacity.

Constraints on Individual Components

Power is transmitted when the line voltage causescurrent to flow in the conductors. The amount ofpower an individual line carries is proportional to theproduct of the current and the voltage. However,every transmission line is limited in the amount ofpower it can transmit by constraints on voltage andcurrent. Flows of reactive power limit both thevoltage and current capacity.

Thermal/Current Constraints

Current flowing causes conductors to heat up. Theamount of heat individual components can toleratelimits the amount of power than can be transmitted.Heating causes the metal conductors to expand andsag. The resulting reduced clearance from theconductor to the ground, towers, and other conduc-tors exacerbates flashover constraints. Excessiveheating may also result in a permanent stretchingand lead to brittleness and a shorter lifespan.Substation equipment is also subject to thermallimitations. Excessive heat can destroy the materialsused in transformers and other terminal equipment.

A thermally overloaded component may reach itscritical temperature within seconds, minutes, orhours, depending on its previous temperature, its

lscas~,za, %hullz & Ass~iates, Inc., Case Studies on Increasing Transmission Access, OTA contractor report, March 1988, p. VA.


Table 4-4-Key Physical Characteristicsof Generating Plants

Table 4-6-Transmission Capability Constraints

Operating costFuel type and costEfficiency

Control rangesReal powerReactive power

Startup costsRamp ratesFuel availabilityExpected equipment failure-availabilityMaintenance requirements and costsEnvironmental impacts and emission requirementsSite availabilityLocation relative to transmission and loadsConstruction cost and lead timeLifetimeSOURCE: Offti of T-1ogy As~ssment, 1889

Table 4-&Key TransmissionPlanning Characteristics

Location and capacity of individual linesEquipment voltage requirementsCapability of VAR support equipmentTransmission lossesEquipment failure ratesMaintenance requirementsEnvironmental impactsRights-of-way and substation site availabilityConstruction costConstruction lead timeLifetimeSOURCE: Office of Technology Assessment, 1989,

physical characteristics, amount of overload, andweather. Therefore it may be able to carry a heavycurrent for a short time, a lesser current for a longertime, and a still lesser one indefinitely. The lattercurrent is the component’s “normal” rating, appli-cable to normal operation. “Emergency” ratings arecurrents that can be carried for shorter times on theassumption that the loadings can be relieved bychanges in generation dispatch or reductions in loadswithin the assigned time period.

Thermal ratings are usually established for load-ings occurring for different periods of time-10minutes, 30 minutes, 4 hours, etc. The ratings aretypically based on current flows rather than theactual temperatures of transmission line equipment.The actual temperature depends not only on current,

Physical constraintsIndividual line constraints

Thermal/current constraintsConductor sagging, equipment lifetime

Voltage constraintsFlashover, corona, and terminal equipment requirements

Reactive power flow and voltage

System operating constraintsDistribution of power flows (parallel path and loop flows)Contingency securityStability (steady-state and transient)

Institutional constraintsInterutility agreementsRegulatory approval

SOURCE: Office of Technology Assessment, 1988.

but on ambient weather conditions including tem-perature, wind speed, and icing. Identical compo-nents may have different ratings due to such factorsas the expected ambient weather conditions and thereduction in equipment life considered acceptableby the utility.

Voltage Constraints

The design of a transmission line specifies theminimum and maximum operating voltages. Volt-ages exceeding the maximum may cause electricityto flashover (i.e., arc between a conductor and theground or the tower), rather than travel along theline. High voltages may also cause corona discharge(i.e., ionized air molecules surrounding the lineresulting from high electric fields), creating noiseand radio interference. The maximum voltage allow-able on a line depends on its height, spacing of theconductors and insulators, and weather (e.g., coronais exacerbated by high humidity, rain, or snow).Excessive voltages may also destroy transformersand other terminal and substation equipment bybreaking down their insulation.

Reactive Power Flows and Voltage

As discussed earlier, reactive power flows maycause voltage to rise or drop significantly alongtransmission lines, particularly long ones. With lowreal power flows on a transmission line, capacitivereactance may create high voltages along the line,exacerbating high voltage constraints.

Similarly, increasing the real power flow on aline also increases the line’s demand for reactivepower. As the flow of reactive power increases, thevoltage along the line drops significantly. If as aresult of reactive power flows, voltages fall belowthe design minimum, transformers will not functionand either power cannot be transferred or the voltageof power delivered to customers will be outside theallowable range. Thus, the ability of generators andother VAR control devices to supply reactive powerlimits the amount of real power which can betransferred. Since the amount of reactive powerrequired by a transmission line increases with linelength, reactive power problems typically affectlong lines.

System Operating Constraints

Parallel Path Flow: Distribution of Power

The flow of power in a transmission network isdictated by the laws of physics. One of the key lawsis that power may flow on all available pathsbetween the generator and the load. This is calledparallel path flow (which is a slight misnomer, sincethe lines are not necessarily parallel).

Generally, the amount of power flowing on anypath of a network is inversely proportional to thatpath’s impedance. The impedance may bethought ofas an “electrical length,” which depends on both theactual length and the voltage of the path. (One mile

Chapter Power System Technology ● 111

of 500-kV line has approximately one-fifth theimpedance of a mile of 230-kV line.) Also, a path’simpedance to power flow does not necessarilyreflect the transfer capacity of that path.

The distribution of power flows and the inabilityto control the flows has two important implicationsfor determining transmission system capability.First, the transmission capacity of a network is notthe sum of the power that could be carried on eachline alone. Rather, the capacity is constrained by theweakest link. The amount of power that can betransferred from one area to another by a transmis-sion system is the smallest power transfer at whichone of the components reaches a thermal or voltagelimit. (See box 4-C.)

Second, the capability of transferring power fromany generator to any loud on the system depends onthe other transfers occurring simultaneously. Thepower flow from a generator to a load divides ontoeach pathway to some extent. Even indirect ordistant lines may receive some of the flow and thushave part of their capacity used up. As a result, thecapacity remaining for additional transfers betweenother generators and loads depends on the othertransfers since they essentially share the sametransmission paths.

Parallel path flows and resulting transmissionproblems can occur both within a single utility andbetween interconnected utilities exchanging power.Parallel path flows crossing the boundaries ofutilities along paths not contracted for, or scheduled,are called loop flows. (See box 4-D.) In intercon-nected systems, such as those in the United States,loop flows are common.

System Stability

In an electrical generating network all generatorsrotate in unison, or synchronism, at the systemfrequency of 60 Hz. The ability to maintain synchro-nism is called stability. Transmission capability maybe limited by instability. Normally, a disturbanceincreasing or decreasing the speed of one generatorwill cause small changes in the unit’s power output,tending to bring that generator back to the commonspeed of the system. Instability is a condition inwhich this stabilizing process does not occur, and

some generators speed up with respect to others,possibly causing the system to fall apart.

Two types of stability can be classified accordingto the magnitude of the disturbance: steady state andtransient. 19

steady state stability refers to the abilityof the system to withstand small changes in loads.Transient stability refers to the ability to withstandlarge disturbances, such as the failure of a transmis-sion line or generating unit. System engineers usecomplex computer programs representing the gen-erators, controls, loads, and the network itself toexamine which operating conditions (e.g., powerflows on the transmission system) are stable andwhich are not.

Contingency Security

A major disturbance such as the failure of agenerating unit or transmission component causeschanges in real and reactive power flows andvoltages around the system. As discussed earlier, then-1 contingency security standard for reliabilityrequires that a system continue functioning withoutcascading failures caused by thermal overloads, orexcessive voltage drops or sags, or instability if anysingle component should fail without notice. Volt-age control devices and generating units are set torespond to a contingency to restore the frequency,voltages, and power flows to acceptable levels.Because of the possibility of a major disturbance, thetransmission system’s capability of carrying poweris limited not only by the actual flows but by theflows that would exist if a given contingency shouldoccur. (See box 4-C.)

The standard approach to avoid such cascadingfailures is to operate in a defensive or preventivemode. In this mode, generation is dispatched andflows are maintained so that sufficient generationand transmission capacity is held in reserve to ensurethat the resulting redistribution of power wouldremain within emergency ratings following the mostsevere single contingency.

While the defensive mode is essential for reli-ability, it may require operating more costly gener-ating units when lower cost units are available.There is an alternative to defensive scheduling ofgeneration and transmission, that of developing

IgInstitute of Electrical and Electronics Engineers (IEEE) Committm RcPofl, ‘‘Proposed llxms and Definitions for Power System Stability, ” IEEETransactions on Power Systems, vol. PAS-101, No. 7, February 1986.


Box 4-C—Example of Transmission Constraints

The transmission capability between two points in a system may be limited by individual componentconstraints or by network constraints. Individual line constraints may be exacerbated by parallel path flows in thenetwork. This highly simplified example shows some of the different constraints and their impact on overalltransmission capability.

Component ConstraintsConsider the very simple transmission network illustrated in figure 4-4. Three parallel transmission lines

connecting a generator to a load. Each line has a capacity of 100 MW, above which it is constrained both thermallyand by voltage. That is, increasing power flows above 100 MW by increasing current would cause overheating andsagging of the line. It would also increase reactive power demand, reducing the voltage at the receiving end belowacceptable levels. Increasing power flows by increasing voltage would cause flashovers, corona discharge, orpossibly destroy terminal equipment, If all three lines could be fully utilized, the system would have the ability totransfer 300 MW.Parallel Path Flows

Because of different path lengths, the power transmitted may divide unequally on the three lines. Assume thatthe division is in the ratios of 10:9:8. Because of the uneven division, when Line A carries its rated power of 100MW, Lines B and C carry only 90 and 80 MW respectively. Any attempt to transmit more power would increasethe loading on all three lines and overload Line A. Thus, due to the impact of parallel path flows, the total capacityis only 270 MW.

Contingency Security ConstraintsUsing the defensive mode to meet the “n-1” contingency security requirement reduces the transmission limit

even more. If Line B were to suddenly fail, perhaps after a lightning strike, Lines A and C would receive theadditional flow and be loaded to 150 MW and 120 MW respectively. To prevent the possibility of an overload, theallowable power transfer is only 180 MW. Then the normal loadings are limited to only 67, 60, and 53 MWrespectively, and the emergency loadings do not exceed the rating of 100 MW. The three 1OO-MW lines form asystem capable of safely carrying only 180 MW, or 60 percent of their total individual ratings.

Transient stability studies of this system may indicate that the two generators will lose synchronous operationafter a disturbance such as the line loss, even though no individual components are overloaded. That is, onegenerator may slow down while the other accelerates, as both respond to the new voltages and power flows. If thisis the case, stability concerns may require further limiting transfers.

SOURCE: Office of Tdmology Assessment, adapted from Casazza-Schulw & Assw]atcs, Inc., Case Studies on /ncrea.iing Transtniss/on Access, OTA contractorreport, March 1988.

special protection systems for responding rapidlyenough to prevent cascading failures after a dis-turbance occurs. Such a mode of action is referred toas corrective or remedial action. Remedial actionmight involve measures including tripping (or rap-idly disconnecting) a remote generator while rapidlyincreasing the output of a nearby generator when aspecific contingency occurs. It is typically usedwhen the preventive mode of operation would entaila heavy economic penalty.

According to the North American Electric Reliabil-ity Council (NERC), corrective action systems are

being considered more frequently as an alternative todefensive generation scheduling.20 NERC notes thatwidespread application of these more complexschemes may affect future power system reliabilitysince system security becomes dependent upon thecorrect functioning of these special protective sys-tems. To provide the same reliability as the defen-sive mode, special protection schemes must either behighly reliable or have built-in redundancy.

In practice, limits caused by contingency securityare challenging to analyze. Circuit configurationsare far more complex than the example of box 4-C,

zONo~ Amefi~~ Electric Rcliabili~ Comcil, 1987 ReliabiliO AssessmenbThe Future of Bulk Electric System Reliability in North America,)987-/996 (Princeion, NJ: October 1987).


Gen

Figure 4-4-Transmission System

t - o

Gen

A 1 0 0

9 0 >B

Load

c 8 0 270 MW

(a)All lines in service.

System loaded without considering contingencies.

Gen

Line A at rated loading.

A 6 7

B 6 0

Gen

Load180 MW

Limitations (all lines rate 100 MW)

Gen

B

c 1 2 0 >

Gen

Load270 MW

(b)Outage of Line B

System loaded without considering contingencies.Lines A and C overloaded.

Gen

A 100

B

Load

c 80 180 MW

(c)All lines in.

System loaded to “n-1 contingency limit”

SOURCE Casazza, Schultz & Associates, Inc., Casa Studies on Increasing Trarrsomwon Access, OTA contractor report, March 1988.

(d)Outage of Line B

System was loaded to “n-1 contingency limit” beforeLine B tripped.

Line A at limit. No overloads.

and the redistribution of power flows after someequipment failure is complicated. Also, both realand reactive power flows and their impact on voltageand thermal limits must be considered. Moreover,following a disturbance, the transition to the newequilibrium state is not instantaneous. Rather, thechange occurs over time as generating units andvoltage control devices react and interact with eachother. Thus, stability analyses must examine notonly whether the new state following a disturbanceis stable, but also whether transition to a new stablestate will occur.

Prospects for Increasing Capability

Assuming that an increase in transmission capa-bility is desired, what can be done? There arepossibilities for mitigating all types of constraints.Options for upgrading both the transmission systemand generators may be useful. Options include:

1, increasing the thermal or voltage capacity ofan individual existing line,

2. improving the control of reactive power andvoltages on a network,


Box 4-D--Loop Flows in the WesternSystem Coordinating Council (WSCC)

Loop flows, the unscheduled use of anotherutility’s transmission resulting from parallel pathflows, is common in the WSCC as in other parts ofthe United States. The WSCC transmission systemhas the general shape of an elongated doughnutincluding a western section of lines joining Oregonand California; an eastern section running generallyfrom Montana to Arizona; a section in the North-west; and a section from southern California toArizona. Figure 4-5 shows a simplified view of theWSCC doughnut.

Because of the shape of the transmission systemand the physical laws of electricity, wheneverpower is sent from one part of the doughnut toanother, the flow is split two ways; some goesclockwise, and some counterclockwise. For exam-ple, if 1,000 MW of power is sent from theMontana-Wyoming area to the Pacific Northwest,only 580 MW of this power flows along therelatively direct counter-clockwise path, as seen inthe figure below; the remaining 420 MW flowsclockwise through California and north through thewestern lines. The flows due to simultaneoustransactions carried on at the time are superimposedon each other depending on their amount anddirection. For example, a sale from the Northwestto California would reduce or reverse the flowbetween the Northwest and Montana-Wyoming,and increase the flow from Montana-Wyoming toCalifornia on the eastern lines.

If the transaction from Montana-Wyoming to theNorthwest used a scheduled or contract pathdirectly joining the two areas and not includingCalifornia, the 420 MW flow not using that path isa clockwise loop flow. WSCC has pursued use offlow control devices called phase shifting trans-formers to reduce loop flows.SOURCE: Casazza-Schultz & Associates, Inc., Cuse Wdies CM /n-

creasing Trunstnission Access, OTA contractor report, March1988.

3. improving the control of real power flows ona network,

4. decreasing the response time of generators andtransmission line switching, and

5. adding new lines.

Figure 4-5--Western Interconnected System

L

ng

DenverA r e a

I

Colorado .

N eWMexico

SOURCE: Casazza+chultz & Assooiakm, Inc., Caw ShJdaIS on lncraas@ Trwmnris-sion Acwss, OTA contractor report March 1988.

Table 4-7 shows how these options relate to thetypes of system limitations.

The costs of increasing transmission capabilityare site-specific, depending on a host of factors suchas terrain, system configuration, type and age ofequipment being upgraded, etc. Often, a transmis-sion line or generator being upgraded must betemporarily taken out of service. Lost use of theequipment creates a highly site-specific cost, par-ticularly significant in cases where a line beingupgraded is in heavy use. As a result, generalizingabout cost and performance is difficult.

Meaningful estimates of the benefits of options toincrease transfer capability are even more difficult todevelop. There are several reasons. First, mostchanges will affect not only transfer capability butthe system operating economics and system reliabil-ity as well. Developing a meaningful combinedmeasure of performance that trades off betweenthese factors has proven elusive. Second, the impactof any measure of system transfer capability, operat-ing economics, and reliability is highly site-specific.The impact of a new transmission circuit in a remotepart of Nevada and an identical circuit parallel to an


Table 4-7-Technologies To IncreaseTransfer Capability

Remedies to individual line constraintsVoltage uprating

Tower extensionsUpgrading insulatorsUpgrading terminal equipment

(circuit breakers, relays, transformers)Current uprating

Dynamic conductor ratingSag assessment and monitoringRestringing (live-line restringing)Changing operating standards

Tower design and new linesConversion to multiple circuit towersHigh-voltage direct current lines

Remedies to steady state system operating constraintsControl of load division

Phase angle regulatorsSeries reactance and capacitanceSystem reconfigurationHVDC control featuresRedispatch of generation

Reactive power management techniquesShunt or series capacitorsShunt reactorsStatic VAR compensatorsSynchronous condensersGenerators as VAR sources

Remedies to contingency security and stability constraints/reproving generation response controls

Generator tripping and fast runbackFast valvingBraking resistors and load switchingAdvanced excitation systems and stabilizersTransient excitation boost

Improving transmission response controlsHigh-speed reclosing and reducing clearing timeRapid adjustment of network impedanceFast acting phase angle regulatorsSectionalizing (adding switching stations)

SOURCE: Adapted horn Po~r Technol@es, “Technical B.w@mJ nd and cOn*ra-tIons In Proc+Osed Whaellm, Transmission Aocess, ti NofI-UfIIIty Genera-tion, ‘ mntrktor report p@rd for the Office of Tec+Inolcgy Assessment,March 1988, p. 6-2.

existing line in the Northeast are considerablydifferent. Also, the impacts vary over time, as loadschange, and as available resources and their costschange.

The following is a summary of some of theapproaches to increasing transfer capability. For amore comprehensive, technical description of solu-

tions to increased transfer capability the reader isreferred to Technical Limits to Transmission SystemOperation, published by the Electric Power Re-search Institute.21

Increasing the Capacityof an Existing Line

Increasing Voltage

Since the acceptable voltage range for operationof transmission circuits without flashovers is deter-mined primarily by the line’s equipment and design,the opportunities are limited to changing equipment.Transmission towers can be extended and insulatorsupgraded to increase the spacing between theconductors, towers, and ground. Terminal equip-ment such as switches, circuit breakers, meters, andtransformers will also need to be upgraded to thehigher voltage ratings.

Increasing Current Ratings

One method to uprate the current carrying capac-ity of a line is to simply increase the allowedtemperature rise, and thereby increase the amount ofcurrent flow allowed. This method has a very lowinitial cost. However, there may be some reductionin equipment lifetime.22

A related technique is to use dynamic line ratings.Normally, a line’s current ratings are based onimprecise and conservatively forecast estimates ofambient weather conditions. Using dynamic linerating, the actual ambient weather and the tem-perature and sag of the conductor are measured,permitting the line to operate closer to its physicallimits on cool or windy days. Dynamic line ratingadds some increased operational complexity andincreases the variability of transfer capability overtime.

Resagging a line to raise it higher off the groundmay also allow increased current flows if the line isconstrained by excessive sagging. This has beendone in some cases while the line is still in service.Another option is restringing-installing a largerconductor with higher current ratings. This may

21power TW~~iogie~, ]nc,, Tec~ical Li~”ts t. Tra~~’ssion ,~ystem ~peration, EPR1 EL-5859, June 1981?.

~~lEEE Standard for Calculation of Bare Overhead Conductor Tcmperaturc and Arnpacity Under Steady-State Condinom. ANSi/’iEEE Standard738-1986, 1986.


require reinforcement of the existing towers butcosts considerably less than adding a new line.

Tower and Circuit Reconfiguration

Voltage and/or current limits may also be ex-tended by more extensive changes in line circuit andtower design. If tower strength permits, a single wireline may be replaced by a bundle of two to fourwires, raising both thermal and corona limits. Inaddition to restringing circuits already in place, itmay be possible to string another separate circuit,Original towers may already have space for newcircuits in anticipation of future need. A newpossibility under investigation is either to restring anold AC line as a new HVDC line, or to add a newHVDC circuit in combination with an existing ACcircuit as space permits.

Controlling Real Power Flows

As explained above, network transfer capability islimited by the most constrained line in the system,so any method which can control or alter flow in thenetwork may have major benefits. There are numberof methods which can be used to control networkflow, but this is done by changing the networkcharacteristics in different places, rather than bychanging the rules of network flow.

The first and foremost technique for controllingnetwork flow is off-economic dispatch of generatingunits. By generating power at particular networklocations, rather than in economic order, networkflow can be kept with reliability and capacity limits.

A second practical, inexpensive, and commonmethod is to alter the network itself by disconnectingone end of a constraining (typically lower voltage)line. The line then carries only the customer load itserves. This method may also be used on one of twoparallel lines under low load conditions, so thatincreased loading on the remaining line is bettermatched to the VAR compensation present. In bothcases, the benefit is purchased at the cost of lowerreliability, as fewer paths between generation andloads remain.

Phase shifting transformers (also called phaseangle regulators) change the phase angle betweeninput and output by advancing or retarding therelative time at which the input and output sine wavevoltage peaks occur. By doing so these devices actas a valve which can increase or decrease the flow ofpower on a line. Phase shifting transformers have notbeen widely used due to past problems with reliabil-ity. They also have the undesirable side effects ofincreasing reactive power losses. There is some hopethat future developments in high-power electronicswill produce devices that can be used with phaseshifting transformers to control individual line flow(discussed below).

A technique called Rapid Adjustment of NetworkImpedance (RANI) can be used to continuously varyVAR compensation to maximize power flow on asingle line. It can also be used to vary the imped-ances on one or more lines in such away as to controlpower flow on the network. Because this control isachieved by varying line impedance, there is thepossibility that it will be paid for in increased linelosses. The cost of such line losses must be balancedagainst improved reliability, reduced need for newpower lines, and other benefits.23

High-voltage direct current (HVDC) power linesare used for high-power, long distance lines and asasynchronous connections between the three maininterconnected regions of the United States.Through application of high-power thyristors usedto convert AC power to DC and back again, thevoltage and hence the power transfer of the DC linecan be directly controlled, possibly enhancing sta-bility as well. While it is currently uneconomic touse DC lines for such power flow control, researchcontinues on multiterminal DC lines and improvingthe cost and capacity of high-power semiconductors.HVDC appears to be the single most powerfulmethod of direct flow control in the battery ofoptions and with economic feasibility could have amajor impact on the industry.

zJThe Bonneville power ~minis~~ion is experimenting with RAN1, with a preliminary study showing it to compare favorably with mechanicallyswitched shunt capacitors, static VAR compensators, and mechanically switched series capacitors on the Pacific AC Intcrtie in providing transient supportand damping and post-disturbance support and stability. In one configuration. loop flow was reduced 43 percent (348 MW) at the cost of 68 MWincremental losses. (Personal communication with Mr. Dean Perry of BPA, July 1988).


Controlling Reactive Power and Voltage

Increasing the availability and control of VARsupplies on the system is one approach to alleviatingvoltage-related transmission limits.24 Generators areone common source of VARs as well as real power.Capacitors may be installed at various points toincrease the available supply of VARs. The capaci-tors can be designed to be switched in and out ofoperation, allowing operators to control VAR sup-plies. Synchronous condensers and static VARcompensators may also be installed on transmissionlines to provide a controllable source of VARs.Inductors may be connected in shunt with the line inspecific instances. Series connection is rare and usedto control excess current in short circuit conditions.Another technique typically used to control VARflows is to control the VAR output of generators, justas real power is controlled.

Increasingly, methods are being developed foroperation and control systems to use computer loadflow models (called optimal power flow models) tosimultaneously dispatch both VARs and real power.It is anticipated that the future will see moreapplications of these methods, The cost of implemen-tation will include metering and communication andcontrol equipment as well as new software.

Stability Response

A utility can increase its transfer capability byshifting its reliability policy from the preventativemode of operation to the remedial mode. There arenumerous special protection schemes being devel-oped and applied. Generation response optionsgenerally increase the ramp rate at which a generatorcan increase or decrease its output or temporarilyincrease its peak output. These options may increasemaintenance costs by increasing operating stresses.

Generator “tripping, “ “fast runback,” and “fastvalving” systems are generator control schemesdesigned to rapidly reduce power fed into the gridwhile continuing to keep the unit on line andavailable, Increased output from generators can beobtained by temporarily turning off auxiliary plantequipment at the generating stations. A 10-percentincrease in output may be temporarily obtained

using these measures. However, operating powerplants in this way may reduce equipment life orincrease the risk of failure.

These methods have been used in specific casesbut have not been widely applied to date. Theyrequire careful study and application if they are toachieve the same level of system reliability asachieved by the defensive scheduling techniques.

Future Trends

A wide variety of techniques for increasingtransfer capability are in use and under developmentin the United States. There are some developmentswhich may have significant long-term effects onsystem operation and transfer capability. Theseinclude developments in high-power semiconductors;ongoing improvements in computing and data process-ing capabilities; and, in the very long term, possiblyeven superconductivity.

In general, developments in these areas will havegradual but increasing impacts. For example, high-power semiconductors are already leading to fast,nonmechanical switching of VAR control devices.Further developments will lead to improved reli-ability and speed for phase shifting transformers andother devices, before direct switching of high-voltage alternating current (HVAC) lines becomespossible.

High-Power Semiconductors

With few exceptions, present-day power systemsuse mechanical control devices. Mechanical circuitbreakers, relays, switches, tap changing transform-ers, and generator controls use moving contacts toclose or open circuits, and are therefore limited in thespeed and number of times they can operate. Theselimitations mean that power systems are neither asresponsive nor as reliable as may often be desired.

Developments in high-power semiconductors toallow electronic rather than mechanical controlpromise to improve the performance of the powersystem in significant and pervasive ways. This is nota sudden revolution, but a continuing trend that hasalready lead to static VAR compensation and HVDCtransmission lines. However, recent research has

24Sec, for example, Scicntifk System, Inc., Optimizatwn of Reactive Volt- Ampere (VAR) Sources in System Planning, EPRI EL-3729, Electric PowerResearch Institute, November 1984; and University of Washington, Reactive Power Management Device Assessment, EPR[ AP-521O, Electric PowerResearch Institute, August 1987.

118 . Electric Power Wheeling and Dealing: Technological Considerations for Increasing Competition

dramatically expanded the prospects for a range offuture uses.25

Controlling Power Flow

There are several possible applications of high-power semiconductors to the general area of control-ling power flows.

26 As described above, there aremany means of relaxing power transfer constraints,and several of them have the opportunity to besignificantly improved by the use of high-powersemiconductors. Static VAR Compensations andRapid Adjustment of Network Impedance (RANI)are both early applications of high-power semi-conductors (thyristors). Both methods rapidly andcontinuously vary the amount of shunt or seriesreactance present to control the total line impedance.Higher power semiconductors will have applica-tions to phase-shifting transformers and variablevoltage tap-changing transformers.

Use of high-power semiconductors as switches, orresettable fuses, to directly reconfigure the actualnetwork will require very high voltage and currentcapabilities. Switches that can handle such highvoltages and currents economically have not yetbeen developed, but the prospect is within sight. Thespeed and control of switches that can turn on and offevery cycle (or more often) without wearing out willmake remedial reliability methods more practicaland economic.

High-power semiconductors are currently used onHVDC power lines to convert AC power to HVDCand back again. The power thyristors used in thisconversion are sufficiently expensive that HVDCpower lines are only practical for lines which arelong enough to bear the high terminal cost or asinterconnections between asynchronous systems.Lower cost and higher capacity semiconductors willmake shorter DC lines economically practicable andallow multiterminal HVDC lines, instead of the twoterminal lines now used. Because the conversionvoltages at both ends of a line can be controlled,HVDC transmission allow essentially completecontrol of network flow.

Power Control of New EnergySources and Storage

In addition to increasing network transfer capabil-ity and reliability, high-power semiconductors willplay a crucial role in power conversion and condition-ing for new power sources and energy storagesystems. Energy sources such as photovoltaics andfuel cells produce relatively low voltage DC power.It is necessary to convert this power to AC in orderto connect the generating unit to the grid. Windpower plants already produce AC power, but themechanical governors needed to regulate frequencyare liable to stress, reduce reliability, and limit themaximum power available from the wind. Solidstate power conversion can be used to let the windturbine generate as much power as possible, convertthe variable frequency power to 60 Hz AC, and alsosupply VARs to compensate for the natural induc-tance of these generators. In addition to generation,various proposed energy storage methods requireconversion of AC to DC power, and back again.Whether as basic as batteries or as esoteric assuperconducting magnetic energy storage, higherpower semiconductors will play a key role in makingthe necessary energy conversion reliable and effi-cient.

One key to increasing semiconductor powercapability is the development of high-purity silicondevices which can handle large currents and voltagewith losses low enough that device temperature isreasonably limited. One such device being devel-oped is the metal oxide semiconductor controlledthyristor (MOS-CT). These devices are light fired, oroptically triggered, by a laser diode either directly orvia fiber optics. Unlike conventional thyristors,these devices can be turned off as well as turned onduring each half cycle. More importantly, thesedevices can combine a microprwessor on the samechip as the power semiconductor to produce integralintelligent control of the high-power switch. Thethyristors may be stacked in parallel or series toincrease current and voltage and this stacking maybe done either on a single chip or by stacking chipstogether.

Zssteitz & As~iates, ~urerl~s and Devicesfor Power Electronic Applicatwns, EPRI AP/EM/EL-S470, Electic Power Researeh Institute, hkch1988.

Z6N.G. Hingoram,“ “High Power Electronics and Flexible AC Transmission System, “ IEEE Power Engineering Review, July 1988, pp. 34.


Development of high-power semiconductors iscurrently underway as a joint effort between industryand government. The Department of Defense isinterested in high-power switching required forStrategic Defense Initiative (SDI) applications, andthe National Aeronautics and Space Administration(NASA) is also interested in civilian space-driveapplications. In October 1987, EPRI opened thePower Electronics Applications Center in Knox-ville, Tennessee. This center and a series of yearlyconferences are intended to promote industrial andconsumer end-use applications of power electronics,rather than basic research. This effort reflects thebelief that recent power electronics developmentshave created opportunities and that applicationshave lagged their potential.

Research is being conducted primarily by GeneralElectric with some work also being performed byWestinghouse. The work is being funded by theArmy (under SDI), Navy, Air Force, NASA, andEPRI. Current funding is about $3.5 million peryear, with about $2.5 million being spent uponhardware/device development and about $1 million(from DARPA) being spent on materials develop-ment. Total government spending is estimated tototal 10 to 40 million dollars, excluding industryinvestment.

Single semiconductor devices rated up 20 kV anda few thousand amps are envisioned within the next10 to 30 years which will meet utility needs with lowlosses, low cost, and fast switching capabilities.Such capabilities make it likely that delivered powerwill flow through high-power semiconductors sev-eral times before reaching the customer.

Improving Computer Capabilities

Continuing trends in data processing and comput-ing capabilities have widespread applications inpower system operations and planning.27 Expertsystems and artificial intelligence continue the trendtowards increasing computer applications in theutility industry and are properly a subset of a widearray of advancing modeling and computer analysiscapabilities. 28

Expert systems essentially codify a subject orsystem of knowledge that is too large or esoteric forthe end user, into software form. The codified rulesthen guide the user interactively by requesting dataand making suggestions. Some of the varied applica-tions for improved computing capabilities includethe following.

System Engineering and Planning

System planners use many specialized computerapplications for such diverse activities as:

. forecasting and modeling loads,

. selecting generation expansion plans,

. selecting transmission capacity expansion plans,

. examining reliability of systems,● investigating system stability, and. real and reactive power flows resulting from

systems changes.

Many of these applications are computationallychallenging and are continuing to benefit from theexpanding abilities of both computer hardware andsoftware.

Control Center Operations

As in system planning, many specialized com-puter applications are used in control centers. Fasterand more powerful hardware and software allowimproved monitoring, analysis, and modeling ofsystem conditions as they change. Areas of applica-tion

●

●

●

●

include:

real-time load flow models allowing optimiza-tion of VAR dispatch and optimization ofpower transfers,improved monitoring of thermal conditions andvoltages on transmission lines allowing dy-namic line ratings,improved response to system emergencies, andreal-time assessment of contingency security.

Transmission and Distribution Automation

The current transmission and distribution systemis largely mechanically controlled and operation isconfined primarily to generation dispatch and emer-

27 SIX for example, “Special Issue on Computers in Power Systcm operations,’” Proceedin~s of the IEEE, December 1987; or Carkm & Fink

Associates, Inc., /nlegrated Power System Analysis Pac&uge ~ Scoping Stud), EPRI EL-4632 VOIS. 1 and 11, Ekmic Power Research lnstilute, July 1986.28univcrsity of Washington, De~eloPme~ of E~efl system av on-Line pow~r ,~~,stem operatio~l Aids, EpR1 EL-5635, Elccwic power Rcsetich

Institute, February 1988. Carnegie-Mellon University, ArriJkial lnrel/igencc Technologies for Power Syslem Operafwns, EPRI EL-4323, ElectricPower Research Institute, January 1986.

120 ● Electric Power Wheeling and Dealing: Technological Consideration for Increasing Competition

gency response. Development of more flexible ACtransmission controls, increased dependence onremedial reliability responses, and distribution automa-tion (including load shedding control) are trendswhich will complicate operation of the transmissionand distribution system. These trends, which shouldbring improved reliability and economy, make useof the advances in communication and data process-ing capabilities.

Power Plant Diagnostics and Monitoring

Advances in power plant diagnostics and controlsystems allow more reliable and efficient operationat the price of complexity. Expert systems can assistthe operator in a range of ways from findingproblems before component failure, to helpingdetermine ramp rates and timing that will optimizethe trade-off between heat rate and plant life.

Operator Training and Simulation

In addition to helping run a generating plant,expert systems can also be used for initial andongoing training of operating personnel. Such sys-tems can be useful in practicing responses toextreme situations which rarely occur, especially innuclear plants. Nuclear plants typically have asimulator operating room for such purposes, butexpert systems can assist in organizing, recognizing,and responding to data instead of just presenting ascenario.

Superconductivity

Superconductors will have a number of obviousand important possible applications in the utilityindustry when further development makes thempractical. These applications include superconductinggenerators, transmission lines, magnetic energystorage, and large inductors.

These possible uses have been recognized for along time and have been researched to a limitedextent using older, conventional low-temperaturemetal superconductors. The recent discovery ofceramics which superconduct above the temperatureof liquid nitrogen (77° K) raises the hope of much

reduced costs and wider applications. The rate andextent of superconductor applications depends uponhow fast and how far it is possible to push threeinterrelated limits to superconductivity: critical tem-perature, magnetic field, and current density. Thecritical temperature is the best known limit anddetermines the extent of thermal losses (whichwould be important in applications such as transmis-sion lines). The current density limit is also impor-tant for power applications, since it is key indetermining size and cost. Cost limitations willprobably be based primarily upon fabrication costsand operating costs.

The most likely actual early use appears to be inenergy storage.29 Economic storage of electricityusing superconducting magnets would be revolu-tionary indeed. The structure and operation of theutility business is built on the fact that electricitycannot be stored. Storage options to date includepumped hydro, compressed air storage, and batter-ies, but are site limited, inefficient, and/or expen-sive. Energy storage would allow supplying electric-ity at a relatively flat, constant rate to meet thechanging daily load curve, which would in turnallow use of efficient, base-load plants only. Reli-ability, system control, and the use of new energysources (such as photovoltaics) would all benefitenormously. 30

The difficulties will include cost, refrigeration,and enormous magnetic stress on brittle ceramicsuperconductors. On a smaller scale, energy storagemay be used to improve system stability andreliability.

Another possible application is in generators,where use of superconductivity would producehigher magnetic fields, smaller size, and lowerlosses. Because of the high efficiencies of conven-tional generators (greater than 98 percent), gainswould be relatively small. Preliminary designs andtesting of prototypes have already been made forsuch generators, using lower temperature metalsuperconductors.31

Nu.s. Conmss, office of ~hnology Assessment, Commercializing High Temperature SuperconductMy, OTA-ITE-388 (Washington, w: U.S.

Government Printing Office, June 1988), p. 161.3~or a ~Wusslon of ~e more gewr~ benefits of storage, see Electric Power Consulting, Inc., DYNAS’TO/?W Computer Modelfor QuanrifYn~

Dynamic Energy Srorage Benefits, EPRI AP-5550, Elecrnc Power Research Institute, December 1987.31 LJ.s. con~ss, office of Technology Assessment, op. cit., footnote 29.


Transmission lines are a possible later use, butnot necessarily as attractive as they might firstappear. Although the line would have no resistance,this would have to be balanced against coolinglosses, total cable and burial costs v. an overheadline, and (presumably) reduced right-of-way re-quirements. More importantly, we have already seenthat pure resistance losses are not the constraininglimits to power transfer, particularly for medium andlong lines. Superconducting cables will not relaxsynchronous stability or voltage support constraints.HVDC circuits would benefit much more fromsuperconducting lines but AC/DC conversion equip-ment costs will still limit use to long lines until theprice of high power semiconductors drops. In eithercase transmission losses typically total in the neigh-borhood of 1 to 3 percent (up to 6 percent for systemswith long lines), so any improvements will not berevolutionary. Reduced siting and environmental

impact of buriedmajor forces evenmarginal.

superconducting lines may beif purely economic benefits are

Some utility applications of superconductivityhave already been designed and tested. Theseinclude:

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●

A 1982 test of superconducting transmissioncable proved a 2,000 MW capacity for a 16inch, liquid helium cooled test cable. This testwas performed by Brookhaven National Labo-ratory in conjunction with EPRI.A superconducting magnet installed and testedby the Bonneville Power Administration foruse in controlling transmission line fluctuationsat its Tacoma substation. This reliability ap-plication of magnetic energy storage is likely toprecede application of large-scale energy stor-age for flattening daily load curves.

Power System Technology

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