MEDIUM-VOLTAGE HIGH-SPEED

SOURCE-TRANSFER SWITCHING SYSTEMS-

A NEW TOOL TO SOLVE POWER QUALITY PROBLEMS

Carl Shapiro

ComEd

Maywood, IL

Ray O’Leary

S&C Electric Company

Chicago, IL

Janos Rajda

Inverpower Controls Limited

Burlington, ON

Presented at the

POWER DELIVERY/POWER-GEN Intenational ’96

Conference Program

Orange county Convention Center

Orlando, Florida

December 6, 1996

621-T65

Medium-Voltage High-Speed Static-Transfer Switching SystemsA New Tool to Solve Power Quality Problems

AbstractThe use of two incoming power sources in a primary-selectivearrangement has been the most commonly applied means forensuring continuity of electrical service in industrial,commercial, and government facilities with critical processes.The sources are often connected to switchgear incorporatingcontrols which can recognize loss of the preferred power sourceand then automatically transfer to the alternate source, thusmaintaining a highly reliable source of power. Such sourcetransfer switching generally requires anywhere from 0.3 to 3 ormore seconds to complete. However, as processes and processcontrols have become increasingly sensitive to not only loss ofthe power source but also to fluctuations in the voltagesupplied, the aforementioned method of source-transfer can betoo slow to achieve the level of power quality required. Thispaper will review a medium-voltage High-Speed Source-Transfer Switching System (HSSTSS) and its ability to solvepower quality problems at a manufacturing plant in theMidwest which had experienced numerous instances of voltagesags and loss of electrical service. These disturbances createdsignificant disruptions to the manufacturing process, resultingin substantially increased operating costs due to productionlosses, as well as costly equipment clean-up and restarting.

In evaluating the problem, it was determined that no commonlyavailable automatic source-transfer equipment would switchquickly enough to allow the manufacturing process to continuewithout shutting down. Various options were reviewed and thedecision was made to procure and install a 15 kV High-SpeedSource-Transfer Switching System. This arrangement consistsof a solid state transfer switch, bypass and isolation switchgear,protection, sensing and power supply – all in a completelyintegrated package. The system accomplishes source-transferin response to voltage fluctuations such as outages, sags, andswells in as little as 4 milliseconds, and provides power to theplant well within the limits of the CBEMA/ITIC curves. Thispaper will describe in detail the problems encountered, thesolutions evaluated, and the performance to date of the newswitching system.

Process disruptions and costsIn the never-ending quest for greater efficiency, manyindustries have developed large complex continuousmanufacturing processes. This trend in manufacturing isubiquitous across industries as diverse as established “smoke-stack” industries, manufacturing goods such as wire and glass,and new “high-tech” industries manufacturing goods such asintegrated circuits. Common threads amongst these industriesinclude a highly automated process which is capable ofoperating 24 hours a day, 365 days a year … and a propensityto come to a screeching halt with power disturbances, which,decades ago, were considered minor. Because of the significantcost of the product which is “in process” at any given time as

well as the production lost during clean-up and restarting, theseshutdowns can be very costly.

As these manufacturing systems evolved, means to mitigate themost costly effects of power system disturbances have beenbrought to bear on the problem. For example, products such asUPSs and motor-generator sets have been employed. As powerelectronic devices and controls develop, new products to solvethese power quality problems are being introduced. Themedium-voltage High-Speed Source-Transfer Switching Systemis the newest of these products, and offers new performance,not available before.

Breakdown of voltage disturbancesVoltage sags and swells are the most common form ofdisturbance, together accounting for almost 90% of the voltagedisturbances seen on typical distribution systems.1 Voltageswells are somewhat less frequent than sags and, if moderate(about 20%), generally do not cause problems with powerusers’ loads. However, sags on the order of 10% and greaterare fairly common and do cause problems. The most commonvoltage sag event is caused by adjacent-feeder faults. Thereason is tied to typical system construction. Most distributionsubstations are built around one to several transformers. Oftenthe transformers are banked and support many feeders off acommon bus. A fault on any of the feeders causes a voltage sagnot only on the faulted feeder, but on all the adjacent feedersfed from the same bus. Consider a typical station bussupporting 10 feeders, it stands to reason that, if all feeders areequally likely to experience a fault, an adjacent-feeder faultwould be nine times more prevalent than a fault on a givenfeeder. Further, if the typical frequency of faults on a feeder isonce a year (10 faults per year for the 10 feeder bus section),the frequency of voltage sags approaches once a month.

Details of sagsThe magnitude of a voltage sag is determined by the location ofthe fault causing the sag. Faults on the line feeding a facilityfall into two categories: those between the facility and thesubstation, (which will drive the voltage at the facility to zero);and those downstream of the facility (which will result in deepsags to near zero voltage). But faults on the line feeding thefacility are much less frequent than adjacent-feeder faults.

Adjacent-faults close to the substation will drive the busvoltage closer to zero than faults which are remote from thesubstation. The reason for this can be readily seen in Figures 1and 2. The bus voltage during a fault is approximatelydetermined by the voltage divider set up between the source

1 National Power Laboratory Study 1990 to present

impedance (mostly transformer impedance) and the lineimpedance between the substation bus and the fault. If thefault location is farther from the substation, there is more lineimpedance and the “voltage divider” yields a higher busvoltage.

Adjacent-feeder fault Z-source

Z line(bus-to-fault)

Substation Bus

Feeders

Figure 1 – Typical feeder layout with the leftmost feederfaulted

Z source.15 kV

..3 12 kAV bus Z line

Z lineZ line Z source

0 1 2 3 4 5 60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

V bus Z line

Z line

Figure 2 – Per unit bus voltage for a line fault as a functionof the line impedance between the bus and the fault

Figure 2 also illustrates the fact that most of the adjacent-feederfaults result in moderate sags. These results are generallyconsistent with measured performance of utility systems. 2

A power quality solution for a plasticsmanufacturerA manufacturer, in the Chicago area, was faced with productionlosses in the order of a several hundred thousand dollars peryear due to “minor” power disturbances. The manufacturer usesa highly automated continuous process to produce plastic filmused to package consumer products. The film is made to aspecific thickness with very tight tolerances. During voltagechanges of sufficient magnitude and duration, the motors usedto move the product change speed at different rates. When thishappens, the thickness tolerance is lost and the manufacturer

2 EPRI Distribution Power Quality (DPQ) Project

has product loss and significant downtime for cleanup andstartup.

The manufacturer had begun to consider their own onsitegeneration. But, before taking on responsibility for powergeneration they asked their power provider, ComEd, to helpevaluate alternatives wich would allow them to reduce powerdisturbance related losses, yet remain on the ComEd system.ComEd committed to a program aimed at customer retentionand a search for a cost effective solution that improves powerquality was undertaken.

Because, the majority of the plant’s voltage disturbanceproblems were caused by voltage sags, and in particular, sagscaused by adjacent-feeder faults, the solutions evaluated had toprimarily address adjacent-feeder faults, and secondarilyaddress plant-feeder faults.

The search involved investigation of the following devices:

1. Superconductive Magnetic Energy Storage2. Rotary UPS3. Motor Generator Set with Flywheel4. Distribution Static Condenser5. Dynamic Voltage Restorer6. Static Series Voltage Regulator7. Low Voltage Static Transfer Switch8. Medium Voltage Static Transfer Switch

The prices and operating voltages for the evaluated solutionsare shown in Table 1.

TABLE 1 – Summary of Power Quality ImprovementSolutions

Device OperatingVoltage

Installed Cost

SuperconductiveMagnetic EnergyStorage

480 V $5,000,000

Rotary UPS 480 V $2,500,000

Motor Generator Setwith Flywheel

480 V $1,300,000

Distribution StaticCondenser

12.5 kV $2,000,000

Dynamic VoltageRestorer

12.5 kV $2,000,000

Static Series VoltageRegulator

480 V $1,450,000

Low Voltage StaticTransfer Switch

480 V $1,100,000

Medium Voltage StaticTransfer Switch

12.5 kV $1,100,000

The plant’s process lines are served from three outdoortransformers, each at a secondary voltage of 480/277 volts.

This means that each device operating at 480 volts wouldrequire one unit at each service point. Two of the servicepoints are in a very congested area on the east side of the plant(see Figure 3) and locating additional equipment in this areawould be difficult. The 480 volt solution also requiredrelocation of the service entrance cables. All 480 volt units,except for the Micro-SMES, require a weather proof enclosureor an indoor location. This causes an additional problembecause there is no indoor space available for the equipment.The lack of space effectively rules out the 480 volt solution.

Plastics Manufacturer

cable pole

N

transformer

Figure 3 – Site layout

The medium voltage static transfer switch addresses the powerquality problem in the most cost effective solution. It providesthe high speed source transfer which the manufacturer needsfor maintaining product tolerance. It provides power to theprocess in the event of a long term 12 kV source outage orreclosing cycle operation that the other medium voltagesolutions do not allow. It also allows the process load to betransferred to the bypass gear for any maintenance on the staticunits and the bypass can function as a standard, lower speedsource transfer. Finally, the training time of the operatingpersonnel is reduced because the bypass is composed ofswitchgear units that are currently used on the distributionsystem.

Application requirementsThe HSSTSS solution obviously requires that two mediumvoltage sources be in position to serve the customer. Althoughthe plant was served by a single 12 kV over-head line, anotherline was in the neighborhood and was easily extended to theplant site to serve as the second source. The two sources mustbe relatively independent; specifically, most conditions which

cause a sag on one feeder, should not affect the other feeder. Inthe case of ComEd’s system, sags on the transmission systemare likely to sag both feeders brought to the customer’s site, buttransmission sags are much less frequent than distributionsystem sags, and the HSSTSS solution remains viable since itis effective for a large majority of the sags experienced by thecustomer.

Power quality measurements indicate that the subject plant hashad problems with voltages which are within the CBEMA/ITICcurves. Specifically, a sag of 10% for a 0.5 second durationcaused some plant equipment to shutdown. The HSSTSS was,therefore, set to provide the more stringent needs of the load.As very moderate sags (such as 10%) are found to be aproblem, one must be careful that the normal day-to-dayvoltage variations, especially under system low-voltage orbrownout conditions, will not bring both sources to a problemlevel.

Finally the voltage regulation at the customer’s site must besufficiently good that when the load is moved from one sourceto the other the regulation does not significantly change sourcevoltages. The worst manifestation of a regulation problemwould be a “hunting” situation, wherein a transfer for a minorsag might cause the alternate source to sag, also in a minorway. With the release of load from the preferred source, thevoltage might increase to the level where it appears to be betterthan the alternate source, and a reverse transfer back to thepreferred source might take place; a series of forward andreverse transfers could result. Dead-bands in the control willavoid this problem, however they must be carefully coordinatedwith the regulation as well as the minimum sag for which theHSSTSS is expected to react. The effect of voltage regulationcan be mitigated by utilizing a split-bus HSSTSS configuration,thereby reducing the amount of load transferred. In the subjectHSSTSS, the split-bus option was used.

The primary functional requirement of the HSSTSS as shownin Figure 4 is to switch the load currents from one feeder toanother through the high speed operation of two source staticswitches, SS1 and SS2 and a bus-tie static switch SS3. Thethree static switches, SS1 and SS2 and SS3, along with theInverpower control are collectively referred to as the statictransfer switch (STS) and form the heart on the HSSTSS. Thebypass and isolation switches (BP 1, 2, & 3 and IS A, B, C, &D in Figure 4) as well the S&C Micro-AT ® control form thebasis of the cooperating switchgear which allows full flexibilityin operation and maintenance of the HSSTSS, includingseamless power continuity to the load, even in the event of aproblem with the STS.

ISA ISD

SS1 SS2SS3

ISB ISC

FEEDERW7418

FEEDERW748

BP1 BP3 BP2

STATIC SWITCH

CONTROLLER

S&CMICRO-AT

Figure 4 – One-line diagram of the ComEd HSSTSS

The high speed STS operation allows the plant load to beessentially free from power quality problems, voltages,especially sags of the distribution system encountered at one ofthe adjacent feeders. Both feeder voltages are continuouslymeasured and appropriate transfers are carried out based uponthe adjustable settings of sag limits.

The operating modes of the STS are:

Start/Stop Sequencing

The high speed transfer feature of the STS may be placed intoor taken out of service through START or STOP sequence.With appropriate safety interlocks, mechanical bypass andisolation with which the STS is closely integrated, willcontinue to feed the load, and provide conventional low speedload transfer capability. The transition from mechanical sourcetransfer to static source transfer is seamless in both directions.Upon the loss of both sources the HSSTSS’s cold restart (boot-strap) is fully automatic and seamless.

Forward Transfer

The forward transfer operation is one in which either sourceSS1 or SS2 switch opens and bus-tie switch, SS3 closes. TheSTS controls constantly monitor the voltage conditions of bothsources and initiates a forward transfer whenever one or moreof the phase voltages is outside a “prime” voltage band eitherin the undervoltage or overvoltage direction.

When a forward transfer is initiated by the complete loss ofeither source, if, after a user adjustable delay time, the sourcedoes not recover, the Inverpower STS control will automaticallyclose the appropriate bypass switches, open the isolationswitches (to minimize the back feed from the energized to theunenergized source) and pass control to the S&C Micro-AT ®.Upon restoration of the source, the STS will automaticallyreturn to service.

Reverse Transfer

When both sources have returned to within the “prime return"voltage band for a adjustable time period the STS control willinitiate an automatic reverse transfer, wherein the closed bus-tie switch, SS3 opens and the open static switch, SS1 or SS2associated with previously faulty source closes.

Better-of- Two Transfers

The STS control allows the user to define multiple voltagebands into which the sources may be classified. If both sourcesare outside of their "prime” band, the STS controlautomatically transfers the bus-tied load to the source whichlies in the more favorable band.

Emergency Stop

The loss of cooling air or steady state overload may lead to anSCR stack over-temperature situation. When an over-temperature signal is received by the STS control, it willperform an emergency STOP sequence: close the appropriatebypass switches, open the isolation switches and pass control tothe S&C Micro-AT®. This will allow STS maintenance underisolated conditions, while also insuring power continuity to theload.

Additional features of the HSSTSS system are:

Environmental design

Special consideration is paid to the enclosure as well as theheating & cooling systems design to ensure reliable operationunder:

• Rain, sleet, snow, outdoor conditions• -40°C to 50°C ambient temperature range• All ranges of relative humidity• Unusual seismic, vibration or tilting conditions

SCR Redundancy

Each SCR stack is able to accommodate the failure of a singleSCR without loosing the functionality. After a single SCRfailure, the STS will issue an alarm; a second SCR failure willinitiate an emergency stop, as described above.

Control Power Supply

The control power supply is ensured from both sources and asmall UPS. Loss of control power will initiate the emergencySTOP without jeopardizing the continuity of the load service.

Electromagnetic Compatibility

The Inverpower STS controls and the S&C Micro-AT® controlmeet ANSI/IEEE C37.90.1 surge voltage and fast transientwithstand capability design tests. The test waves were appliedin both common mode (between line and ground) anddifferential mode (between line inputs) at each control I/O andcontrol power line.

System Diagnostics

The status of the three static switches are continuouslymonitored and key system data are stored in an event log ateach transfer. This information is useful in maintenance andverification of transfer performance.

Performance to dateThe first commercial installation of a HSSTSS went intoservice on August 14, 1996. Two of the customer’s three 2,500

kVA transformers were initially cut-over. Cut-over of the thirdtransformer was delayed until the production line fed by thattransformer could be shutdown. On August 15, an adjacent-feeder fault, involving a tree, caused the transfer of load fromthe left (sagged) source to the right (healthy) source. The twoproduction lines protected by HSSTSS rode through thedisturbance; the third production line not yet protected byHSSTSS shutdown. Interestingly, prior history shows that thetwo production lines protected by HSSTSS had been moresensitive to sags than the third one which shutdown.