DVR

PERFORMANCE EVALUATION OF A FAST DYNAMIC CONTROL SCHEME FOR CAPACITOR- SUPPORTED INTERLINE DYNAMIC VOLTAGE RESTORER

1. INTRODUCTION

1.1. INTRODUCTION

Electronic systems operate properly as long as the supply voltage stays within a

consistent range. There are several types of voltage fluctuations that can cause the systems to

malfunction, including surges and spikes, sags, harmonic distortions, and momentary

disruptions. Among them, voltage sag is the major power-quality problem. It is an unavoidable

brief reduction in the voltage from momentary disturbances, such as lightning strikes and wild

animals, on the power system. Although most of them last for less than half a second, this is

often long enough for many types of loads to drop out. Typical examples include variable

speed drives, motor starter contactors, control relays, and programmable logic controllers.

Such an unplanned stoppage can cause the load to take a long time to restart and can lead to

high cost of lost production.

Dynamic voltage restorer (DVR) is presently one of the most cost-effective and

thorough solutions to mitigate voltage sags by establishing proper quality voltage level for

utility customers. Its function is to inject a voltage in series with the supply and compensate

for the difference between the nominal and sagged supply voltage. The injected voltage is

typically provided by an inverter, which is powered by a dc source, such as batteries,

flywheels, externally powered rectifiers, and capacitors.

Voltage restoration involves determining the amount of energy and the magnitude of

the voltage injected by the DVR. Conventional voltage-restoration technique is based on

injecting a voltage being in-phase with the supply voltage. The injected voltage magnitude will

be the minimum, but the energy injected by the DVR is non minimal.

In order to minimize the required capacity of the dc source, a minimum energy

injection (MEI) concept is proposed in. It is based on maximizing the active power delivered

by the supply mains and the reactive power handled by the DVR during the sag. Determination

of the injected-voltage magnitude is based on a real-time iterative method to minimize the

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active power injection by the DVR. This can then enhance the ride-through capability.

However, the operation of each phase is individually controlled. There is no energy interaction

between the un sagged phase(s) and the sagged phase(s), in order to enhance the voltage

restoration. Moreover, as the computation method is purely based on sinusoidal waveforms,

the implementation is complicated in distribution network with nonlinear load. A sliding

window over one line cycle of the fundamental frequency is proposed to determine the active

and reactive power at the fundamental frequency. The lengthy computation time of the injected

voltage phasor will cause output distortion after voltage sag.

Instead of using an external energy storage device, the methods are taking the active

power for the inverter from the transmission system via a shunt-connected rectifier. The series

inverter by its self has the capability to provide real-series compensation to the line. The

rectifier-based source requires a separate service supply, while the battery-based source

requires regular maintenance and is not environment friendly. No external source is required in

the DVR. The required phase and magnitude of the inverter load-voltage phasor are derived by

considering the energy balance between the supply and the load, but the advantage is offset by

the lengthy digital transformation and inversion of symmetrical components and calculation of

the power consumption. The wave shape of the load voltage will be distorted and will take a

relatively long settling time. As the supply voltage is assumed to be sinusoidal in the

calculation, the load voltage will be affected and distorted with non sinusoidal supply voltage

and load current. A single-phase capacitor-supported DVR can ideally revert the load voltage

to steady state in two switching actions after voltage sags. By extending the concept, an

analog-based three-phase capacitor-supported interline DVR is presented.

1.2. OBJECTIVE

Dynamic voltage restorer features the following operational characteristics:

a) By extending the fast transient control scheme the load voltage can ideally be

restored in two switching actions after a voltage sag. This can assure near-seamless

load voltage and power.

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b) The load voltage balance and quality can be maintained, even if the supply is

unbalanced and/or has harmonic distortion, and the load is nonlinear and/or

unbalanced.

c) As the three inverters share the same dc link and the controller acts on the three

inverters together, the un sagged phase(s) will help restore the sagged phase(s)

during the voltage sag.

The simulation model has been built and tested. The steady-state and dynamic

behaviors of the simulation model are provided.

1.3 RESEARCH METHODOLOGY

1.3.1. Literature Survey:

Brief survey of project work analysis is done in the following areas:

1. Understanding Power Quality Problems, Voltage Sags and Power interruption costs

to industrial and commercial consumers of electricity in power quality issue.

2. Experience with an inverter based dynamic voltage restorer is developed and

Experimental investigation of DVR with dynamic control is analyzed.

3. Open loop and closed loop control methods for the compensation of voltages in

dynamic voltage restorer technique is carried out.

4. Closed-loop state variable controls of dynamic voltage restorers with fast

compensation characteristics are developed.

5. Power quality ensured by dynamic voltage correction is flexible and accurate.

6. Simulation and analysis of series voltage restorer (SVR) for voltage sag relief.

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7. Optimized control strategy for a medium-voltage DVR-Theoretical investigations

and experimental results are obtained.

8. A new approach of DVR control with minimized energy injection is provided in the

system.

9. Interline dynamic voltage restorer: A economical approach for multiline power

quality compensation is analyzed.

10. Design of a capacitor-supported dynamic voltage restorer (DVR) for

unbalanced and distorted loads.

11. Design and practical simulink model of a fast dynamic control scheme for

capacitor-supported dynamic voltage restorers is developed.

12. A comparative study of the boundary control of second-order switching surfaces is

obtained.

13. A survey of distribution system power quality-Preliminary results, Voltage sags &

Voltage swell effects and their mitigation is done.

1.3.2. Organization of Thesis:

1. Chapter 1 directs the basics definitions of power quality and their occurrence will

be discussed.

2. The chapter 2 deals with the different types of power quality problems and their

time of occurrence and how to overcome the power quality issues.

3. The chapter 3 directs us with voltage sag and the improvement methods to

overcome the issue.

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4. Chapter 4 explains the concept related to voltage swell their occurrence and the

methods required to limit the voltage swell.

5. Chapter 5 illustrates about the concept of basic FACTS devices that are used to

regulate the real power and classifies different FACTS devices.

6. Chapter 6 explains about Dynamic Voltage Restorer and components involved in

the designing the simulink model and importance to regulate the uneven conditions

of voltage levels to nominal value with the energy stored in capacitors.

7. Chapter 7 illustrate the inverter concepts, which is used to either convert AC to DC

i.e. acts converter as well as DC to AC i.e. acts as inverter for different voltage sag

and swell condition.

8. Chapter 8 deals with the modeling of the fast dynamic control mechanism of

dynamic voltage restorer, their design procedures, equations and results.

Developing a simulink model and tested it with MAT LAB/SIMULINK library.

Developing the DVR model using the concept of second order switching and PLL.

1.4. SUMMARY

The power quality is the main problem in power systems and the different

methodologies required to overcome the phenomenon of uneven voltage unbalances that result

in the power system. The literature survey that is requires to build the model and the steps that

are required to get analyzed with the concept of fast dynamic control scheme to regulate the

voltage to nominal values.

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2. POWER QUALITY

2.1. INTRODUCTION

The contemporary container crane industry, like many other industry segments, is often

enamored by the bells and whistles, colorful diagnostic displays, high speed performance, and

levels of automation that can be achieved. Although these features and their indirectly related

computer based enhancements are key issues to an efficient terminal operation, we must not

forget the foundation upon which we are building. Power quality is the mortar which bonds the

foundation blocks. Power quality also affects terminal operating economics, crane reliability,

our environment, and initial investment in power distribution systems to support new crane

installations.

To quote the utility company newsletter which accompanied the last monthly issue of

my home utility billing: ‘Using electricity wisely is a good environmental and business

practice which saves you money, reduces emissions from generating plants, and conserves our

natural resources.’ As we are all aware, container crane performance requirements continue to

increase at an astounding rate. Next generation container cranes, already in the bidding

process, will require average power demands of 1500 to 2000 kW – almost double the total

average demand three years ago. The rapid increase in power demand levels, an increase in

container crane population, SCR converter crane drive retrofits and the large AC and DC

drives needed to power and control these cranes will increase awareness of the power quality

issue in the very near future.

2.2. POWER QUALITY PROBLEMS

Power quality problem can be defined as:

“Any power problem that results in failure or misoperation of customer equipment, manifests

itself as an economic burden to the user, or produces negative impacts on the environment.”

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When applied to the container crane industry, the power issues which degrade power

quality include:

1) Power Factor

2) Harmonic Distortion

3) Voltage Transients

4) Voltage Sags or Dips5) Voltage Swells

The AC and DC variable speed drives utilized on board container cranes are significant

contributors to total harmonic current and voltage distortion. Whereas SCR phase control

creates the desirable average power factor, DC SCR drives operate at less than this. In

addition, line notching occurs when SCR’s commutate, creating transient peak recovery

voltages that can be 3 to 4 times the nominal line voltage depending upon the system

impedance and the size of the drives. The frequency and severity of these power system

disturbances varies with the speed of the drive.

Harmonic current injection by AC and DC drives will be highest when the drives are

operating at slow speeds. Power factor will be lowest when DC drives are operating at slow

speeds or during initial acceleration and deceleration periods, increasing to its maximum value

when the SCR’s are phased on to produce rated or base speed. Above base speed, the power

factor essentially remains constant. Unfortunately, container cranes can spend considerable

time at low speeds as the operator attempts to spot and land containers. Poor power factor

places a greater kVA demand burden on the utility or engine-alternator power source. Low

power factor loads can also affect the voltage stability which can ultimately result in

detrimental effects on the life of sensitive electronic equipment or even intermittent

malfunction. Voltage transients created by DC drive SCR line notching, AC drive voltage

chopping, and high frequency harmonic voltages and currents are all significant sources of

noise and disturbance to sensitive electronic equipment.

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It has been our experience that end users often do not associate power quality problems

with Container cranes, either because they are totally unaware of such issues or there was no

economic Consequence if power quality was not addressed. Before the advent of solid-state

power supplies, Power factor was reasonable, and harmonic current injection was minimal.

Not until the crane Population multiplied, power demands per crane increased, and static

power conversion became the way of life, did power quality issues begin to emerge. Even as

harmonic distortion and power Factor issues surfaced, no one was really prepared. Even today,

crane builders and electrical drive System vendors avoid the issue during competitive bidding

for new cranes. Rather than focus on Awareness and understanding of the potential issues, the

power quality issue is intentionally or Unintentionally ignored. Power quality problem

solutions are available. Although the solutions are not free, in most cases, they do represent a

good return on investment. However, if power quality is not specified, it most likely will not

be delivered.

2.3. IMPROVEMENT OF POWER QUALITY

The methods used to improve the power quality are as follows:

1) Power factor correction

2) Harmonic filtering

3) Special line notch filtering

4) Transient voltage surge suppression

5) Proper earthing systems

In most cases, the person specifying and/or buying a container crane may not be fully

aware of the potential power quality issues. If this article accomplishes nothing else, we would

hope to provide that awareness. In many cases, those involved with specification and

procurement of container cranes may not be cognizant of such issues, do not pay the utility

billings, or consider it someone else’s concern. As a result, container crane specifications may

not include definitive power quality criteria such as power factor correction and/or harmonic

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filtering. Also, many of those specifications which do require power quality equipment do not

properly define the criteria.

Early in the process of preparing the crane specification:

a) Consult with the utility company to determine regulatory or contract requirements that

must be satisfied, if any.

b) Consult with the electrical drive suppliers and determine the power quality profiles that

can be expected based on the drive sizes and technologies proposed for the specific

project.

c) Evaluate the economics of power quality correction not only on the present situation,

but consider the impact of future utility deregulation and the future development plans

for the terminal.

2.4. BENEFITS OF POWER QUALITY

Power quality in the container terminal environment impacts the economics of the

terminal operation, affects reliability of the terminal equipment, and affects other consumers

served by the same utility service. Each of these concerns is explored in the following

paragraphs.

2.4.1. Economic Impact:

The economic impact of power quality is the foremost incentive to container terminal

operators. Economic impact can be significant and manifest itself in several ways:

a. Power Factor Penalties:

Many utility companies invoke penalties for low power factor on monthly billings.

There is no industry standard followed by utility companies. Methods of metering and

calculating power factor penalties vary from one utility company to the next. Some utility

companies actually meter kVAR usage and establish a fixed rate times the number of kVAR-

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hours consumed. Other utility companies monitor kVAR demands and calculate power factor.

If the power factor falls below a fixed limit value over a demand period, a penalty is billed in

the form of an adjustment to the peak demand charges.

b. System Losses:

Harmonic currents and low power factor created by nonlinear loads, not only result in

possible power factor penalties, but also increase the power losses in the distribution system.

These losses are not visible as a separate item on your monthly utility billing, but you pay for

them each month. Container cranes are significant contributors to harmonic currents and low

power factor. Based on the typical demands of today’s high speed container cranes, correction

of power factor alone on a typical state of the art quay crane can result in a reduction of system

losses that converts to a 6 to 10% reduction in the monthly utility billing. For most of the

larger terminals, this is a significant annual saving in the cost of operation.

c. Power Service Initial Capital Investments:

The power distribution system design and installation for new terminals, as well as

modification of systems for terminal capacity upgrades, involves high cost, specialized, high

and medium voltage equipment. Transformers, switchgear, feeder cables, cable reel trailing

cables, collector bars, etc. must be sized based on the kVA demand. Thus cost of the

equipment is directly related to the total kVA demand. As the relationship above indicates,

kVA demand is inversely proportional to the overall power factor, i.e. a lower power factor

demands higher kVA for the same kW load. In the absence of power quality corrective

equipment, transformers are larger, switchgear current ratings must be higher, feeder cable

copper sizes are larger, collector system and cable reel cables must be larger, etc.

2.4.2. Equipment Reliability:

Poor power quality can affect machine or equipment reliability and reduce the life of

components. Harmonics, voltage transients, and voltage system sags and swells are all power

quality problems and are all interdependent. Harmonics affect power factor, voltage transients

can induce harmonics, the same phenomena which create harmonic current injection in DC

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SCR variable speed drives are responsible for poor power factor, and dynamically varying

power factor of the same drives can create voltage sags and swells. The effects of harmonic

distortion, harmonic currents, and line notch ringing can be mitigated using specially designed

filters.

2.4.3. Power System Adequacy:

When considering the installation of additional cranes to an existing power distribution

system, a power system analysis should be completed to determine the adequacy of the system

to support additional crane loads. Power quality corrective actions may be dictated due to

inadequacy of existing power distribution systems to which new or relocated cranes are to be

connected. In other words, addition of power quality equipment may render a workable

scenario on an existing power distribution system, which would otherwise be inadequate to

support additional cranes without high risk of problems.

2.5. Environment

No issue might be as important as the effect of power quality on our environment.

Reduction in system losses and lower demands equate to a reduction in the consumption of our

natural nm resources and reduction in power plant emissions. It is our responsibility as

occupants of this planet to encourage conservation of our natural resources and support

measures which improve our air quality.

2.5. Summary

Power quality is the main criteria for the customers and the utility companies. So, there

is a need to maintain the supply voltage profile to nominal values, to achieve voltage stability

basic improvement methods stated above must be implemented to improve the power factor to

reduce the KVA ratings, size and cost. The series and shunt capacitors must be used to

improve the voltage profile and power factor, these methods improves the customer equipment

reliability. To avoid the environmental issues related to fossil fuels operated power plants we

need to opt for the renewable energy sources like wind, hydro, geothermal, solar energies

which are eco friendly.

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3. VOLTAGE SAG

3.1. INTRODUCTION

Voltage sags and momentary power interruptions are probably the most

important PQ problem affecting industrial and large commercial customers. These events are

usually associated with a fault at some location in the supplying power system. Interruptions

occur when the fault is on the circuit supplying the customer. But voltage sags occur even if

the faults happen to be far away from the customer's site. Voltage sags lasting only 4-5 cycles

can cause a wide range of sensitive customer equipment to drop out. To industrial customers,

voltage sag and a momentary interruption are equivalent if both shut their process down.

Fig 3.1.A typical example of voltage sag

3.2. CHARACTERISTICS OF VOLTAGE SAGS:

Voltage sags which can cause event impacts are caused by faults on the power system.

Motor starting also results in voltage sags but the magnitudes are usually not severe enough to

cause equipment misoperation. The one line diagram given below in fig.3.2 can be used to

explain this phenomenon. Consider a customer on the feeder controlled by breaker 1. In the

case of a fault on this feeder, the customer will experience voltage sag during the fault and

an interruption when the breaker opens to clear the fault. For temporary fault, enclosure may

be successful. Anyway, sensitive equipment will almost surely trip during this interruption.

Another kind of likely event would be a fault on one of the feeders from the

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substation or a fault somewhere on the transmission system. In either of these cases, the

customer will experience a voltage sag during the actual period of fault.

Fig.3.2 One line diagram of Power System

As soon as breakers open to clear the fault, normal voltage will be restarted at the

customer's end. The customer voltage during a fault on a parallel feeder circuit that is cleared

quickly by the substation breaker i.e total duration of fault is 150m sec. The voltage during a

fault on a parallel feeder will depend on the distance from the substation to fault point. A fault

close to substation will result in much more significant sag than a fault near the end of feeder.

A single line to ground fault condition results in a much less severe voltage sag than 3-

phase fault Condition due to a delta--star transformer connection at the plant. Transmission

related voltage sags are normally much more consistent than those related to distribution.

Because of large amounts of energy associated with transmission faults, they are cleared as

soon as possible. This normally corresponds to 3-6 cycles, which is the total time for fault

detection and breaker operation Normally customers do not experience an interruption for

transmission fault. Transmission systems are looped or networked, as distinct from radial

distribution systems. If a fault occurs as shown on the 115KV system, the protective

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relaying will sense the fault and breakers A and B will open to clear the fault. While the fault

is on the transmission system, the entire power system, including the distribution system will

experience Voltage sag. In general the magnitude of measured voltage sags at an industrial

plant supplied from a 115 kV system. Most of the voltages were 10-30% below nominal

voltage, and no momentary interrupts were measured at the plant during the monitoring period

(about a year). This is a convenient way to completely characterize the actual or expected

voltage sag conditions at a site. Evaluating the impact of voltage sags at a customer plant

involves estimating the member of voltage sags that can be expected as a function of the

voltage sag magnitude and then comparing this with equipment sensitivity. The estimate

of voltage sag performance are developed by performing short-circuit simulations to determine

the plant voltage as a function of fault location throughout the power system. Total circuit

miles of line exposure that can affect the plant (area of vulnerability) are determined for a

particular sag level.

3.3. VOLTAGE-SAG ANALYSIS

3.3.1. Load Flow:

A load flow representing the existing or modified system is required with an accurate

zero- sequence representation. The machine reactance Xd" or Xd' is also required. The

reactance used is dependent upon the post fault time frame of interest. The machine and zero-

sequence reactance are not required to calculate the voltage sag magnitude.

3.3.2. Voltage Sag Calculation:

Sliding faults which include line-line, line to ground, line to line- to ground and three

phases are applied to all the lines in the load flow. Each line is divided into equal sections and

each section is faulted.

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3.3.3. Voltage Sag Occurrence Calculation:

Based upon the utilities reliability data (the number of times each line section will

experience a fault) and the results of load flow and voltage sag calculations, the number of

voltage sags at the customer site due to remote faults can be calculated. Depending upon the

equipment connection, the voltage sag occurrence rate may be calculated in terms of either

phase or line voltages dependent upon the load connection. For some facilities, both line and

phase voltages may be required. The data thus obtained from load flow, Voltage sag

calculation, and voltage sag occurrence calculation can be sorted and tabulated by sag

magnitude, fault type, location of fault and nominal system voltage at the fault location.

3.3.4. Study of Results of Sag- Analysis:

The results can be tabulated and displayed in many different ways to recognize difficult

aspects. Area of vulnerability can be plotted on a geographical map or one - line diagram.

These plots can be used to target transmission and distribution lines for enhancements in

reliability. Further bar charts, and pie-charts showing the total number of voltage sags with

reference to voltage level at fault point, area/zone of fault, or the fault type can be

developed to help utilities focus on their system improvements. Possible such system

structural changes that can be identified include. Reconnection of a customer from one voltage

level to another, Installation of Ferro-resonant transformers or time delayed under voltage,

drop out relay to facilitate easy ride - through the sag Application of static transfer switch and

energy storage system., Application of fast acting synchronous condensers, Neighborhood

generation capacity addition, Increase service voltage addition through transformer tap

changing, By enhancement of system reliability.

A) Equipment Sensitivity Studies:

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Process controllers can be very sensitive to voltage sags. An electronic

component manufacturer was experiencing problems with large chiller motors tripping

off-line during voltage sag conditions. A 15VA process controller which regulates

water temperature was thought to be causing individual chillers to trip. This controller

was tested using a voltage sag simulator for voltage sags from 0.5-1000 cycles

in duration. The controller was found to be very sensitive to voltage sags tripping at

around 80% of voltage regardless of duration.

B) Chip Testers:

Electronic chip testers are very sensitive to voltage variations, and because of the

complexity involved, often require 30 minutes or more to restart. In addition, the chips

involved in the testing process can be damaged and several days' later internal electronic

circuit boards in the testers may fail. A chip tester consists of a collection of electronic

loads, printers, computers, monitors etc. If any one component of the total package goes

down, the entire testing process is disrupted. The chip testers can be 50KVA or larger in

size.

C) DC Drives:

DC drives are used in many industrial processes, including printing presses and

plastics manufacturing. The plastic extrusion process is one of the common applications

where voltage sag can be particularly important. The extruders melt and grind plastic

pellets into liquid plastic. The liquid plastic may then be blowup into a bag or processed

in some other way before winder winds the plastic into spools. During voltage sag, the

controls to the D.C. Drives and winders may trip. These operations are typically

completely automated and an interruption can cause very expensive cleanup and restarting

requirements. Losses may be of the order of Rs. 15 lakhs / event and a plant fed from a

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distribution system is likely to experience at least one event per month. Extra ders begin to

have problems when the voltage sags to only 88% of normal, which indicates a very high

level of sensitivity. Faults May miles away from the plant will cause voltage sags down to

88% level. Even protecting only the winders and controls does not serve the purpose

always. When they are protected and voltage sag occurs, the controls and winders

continue to work properly. However, the dc drives slow down. For severe voltage dips, the

slowing down are so much that the process is interrupted. Therefore D.C. drives also need

to be helped to ride through all voltage sags.

D) Programmable Logic Controllers:

Their overall sensitivity to voltage sags varies greatly by portions of an overall PLC

system have been found to be very sensitive The remote I/O units have been found to

trip for voltages as high as 90% for a few cycles.

E) Machine Tools:

Robots or complicated machines used in cutting, drilling and metal processing can

be very sensitive to voltage variation. Any variation in voltage can affect the quality of the

part that is being machined. Robots generally need very constant voltage to operate

properly and safely. Any voltage fluctuations especially sag. May cause unsafe operation

of robot. Therefore these types of machines are often set to trip at voltage levels of only

90%.

3.4. SOLUTION TO VALTAGE SAGS

Efforts by utilities and customers can reduce the number and severity of sags.

A. Utility solutions:

Utilities can take two main steps to reduce the detrimental effects of sags

(1) Prevent fault (2) Improve fault clearing methods

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Fault prevention methods include activities like tree trimming, adding line

arrests, washing insulators and installing animal guards. Improved fault clearing

practices include activities like adding line recloses, eliminating fast tripping, adding

loop schemes and modifying feeder design. These may reduce the number and /or

duration of momentary interruptions and voltage sags but faults cannot be eliminated

completely.

B. Customer solutions:

Power conditioning in power utilization helps to:

1. Isolate equipment from high frequency noise and transients.

2. Provide voltage sag ride through capability.

The following are some of the solutions available to provide ride - through capability to

critical loads:

a) Motor generator sets (M-G sets)

b) Uninterruptible Power supply (UPS's)

c) Ferro resonant, constant voltage transformers (CVT's)

d) Magnetic synthesizers

e) Super conducting storage devices (SSD's)

MG sets usually utilize flying wheels for energy storage. They completely decouple

the loads from electric power system Relational energy in the flywheel provides voltage

regulation and voltage support during under voltage conditions. MG sets have relatively high

efficiency and low initial cost. UPS's (Fig.3.3) Utilize batteries to store energy which is

converted to usable form during an outage or voltage sag UPS technology is well established

and there are many UPS configurations to choose.

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Fig.3.3 UPS Configurations

3.5. ECONOMIC EVALUATION

If the less-expensive solutions mentioned in this brief are not effective, the next step is

to evaluate the life-cycle costs and effectiveness of voltage sag mitigation technologies. This

task can be very challenging and tends to be beyond the expertise of most industrial facility

managers. This type of evaluation requires an analysis of the costs of your voltage sag

problems in terms of downtime and lost production, the costs of the devices, and an

Understanding of how the mitigation devices work, including partial solutions. A good place

to start in performing this type of analysis is to ask your utility or a power quality consultant

for assistance. Many utilities offer power quality mitigation services or can refer you to outside

specialists.

3.6. SUMMARY

Voltage sag is the severe power quality issue running at the utility and consumers. In

general due to large number of loads connected to a substation transformers will cause sudden

decrease of voltage profile which lasts for 0.5-1 sec which is enough to drop out large amout

of loads i.e. discontinuity of power supply to consumers. To avoid sag in voltages prior

voltage as well as load flow analysis is carried out. This mitigation has also considered the

economic aspects from the customer point of view.

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4. VOLTAGE SWELL

4.1. INTRODUCTION

A swell is the reverse form of Sag, having an increase in AC Voltage for duration of

0.5 cycles to 1 minute's time. For swells, high-impedance neutral connections, sudden large

load reductions, and a single-phase fault on a three phase system are common sources. Swells

can cause data errors, light flickering, electrical contact degradation, and semiconductor

damage in electronics causing hard server failures. Our power conditioners and UPS Solutions

are common solutions for swells.

It is important to note that, much like sags, swells may not be apparent until results are

seen. Having your power quality devices monitoring and logging your incoming power will

help measure these events.

A) OVER-VOLTAGE:

Over-voltages can be the result of long-term problems that create swells. Think of an

overvoltage as an extended swell. Over-voltages are also common in areas where supply

transformer tap settings are set incorrectly and loads have been reduced. Over-voltage

conditions can create high current draw and cause unnecessary tripping of downstream circuit

breakers, as well as overheating and putting stress on equipment. Since an overvoltage is a

constant swell, the same UPS and Power Conditioners will work for these. Please note

however that if the incoming power is constantly in an overvoltage condition, the utility power

to your facility may need correction as well. The same symptoms apply to the over-voltages

and swells however since the overvoltage is more constant you should expect some excess

heat. This excess heat, especially in data center environments, must be monitored.

B) SWELL CAUSES:

As discussed previously, swells are less common than voltage sags, but also usually

associated with system fault conditions. A swell can occur due to a single line-to ground fault

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on the system, which can also result in a temporary voltage rise on the unfaulted phases. This

is especially true in ungrounded or floating ground delta systems, where the sudden change in

ground reference result in a voltage rise on the ungrounded phases. On an ungrounded system,

the line-to ground voltages on the ungrounded phases will be 1.73 pu during a fault condition.

Close to the substation on a grounded system, there will be no voltage rise on unfaulted phases

because the substation transformer is usually connected delta-wye, providing a low impedance

path for the fault current. Swells can also be generated by sudden load decreases. The abrupt

interruption of current can generate a large voltage, per the formula: v = L di/dt, where L is the

inductance of the line, and di/dt is the change in current flow. Switching on a large capacitor

bank can also cause a swell, though it more often causes an oscillatory transient.

C) MONITORING & TESTING:

As with other technology-driven products, the power quality monitoring products have

rapidly evolved in the last fifteen years. Increased complexity and performance of VLSI

components, particularly microprocessor, digital signal processors, programmable logic, and

analog/digital converters, have allowed the manufacturer's of power quality monitoring

instruments to include more performance in the same size package for the same or reduced

price. Different types of monitoring equipment are available, depending on the user's

knowledge base and requirements.

The four basic categories of power quality monitors (also known as power line

disturbance monitors) are: event indicators, text monitors, solid state recording volt/ammeters,

and graphical monitors. While all of these devices can be used to measure/monitor sags and

swells, the effectiveness of each depends on what information the user wants to gain. Since

sags and swells are relatively slow events (as opposed to microsecond duration transients), the

wide variety of instruments are generally capable of capturing a sag or swell with reasonable

reliability. Event indicators are usually on the lower price end of the market. They indicate to

the user that a sag or swell has occurred through visual means, such as indicator lights or

illuminated bar graphs. Some products will store the worst case amplitudes of such and/or the

21


number of occurrences of the type of event. Most such device do not provide an indication of

the time of occurrence or the duration. The voltage limit detectors may be preset or

programmable, with the accuracy being in the 2-5% range. Textual-based monitors were

actually the first dedicated power quality monitors, produced back in 1976. The function of

these instruments is similar to the event indicators, except the output is in alphanumeric format

Additional information, such as duration and time-of-occupance is often included. Some of

these products allow for the correlation of other information (such as environmental

parameters and system status levels) to assist the user in determining the cause of the event.

Solid state recording volt/ammeters have replaced the older pen-and-ink chart recorders as a

means of providing a graphical history of an event.

These devices typically lack the resolution necessary for monitoring fault-clearing

sags. Sampling techniques range from average of several cycles to samples over 2-30 cycles.

The averaging over several cycles may mask the sag or swell, as well as result in misleading

amplitudes. Sampling over multiple cycles will not properly represent the event either.

Graphical monitors provide the most information about sag or swell. Most graphical monitors

provide a cycle-by-cycle picture of the disturbance, as well as recording minimum/maximum

values, duration, and time-of-occurrence. The three-phase voltage graphs, coupled with graphs

of neutral to ground voltage, phase currents, neutral current (in wye), and ground currents, will

usually provide the user with enough information to determine if the fault occurred upstream

or downstream.

The timing and magnitude information can often identify the source of the fault. For

example, if the phase current levels of the load did not change prior to the voltage sag; the

fault is more likely upstream. If the magnitude of the sag is down to 20% of nominal, it is

likely that the fault was close by. If the sag duration was less than four cycles, it was most

likely a transmission system fault. If the swell waveform is preceded by a oscillatory transient,

it may be the result of a power factor correction capacitor being switched on. Line-to-neutral

voltage sag is often accompanied by a neutral-to-ground voltage swell. The location of the

monitor, power supply wiring, measurement input wiring, and immunization from RFI/EMI is

22


especially critical with the higher performance graphical monitors. The monitor itself must

also be capable of riding through the sag and surviving extended duration swells. The

functionality of the monitor should be thoroughly evaluated in the laboratory, under simulated

disturbances, before placing out in the field. Just because it didn't record it, does not mean it

didn't happen. Unless there is significant information pointing to the cause of the disturbance

before the monitoring begins, it is common practice to begin at the point of common coupling

with the utility service as the initial monitoring point. If the initial monitoring period indicates

that the fault occurred on the utility side of the service transformer, then further monitoring

would not be necessary until attempting to determine the effectiveness of the solution. If the

source of the disturbance is determined to be internal to the facility, the placing multiple

monitors on the various feeds within the facility would most likely produce the optimal answer

in the shortest time period.

Otherwise, the monitor must be moved from circuit to circuit, with particular attention

to circuits powering suspected sources, and the circuits of the susceptible devices. Recent

developments in artificial intelligence tools, especially fuzzy logic, have allow software

vendors to develop products that allow knowledge and reasoning patterns to be stored in the

software program. Further analysis of the event, beyond the IEEE 1159 classifications, is

possible. These include the severity of the event, relative to the type of equipment that would

be effected, and probability factors on the cause of the disturbance. Multiple, successive sags

that return to nominal for an adequate time for the power supply capacitors to recharge may

not be as severe as a longer duration sag of a higher amplitude.

4.2. SOLUTIONS

The first step in reducing the severity of the system sags is to reduce the number of

faults. From the utility side, transmission-line shielding can prevent lighting induced faults. If

tower-footing resistance is high, the surge energy from a lightning stroke is not absorbed

quickly into the ground. Since high tower-footing resistance is an import factor in causing

back flash from static wire to phase wire, steps to reduce such should be taken. The probability

23


of flashover can be reduced by applying surge arresters to divert current to ground. Tree-

trimming programs around distribution lines are becoming more difficult to maintain, with the

continual reductions in personnel and financial constraints in the utility companies. While the

use of underground lines reduces the weather-related causes, there are additional problems

from equipment failures in the underground environment and construction accidents. The

solutions within the facility are varied, depending on the financial risk at stake, the

susceptibility levels and the power requirements of the effected device. Depending on the

transformer configuration, it may be possible to mitigate the problem with a transformer

change. "It is virtually impossible for an SLTG condition on the utility system to cause a

voltage sag below 30% at the customer bus, when the customer is supplied through a delta-

wye or wye-delta transformer."

TABLE 4.1 TRANSFORMER SECONDARY VOLTAGES (PU)

24


For wye-wye and delta-delta connections two phase-to-phase voltages will drop to 58%

of nominal, while the other phase-to-phase is unaffected. However, for delta-wye and wye-

delta connections, one phase-to-phase voltage will be as low as 33% of nominal, while the

other two voltages will be 88% of nominal. It is the circulating fault current in the delta

secondary windings that results in a voltage on each winding. Another possible solution is

through the procurement specification.

If a pre-installation site survey is done, the distribution curve and probability of the

sags and/or swells can be determined. The user then specifies such information in the

equipment procurement specifications.

4.2.1. FERRORESONANT TRANSFORMERS:

Ferro resonant transformers, also called constant-voltage transformers (CVT), can

handle most voltage sags. Ferro resonant transformers can have separate input and output

windings, which can provide voltage transformation and common-mode noise isolation as well

as voltage regulation. While CVTs provide excellent regulation, they have limited overload

capacity and poor efficiency at low loads. At a load of 25% of rating, they require an input of a

minimum of 30% of nominal to maintain a +3/-6% output. At 50% load of rating, they

typically require 46% of nominal input for regulation, which goes to 71% of nominal input at

full load.

4.2.2. MAGNETICALLY CONTROLLED VOLTAGE REGULATORS:

Magnetic synthesizers use transformers, inductors and capacitors to synthesize 3- phase

voltage outputs. Enough energy is stored in the capacitors to ride through one cycle. They use

special autotransformers, with buck-boost windings to control the voltage. The effect of the

buck-boost windings is varied by a control winding with DC current that affects the saturation

of the core. The control-winding current is produced by electronic sensing and control circuits.

The response time is relatively slow (3-10 cycles).

25


4.2.3. TAP SWITCHING TRANSFORMERS:

Electronic tap-switching transformers have the high efficient, low impedance, noise

isolation, and overload capacity of a transformer. These regulators use solid state switches

(thyristors) to change the turn’s ratio on a tapped coil winding. The switching is controlled by

electronic sensing circuits, and can react relatively quickly (1-3 cycles).

Thyristor switching at zero voltage is easier and less costly than at zero current, but can

cause transient voltages in the system, as the current and voltage are only in phase at unity

power factor. Thus, switching at zero-current is preferred. The voltage change is in discrete

steps, but the steps can be small enough so as not to induce additional problems.

4.2.4. STATIC UPS:

A UPS can provide complete isolation from all power line disturbances, in addition to

providing ride-through during an outage. A static UPS consist of a rectifier AC to DC

converter, DC bus with a floating battery, DC to AC inverter, and solid state bypass switch.

The rectifier converts the raw input power to DC, which keeps the floating battery fully

charged and supplies power to the inverter section. The inverters generate 6 or 12 step waves,

pulse-width modulated waves, or a combination of the two, to create a synthetic sine-wave

output.

Inverter output should be a stable, low-distortion sine wave, provided there is adequate

filtering in the output stage. The batteries supply the DC bus voltage when the AC voltage is

reduced. There units range from a few hundred VA to 750kVA or higher. Since they are

constantly running, there is no switch-over time, except when the bypass switch is activities.

The capacity of the battery banks determines the length of ride-through.

26


4.2.5. ROTARY UPS/MOTOR GENERATORS:

Motor generator sets can also provide power conditioning by fully isolating the output

power of the generator from disturbances of the input power (except for sustained outages).

Various configurations are possible, including single shaft synchronous MG, DC motor driven

MG, 3600 rpm induction motor with a flywheel driving a 1800 rpm generator, synchronous

MG with an additional DC machine on same shaft, which powers AC generator with AC fails;

or, variable speed, constant frequency synchronous MG (varies number of poles so that

frequency remains the same. The inertia of an MG set, (especially if supplemented by a

flywheel), can ride-through several seconds of input power interruption. Since the generator

output can be of different voltage and frequency from the motor input, conversion from 60 Hz

to 400 Hz is possible.

4.3. SUMMARY

Swell in voltage results when large kva rating of loads are drop out. Time to time

monitoring and preventive measures must be taken while desingning the electrical equipments

like ferro resonant transformers, magnetically controlled regulators, tab changing switching

transformer, ups, motor generator sets. To overcome the aspect of voltage swell at houses it is

better to pprefer ups configurations which eliminates voltage flickering and saves the data.

27


5. FLEXIBLE AC TRANSMISSION SYSTEMS

5.1 INTRODUCTION

Flexible AC Transmission Systems, called FACTS, got in the recent years a well

known term for higher controllability in power systems by means of power electronic devices.

Several FACTS-devices have been introduced for various applications worldwide. A number

of new types of devices are in the stage of being introduced in practice. In most of the

applications the controllability is used to avoid cost intensive or landscape requiring extensions

of power systems, for instance like upgrades or additions of substations and power lines.

FACTS-devices provide a better adaptation to varying operational conditions and improve the

usage of existing installations.

The basic applications of FACTS-devices are:

1. Power flow control

2. Increase of transmission capability

3. Voltage control

4. Reactive power compensation

5. Stability improvement

6. Power quality improvement

7. Power conditioning

8. Flicker mitigation

9. Interconnection of renewable and distributed generation and storages

Fig.5.1 shows the basic idea of FACTS for transmission systems. The usage of lines for

active power transmission should be ideally up to the thermal limits. Voltage and stability

limits shall be shifted with the means of the several different FACTS devices. It can be seen

that with growing line length, the opportunity for FACTS devices gets more and more

important. The influence of FACTS-devices is achieved through switched or controlled shunt

compensation, series compensation or phase shift control. The devices work electrically as fast

28


current, voltage or impedance controllers. The power electronic allows very short reaction

times down to far below one second.

Devices for high power levels have been made available in converters for high and

even highest voltage levels. Figure 5.1 shows a number of basic devices separated into the

conventional ones and the FACTS-devices. For the FACTS side the taxonomy in terms of

'dynamic' and 'static' needs some explanation. The term 'dynamic' is used to express the fast

controllability of FACTS-devices provided by the power electronics. This is one of the main

differentiation factors from the conventional devices.

The term 'static' means that the devices have no moving parts like mechanical switches

to perform the dynamic controllability. Therefore most of the FACTS-devices can equally be

static and dynamic. The left column in Figure 5.1 contains the conventional devices build out

of fixed or mechanically switch able components like resistance, inductance or capacitance

together with transformers. The FACTS-devices contain these elements as well but use

additional power electronic valves or converters to switch the elements in smaller steps or with

switching patterns within a cycle of the alternating current. The left column of FACTS-devices

uses Thyristor valves or converters. These valves or converters are well known since several

years. They have low losses because of their low switching frequency of once a cycle in the

converters or the usage of the Thyristors to simply bridge impedances in the valves.

The right column of FACTS-devices contains more advanced technology of voltage

source converters based today mainly on Insulated Gate Bipolar Transistors (IGBT) or

Insulated Gate Commutated Thyristors (IGCT). Voltage Source Converters provide a free

controllable voltage in magnitude and phase due to a pulse width modulation of the IGBTs or

IGCTs. High modulation frequencies allow to get low harmonics in the output signal and even

to compensate disturbances coming from the network. The disadvantage is that with an

increasing switching frequency, the losses are increasing as well. Therefore special designs of

the converters are required to compensate this.

29


Fig 5.1 Basic FACTS devices

5.2. CONFIGURATIONS OF FACTS-DEVICES:

5.2.1. SHUNT DEVICES:

The most used FACTS-device is the SVC or the version with Voltage Source

Converter called STATCOM. These shunt devices are operating as reactive power

compensators.

The main applications in transmission, distribution and industrial networks are:

a) Reduction of unwanted reactive power flows and therefore reduced network losses.

b) Keeping of contractual power exchanges with balanced reactive power.

c) Compensation of consumers and improvement of power quality especially with huge

demand fluctuations like industrial machines, metal melting plants, railway or

underground train systems and improvement of static or transient stability.

30


d) Compensation of Thyristor converters e.g. in conventional HVDC lines.

Almost half of the SVC (Fig 5.2) and more than half of the STATCOMs (Fig 5.3) are

used for industrial applications. Industry as well as commercial and domestic groups of users

require power quality. Flickering lamps are no longer accepted, nor are interruptions of

industrial processes due to insufficient power quality. Railway or underground systems with

huge load variations require SVCs or STATCOMs. Applications of the SVC and STATCOMs

systems in transmission systems:

a. To increase active power transfer capacity and transient stability margin

b. To damp power oscillations

c. To achieve effective voltage control

Fig 5.2 SVC design blocks Fig 5.3 STATCOM

5.2.2. SERIES DEVICES:

31


Series devices have been further developed from fixed or mechanically switched

compensations to the Thyristor Controlled Series Compensation (TCSC) or even Voltage

Source Converter based devices.

The main applications are:

a) Reduction of series voltage decline in magnitude and angle over a power line.

b) Reduction of voltage fluctuations within defined limits during changing power

Transmissions and improvement of system damping resp. damping of

oscillations,

c) Limitation of short circuit currents in networks or substations,

5.2.3. SHUNT AND SERIES DEVICES

A) DYNAMIC POWER FLOW CONTROLLER:

A new device in the area of power flow control is the Dynamic Power Flow Controller

(DFC). The DFC is a hybrid device between a Phase Shifting Transformer (PST) and switched

series compensation.

The Dynamic Flow Controller consists of the following components:

1. A standard phase shifting transformer with tap-changer (PST)

2. series-connected Thyristor Switched Capacitors and Reactors (TSC / TSR)

3. A mechanically switched shunt capacitor (MSC)

Based on the system requirements, a DFC might consist of a number of series TSC or

TSR. The mechanically switched shunt capacitor (MSC) will provide voltage support in case

of overload and other conditions. Normally the reactance of reactors and the capacitors are

selected based on a binary basis to result in a desired stepped reactance variation. If a higher

power flow resolution is needed, a reactance equivalent to the half of the smallest one can be

added. The switching of series reactors occurs at zero current to avoid any harmonics.

32


However, in general, the principle of phase-angle control used in TCSC can be applied for a

continuous control as well.

B) UNIFIED POWER FLOW CONTROLLER:

The UPFC is a combination of a static compensator and static series compensation. It

acts as a shunt compensating and a phase shifting device simultaneously. The UPFC consists

of a shunt and a series transformer, which are connected via two voltage source converters

with a common DC-capacitor. The DC-circuit allows the active power exchange between

shunt and series transformer to control the phase shift of the series voltage which provides the

full controllability for voltage and power flow. The series converter needs to be protected with

a Thyristor bridge. Due to the high efforts for the Voltage Source Converters and the

protection, an UPFC is getting quite expensive, which limits the practical applications where

the voltage and power flow control is required simultaneously.

Fig 5.4 UPFC controller

The UPFC has many possible operating modes. In particular, the shunt inverter is

operating in such a way to inject a controllable current, ish into the transmission line.

The shunt inverter can be controlled in two different modes:

a) VAR Control Mode: The reference input is an inductive or capacitive VAR request.

The shunt inverter control translates the VAR reference into a corresponding shunt

current request and adjusts gating of the inverter to establish the desired current.

33


For this mode of control a feedback signal representing the dc bus voltage, Vdc, is

also required.

b) Automatic Voltage Control Mode: The shunt inverter reactive current is

automatically regulated to maintain the transmission line voltage at the point of

connection to a reference value. For this mode of control, voltage feedback signals

are obtained from the sending end bus feeding the shunt coupling transformer.

The series inverter controls the magnitude and angle of the voltage injected in series

with the line to influence the power flow on the line. The actual value of the injected voltage

can be obtained in several ways.

a) Direct Voltage Injection Mode: The reference inputs are directly the magnitude and

phase angle of the series voltage.

b) Phase Angle Shifter Emulation mode: The reference input is phase displacement

between the sending end voltage and the receiving end voltage. Line Impedance

Emulation mode: The reference input is an impedance value to insert in series with

the line impedance

c) Automatic Power Flow Control Mode: The reference inputs are values of P and Q

to maintain on the transmission line despite system changes.

5.3. SUMMARY

Flexible AC transmission systems has the higher controllability and reliability in

regulated power devices used to compensate the real powers and reactive powers that get

distracted when transmitted from sending to receiving end. The FACTS devices require lesser

cost for installation for lesser MVA rated mechanisms and facilitates the fast response and

easier to operate and control. The various FACTS devices used in power system are used

accordingly, such as the series controller are used to inject the currents and shunt controllers

are to inject the voltages and a combination of both are used to facilitate controller either injet

the voltages or currents.

34


6. DYNAMIC VOLTAGE RESTORER

6.1. INTRODUCTION

The major objectives are to increase the capacity utilization of distribution feeders (by

minimizing the rms values of the line currents for a specified power demand), reduce the

losses and improve power quality at the load bus. The major assumption was to neglect the

variations in the source voltages. This essentially implies that the dynamics of the source

voltage is much slower than the load dynamics.

When the fast variations in the source voltage cannot be ignored, these can affect the

performance of critical loads such as (a) semiconductor fabrication plants (b) paper mills (c)

food processing plants and (d) automotive assembly plants.

The most common disturbances in the source voltages are the voltage sags or swells

that can be due to

(i) Disturbances arising in the transmission system

(ii) Adjacent feeder faults and

(iii) Fuse or breaker operation.

Voltage sags of even 10% lasting for 5-10 cycles can result in costly damage in critical

loads. The voltage sags can arise due to symmetrical or unsymmetrical faults. In the latter case,

negative and zero sequence components are also present. Uncompensated nonlinear loads in

the distribution system can cause harmonic components in the supply voltages. To mitigate the

problems caused by poor quality of power supply, series connected compensators are used.

These are called as Dynamic Voltage Restorer (DVR) in the literature as their primary

application is to compensate for voltage sags and swells. Their configuration is similar to that

of SSSC. However, the control techniques are different. Also, a DVR is expected to respond

fast (less than 1/4 cycle) and thus employs PWM converters using IGBT or IGCT devices. The

first DVR entered commercial service on the Duke Power System in U.S.A. in August 1996. It

35


has a rating of 2 MVA with 660 kJ of energy storage and is capable of compensating 50%

voltage sag for a period of 0.5 second (30 cycles). It was installed to protect a highly

automated yarn manufacturing and rug weaving facility. Since then, several DVRs have been

installed to protect microprocessor fabrication plants, paper mills etc. Typically, DVRs are

made of modular design with a module rating of 2 MVA or 5 MVA. They have been installed

in substations of voltage rating from 11 kV to 69 kV. A DVR has to supply energy to the load

during the voltage sags. If a DVR has to supply active power over longer periods, it is

convenient to provide a shunt converter that is connected to the DVR on the DC side. As a

matter of fact one could envisage a combination of DSTATCOM and DVR connected on the

DC side to compensate for both load and supply voltage variations. In this section, we discuss

the application of DVR for fundamental frequency voltage. The voltage source converter is

typically one or more converters connected in series to provide the required voltage rating. The

DVR can inject a (fundamental frequency) voltage in each phase of required magnitude and

phase. The DVR has two operating modes

A. Standby (also termed as short circuit operation (SCO) mode) where the voltage injected

has zero magnitude.

B. Boost (when the DVR injects a required voltage of appropriate magnitude and phase to

restore the prefault load bus voltage).

The power circuit of DVR shown in Fig. 6.1 has four components listed below

6.1.1. Voltage Source Converter (VSC):

This could be a 3 phase - 3 wire VSC or 3 phase - 4 wire VSC. The latter permits the injection

of zero-sequence voltages. Either a conventional two level converter (Graetz bridge) or a three

level converter is used.

6.1.2. Boost or Injection Transformers:

Three single phase transformers are connected in series with the distribution feeder to

couple the VSC (at the lower voltage level) to the higher distribution voltage level. The three

single transformers can be connected with star/open star winding or delta/open star winding.

36


The latter does not permit the injection of the zero sequence voltage. The choice of the

injection transformer winding depends on the connections of the step down transformer that

feeds the load. If ac Y connected transformer (as shown in Fig. 6.1) is used, there is no need to

compensate the zero sequence voltages. However if Y | Y connection with neutral grounding

is used, the zero sequence voltage may have to be compensated. It is essential to avoid the

saturation in the injection transformers.

Fig. 6.1 Power circuit of DVR

6.1.3. Passive Filters:

The passive filters can be placed either on the high voltage side or the converter side of

the boost transformers. The advantages of the converter side filters are (a) the components are

rated at lower voltage and (b) higher order harmonic currents (due to the VSC) do not °own

through the transformer windings. The disadvantages are that the filter inductor causes voltage

drop and phase (angle) shift in the (fundamental component of) voltage injected. This can

affect the control scheme of DVR. The location of the filter o the high voltage side overcomes

the drawbacks (the leakage reactance of the transformer can be used as a filter inductor), but

results in higher ratings of the transformers as high frequency currents can °ow through the

windings.

37


6.1.4. Energy Storage:

This is required to provide active power to the load during deep voltage sags. Lead-

acid batteries, flywheel or SMES can be used for energy storage. It is also possible to provide

the required power on the DC side of the VSC by an auxiliary bridge converter that is fed from

an auxiliary AC supply.

6.2. CONTROL STRATEGY

There are three basic control strategies as follows:

6.2.1. Pre-Sag Compensation:

The supply voltage is continuously tracked and the load voltage is compensated to the

pre-sag condition. This method results in (nearly) undisturbed load voltage, but generally

requires higher rating of the DVR. Before a sag occur, VS = VL = Vo. The voltage sag results

in drop in the magnitude of the supply voltage to VS1. The phase angle of the supply also may

shift see Fig. 6.2. The DVR injects a voltage VC1 such that the load voltage (VL =

VS1 + VC1) remains at Vo (both in magnitude and phase). It is claimed that some loads are

sensitive to phase jumps and it is necessary to compensate for both the phase jumps and the

voltage sags.

Fig 6.2 Pre-Sag Compensation Phasor diagram

6.2.2. In-phase Compensation:

The voltage injected by the DVR is always in phase with the supply voltage regardless

of the load current and the pre-sag voltage (Vo). This control strategy results in the minimum

value of the injected voltage (magnitude). However, the phase of the load voltage is disturbed.

38


For loads which are not sensitive to the phase jumps, this control strategy results in optimum

utilization of the voltage rating of the DVR. The power requirements for the DVR are not zero

for these strategies.

6.2.3. Minimum Energy Compensation:

Neglecting losses, the power requirements of the DVR are zero if the injected voltage

(Vc) is in quadrature with the load current. To raise the voltage at the load bus, the voltage

injected by the DVR is capacitive and Vl leads VS1 (see Fig. 6.3). Fig. 14.3 also shows the in-

phase compensation for comparison. It is to be noted that the current phasor is determined by

the load bus voltage phasor and the power factor of the load.

Fig 6.3 Minimum Energy Compensation Phasor diagram

Implementation of the minimum energy compensation requires the measurement of the

load current phasor in addition to the supply voltage. When VC is in quadrature with the load

current, DVR supplies only reactive power. However, full load voltage compensation is not

possible Unless the supply voltage is above a minimum value that depends on the load power

factor.

When the magnitude of VC is not constrained, the minimum value of VS that still

allows full compensation is where Á is the power factor angle and Vo is the required

magnitude of the Load bus voltage. If the magnitude of the injected voltage is limited (V max

C ), the minimum supply voltage that allows full compensation is given by the expressions

39


figures (6.1) and (6.2). Note that at the minimum source voltage, the current is in phase with

VS for the case (a).

6.3. CONTROL AND PROTECTION

The control and protection of a DVR designed to compensate voltage sags must

consider the following functional requirements.

1. When the supply voltage is normal, the DVR operates in a standby mode with zero

voltage injection. However if the energy storage device (say batteries) is to be charged, then

the DVR can operate in a self- charging control mode.

2. When a voltage sag/swell occurs, the DVR needs to inject three single phase

voltages in synchronism with the supply in a very short time. Each phase of the injected

voltage can be controlled independently in magnitude and phase. However, zero sequence

voltage can be eliminated in situations where it has no effect. The DVR draws active power

from the energy source and supplies this along with the reactive power (required) to the load.

3. If there is a fault on the downstream of the DVR, the converter is by- passed

temporarily using thyristor switches to protect the DVR against over currents. The threshold is

determined by the current ratings of the DVR.

The overall design of DVR must consider the following parameters:

1. Ratings of the load and power factor

2. Voltage rating of the distribution line

3. Maximum single phase sag (in percentage)

4. Maximum three phase sag (in percentage)

5. Duration of the voltage sag (in milliseconds)

6. The voltage time area (this is an indication of the energy requirements)

7. Recovery time for the DC link voltage to 100%

40


8. Over current capability without going into bypass mode.

Typically, a DVR may be designed to protect a sensitive load against 35% of three

phase voltage sags or 50% of the single phase sag. The duration of the sag could be 200 ms.

The DVR can compensate higher voltage sags lasting for shorter durations or allow longer

durations up to 500 ms for smaller voltage sags. The response time could be as small as 1 ms.

6.4. SUMMARY

DVR is used in the power system network mainly to mitigate the power quality issues

related to voltage sags and voltage swells. It contains fast dynamic switching devices like

IGBT to facilitate fast switching action and has a frequency of 10 KHz. The voltage source

inverters converts AC to DC supply to charge the capacitor banks when a swell occurs and

converts DC to AC supply and injects a voltage in series with the line to overcome the effect

of voltage sag. The components used in the DVR model are VSC, boosting transformer,

passive filters to overcome the harmonics which while switching the IGBTs. To reduce the

cost required by the storage devices we can opt for a closed loop control to charge the

capacitor banks from the supply itself irrespective of sagged or swelled conditions.

41


7. INVERETER

7.1. INTROCTION

The main objective of static power converters is to produce an ac output waveform

from a dc power supply. These are the types of waveforms required in adjustable speed drives

(ASDs), uninterruptible power supplies (UPS), static var compensators, active filters, flexible

ac transmission systems (FACTS), and voltage compensators, which are only a few

applications. For sinusoidal ac outputs, the magnitude, frequency, and phase should be

controllable. According to the type of ac output waveform, these topologies can be considered

as voltage source inverters (VSIs), where the independently controlled ac output is a voltage

waveform.

These structures are the most widely used because they naturally behave as voltage

sources as required by many industrial applications, such as adjustable speed drives (ASDs),

which are the most popular application of inverters; see Fig 7.1. Similarly, these topologies

can be found as current source inverters (CSIs), where the independently controlled ac output

is a current waveform. These structures are still widely used in medium-voltage industrial

applications, where high-quality voltage waveforms are required. Static power converters,

specifically inverters, are constructed from power switches and the ac output waveforms are

therefore made up of discrete values. This leads to the generation of waveforms that feature

fast transitions rather than smooth ones. For instance, the ac output voltage produced by the

VSI of a standard ASD is a three-level

Fig 7.1 Inverter DC link

42


7.1.1. Basic designs:

In one simple inverter circuit, DC power is connected to a transformer through the

centre tap of the primary winding. A switch is rapidly switched back and forth to allow current

to flow back to the DC source following two alternate paths through one end of the primary

winding and then the other. The alternation of the direction of current in the primary winding

of the transformer produces alternating current (AC) in the secondary circuit.

The electromechanical version of the switching device includes two stationary contacts

and a spring supported moving contact. The spring holds the movable contact against one of

the stationary contacts and an electromagnet pulls the movable contact to the opposite

stationary contact. The current in the electromagnet is interrupted by the action of the switch

so that the switch continually switches rapidly back and forth. This type of electromechanical

inverter switch, called a vibrator or buzzer, was once used in vacuum tube automobile radios.

A similar mechanism has been used in door bells, buzzers and tattoo. As they became

available with adequate power ratings, transistors and various other types

of semiconductor switches have been incorporated into inverter circuit designs.

Fig 7.2 switching of MOSFET

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7.1.2. Output waveforms:

The switch in the simple inverter described above, when not coupled to an output

transformer, produces a square voltage waveform due to its simple off and on nature as

opposed to the sinusoidal waveform that is the usual waveform of an AC power supply.

Using Fourier analysis, periodic waveforms are represented as the sum of an infinite series of

sine waves. The sine wave that has the same frequency as the original waveform is called the

fundamental component. The other sine waves, called harmonics, that are included in the

series have frequencies that are integral multiples of the fundamental frequency. The quality of

the inverter output waveform can be expressed by using the Fourier analysis data to calculate

the total harmonic distortion (THD) Fig7.3. The total harmonic distortion is the square root of

the sum of the squares of the harmonic voltages divided by the fundamental voltage:

THD = …7.1

Fig 7.3 Total harmonic distortion

7.2. SINGLE-PHASE VOLTAGE SOURCE INVERTER

Single-phase voltage source inverters (VSIs) can be found as half-bridge and full-

bridge topologies. Although the power range they cover is the low one, they are widely used in

power supplies, single-phase UPSs, and currently to form elaborate high-power static power

topologies, such as for instance, the multi cell configurations that are reviewed.The main

features of both approaches are reviewed and presented in the following.

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7.2.1. Types of VSI:

A. Half-Bridge VSI:

The power topology of a half-bridge VSI, where two large capacitors are required to

provide a neutral point N, such that each capacitor maintains a constant voltage=2. Because

the current harmonics injected by the operation of the inverter are low-order harmonics, a set

of large capacitors (C. and Cÿ) is required. It is clear that both switches S. and Sÿ cannot be on

simultaneously because short circuit across the dc link voltage source V i would be produced.

In order to avoid the short circuit across the dc bus and the undefined ac output voltage

condition, the modulating technique should always ensure that at any instant either the top or

the bottom switch of the inverter leg is on.

Fig 7.4 Half-bridge inverter

shows the ideal waveforms associated with the half-bridge inverter shown in Fig 7.4. The

states for the switches S. and Sÿ are defined by the modulating technique, which in this case is

a carrier-based PWM. The Carrier-Based Pulse width Modulation (PWM) Technique: As

mentioned earlier, it is desired that the ac output voltage. Va,N follow a given waveform (e.g.,

sinusoidal) on a continuous basis by properly switching the power valves. The carrier-based

PWM technique fulfils such a requirement as it defines the on and off states of the switches of

one leg of a VSI by comparing a modulating signal Vc (desired ac output voltage) and a

triangular waveform Vd (carrier signal). In practice, when Vc > Vd the switch S. is on and the

switch is off; similarly, when Vc < Vd the switch S. is off and the switch Sÿ is on. A special

case is when the modulating signal Vc is a sinusoidal at frequency fc and amplitudeVc, and the

triangular signal Vd is at frequency fD and amplitude Vd. This is the sinusoidal PWM (SPWM)

45


scheme. In this case, the modulation index ma (also known as the amplitude-modulation ratio)

is defined as

= …7.2

And the normalized carrier frequency mf (also known as the frequency-modulation ratio) is

= …7.3

. VaN is basically a sinusoidal waveform plus harmonics, which features: (a) the

amplitude of the fundamental component of the ac output voltage ^vo1 satisfying the

following expression: = = ...7.4

46


Fig 7.5.Pulse width modulation

will be discussed later); (b) for odd values of the normalized carrier frequency of the

harmonics in the ac output voltage appear at normalized frequencies fh centered around mf

and its multiples, specifically,

h=lmf ± k where l=1,2,3…. …7.5

Where k . 2; 4; 6; . . . for l . 1; 3; 5; . . . ; and k . 1; 3; 5; . . . for l. 2; 4; 6; . . . ; (c) the

amplitude of the ac output voltage harmonics is a function of the modulation index ma and is

independent of the normalized carrier frequency mf form f > 9; (d) the harmonics in the dc

link current (due to the modulation) appear at normalized frequencies fp centered around the

normalized carrier frequency mf and its multiples, specifically,

h=lmf ± k ± 1 where l=1,2,3…. …7.6

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Where k . 2; 4; 6; . . . for l . 1; 3; 5; . . . ; and k . 1; 3; 5; . .for l . 2; 4; 6; . . . . Additional

important issues are: (a) for small values of mf (mf < 21), the carrier signal vD and the

modulating signal Vc should be synchronized to each other(mf integer), which is required to

hold the previous features; if this is not the case, sub harmonics will be present in the ac output

voltage; (b) for large values of mf (mf > 21), the sub harmonics are negligible if an

asynchronous PWM technique is used, however, due to potential very low-order sub

harmonics, its use should be avoided;

Fig.7.6. Modulation regionfinally (c) in the over modulation region (ma > 1) some intersections between the carrier and

the modulating signal are missed, which leads to the generation of low-order harmonics but a

higher fundamental ac output voltage is obtained; unfortunately, the linearity between ma and

^vo1achieved in the linear region does not hold in the over modulation region, moreover, a

saturation effect can be observed.

The PWM technique allows an ac output voltage to be generated that tracks a given

modulating signal. A special case is the SPWM technique (the modulating signal is a

sinusoidal) that provides in the linear region an ac output voltage that varies linearly as a

function of the modulation index and the harmonics are at well-defined frequencies and

amplitudes.

48


These features simplify the design of filtering components. Unfortunately, the

maximum amplitude of the fundamental ac voltage is V i=2 in this operating mode. Higher

voltages are obtained by using the over modulation region (ma > 1); however, low-order

harmonics appear in the ac output voltage.

Fig7.7 Voltage wave forms

B. Square-Wave Modulating Technique:

Both switches S. and Sÿ are on for one-half cycle of the ac output period. This is

equivalent to the SPWM technique with an infinite modulation index ma. Figure 7.5 shows the

following: (a) the normalized ac output voltage harmonics are at frequencies h 3; 5; 7; 9; . . . ,

and for a given dc link voltage; (b) the fundamental ac output voltage features an amplitude

given by

…7.7

and the harmonics feature an amplitude given by equation (7.8)

C. Full-Bridge VSI:

The power topology of a full-bridge VSI. This inverter is similar to the half-bridge

inverter; however, a second leg provides the neutral point to the load. As expected, both

switches S1. and S1ÿ (or S2. and S2ÿ) cannot be on simultaneously because a short circuit

across the dc link voltage source vi would be produced. The undefined condition should be

avoided so as to be always capable of defining the ac output voltage. In order to avoid the

short circuit across the dc bus and the undefined ac output voltage condition, the modulating

technique should ensure that either the top or the bottom switch of each leg is on at any instant.

49


It can be observed that the ac output voltage can take values up to the dc link value vi which is

twice that obtained with half-bridge VSI topologies.

Several modulating techniques have been developed that are applicable to full-bridge

VSIs. Among them are the PWM (bipolar and unipolar) techniques.

Fig 7.8 Full-bridge inverter

D. Bipolar PWM Technique:

States 1 and 2 are used to generate the ac output voltage in this approach. Thus, the ac

output voltage waveform features only two values, which are V i and ÿ*vi. To generate the

states, a carrier-based technique can be used a sine half-bridge configurations where only one

sinusoidal modulating signal has been used. It should be noted that the on state in switch S. in

the half-bridge corresponds to both switches S1. and S2ÿ being in the on state in the full-

bridge configuration.

Similarly, Sÿ in the on state in the half-bridge corresponds to both switches S1ÿ

andS2. being in the on state in the full-bridge configuration. This is called bipolar carrier-based

SPWM. The ac output voltage waveform in a full-bridge VSI is basically a sinusoidal

waveform that features a fundamental component of amplitude υo1 that satisfies the expression

υo1 = υab1 = (υi * ma) …7.9

In the linear region of the modulating technique (ma 1), which is twice that obtained in

the half-bridge VSI. Identical conclusions can be drawn for the frequencies and amplitudes of

the harmonics in the ac output voltage and dc link current, and for operations at smaller and

50


larger values of odd mf(including the over modulation region (ma > 1)), than in half bridge

VSIs, but considering that the maximum ac output voltage is the dc link voltage υi .

Thus, in the over modulation region the fundamental component of amplitude ^vo1

satisfies the expression

…7.10

In contrast to the bipolar approach, the unipolar PWM technique uses the states 1, 2, 3,

and to generate the ac output voltage. Thus, the ac output voltage waveform can

instantaneously take one of three values, namely υi, -υi . The signal υc is used to generate van,

and –υc is used to generate υbN ; -υbN1 = υaN1. On the other hand,

υo1=υaN1-υbN1=2*υaN1 thus υo1=2*υaN1=ma*υi.

This is called unipolar carrier-based PWM. Identical conclusions can be drawn for the

amplitude of the fundamental component and harmonics in the ac output voltage and dc link

current, and for operations at smaller and larger values of mf , (including the over modulation

region (ma > 1)), than in full-bridge VSIs modulated by the bipolar SPWM. However, because

the phase voltages υaN and υbN are identical but 180 out of phase, the output voltage υo =

υaN - υbN = υab will not contain even harmonics. Thus, if mf is taken even, the harmonics in

the ac output voltage appear at normalized odd frequencies fh centered around twice the

normalized carrier frequency mf and its multiples. Specifically, h=lmf±k where l=2,4….

Where k=1,3,5… and the harmonics in the dc link current appear at normalized frequencies fp

centered around twice the normalized carrier frequency mf and its multiples. Specifically,

h=lmf ± k ± 1 where l=2,4…. Where k=1,3,5…This feature is considered to be an advantage

because it allows the use of smaller filtering components to obtain high-quality voltage and

current waveforms while using the same switching frequency as in VSIs modulated by the

bipolar approach.

7.3. APPLICATIONS

51


The various applications of voltage source inverter are as follows

1. DC power source utilization:

An inverter converts the DC electricity from sources such as batteries, solar

panels, or fuel cells to AC electricity. The electricity can be at any required voltage; in

particular it can operate AC equipment designed for mains operation, or rectified to

produce DC at any desired voltage. Grid tie inverters can feed energy back into the

distribution network because they produce alternating current with the same wave

shape and frequency as supplied by the distribution system. They can also switch off

automatically in the event of a blackout.

2. Uninterruptible power supplies:

Inverters convert low frequency main AC power to a higher frequency for use

in induction heating. To do this, AC power is first rectified to provide DC power. The

inverter then changes the DC power to high frequency AC power’.

3. HVDC power transmission & Variable-frequency drives:

With HVDC power transmission, AC power is rectified and high voltage DC

power is transmitted to another location. At the receiving location, an inverter in

a static inverter plant converts the power back to AC. A variable-frequency

drive controls the operating speed of an AC motor by controlling the frequency and

voltage of the power supplied to the motor. An inverter provides the controlled power.

In most cases, the variable-frequency drive includes a rectifier so that DC power for the

inverter can be provided from main AC power. Since an inverter is the key component,

variable-frequency drives are sometimes called inverter drives or just inverters.

4. Electric vehicle drives:

Adjustable speed motor control inverters are currently used to power

the traction motors in some electric and diesel-electric rail vehicles as well as

some battery electric vehicles and hybrid electric highway vehicles such as the Toyota

Prius. Various improvements in inverter technology are being developed specifically 52

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for electric vehicle applications. In vehicles with regenerative braking, the inverter also

takes power from the motor (now acting as a generator) and stores it in the batteries.

5. Air conditioning:

A transformer allows AC power to be converted to any desired voltage, but at

the same frequency. Inverters, plus rectifiers for DC, can be designed to convert from

any voltage, AC or DC, to any other voltage, also AC or DC, at any desired frequency.

The output power can never exceed the input power, but efficiencies can be high, with

a small proportion of the power dissipated as waste heat.

7.4. SUMMARY

The voltage source converter consists of IGBT switches to provide high speed

switching actions as they operate for higher frequency around 10 KHz. The voltage source

inverters converts AC to DC supply to charge the capacitor banks when a swell occurs and

converts DC to AC supply and injects a voltage in series with the line to overcome the effect

of voltage sag. The pulse width modulation technique is used to generate the firing pulses to

turn on the IGBTs in half bridge and full bridge configurations.

8. MODELING OF DYNAMIC VOLTAGE RESTORER

8.1. PRINCIPLES OF OPERATION

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Fig.8.1. Structure of the proposed interlines DVR and its connection to the utility.

Fig. 8.1 shows the circuit schematic of the DVR with three phase transmission system.

It consists of inverters, output LC filters, and injection transformers. The dc side is connected

to a capacitor bank, formed by two capacitors. Each capacitor has the value of . The inverter

shown in Fig. 8.1 is a half bridge configuration. However, the operation is similar in the full-

bridge configuration. The DVR is operated as a controllable voltage source , where n

represents either phase a, b, or c. It is connected between the supply and the load. The

relationship among the supply voltage , the load voltage υd,n and υo,n .

54


υd,n(t) = υs,n(t) – υo,n(t) …8.1

Fig. 8.3 shows the phasor diagrams of υs,n , υo,n , υd,n , and the load current in

voltage sag and unbalanced conditions. Fig. 8.2 shows the control block diagram of proposed

control scheme. The control scheme consists of two main loops. The first control loop, namely

inner loop, is formed in each phase. It is used to generate the gate signals for the switches in

the inverter, so that will follow the DVR output reference . This loop has fast dynamic

response to external disturbances. Its operating principle is based on extending the boundary

control technique with second-order switching surface in. The second loop, namely outer loop,

is used to generate .Based on (Eq. 8.1)

υ*d,n(t) = υs,n(t) – υ*

o,n(t) …8.2

Where is the load-voltage reference and is generated by the phase-lock loop (PLL)

The outer loop is used to regulate the dc-link voltage by adjusting the phase of the

inverter load voltage with respect to the load current. Its bandwidth is set much lower than the

line frequency. The purpose is to attenuate the undesirable signals, which are due to the ac

component on the dc-link voltage and the load current, getting into the loop. Since the inner

and outer loops have different dynamic behaviors, the controller will react differently in the

voltage sags of short and long durations. If the duration of the voltage sag is short, typically

less than 0.5 s, the inner loop will react immediately and maintain the wave shape of the load

voltage. The outer loop is relatively inert during the period. The sagged phase(s) will be

supported purely by the capacitor bank. The capacitor voltage will decrease. After the voltage

sag, the outer loop will start reacting to the decrease in the capacitor voltage. The capacitor

will be charged up from the supply by adjusting the phase angle of the inverter output. If the

duration of the voltage sag is long, the inner loop will keep the wave shape of the load voltage

and the outer loop will regulate the dc-link voltage. Thus, the sagged phase(s) will be

supported by the dc link, while electric energy will also be extracted from the un sagged

phase(s) through the dc link. In the steady-state operation, as the frequency response of the

55


inner loop is very fast, the wave shape of the load voltage can be kept sinusoidal, even if there

are harmonic distortions in the supply voltage and the load current. Moreover, with the

interline energy flow in the outer-loop control; the output quality can be maintained, even if

there is an unbalanced supply voltage. At any time, the amplitude of is fixed because the

load voltage is regulated at the nominal value. Has the same frequency as , and the

phase angle β between and is controlled by the signal shown in Fig. 8.3. The supply

voltages can be expressed as follows:

υs,a (t) = Vsm,a cos(ωt) …8.3

υs,b (t) = Vsm,b cos(ωt - 120˚) …8.4

υs,c (t) = Vsm,c cos(ωt + 120˚) …8.5

Where , , and are the peak values of the supply voltages of the phases a, b, and c,

respectively, and ω is the angular line frequency.

Fig.8.2 Block diagram of the proposed controller

56


Fig.8.3. Steady-state vector diagrams with different sagged conditions. (a) vs,a is changed

from 220 to 120 Vrms . (b) Unbalanced three-phase voltages with vs,a = 210 Vrms ,

vs,b = 190 Vrms, and vs,c = 240 Vrms

57


8.2. REVIEW OF INNER LOOP

The inner-loop control strategy is by using boundary control with second-order

switching surface. A brief review of the inner loop will be given in this section. In one leg of

the three-phase half-bridge inverter, which is shown in Fig. 8.1, as S1,n and S2,n are operated

in anti phase and the output inductor current is continuous, two possible switching modes are

derived, and their state-space equations are shown as follows.

When S1,n is OFF and S2,n is ON

n = xn + un …8.6

υd,n = xn …8.7

When S1,n is ON and S2,n is OFF

n = xn + un ...8.8

υd,n = xn ...8.9

Where xn = [ iL,n υC,n ]T ; un = [ Vdc Vs,n ]T …8.10

58


And RL,n is the fictitious resistance showing the ratio between the load voltage and load

current.

Fig. 8.4(a) and (b) shows the equivalent circuits of the two modes corresponding to Eq.

8.6 to 8.10. By solving Eq. 8.6 to 8.10 with different initial values, families of state-plane

trajectories shown in Fig. 8.5 can be derived. The solid lines are named as the positive

trajectories, while the dotted lines are named as the negative trajectories. The positive

trajectories show the trajectories of the state variables when S1,n is ON and S2,n is OFF. The

negative trajectories show the trajectories of the state variables when S1,n is OFF and S2,n is

ON.

Fig.8.4. Equivalent circuits of one phase in the inverter system. (a) S1,n is OFF and S2,n is

ON. (b) S1,n is ON and S2,n is OFF.

59


Fig.8.5. State-plane trajectories of the inverter

The tangential component of the state-trajectory velocity on the switching surface

determines the rate at which successor points approach or recede from the operating point in

the boundary control of the switches. An ideal switching surface that gives fast dynamics

should follow the only trajectory passing through the operating point. Although can ideally

go to the steady state in two switching actions during a disturbance, its shape is load dependent

and requires sophisticated computation to determine the only positive and negative trajectories

that pass through the operating point. A second-order switching surface , which is close to

around the operating point, is derived. The concept is based on estimating the state

trajectory after a hypothesized switching action. As the switching frequency of the switches is

much higher than the signal frequency, the load current io is relatively constant over a

switching cycle. The gate signals to the switches are determined by the following criteria. For

the sake of simplicity and without loss of generality, υo,n and υd,n with Fig 8.1.

A. Criteria for Switching S1,n OFF and S2,n ON:

60


S1,n and S2,n are originally ON and OFF, respectively. S1,n and S2,n are going to

switch OFF and ON, respectively. The criteria for switching S1,n OFF and S2,n ON are as

follows:

υd,n(t1) υd,n,max - [ 1 / (υDC(t1)/2) + υd,n(t1) ] i2C,n (t1) …8.11

iC,n(t1) 0 …8.12

B. Criteria for Switching S1,n ON and S2,n OFF:

S1,n and S2,n are originally OFF and ON, respectively. S1,n and S2,n are going to

switch ON and OFF, respectively. The criteria for switching S1,n ON and S2,n

OFF are as follows: υd,n(t3) υd,n,min + [ 1 / (υDC(t3)/2) - υd,n(t3) ] i2C,n (t3) …8.13

iC,n(t3) 0 …8.14

Based on (6)–(9) and υd,n,min = υd,n,max = υ*d,n , the general form of σ2 is defined as follows:

σ2 [ iL,n (t), υd,n(t) ] = [υd,n(t) – υ*

d,n(t)] * [ + [sgn[iC,n(t)] υd,n(t)]] + sgn[iC,n(t)]i2C,n(t)

Where k = L/2C …8.15

Fig.8.5 shows the load line and the second-order switching surface (σ2 − 127 V) when

the required output voltage of the DVR is − 127 V. Fig. 8.6 shows the simulated state-plane

trajectory of the DVR for compensating changing from 220 to 130V. The states of the

first and the second switching actions are labeled. The switching surface described in (Eq.8.15)

is implemented with analog circuitry.

8.3. CHARACTERISTICS OF OUTER LOOP

A. Steady-State Characteristics:

61


The function of the outer loop is to regulate υdc at the reference value of υ*dc. By

applying the conservation of energy, the DVR will ideally have zero-average real power flow

at the steady state. Fig.8.6. Simulated state trajectory of the DVR for v(s,n) changing from 220

to 130 V.

Fig.8.6 Simulated state trajectory of the DVR

PS,a + PS,b+ PS,c = Po,a + Po,b + Po,c …8.16

Pa = Pb = Pc = 0 …8.17

Ps,n = υs,n io,n cos (θn - β) …8.18

Po,n = υo,n io,n cos (θn) ; Pn = υd,n io,n cos(φn) …8.19

where , , and are the input, output powers, and power transferred of the each

phase, respectively, is the load current of each phase, is the phase angle between and

, β is the phase angle between and , and is the phase angle between and .

Under supply-voltage interruption, the outer loop will adjust the value of β. The DVR will

generate the required magnitude and phase of in each phase individually. The DVR will

then absorb (deliver) electric energy from (to) the dc link. As the adjustment of β is common

to the three phases, the sagged phase(s) will be supported by the capacitor bank and the un

sagged phase(s). The corresponding equations of are as follows:

62


υo,a (t) = Vom,a cos(ωt - β) …8.20

υo,b (t) = Vom,b cos(ωt - 120˚ - β) …8.21

υo,c (t) = Vom,c cos(ωt + 120˚ - β) …8.22

Where is the peak load voltage of phase n.

Fig. 8.3 shows the steady-state phasor diagrams with one-phase sagged and three-phase

voltage balancing, respectively. The parameters used are based on Tables 8.1-8.3. In Fig.

8.3(a), is reduced to 120 V, while the other phases are at the nominal value of 220 V. By

increasing the value of β, is established by the DVR and can be kept at 220 V. Thus,

part of the energy supplied to phase-a load is supported by phase’s b and c.

TABLE 8.1 SPECIFICATIONS OF THE DVR

TABLE 8.2 PARAMETERS OF THE PLL AND TRANSDUCER GAINS

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TABLE 8.3 COMPONENT VALUES OF THE DVR

In Fig. 8.3(b), all three phases are unbalanced, where = 210 V, = 190 V, and

= 240 V. Again, by adjusting the value of β, , , and are regulated at 220 V. For the

phase transformation between β and

α = - (β – φn- θn) …8.23

Where is the phase difference between and .

Based on Fig. 8.3(a) and by using Equations (8.19) to (8.23), it can be shown that the steady-

state power-transfer equation can be expressed as follows:

64


… 8.24

Detailed derivation of (34) is given in the Appendix. The values of , ,

, and

are different in each phase. Thus, the power flow of each phase inverter is different. β is

controlled by the outer loop in order to achieve power equilibrium in the system, and thus,

satisfy (8.16) and (8.17).

B. Small Signal Modeling:

Fig. 8.7 shows the small-signal model of the outer loop. It consists of the transfer

characteristics of the inner loop, inverter, PLL, and power-flow controller. As the inner loop

has much faster dynamic response than the outer loop, the small-signal transfer function of the

inner loop is unity. The transfer function of the inverter describes the small-signal behaviors

between β and Vdc. The functional blocks of power stage and controller are derived as

follows.

Fig.8.7. Small-signal model of the outer loop

65


1) Relationship Between and : The small-signal dclink voltage to dc-power-transfer

function Tp (s) is as follows:

…8.25

Where is the steady-state values of .

2) Relationship between Power Flow and the Phase of With Respect to io,n in Each Phase:

The small-signal DVR phase- to-power transfer function Tr,n (s) equals

Tr,n(s) = [Δpn(s)/Δφn(s)] = - Vd,n io,n … 8.26

Where Vd is the steady-state values of vd .

3) Phase Transformation Between β and φ in Each Phase: The transfer function Tpt,n (s)

representing the transformation between β and φ is

...8.27

Where Vs , Vo , and B are the steady-state values of vs , vo , and β, respectively.

4) Phase Transformation Between β and vdc in Inverter: The transfer function Tinv (s) of the

inverter is as follows:

...8.28

Detailed derivation of (8.25)–(8.28) is given in the Appendix.

66


Fig.8.8. Circuit schematic of the power-flow controller

5) PLL: The PLL consists of three components, including the phase detector (PD), loop filter

(LF), and the voltage controlled oscillator (VCO).Based on the small-signal model of the PLL

is as follows:

…8.29

Where A = - (Rl / (RcKpd)) ; ξ = 0.5 ( τKpdKlKvco) ; ωn = (KpdKlKvco/τ) …8.30

, , are the constant gains of PD, LF and VCO, respectively.

6) Power-Flow Controller

The function of the power-flow controller is to regulate vdc at the reference voltage

Vdc, which is determined by the voltage ratings of the capacitor and the switches. Charging or

discharging the capacitor Cdc is achieved by adjusting φn in three phases individually. The

regulation action is performed by the error amplifier shown in Fig. 8.8. The transfer function

TC (s) can be shown in (Eq.8.29-8.30), at the bottom of the page.

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8.4. SIMPLIFIED DESIGN PROCEDURES

The values of L, C, and Cdc in the inverter, R1 , R2 , C1 , and C2 in the power-flow

controller are designed as follows.

A. Design of L and C in the Inverter:

The values of L and C in the output filters are determined by considering the maximum

voltage drop across the inductor vL, D at the maximum line current Io, max, angular line

frequency ω, maximum ripple current I ripple , and angular switching frequency ωsw . As

most of the load current is designed to flow through L, the value of L is determined by

considering that its voltage drop vL, D is small at the maximum line current Io, max.

Thus ωLIo,max < υL,D = L < (υL,D/ωIo,max) …8.31a

As the inverter output consists of high-frequency harmonics, the fundamental component of

the ripple current through the filter is designed to be less than I ripple. For the sake of

simplicity In the calculation, the load impedance at the switching frequency is assumed to be

infinite. Thus

& …8.31b

The nominal switching frequency is chosen to be a few hundred times the line

frequency. vL,D is chosen to be 1% of the line voltage, and Iripple is chosen to one half of the

peak of the line current. As shown in Table I, ωsw = 300 ω, vL,D = 2 V, and Iripple = 2.6 A

for the designed prototype. Based on (Eq.8.31) and stated criteria, the values of L and C in the

output filters are determined.

B. Design of Cdc in the Inverter:

The value of Cdc is determined by

...8.32

Where vo, nor and vs, min are nominal value of load voltage and minimum voltage of

supply voltage in specification, respectively, tres is duration of restoration.

C. Design of R1 , R2 , C1 , and C2 in the Power-Flow Controller:

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The pole and zeros are designed as follows:

log ωp = Log ωz2 – (ψ/20) …8.33

Typically, ψ = 20 is chosen, and the ratio of ωz1 and ωz2 is chosen to be at least 100,

in order to avoid overlapping in the two zeros. Therefore

log ωz = log ω2z – 2 …8.34

Based on Eq. (8.35)–(8.37), R1 , R2 , C1 , and C2 are designed by putting a value into one of

them. The practical simulation model of DVR is shown in Fig 8.9. The loop gain TOL(s), it is

based on the specifications and designed component values listed in Tables 8.2 and 8.3. The

bode plot shows operation range, the frequency between ωcross, min, and ωcross, max within

the stable regions.

Based on Eq. (8.30), we have

ωz1 + ωz1 = (1/R2C2) + (1/R2C1) + (1/R1C2) …8.35

ωz1* ωz1 = (1/R1 R2 C1 C2) …8.36

ωP = (C1 + C2)/(C1 C2 R1 ) …8.37

8.5 RESULTS

8.5.1. Result analysis without using DVR

a) When large amount of loads are added or removed at the load centre’s a dip in voltage level will take place and that can abrupt performance of the customer’s equipment. The above mentioned are two major power quality issues that existing in the power systems. Due these effects the total harmonic distortion that is transmitted in the line is 33.54% for one complete cycle of the waveform and the transmitted signals will affect the performance of the line.

b) Result without using DVR

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Fig. 8.9 The voltage and harmonic analysis of wave forms without DVR

8.5.2. Result analysis with DVR

a. Description

When large amount of loads are connected the dip in voltage will rise which can be reduced by using a dynamic voltage restorer. The sag that is reduced by using the DVR is shown in the below figure. The storage element in the DVR i.e. capacitor stored energy will be used to bounce back the sagged phases to nominal values and the total harmonic distortion can be reduce to1.37% from a value 33.54%

b. Result with DVR

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Fig.8.10 The voltage and harmonic analysis of wave forms with DVR

The voltage sag and swell cases that are compensated by the dynamic voltage restorer are

shown in the fig. (11-15).

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72


Fig.8.11 Waveforms at three sagged phases under condition. Vs,a , Vs,b , and Vs,c are

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changed from 220 to 120 Vrms .

Fig.8.12 Waveforms at single phase sagged condition: Vs,a is changed from 220 to 120 Vrms.

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Fig.8.13 Waveforms at single phase swell condition. Vs,a is changed from 220V to 260V Vrms

75


76


Fig.8.14 Waveforms at three swell conditions. (a) Vs,a , Vs,b , and Vs,c are changed from

220V to 260V Vrms.77


8.6. COMPARISION OF RESULTS WITH AND WITHOUT DVR

The effect of voltage sag and swell has a greater impact on the power that is transmitted from sending end to recieveing end. The real power that is transmitted gets varied due to the addition of more non-linear loads or sudden shutdown of large rating loads. The voltage sag and swell cases cause customer appliances to get abrupt by reducing their life span. The power quality at the load centre are mostly electonic switched ones, so due to the switching actions of these non-linear loads the level of harmonics injected into the supply will be more and the cost of the capacitors required to compensate the harmonics will increase there by increasing the overall cost.

When an automatically controlled regulators are used then the chances of overcoming the voltage sag or swell cases can be aoided. The dynamic voltage restorer operates at desired levels to regulate voltages as well as eliminates the harmonics. The entire mechanism uses capacitor banks to restore the voltage to nominal values and the energy stored in the capacitor can be charged from the supply it self. Under normal operated conditions the MOSFET switches which operate at 10 KHz frequency nor inject nor charge the capacitor. When a voltage sag occur the stored enrgy restores the voltage to nominal value by discharging the energy. When a voltage swell occur in any of the phases, the capacitor uses the swelled phase to charge its energy to bounce to normal full capacity.

The total harmonic content that will be available when a power system is operated under sagged condition will be 33.54% i.e without using the DVR. When a DVR is used in to back up the sagged voltage to nominal value it also eliminates the total harmonic content to a lower value of 1.37%.

CONCLUSION & FUTURE SCOPE78


10.1. CONCLUSION

A control scheme for three-phase capacitor-supported interline DVR has been

presented. By integrating a recently proposed boundary-control method with second-order

switching surface (inner loop), the dynamic response has been minimized to two switching

actions. The voltage sag, swell, and voltage harmonic distortion have been compensated by

this DVR. Moreover, by using series bidirectional inverter as DVR, capacitor banks are used

to support the dc link and the sagged phase(s) could be supported by the un sagged phase(s).

Long-duration voltage sags and swell and three-phase voltage unbalance could be overcome

by the proposed power-flow controller (outer loop). The MOSFET switches that are used in

the voltage source converter operate at a variable frequency range i.e. 10 KHz. This has

facilitated for the fast switching action and provides dynamic response in turn maintains the

stability of the system. The designed model of dynamic voltage restorer also reduces the

harmonic percentages that resulted due to MOSFET switching actions. The gating pulse to the

switch is generated by the pulse width modulation technique. The method has been verified

with a 1.5-kVA system. The performances of the DVR have been demonstrated and evaluated

with different power-quality disturbances. Experimental measurements are favorably verified

with theoretical results. The power quality problem issues related to voltage sag and swell as

well as total harmonic distortion can be compensated effectively by using the dynamic voltage

restorer.

10.2. FUSTURE SCOPE

The Dynamic voltage restorer technology is used to reduce uneven voltage unbalances

and harmonic distortion levels. These are extensively used in small rating power supplies to

large rating machines for successful operation and desired response. There is consistent new

development of additional circuit configurations of dynamic voltage restorer to provide cost-

effective and improved performance of power system.

79


Some of new concepts, such as reducing the rating of passive filter element in shunt

and series filters and eliminating the drawbacks of passive filters such as resonance and fixed

compensation for improved performance of filters.

The artificial neural networks and fuzzy logic control methodology are yet getting to

operate the DVR operated power systems to regulate the power more efficiently and reduces

the on state power loss. The harmonic levels by using ANNFL can be reduced much to lower

values compared with any other controllers.

Use of an improved control algorithm reduces the requirements of sensors and provides

the fast response of the system. Development of dedicated application specific integrated

circuits (ASICs) and new series of DSPs and microcontrollers is providing cost-effective and

compact size in filters. Improved switching devices with reduced conduction losses and high

permissible switching frequency and better gating requirement will improve the power quality

issues.

New development in magnetic such as filter magnetic materials will reduce the losses

and size of passive filter elements such as transformers, inductors, etc., and thus cost and

weight of dynamic voltage restorer.

80


REFERENCES

[1] M. Bollen, Understanding Power Quality Problems, Voltage Sags and Interruptions. New

York: IEEE Press, 2000.

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Nov. 1997.

[3] N. Woodley, L. Morgan, and A. Sundaram, “Experience with an inverterbased dynamic

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[5] S. Lee, H. Kim, and S. K. Sul, “A novel control method for the compensation voltages in

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[30] H. Awad, J. Svensson, and M Bollen, “Mitigation of unbalanced voltage dips using static

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[32] M. Ordonez, J. E. Quaicoe, and M. T. Iqbal, “Advanced boundary control of inverters

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Electron., vol. 23, no. 6, pp. 2915–2930, Nov. 2008.

[33] S. Chen, Y. M. Lai, S. C. Tan, and C. K. Tse, “Boundary control with ripple-derived

switching surface for DC–AC inverters,” IEEE Trans. Power Electron., vol. 24, no. 12, pp.

2873–2885, Dec. 2009.

[34] M. Rashid, Power Electronics, Circuits, Devices, and Applications, 3rd ed. Englewood

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[35] C. Ho, K. Au, and H. Chung, “Digital implementation of boundary control with second-

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