UPFC (2)

TITLE

1

CERTIFICATE

2

.

3

ACKNOWLEDGMENTS

4

ABSTRACT

Analysis of Unified Power Flow Controller (UPFC)

The focus of this project is a FACTS device known as the Unified Power Flow

Controller (UPFC). With its unique capability to control simultaneously real and reactive

power flows on a transmission line as well as to regulate voltage at the bus where it is

connected, this device creates a tremendous quality impact on power system stability.

These features become even more significant knowing that the UPFC can allow loading of

the transmission lines close to their thermal limits, forcing the power to flow through the

desired paths. This will give the power system operators much needed flexibility in order to

satisfy the demands that the deregulated power system will impose.

The most cost-effective way to estimate the effect the UPFC has on a specific power

system operation is to simulate that system together with the UPFC by using MATLAB.

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TABLE OF CONTENTS

Title Page……………………………………………………………………………………… iCertificate……………………………………………………………………………………… iiAcknowledgement……………………………………………………………………………...iiiAbstract…………………………………………………………………………………………ivTable of contents………………………………………………………………………………..v

1. Introduction2. Literature Survey

2.1 Power System Operation 2.2 Power Flow Control 2.3 Power System Limitations 2.4 Power Controlling Devices 2.5 Flexible Alternating Current Transmission Systems

3. UPFC Basic Operation And characteristics 3.1 Basics of Voltage Source Converters and Pulse Width Modulation Technique 3.2 UPFC Description 3.3 Operating Modes of UPFC

4. Case Study 4.1 Modeling Of UPFC On A Transmission System 4.2 Explanation Of Single Line Diagram 4.3 Model Block of Single Line Diagram 4.4 Power Flow Control with the UPFC 4.5 Simulations

5. Conclusion

References

7

Chapter 1

INTRODUCTION

The power system is an interconnection of generating units to load centers through high

voltage electric transmission lines and in general is mechanically controlled. It can be

divided into three subsystems: generation, transmission and distribution subsystems. Until

recently all three subsystems were under supervision of one body within a certain

geographical area providing power at regulated rates. In order to provide cheaper electricity

the deregulation of power system, which will produce separate generation, transmission and

distribution companies, is already being performed. At the same time electric power demand

continues to grow and also building of the new generating units and transmission circuits is

becoming more difficult because of economic and environmental reasons. Therefore, power

utilities are forced to relay on utilization of existing generating units and to load existing

transmission lines close to their thermal limits. However, stability has to be maintained at all

times. Hence, in order to operate power system effectively, without reduction in the system

security and quality of supply, even in the case of contingency conditions such as loss of

transmission lines and/or generating units, which occur frequently, and will most probably

occur at a higher frequency under deregulation, a new control strategies need to be

implemented.

In the late 1980s the Electric Power Research Institute (EPRI) has introduced a new

technology program known as Flexible AC Transmission System (FATCS). The main

idea behind this program is to increase controllability and optimize the utilization of the

existing power system capacities by replacing mechanical controllers by reliable and high-

speed power electronic devices.

The latest generation of FACTS controllers is based on the concept of the solid state

synchronous voltage sources (SVSs) introduced by L. Gyugyi in the late 1980s. The SVS

behaves as an ideal synchronous machine, i.e. generates fundamental frequency three-phase

balanced sinusoidal voltages of controllable amplitude and phase angle. It can internally8

Generate both inductive and capacitive reactive power. If coupled with an appropriate energy

storage device, i.e. dc storage capacitor, battery, etc, SVS is able to exchange real power

with the ac system. The SVS can be implemented by the use of the voltage sourced-

converters (VSC).

The SVS can be used as shunt or series compensator. If operated as a reactive shunt

compensator it is called static condenser (STATCON), operated as a reactive series

compensator it is called static synchronous series compensator (SSSC). A

s p e c i a l arrangement of two SVSs, one connected in series with the ac system and the

other one connected in shunt, with common dc terminals is called Unified Power Flow

Controller (UPFC). It represents series - shunt type of controller. The Interline Power Flow

Controller (IPFC) is recently introduced series-series type of controller. It consists of two

or more SSSCs coupled through a common DC link. IPFC provides independently

controllable reactive series compensation of each selected line as well as transfer of real

power between the compensated lines.

The advantages of SVS based compensators over mechanical and conventional thyristor

compensators are

• improved operating and performance characteristics

• Uniform use of same power electronic device in different compensation and control

applications

• reduced equipment size and installation labor.

The objective of this project is to develop a UPFC model, design its controls,

incorporate the model and its controls in the MATLAB based commercial power

system simulation software Power System Toolbox (PST), and use the UPFC to

enhance operation and control of electric power systems.

To demonstrate the performance of the UPFC under dynamic conditions, a power

system, extensively used in the literature, consisting of two-areas, each with two generating

plants, is used. Simulation results show that the UPFC can significantly enhance power

system operation and performance.

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Chapter 2

LITERATURE SURVEY

In this chapter a literature survey of topics related to Power Flow Control In Power

Systems, UPFC operation, modeling and control will be given.

Power Flow Control in Power Systems

2.1 Power System Operation

A power system is a large interconnected network with components converting nonelectrical

energy into the electrical form to meet the demanded high quality power supply to the end

users. A power system is an electrical network divided into three sub-systems. The three

sub-systems are the generation stations, the transmission systems and the distributed

systems. Electric power produced by a generator unit transmitted from generators to

loads by transmission system. The transmission systems are the connecting link between

generating stations and the distributed systems that leads to other power system over

interconnections as shown in the block diagram in Figure 2.1 .

Figure 2.1: Block Diagram of Power System Operation

10

Generation Unit: The generation unit includes the generating plants that produce

energy fed through the transformers to a high voltage transmission network

interconnecting to other generating plants. It converts non-electrical energy such as coal,

water, natural gas, hydroelectric, solar and geothermal sources etc., into electrical

energy.

Transmission S y s te ms : In t r a ns mi s s io n s y s t e m s , e l e c t r i c i t y g e n e r a t e d

f rom power generation unit is transferred to substations. It performs voltage

transformation, power switching, measurement and control.

Distribution Systems: This is the final stage in the delivery of electricity to the end

users. A distribution system carries electricity from the transmission system and

delivers it to the end users.

2.2 Power Flow Control

Today most of the electrical power systems in the world are widely interconnected due to

economic reasons to reduce the cost of electricity and to improve system stability and

reliability. Because of the increasing complexity of power system design, the challenge to

meet the high quality power supply in a power system is highly desirable. The factors

considered for the smooth functionality of power system operation and control as follows:

- Power system operating in a synchronous mode maintains the power quality with a

controlled phase between all the interconnected networks.

- The voltage level in a power system should maintain within limits. Any variations in

the voltage level cause damage to electric motors and dielectric components, which is 11

not acceptable and leads to overloading of many electric components.

- Transmission lines of power systems should operate with minimum losses by using

the most efficient transmission paths capable of handling the loads.

- In practical power engineering it is not possible to operate a power system without

any single faults. When faults naturally occur protective relaying systems are used to

detect the faults and restore the system operation.

- Whenever there are disturbances in the power system because of system failure, the

part of the system that remains operable may not have sufficient capacity to serve the

loads without becoming overloaded. In such cases, the major control objective of

power system is to manage the overloads that may results from disturbances to the

normal operation of the system.

2.3 Power System Limitations

Theoretically, power engineers have taken lot of measures to avoid the limitations and

maintain the power system to work with stability and reliability. However, it is very hard

to predict the power system limitations that affect the system operation. Following are the

some of the limitations considered in power system: Thermal, Voltage and Transient

Stability limits.

Thermal limit: Thermal limits are due to the thermal capability of power systems. As

power transfer increases, current magnitude increases which is key to thermal damage.

For example, in a power system, the sustained operation of units beyond the maximum

operation limits will result in thermal damage.

Voltage limit: Power systems are designed to operate at a nominal supply 12

voltage. Variations in nominal voltage can adversely affect the performance as well as

cause serious damage to the system. Current flowing through the transmission lines

may produce an unacceptably large voltage drop at the receiving end of the power

system. This voltage drop is primarily due to the large reactive power loss, which

occurs as the current flows through the systems.

Transient Stability: It is defined as the ability of power system to maintain

synchronism when it is subjected to severe transient disturbance. In general, power

systems with long transmission lines are most susceptible to transient instability. The

best way to analyze the transient stability limit is to study the change of rotor angle

of all synchronous machines connected to the system after the system subjected to large

disturbance.

2.4 Power Controlling Devices

To overcome the above limitations, power system engineers introduced the concept of

advanced controller devices that provide techniques to maintain system stability and reduce

losses. Different types of power controlling devices are as follows:

2.4.1 Phase Shifting Transformer (PST): Generally, transformers transport electric

power between different voltage levels of a power system. It may also used to control

the phase displacement between the input voltage and current phase by an angle adjusted

by means of a tap changer. Such special transformers are termed as Phase Shifting

Transformer (PST). PSTs used to control the power flow through a specific line and

line losses in a complex transmission network.

Disadvantages: The speed of the phase shifting transformers to change the phase angle of

13

the injected voltage is very slow and limited to issues with short-circuit current

protection. In conclusion, PSTs applied in power system are very limited with slow

requirements under steady state system conditions.

Figure 2.2: Phase Shifting Transformer (PST)

2.4.2 High Voltage Direct Current (HVDC): HVDC systems introduced in 1950’s play

an important role to improve the reliability of the power system in addition to the power

transfer operations. It is the feasible way to interconnect two asynchronous networks,

reduce fault currents, power system reliability and utilize long cable circuits. Basic

functionality of HVDC system is to convert electrical current from AC to DC terminal at

the transmitting end and from DC to AC terminal at the receiving end. Converting AC to

DC terminal referred as rectifier and DC to AC terminal referred as inverter terminal.

Figure 2.3 taken from reference gives us the idea of how HVDC converters connected in

the interconnected systems.

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Figure 2.3: HVDC Converter Station

HVDC Applications: These provide high power flow transfers over long distance using

fewer transmission lines than AC transmission lines, with lower system losses by

increasing the dc voltage level. HVDC underground cables have no restricted limitation

over the distance as in case of ac cables. HVDC cables used with voltage source

converter based HVDC transmission systems are lighter and more flexible.

HVDC transmission systems used in interconnections between asynchronous networks

provides more reliable system operation. Many asynchronous interconnections exist in

North America between the eastern and western interconnected systems, between the

Electric Reliability Council of Texas.

Disadvantages: HVDC system generates harmonics that effect on the power quality of a

power system. Normal operation of HVDC requires a reactive power to support hence

large reactive source should be installed at the converter stations .

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2.4.3 Flexibility of AC Transmission Systems (FACTS): The world’s electrical power

systems today are widely interconnected due to economic reasons to reduce the cost of

electricity and to improve the reliability of the system. These interconnected networks are

difficult to operate and cannot utilize the full potential of a transmission system. In order

to overcome these limitations, power systems came up with the concept of mechanical

controllers in the past but these mechanical controllers had numerous intrinsic problems.

Later power system engineers introduced the concept of power electronic devices to

control the power system limitations known as Flexible AC Transmission System

(FACTS) devices.

FACTS Applications: In interconnected as well as in long transmission power systems

technical problems occur which limits the load ability and reliability of the system.

The best devices for the use in complex systems are the phase angle regulator, the

controlled series compensator, especially when gate turn of thyristor technology with

unified power flow controller. In long-distance transmission, TCSC or SSSC offers

advantages comparing effectiveness against the rating, complexity and costs.

Disadvantages:

Of all the power-controller devices discussed above, FACTS controllers are the most

recent and commonly used application in power system operation. In the next chapter, I

discussed in detail about the different FACTS Controllers and their characteristics.

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2.5 Flexible Alternating Current Transmission Systems

According to the IEEE definition, FACTS is defined as “The Flexible AC

Transmission System(FACTS) is a new technology based on power electronic devices

which offers an opportunity to enhance controllability, stability and power transfer

capability of AC Transmission Systems” .

Power systems today are highly complex and the requirements to provide a stable, secure,

controlled and economic quality of power are becoming vitally important with the rapid

growth in industrial area. To meet the demanded quality of power in a power system it is

essential to increase the transmitted power either by installing new transmission lines or

by improving the existing transmission lines by adding new devices. Installation of new

transmission lines in a power system leads to the technological complexities such as

economic and environmental considerations that includes cost, delay in construction as so

on. Considering these factors power system engineers concentrated the research process

to modify the existing transmission system instead of constructing new transmission

lines. Later they came up with the concept of utilizing the existing transmission line just

by adding new devices, which can adapt momentary system conditions in other words,

power system should be flexible.

In this research process, in late 1980s Electric Power Research Institute (EPRI) came up

with the concept of Flexible AC Transmission Systems (FACTS) technology, which

enhances the security, capacity and flexibility of power transmission systems. It was the

new integrated concept based on power electronic switching device and dynamic

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controllers to enhance the system utilization and power transfer capacity as well as the

stability, security, reliability and power quality of AC transmission Systems. The

controllers designed based on the concept of FACTS technology known as FACTS

controllers.

2.5.1 Introduction to FACTS controllers:

The controllers that are designed based on the concept of FACTS technology to improve

the power flow control, stability and reliability are known as FACTS controllers. These

controllers were introduced depending on the type of power system problems. Some of

these controllers were capable of addressing multiple problems in a power system but

some are limited to solve for a particular problem. All these controllers grouped together

as a family of FACTS controllers categorized as follows:

First Generation of FACTS Controllers: Static Var Compensator (SVC) and Thyristor

Controlled Series Compensator (TCSC)

Second Generation of FACTS Controllers: Static Synchronous Series Compensator

(SSSC) and Static Synchronous Compensator (STATCOM)

Third Generation of FACTS Controllers: Unified Power Flow Controller (UPFC)

Fourth Generation of FACTS Controllers: Interline Power Flow Controller (IPFC) and

Generalized Power Flow Controller (GUPFC)

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Figure 2.4: Block Diagram of FACTS Controllers

2.5.2 Different FACTS Controllers

First Generation of FACTS Controllers: These categories of controllers are

designed based on thyristor based FACTS technology.

Static Var Compensator (SVC): It is the first device in the first generation of

FACTS controller introduced to provide fast-acting reactive power compensation in

the transmission network.

Circuit Description: Static Var Compensator as shown in Fig 2.5 composed of thyristor

controlled reactor (TCR), thyristor switched capacitor (TSC) and harmonic filters

connected in parallel to provide dynamic shunt compensation. The current in the thyristor19

Controlled reactor is controlled by the thyristor valve that controls the fundamental

current by changing the fire angle, ensuring the voltage limited to an acceptable range at

the injected node. Current harmonics are inevitable during the operation of thyristor

controlled rectifiers, thus it is essential to have filters to eliminate harmonics in the SVC

system. The filter banks not only absorb the risk harmonics but also produce the

capacitive reactive power.

Figure 2.5: Circuit Diagram of Static Var Compensator (SVC)

Characteristics of SVC: SVC placed in a transmission network provides a dynamic

voltage control to increase the transient stability, enhancing the damping power

oscillations and improve the power flow control of the power systems.

In real time scenario, it effectively controls the reactive power, improves the power

factor, reduces the voltage levels caused by the nonlinear loads, improves the power

quality and reduces the energy consumption.

20

The main advantage of SVC application is to maintain bus voltage approximately near a

constant level in addition used to improve transient stability. It is widely used in

metallurgy, electrified railway, wind power generation etc.

Thyristor Controlled Series Compensator (TCSC): It is designed based on the

thyristor based FACTS technology that has the ability to control the line impedance

with a thyristor-controlled capacitor placed in series with the transmission line. It is

used to increase the transmission line capability by installing a series capacitor that

reduces the net series impedance thus allowing additional power to be transferred.

Circuit Description: TCSC device consists of three main components: Capacitor bank,

bypass inductor and bidirectional thyristors SCR1 and SCR2 as shown in the Fig 2.6.

Figure 2.6: Circuit Diagram of Thyristor Controlled Series Compensator (TCSC)

Characteristics of Thyristor Controlled Series Compensator (TCSC): TCSC placed in

a transmission network provides the power flow control in a power system improving

the damping power oscillation and reduces the net loss providing voltage support.

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The thyristors in TCSC device offers a flexible adjustment with the ability to control the

continuous line compensation. TCSC controllers effectively used for solving power

system problems of transient stability, dynamic stability, steady state stability and voltage

stability in long transmission lines.

Second Generation of FACTS Controllers:

These categories of controllers are designed based on voltage source converter FACTS

Technology.

Static Synchronous Series Compensator (SSSC): Static Synchronous Series

Compensator is based on solid-state voltage source converter designed to generate the

desired voltage magnitude independent of line current.

Circuit Description: SSSC consists of a converter, DC bus (storage unit) and coupling

transformer as shown in Figure 2.7. The dc bus uses the inverter to synthesize an ac

voltage waveform that is inserted in series with transmission line through the transformer

with an appropriate phase angle and line current. If the injected voltage is in phase with

the line current it exchanges a real power and if the injected voltage is in quadrature with

line current it exchanges a reactive power. Therefore, it has the ability to exchange both

the real and reactive power in a transmission line.

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Figure 2.7: Block Diagram of Static Synchronous Series Compensator (SSSC)

Characteristics of SSSC: SSSC in a transmission network generates a desired

compensating voltage independent of the magnitude of line current, by modulating

reactive line impedance and combining real and reactive compensation it can provide

high damping of power oscillation.

The capability of SSSC to exchange both active and reactive power makes it possible to

compensate both the reactive and the resistive voltage drop thereby maintains a high

effective X/R ration independent of degree of series oscillation.

All the above features of SSSC attract the FACTS device for power flow control,

damping of power oscillations and transient stability.

Static Synchronous Compensator (STATCOM): It is designed based on Voltage

source converter (VSC) electronic device with Gate turn off thyristor and dc capacitor

coupled with a step down transformer tied to a transmission line as shown in Fig 2.8. It

converts the dc input voltage into ac output voltages to compensate the active and

reactive power23

of the system. STATCOM has better characteristics than SVC and it is used for voltage

control and reactive power compensation.

Figure 2.8: Circuit Diagram of Static Synchronous Compensator (STATCOM)

Characteristics of Static Synchronous Compensator (STATCOM): STATCOM placed on

a transmission network improve the voltage stability of a power system by controlling the

voltage in transmission and distribution systems, improves the damping power oscillation

in transmission system, provides the desired reactive power compensation of a power

system.

Third Generation of FACTS Controllers:

The third generation of FACTS controllers is designed by combining the features of

previous generation’s series and shunt compensation FACTS controllers.

Unified Power Flow Controller (UPFC):

It is designed by combining the series compensator (SSSC) and shunt compensator

(STATCOM) coupled with a common DC capacitor. It provides the ability to

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simultaneously control all the transmission parameters of power systems, i.e. voltage,

impedance and phase angle.

Circuit Description: As shown in Fig 2.9 it consists of two converters – one connected in

series with the transmission line through a series inserted transformer and the other one

connected in shunt with the transmission line through a shunt transformer. The DC

terminals of the two converters are connected together with a DC capacitor. The series

converter control to inject voltage magnitude and phase angle in series with the line to

control the active and reactive power flows on the transmission line. Hence the series

converter will exchange active and reactive power with the line.

Figure 2.9: Circuit Diagram of Unified Power Flow Controller (UPFC)

Characteristic of UPFC: The concept of UPFC makes it possible to handle practically

all the power flow control and transmission lines compensation problems using solid-

state controllers that provide functional flexibility which are generally not obtained by

thyristor-controlled controllers.

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Convertible Static Compensator (CSC): It is the latest generation and most recent

development in the field of FACTS controllers. It has the ability to increase the power

transfer capability and maximize the use of existing transmission line .

Interline Power Flow Controller (IPFC): It is designed based on Convertible Static

Compensator (CSC) of FACTS Controllers. As shown in Fig 2.10, IPFC consists of

two series connected converters with two transmission lines. It is a device that provides

a comprehensive power flow control for a multi-line transmission system and consists

of multiple number of DC to AC converters, each providing series compensation for a

different transmission line. The converters are linked together to their DC terminals and

connected to the AC systems through their series coupling transformers. With this

arrangement, it provides series reactive compensation in addition any converter can be

controlled to supply active power to the common dc link from its own transmission line

Figure 2.10: Circuit Diagram of Interline Power Flow Controller

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Characteristics of IPFC: To avoid the control of power flow problem in one system with

synchronous of power in other system, installation of IPFC system in additional parallel

inverter is required to meet the active power demand.

Generalized Unified Power Flow Controller (GUPFC): It has been proposed to

realize the simultaneous power flow control of several transmission lines. It is designed

by combining three or more dc to ac converters working together extending the

concepts of voltage and power flow control of the known two-converter UPFC

controller to multi voltage and power flow control. The GUPFC shown in Fig 2.11

consists of three converters, one

Figure 2.11: Circuit Diagram of Generalized UPFC

2.5.3 Advantages of FACTS controllers in Power Systems:

Power system stability: Instabilities in power system are created due to long

length of the transmission lines, interconnected grid, changing system loads and

line faults in the system. These instabilities results in reduced transmission line

27

flows or even tripping of the transmission. FACTS devices stabilize transmission

systems with increased transfer capability and reduced risk of transmission line

trips.

Power Quality and Reliability: Modern power industries demand for the high

quality of electricity in a reliable manner with no interruptions in power supply

including constant voltage and frequency. The change in voltage drops, frequency

variations or the loss of supply can lead to interruptions with high economic

Losses. Installation of TCSC at the distribution system without increasing the

short circuit current level considerably increases the reliability for the consumer.

Environmental Benefits: The construction of new transmission line has negative

impact on the economical and environmental factors. Installation of FACTS

devices in the existing transmission lines makes the system more economical by

reducing the need for additional transmission lines. For example, In Sweden, eight

400 kV systems run in parallel to transport power from north to south. Each of the

transmission systems are equipped with FACTS. Studies show that four additional

400kV transmission systems would be necessary if FACTS were not utilized on

the existing system .

Flexibility: The construction of new transmission lines take several years but the

installation of FACTS controllers in a power system requires only 12 to 18

months. It has the flexibility for future upgrades and requires small land area.

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Reduced maintenance cost: Maintenance cost of FACTS controllers are less

compared to the installation of new transmission lines. As the number of

transmission line increases, probability of fault occurring in a line also

increases resulting in system failure. By utilizing the FACTS controllers in a

transmission network, power system minimizes the number of line faults thus

reducing the maintenance cost

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Chapter 3

UPFC BASIC OPERATION AND CHARACTERISTICS

The UPFC, which was proposed by L. Gyugyi in 1991, is one of the most complex

FACTS devices in a power system today. It is primarily used for independent control of

real and reactive power in transmission lines for a flexible, reliable and economic operation

and loading of power system. Until recently all four parameters that affect real and reactive

power flow on the line, i.e. the line impedance, voltage magnitudes at the terminals of the

line or power angle, were controlled separately using either mechanical or other FACTS

devices such as a Static Var Compensator (SVC), a Thyristor Controlled Series

Capacitor (TCSC), a phase shif ter , e tc . However, the UPFC al lows s imul taneous

or independent control of these parameters with transfer from one control scheme to

another in real time. Also, the UPFC can be used for voltage support, transient stability

improvement and damping of low frequency power system oscillations. Because of its

attractive features, modeling and controlling an UPFC have come into intensive

investigation in the recent years.

Several references in technical literature can be found on development of UPFC steady

state, dynamic and linearized models.

UPFC dynamic model known as a fundamental frequency model consists of two

voltage sources one connected in series and the other one in shunt with the power network

to represent the series and the shunt voltage source inverters. Both voltage sources are

modeled to inject voltages of fundamental power system frequency only.

This chapter will explain basic operation and characteristics of the UPFC. Since UPFC

consists of two voltage-sourced converters (VSCs), basics of VSCs will be briefly discussed

at the beginning of the chapter.

30

3.1 Basics of Voltage Source Converters and Pulse Width Modulation Technique

Typical three-phase VSC is shown in Fig. 3.1 .+VDC/2

1 1' 3 3' 5 5'

VDC N or

4 4' 6 6'

2 2'

-VDC/2

Fig. 3.1 Three-phase voltage sourced-

converter

It is made of six valves each consisting of a gate turn off device

(GTO) paralleled with a reverse diode, and a DC capacitor. An AC

voltage is generated from a DC voltage through sequential switching

of the GTOs. The DC voltage is unipolar and the DC current can

flow in either direction.

Controlling the angle of the converter output voltage with

respect to the AC system voltage controls the real power exchange

between the converter and the AC system. The real power flows from

the DC side to AC side (inverter operation) if the converter output

voltage is controlled to lead the AC system voltage. If the converter

output voltage is made to lag

the AC system voltage the real power will flow from the AC side to DC side (rectifier

31

operation). Inverter action is carried out by the GTOs while the rectifier action is carried out

by the diodes. Two switches on the same leg cannot be on at the same time.

Controlling the magnitude of the converter output voltage controls the reactive power

exchange between the converter and the AC system. The converter generates reactive power

for the AC system if the magnitude of the converter output voltage is greater than the

magnitude of the AC system voltage. If the magnitude of the converter output voltage is less

than that of the AC system the converter will absorb reactive power.

The converter output voltage can be controlled using various control techniques. Pulse

Width Modulation (PWM) techniques can be designed for the lowest harmonic content. It

should be mentioned that these techniques require large number of switching per cycle

leading to higher converter losses. Therefore, PWM techniques are currently considered

unpractical for high voltage applications. However, it is expected that recent developments

on power electronic switches will allow practical use of PWM controls on such applications

in the near future. Due to their simplicity many authors, i.e. have used PWM control

techniques in their UPFC studies. Hence, the same approach will be used in this project.

When sinusoidal PWM technique is applied turn on and turn off signals for GTOs are

generated comparing a sinusoidal reference signal Vr of amplitude Ar with a sawtooth carrier

waveform Vc of amplitude Ac as shown in Fig. 3.2 (b) . The frequency of the sawtooth

waveform establishes the frequency at which GTOs are switched.

Consider a phase-leg as shown in Fig. 3.2 (a).

In this case

Vr>VC results in a turn on signal for the device one and gate turn off signal for the

device four

and Vr<VC results in a turn off signal for the device one and

gate turn on signal for the device four.

3

vr vc

2

1

+VDC/2

0t

1 1'-1

a -2

VDC N

4 4'-3

0 0. 5 1 1. 5 2 2. 5 3 3. 5 4 4. 5 5

va

N

-VDC/21 1 1 1 1

+VDC/2

t

-VDC/2

4 4 4 4 4

(a) (b)

Fig. 3.2 PWM converter (a) A phase-leg (b) PWM waveforms

The fundamental frequency of the converter output voltage

is determined by the frequency of the reference signal. Controlling

the amplitude of the reference signal controls

the width of the pulses. The amplitude modulation index is defined as ratio of Ar to Ac

m = A r

Ac(3.1)

For m≤1 the peak magnitude of the fundamental frequency component of the converter

output voltage can be expressed as

V = m VDC

2(3.2

)

3.2 UPFC Description

The UPFC is a device placed between two busses referred to as

the UPFC sending bus and the UPFC receiving bus. It consists of two

Voltage-Sourced Converters (VSCs) with a common DC link. For the

fundamental frequency model, the VSCs are replaced by two controlled

voltage sources as shown in Fig. 3.3. The voltage source at the sending

bus is

connected in shunt and will therefore be called the shunt voltage source. The second source,

n SH

:1

n SE:

1

the series voltage source, is placed between the sending and the receiving busses. The UPFC

is placed on high-voltage transmission lines. This arrangement requires step-down

transformers in order to allow the use of power electronics devices for the UPFC.

sending bus

S IS

VS

ILine

I

zSE

TSE

receiving bus

R

VR

TSH

PSH

zSH

shunt converter +

Idc

series converter PSE

VSH

V- VSE

mSH ϕSH mSE ϕSE

Fig. 3.3 Fundamental frequency model of UPFC

Applying the Pulse Width Modulation (PWM) technique to the

two VSCs the following equations for magnitudes of shunt and series

injected voltages are obtained

where:

VSH = mSH 2

VSE = mSE 2

VDC

2n SH

VB

VDC

2n SE

V

B

(3.3)• mSH – amplitude modulation index of the shunt VSC control signal• mSE – amplitude modulation index of the series VSC control signal• nSH – shunt transformer turn ratio• nSE – series transformer turn ratio• VB – the system side base voltage in kV

• VDC – DC link voltage in kV

3.3 Operating Modes of UPFC

The UPFC has many possible operating modes. In particular,

the shunt inverter is operating in such a way to inject a controllable

current, into the transmission line. This current consists of two

components with respect to the line voltage: the real or direct

component, which is in phase or in opposite phase with the line

voltage, and the reactive or quadrature component, which is in

quadrature. The direct component is automatically determined by the

requirement to balance the real power of the series inverter. The

quadrature component, instead, can be independently set to any

desired reference level (inductive or capacitive) within the capability

of the inverter, to absorb or generate respectively reactive power

from the line. The shunt inverter can be controlled in two different

modes:

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. For this mode

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

also required.

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.

Direct Voltage Injection Mode: The reference inputs are

directly the magnitude and phase angle of the series voltage.

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.

Automatic Power Flow Control Mode: The reference

inputs are values of P and Q to maintain on the transmission line

despite system changes.

Chapter 4

CASE STUDY

4.1 Modeling Of UPFC On A Transmission System

Using the concept of the control system a power system is taken to implement the use of

UPFC. The two modes i.e. the power flow control and the voltage injection mode are

simulated in SIMULINK to see the effect of UPFC on a power system. Study is

carried out to verify the utility of FACT device. The figure below illustrates

application study the steady-state and dynamic performance of a unified power flow

controller (UPFC) used to relieve power congestion in a transmission system. The load

flow analysis and the single line diagram simulation are done on power flow simulator.

This software helps to calculate the power flow, the voltage at each bus and the cost

effectiveness of the system

Fig4.1 500kV/230kV transmission system

. 4.2 Explanation Of Single Line Diagram

A UPFC is used to control the power flow in a 500 kV /230 kV transmission systems. The

system, connected in a loop configuration, consists essentially of five buses (B1 to B5)

interconnected through three transmission lines (L1, L2, L3) and two 500 kV/230 kV

transformer banks Tr1 and Tr2. Two power plants located on the 230 kV system generate a

total of 1500 MW which is transmitted to a 500 kV, 15000 MVA equivalent and to a 200

MW load connected at bus B3. Each plant model includes a speed regulator, an

excitation system as well as a power system stabilizer (PSS). In normal operation, most

of the 1200 MW generation capacity of power plant #2 is exported to the 500 kV

equivalents through two 400 MVA transformers connected between buses B4 and B5. For

this illustration we consider a contingency case where only two transformers out of three

are available (Tr2= 2*400 MVA = 800 MVA). The load flow shows that most of the power

generated by plant #2 is transmitted through the 800 MVA transformer bank (899 MW out of

1000 MW) and that 96 MW is circulating in the loop. Transformer Tr2 is therefore

overloaded by 99 MVA. This will now illustrate how a UPFC can relieve this power

congestion. The UPFC located at the right end of line L2 is used to control the active and

reactive powers at the 500 kV bus B3, as well as the voltage at bus B_UPFC. The UPFC

consists of two 100 MVA, IGBT-based, converters (one shunt converter and one series

converter interconnected through a DC bus). The series converter can inject a maximum of

10% of nominal line-to-ground voltage (28.87 kV) in series with line L2

4.3 Model Block of Single Line Diagram

The single line diagram illustrated in Figure 4.1 is implemented on MATLAB SIMULINK

to check the validity of the UPFC controller. The Model of UPFC will generate two kinds of

results. First is based upon the simulations at power flow control mode and second on

voltage injection Mode.

Fig 4.2 Simulink Model of UPFC

The important keys to note in the block diagram are,1. Use of Bypass breaker – Used to connect or disconnect UPFC Block from Power System

2. The reference power inputs [P Qref] – Reference for power flow control

3. The reference voltage Vdref – Reference for voltage injection

4. Power flow analysis at load flow indicated by arrows – Comparison with & without UPFC

Various Blocks of UPFC Simulink Model:

Power Plant:It consists of speed regulator with generic power system stabilizer, a

three phase synchronous machine, a three phase transformer and connecting ports.

Fig 4.3 Power Plant Simulink Model

Speed Regulator With generic Power System Stabilizer:

It further consists of system stabilizer, excitation block, stator and rotor voltage and speed controls.

Fig 4.4 Speed Regulator With generic Power System Stabilizer

UPFC Measurements Block:The UPFC measurement block holds the required logical

mechanisms for the outputs of the UPFC block.

Fig 4.5 UPFC measurement blockVoltage, Active Power, Reactive Power Block:

This block holds the required logical mechanisms for the outputs of the V,P,Q block.

Fig 4.6 Voltage, Active Power, Reactive Power Block

4.4 Power Flow Control with the UPFC:

Parameters of the UPFC are given in the dialog box. In the Power data parameters that the

series converter is rated 100 MVA with a maximum voltage injection of 0.1 pu. The shunt

converter is also rated 100 MVA. Also, in the control parameters, that the shunt converter is

in Voltage regulation mode and that the series converter is in Power flow control mode.

The UPFC reference active and reactive powers are set in the magenta blocks labeled

Pref(pu) and Qref(pu). Initially the Bypass breaker is closed and the resulting natural power

flow at bus B3 is 587 MW and -27 Mvar. The Pref block is programmed with an

initial active power of 5.87 pu corresponding to the natural power flow. Then, at t=10s,

Pref is increased by 1 pu (100 MW), from 5.87 pu to 6.87 pu, while Qref is kept constant at

-0.27 pu.

4.5 Simulations:

Without UPFC:

With UPFC:

Power Flow Control with UPFC:

Chapter 5

CONCLUSION

This project deals with the case study of power flow control with the Unified Power

Flow Controller (UPFC) that is used to maintain and improve power system operation and

stability. This paper presents the power flow operation of power systems and its

limitations, different devices to control the power flow with the existing transmission lines,

types of FACTS controllers used in the power system, basic characteristics and operation

of UPFC. The Unified Power Flow Controller provides simultaneous or individual controls of

basic system parameters like transmission voltage, impedance and phase angle there

by controlling the transmitted power.

In power system transmission, it is desirable to maintain the voltage magnitude, phase

angle and line impedance. Therefore, to control the power from one end to another end,

this concept of power flow control and voltage injection is applied. Modeling the system

and studying the results have given an indication that UPFC are very useful when it comes

to organize and maintain power system. Following conclusions are made-

1. Power flow control is achieved and congestion is less.

2. Transient stability is improved.

3. Faster Steady State achievement.

4. Improved Voltage Profile

REFERENCES

[1] Modeling and Control of the Unified Power Flow Controller, Azra Hasanovic, The College of Engineering and Mineral Resources, West VirginiaUniversity.

[2] Study The Power Flow Control Of A Power System With Unified Power Flow Controller, Vakula Peesari, California State University.

[3] Performance Analysis of Fuzzy Logic Based Unified Power Flow Controller, Lütfü Saribulut, Mehmet Tümay, and Đlyas Eker

[4] UPFC Simulation and Control Using the ATP/EMTP and MATLAB/Simulink Programs,R.Orizondo ,R.Alves,Member,IEEE.

[5] Study and Effects of UPFC and its Control System for Power Flow Control and Voltage Injection in a Power System, Vibhor Gupta / International Journal of Engineering Science

And technology, Vol.2(7),2010

[6] www.google.com

[7] www.wikipedia.org