upfc (2)
TRANSCRIPT
TITLE
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CERTIFICATE
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.
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ACKNOWLEDGMENTS
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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
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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
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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
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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.
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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
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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.
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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
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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