CHAPTER- 2

FACTS CONTROLLERS

CHAPTER 2

FACTS CONTROLLERS

2.1 OVER VIEW OF FACTS:

The rapid development of power electronics technology provides exciting

opportunities to develop new power system equipment for better utilization of existing

systems. Since 1990, a number of devices under the term FACTS (flexible AC transmission

systems) technology have been proposed and implemented [I ] .

Flexible AC Transmission Systems (FACTS) are the name given to the application of

power electronics devices to control the power flows and other quantities in power systems.

IEEE Definitions FACTS are AC transmission systems incorporating the power

electronic-based and other static controllers to enhance controllability and increase power

transfer capability.

FACTS devices can be effectively used for power flow control, load sharing among

parallel corridors, voltage regulation, and enhancement of transient stability and mitigation

of system oscillations. By giving additional flexibility, FACTS controllers can enable a line

to carry power close to its thermal rating. Mechanical switching has to be supplemented by

rapid response power electronics. It may be noted that FACTS is enabling technology, and

not a one-on-one substitute for mechanical switches [2-3).

FACTS employs high speed thyristor for switching in or out transmission line

components such as capacitors, reactors or phase shifting transformers for desirable

performance of systems. The FACTS technology is not a single high power controller, but

rather a coilection of controllers, which can be applied individually or in coordination with

others to control one or more of system parameters. It started with the High Voltage DC

Current (HVDC) transmission, and Static Var Compensator (SVC) systems were employed

later for the reactive power compensation of power transmission lines. Subsequently,

devices like Thyristor Controlled Series Compensator (TCSC), Static Compensator

(STATCOM), Static Synchronous Series Compensator (SSSC) and Unified Power Flow

Controller (UPFC) are proposed and installed under the generic name of Flexible AC

Transmission Systems (FACTS) controllers 141.

2.2. RELATIVE CONTROLLABLE PARAMETERS OF FACTS 151

Control of the line impedance X (with a thyristor controlled series capacitor) can

provide a powerful means of current control.

When the angle is not large, control of X or the angle substantially provides the

control of active power.

Control of angle (with a phase angle regulator), which in turn controls the driving

voltage, provides a powerful means of controlling the current flow and hence

provides active power flow when the angle is not large.

111-jecting the voltage in series with the line, and perpendicular to the current flow,

can increase or decrease the magnitude of current flow. Since the current flow lags

the driving voltage by 90°, the injection of reactive power in series, can provide a

powerful means of controlling the line current.

Injecting voltage in series with the line and with any phase angle with respect to the

driving voltage can control the magnitude and the phase of the line current. This

means that injecting a voltage phasor with the variable phase angle can provide a

powerful means of precisely controlling the active and reactive power flow. This

requires injection of both active and reactive power in series.

Because the per unit line impedance is usually a small fraction of the line voltage,

the MVA rating of a series controller will often be a small fraction of the

throughout line MVA.

When the angle is not large, controlling the magnitude of one or more line voltages

can be a very cost-effective means for the control of reactive power flow through

the interconnection.

Combination of the line impedance control with a series controller and voltage

regulation with a shunt controller can also provide a cost-effective means to control

both the active and reactive power flow between the two systems.

2.3. BASIC TYPES OF FACTS CONTROLLERS

In general, FACTS controllers can be divided into four categories 16-81

Series Control.lers

Shunt Controllers

Combined series-series Controllers

Combined series-shunt Controllers

2.3.1 Series Controllers

The series controller could be variable impedance, such as a reactor, capacitor, etc.,

or power electro~iics based variable source of main frequency, sub synchronous and

harmonic frequencies to serve the desired need. In principle, all series controllers inject

voltage in series with the line. As long as the voltage is in phase quadrature with the line

current, the series controller only supplies or consumes variabte reactive power. Any other

relationship will involve handling of real power too.

2.3.2 Shunt Controllers

As in the case of series controllers, the shunt controllers may be variable

impedance, variabfe source, or a combination of these. In principle, all shunt controllers

inject current into the system at the point of connection. Even a variable shunt impedance

connected to the line voltage causes a variable current flow and hence represents injection

of the current into the line. As long as the injected current is in phase quadrature with the

line voltage, the shunt controller only supplies or consumes variable reactive power. Any

other phase relationship will involve handling of real power too.

2.3.3 Combined Series-Series Controllers

This could be a separate series controllers, which are controlled in a coordinated

manner, in a multifine transmission system, or it could be a unified controller. Series

controllers provide independent series reactive compensation for each line and also transfer

real power among the lines via the power link. The real power transfer capability of

the unified series-series controller, referred to as Inter Line Power Flow Controller, makes

it possible to balance both the real and reactive power flow in the lines and thereby

maximize the utilization of the transmission system,

2.3.4 Combined Series-Shunt Controllers

There could be a combination of separate shunt and series controllers, which are

controlled in a coordinated manner or a Unified Power Flow Controller with series and

shunt elements. In principle, combined shunt and series controllers inject current into the

system with the shunt part of the controller taking case of the current while voltage in

series in the line with the series part of the controller. However, when the shunt and series

controllers are unified there can be real power exchange between the series and shunt

controllers via the power link

2.4. BENEFITS OF UTILIZING FACTS DEVICES

The benefits of utilizing FACTS devices in electrical transmission systems can be

summarized as follows [9 ] :

2.4.1 Technical Benefits

Better utilization of existing transmission system assets

Increased transmission system reliability and availability

Increased dynamic and transient grid stability and reduction of loop flows

Increased quality of supply for sensitive industries

Control of power flow as ordered. The use of control of the power flow may be to

follow a contract, meet the utilities own needs, and ensure optimum power flow.

Increase the loading capability of lines to their thermal capabilities, including short

tern and seasonal.

Increase the system security by raising the transient stability limit, limiting short -

circuit currents and overloads.

Provide secure tie line connections to neighbouring utilities and regions thereby

decreasing overall generation reserve requirements on both sides.

Provide greater flexibility in siting new generation.

Upgrade of lines.

Reduce reactive power flows, thus allowing the lines to carry more active power.

Reduce koop flows.

Increase utilization of lowest cost generation.

2.4.2 Financial Benefits of FACTS Devices

There are three areas where the financial benefits can be calculated easily.

Additional sales due to increased transmission capability.

Additional wheeling charges due to increased transmission capability.

Avoiding or delaying of investments in high voltage transmission lines or even new

power generation. Maintenance of FACTS devices is minimal and similar to that

required for shunt capacitors, reactors and transformers. It can be performed by

normal substation personnel with no special procedures. The amount of

maintenance ranges from 150-250 man hours per year and depends upon the size of

the installation and the local ambient (pollution) conditions.

2.4.3 Environmental Benefits

FACTS devices are environmentally friendly. They contain no hazardous materials

and produce no waste or pollutants. FACTS help the distribution of the electrical energy

more economically through better utilization of existing installations, thereby reducing the

need for additional transmission lines.

2.5 NEW TRANSMISSION TECHNOLOGIES

Electrically restructuring envisions the transmission system as flexible, reliable, and

open to all exchange no matter where the suppliers and consumers of energy are located.

Today, however, the transmission system cannot fully support such diverse and open

exchange. Desirable market transactions today are quite different from those envisianed

when the transmission system was designed; executing them may stress the limits of safe

operation. The risks posed by such transactions may not be recognized in time to avert

major system was designed; executing them may stress the limits of safe operation, the

risks posed by such transactions may not be recognized in time to avert major system

emergencies. It is also increasingly common for one transmission to interfere with the safe

execution of others, producing "congestion" in the system. Militating congestion is

technically dificult and poses new administrative challenges when the paths are long

enough to span regions that have not to coordinate such operations in the past. In addition,

the technical infrastructure required to manage operations reliably may be overwhelmed by

the sheer volume of data and information concerning system conditions, transactions, and

events.

These problems can be remedies in part by physical reinforcements to the

transmission system, sometimes through new construction but increasingly through broader

use of improved hardware technologies. However, the greater strategic is needed for

indirect reinforcements to the general infrastructure for grid operations and planning. It is

at this level that transmission needs are recognized, options for countering those needs are

assessed and a balance is achieved between new transmission assets and new operating

methods. Timely development and deployment of requisite technology is a major element

throughout this process. But requisite technology is not exclusively a new one, There is

today a backlog of prototype technologies that can be adapted to power system

applications, given the incentive and the means to do so.

Progress in transmission system reinforcements has, for many years, been hampered

by yet another aspect of electricity restructuring. The boundaries of operation and planning

responsibilities are no longer clearly delineated. This process is far from complete, and it

has greatly weakened the essential dialog between technology developer and technology

users. Development of new technology must be clasely linked to its actual deployment for

operational use. Together, both activities should reflect, serve and keep pace with the

evolving in fraction needs of transmission organizations. The present level of uncertainty

precludes this once orderly process.

Neither the details nor the needs of the future in fracture for grid management are

well known, and all parties are understandably averse to investments that may not be

promptly and directly beneficial to them. Some utilities are concerned that transmission

investments may be greater benefit to their competitors than to themselves. In the near

term, relief of congestion may harm their business, As a result of such faces many

promising technologies are stranded at various points in the "pipeline" from concept to

practical use. Included among them are superconducting equipment, large scale devices for

routing power flow on the grid, real time operating tools for enhanced management of grid

assets, and a new generation of system planning methods that are against the many kinds of

uncertainty that are emerging in the new power system.

Another important issue is that some enabling technologies for healthy and reliable

energy commerce are not perceived as profitable enough to attract the interest of

commercial developers. Some means should be found for developi~~g and deploying such

technologies in furtherance of the public good [93.

2.5.1 Technology issues

Assuming that sufficient generated power is available, the challenge is to ensure the

reliable operational performance of the delivery system. Reliable system requires

coordinated management of both generation and transmission assets, since the pattern of

generation strongly influences "loadability" of the transmission lines. Restructuring has

greatly reduced the degree to which grid operators can manage the generation side of this

relationship, so the emphasis here is upon enhanced system performance through

improvements in transmission capabilities alone. Focus items include the following [lo]:

Q Passive reinforcements through new alternating current device

configurations and materials, to include superconducting technology,

f Indirect information reinforcements through improved models, methods

and toolsets for estimating the limits of safe operation and operating

within them,

*:* Direct information reinforcements through real-time monitoring and

performance assessment of critical devices and overall system behavior,

*:* Controllability reinforcements through flexible AC Transmission System

(FACTS) device, conventional high voltage direct current (HVDC), and

lower power level HVDC ("DC lite"),

*:* Operations based reinforcements that manually or automatically regulate

HVDC FACTS and other controllable devices in order to best manage

network power flows,

*:+ Robust design methods that combine the above reinforcements to achieve

high levels of system performance together with sufficient operational

reliability and

Performance certification methods which all major devices connected

to the transmission system are determined to operate according to the

information provided to system planning and operations staff, and according

to established engineering practice.

The electric utilities system for transmitting and distributing power is to be fine

tuned, to an unprecedented degree, by the application of power electronics,

microprocessors and thyristors in general. With these new technologies transmission and

distribution of electricity will be more reliable, more controllable and more efficient. The

flexible transmission system is akin to HVDC, related to thyristor developments and

designed to overcome the limitations of present mechanically controlled AC power

transmission systems.

By using reliable high speed power electronic controllers, the technology offers five

unique advantages over other systems:

k Greater control of power, so that it flows on the prescribed transmission

routs,

P Secure loading (but not over1oading)of transmission lines to levels nearer to

their thermal limits,

P Greater ability to transfer power between controlled areas so that the

generation reserve margin may be reduced to 15% from 18%,

P Prevention of cascading outages by limiting the effects of faults and

equipment failure and

k Damping of power system oscillations, which could be of faults and

equipments failures.

The flexible system owes tighter transmission control to its ability to manage the

interrelated parameters that constrain today's system, including series impedance, shunt

impedance, phase angle, and the occurrence of oscillations at various frequencies below the

rated frequency. By adding to flexibility in this way, the controllers enable transmission

line to function nearer to its thermal rating; it is not often possible means alone, as tap

changes, phase shifters, and switched capacitors and i~~ductors (reactors). The new

technology is not a single, high cost efficient power electronic controller that wipes out all

conventionat methods and their disadvantage, but rather a collection of controllers, which

can be applied individually or collectively in a specific power system to control the five

interrelated functions already mentioned the thyristor is the basic power electronic

controller which has become the magic word in flexible AC systems just like a transistor in

integrated circuitry.

Electric power networks integrate generation and load centers within each utility

system and through interconnections among neighboring systems, share power with vast

regional grids. ?'he purpose of this is to take advantage of the diversity of loads, changes in

peak demand due to time differences, the availability of different generation reserves in

various geographic regions, power sharing arrangements among utilities, shifts in fuel

prices, regulatory changes, and other discrepancies.

By facilitating bulk power transfers, these interconnected networks help minimize

the need to enlarge power plants and enable neighboring utilities and regions to buy and

seH power among themselves, Thus, the electric power transmission will result in less

reliable more costly power[l 01.

2.5.2 Regulating the Flow of Power

At present many transmission facilities confront one or more limiting network

parameters plus the inability to direct power ftow at wilt. We know that the power low

between two points is equal to the product of the voltages at the two points, times the sine

of the phase angte difference between the two points divided by the transmission line's

reactance.

Figure 2.1 : Power flow relation

Power transmissiorl Power a series compensated tie line is governed by the expression

U,U, sin (// P(t) =

x, - x,. By proper control of the TCSC, the overall transfer reactance i s modulated

X-time in such a way that the power oscillations are damped out.

In international power systems, the actual power flow from one region to another

might take unintended routes depending on impedances of transmission lines connecting

the areas. Controlled series compensation is a useful means for optimizing power flow

between regions for varying loading and network configurations. It becomes possible to

control power flows in order to achieve a number of goals [19]:

Minimizing of system losses

> Reduction of loop flows

P Elimination of line overloads

k Optimizing of load sharing between parallel circuits

l+ Directing of power flows along contractual paths

The following illustration explains the power in a normal AC transmission system and in

that of flexible system.

Figure 2.3 Figure 2.4

There are two generators (A&B) sending power to load centre through a network

consisting of three lines in a meshed connection. The lines AB, BC and AC have

continuous ratings of 1000 M W, respectively, with emergency rating of twice those figures.

For the impedances shown, the three lines would carry 600 MW, 1600 MW, and 1400 MW

respectively. Such a situation would overload one line. Power flows in accordance with

transmission line impedances that bear no relationship with transmission ownership,

contracts or thermal limits. If the owner of generator A had a contract with utilize C to

deliver power of 1500MW, he would supply 1 OOMW short of the contract as shown in the

power flow diagram effectively C but still receives the required power through the other

loop ABC virtually from A.

If however, a capacitor whose reactance is j5 ohms at the synchronous frequency is

inserted in one line (as shown in figure 4), it reduces the line's impedance from 10 ohms to

5 ohms, so that the power flow through the lines would be 250MW, 1250 MW, and 1750

MW respectively. By adjusting the value of capacitor properly one can exactly determine

the quantity of power flow through the line in accordance with contracts and ownership.

Similar results may be obtained by increasing the irnpendence of one of the lines in the

same meshed configuration by inserting a 7 ahm in series with the line (as shown in

Fig. 2.5).

In either case a thyristor controlled phase angle regulator could be installed instead

of a series or a series inductor in any of the three lines to serve the same purpose. The

inductor and the phase angle do not contribute to sub synchronous resonance. A phase

angle installed in line AC reduces the phase angle difference from 8.5 degrees to 4.2

degrees Fig.2.6 [20].

&,s n I750 MN

LOOII M W

LOAD

Figure 2.5 Figure 2.6

Generally a combination of mechanicalty controlled and thyristor controlled series

capacitor, series inductor or phase angle regulator is used to counter the system constraints

at minimum capital cost. Several thyristor based controllers are presently being used for

flexihle transmission systems. The first generation controllers had two thyristor based

systems that have found use in some systems.

Series capacitors have been successfully utilized for many years in electric power

networks. With series compensation, it is possible to increase the transfer capability of

power transmission systems at a favorable investment cost and with a short installation

time compared to the building of additional lines; this is due to the inherent ability of series

capacitors to achieve:

P Increased dynamic stability of power transmission systems

> Improved voitage regulation and reactive power balance

> Improved load sharing between parallel lines

The capacitor should be modular and mechanically switched, but the number of

operations would be severely limited by wear on the mechanical components. Other

complications may also arise. A series capacitor in a line may lead to sub synchronous

response at 15-30 Hz. This response occurs when the mechanical response frequency of the

shaft of the generator at risk c~incides with 50Hz (60Hz in America) minus the electrical

response frequency of the capacitor in series with the total system impedance. If such

response persists, it soon damages the shaft of the generator. This eventually causes the

power lines to operate at their emergency ratings and carry higher power loads producing

power flow oscillations which put the generators out of synchronism, perhaps prompting

the system's collapse [21].

Uprathg of series cwacitor into TCSC (wical d u e s )

Figure 2.7

With the advent of thyristor control, the concept of series compensation has been

broadened and its use has been increased further. Thyristor controlled series compensation

(TCSC) introduces a number important new benefit in the application of series capacitors:

k Mitigation of sub synchronous resonance risks

> Damping of active power oscillations

k Post-contingency stability improvement

k Dynamic Power Flow Control

Applicable in new as well as existing, the benefits of thyristor controlled series

compensation are by no means attainable only for installations starting from scratch. It is

fully possible and practicable also to upgrade existing series capacitors by making all of

them thyristor controlled, thereby extending their impact and usefulness in the grid most

considerably. This has been practiced in reality already 14-51.

2.5.3 Mitigation of SSR

The phenomenon of Sub Synchronous Resonance (SSR) has caused concern in the

past in situations where the risk for occurrence of SSR has acted as an impediment to the

use of series compensation in cases where the technology would otherwise have offered the

best and most economical solution. With the advent of TCSC, no such concerns need be

entertained any longer, and series compensation can be used to its fullest merit [ I 1.

The SSR risk used to be linked to the utilization of series compensation of

transmission lines fed by thermal generation, particularly in case of high degrees of

compensation, where analysis showed that the complementary series resonance frequency

of the compensated line(s) coincided with some damped tensional vibration frequency of

the turbo-generator shaft, and could hence induce increased mechanical stresses in the

shafis[2].

The TCSC acts to eliminate this risk for coinciding resonance frequencies by

making the series capacitor(s) act inductive in the sub synchronous frequency hand, thereby

rendering the occurrence of series resonance in the transmission system for sub

synchronous frequencies altogether impossible. This inductive character of the TCSC is

made possible by the use of a thyristor controlled inductor in parallel with the series

capacitor. The system is governed by an patented control scheme called SVR (Synchronous

Voltage Reversal)[S-63.

2.5.4 Damping of Power Oscillations

Oscillations of active power in power transmission systems may arise in corridors

between generating areas as a result of poor damping of the interconnections, particularly

during heavy power transmission. Such oscillations can be excited by a number of reasons

such as line faults or a sudden change of generator output. The presence of active power

oscillations acts to limit the power transmission capacity of interconnections between areas

or regions or even countries. It is often possible to find remedy by building additional lines

or upgrading existing lines, but this cost a lot of money and takes a lot of time, if not

rendered impossible altogether by lack of the necessary permits. In some cases, it may also

be possible to introduce power system stabilizers(PSS) on generators, but this will not

always work, particularly not for inter-area power osciHations which tend to be of a low

frequency (typically 0, 2 Hz to 0, 7 Hz). In either case, TCSC will be an attractive

alternative to consider. It offers a cost-effective, robust power oscillation damper,

insensitive to its location in the system and nano interacting with local oscillation models.

In a number of cases, it will turn out to be the best practicable solution [32J.

Figure: 2.8 Power oscillations damped out by means of TCSC

2.6 OPERATION OF FACTS DEVICES

Normally FACTS devices are operated automatically. They can be located in

unmanned substations. Changing af set-points or operation modes can be done locally and

remotely (example from the substation control room, from a regional control center, or

from a national control center) [ 1-21.

2.6.1 Steps for the identification of facts projects

The first step should always be to conduct a detailed network study to

investigate the critical conditions of grid or grids connections. These conditions

can include risks of voltage problems or even voltage collapse, undesired power

flows, as well as the potential for power swings or sub synchronous resonances.

For a stable grid, the optimized utilization of the transmission lines, example

increasing the energy transfer capability can be investigated.

8 If there is a potential for improving the transmission system, either through

enhanced stability or energy transfer capability, the appropriate FACTS device

and its required rating can be determined.

Based on this technical information, an economical study can be performed to

compare costs of FACTS devices are conventional solutions with the achievable

benefits.

2.6.2 Performance verification

The design of all FACTS devices should be tested in a Transient Network Analyzer

(TNA) under all possibte operational conditions and fault scenarios. The results of the TNA

tests should be consistent with the results of the network study, which was performed at the

start of the project. The results of the TNA study also provide the criteria for the evaluation

of the site commissioning tests.

2.7. POWER ELECTRONIC APPLICATIONS IN POWER TRANSMISSION

SYSTEM

Facts Controllers A power electronic based system & other static equipment that

provide control of one or more AC transmission parameters.

The development of FACTS controllers has followed two different approaches. The

first approach employs reactive impedances or a tap changing transformer with thyristor

switches as the controlled elements, the second approach employs self cornmutated static

converters as voltage sources. In general these are categories [ I -21

In series with the power system (series compensation)

In shunt with the power system (shunt compensation)

Both in series and in shunt with the power system

Types of FACTS Controllers

I FACTS I

Thyristor I Static Var controlled Series Compensator Capacitor (SVC)

Crcsc>

Static 0 Static Synchronous Synchronous Series Compensator Compensator (STATC Ohll) (SSSC)

4

Series - Shunt Series - Series

Unified Power Interline Power Flow Controller Flow Controller EUPFC) (IPFC)

I Thyristor-based FACTS Controllers TCSC, SVC, etc,. I SSSC, STATCOM

UPFC. IPFC

Figure 2.9 Overview of FACTS controllers

2.7.1. Series compensation

In series compensation, FACTS is connected in series with the power system. It

works as a controllable voltage source. In series compensation generally inductors are

connected in series with the transmission line. In case of long transmission line, due to

series inductance when a large current flows through it, it causes a large voltage drop. To

compensate that large voltage drop due to inductance, series capacitances are connected.

All series controllers inject voltage in series with the line. If the voltage is in phase

quadrature with the line, series controlier only supplies or consumes variable reactive

power. Any other phase relationship will involve real power also [30-3 I}.

Tasks of dynamic series compensation:

Reduction of load dependent voltage drops

Reduction of system transfer impedance

Reduction of transmission angle

Increase of system stability

Load flow control for specified power paths

Damping of active power oscillations

2.7.1.1 Static Synchronous Series Compensator (SSSC)

The series reactive compensation scheme, using a switching power converter

(voltage source converter) as a synchronous voltage source to produce a controllable

voltage in quadrature with the line current can be defined as Static Synchronous Series

Compensator (SSSC). It can be defined by the equation j 101

Where, V, (0 is the magnitude of the injected compensating voltage (0 I V, (6) r: V,,,,) and <i is the control parameter. The SSSC consists of a series insertion transformer,

voltage sourced converter. The SSSC model is shown in the Fig. 2.10. SSSC is one of the

most important FACTS controllers [lo}.

Figure 2.10 Basic two machine system with synchronous voltage source.

It is like STATCOM, except that the output is in series with the line. It can be based

on a voltage sourced converter or a current sourced converter. Usually the injected voltage

in series would be quite small compared to the line voltage, and the insulation to the

ground would be quite high. With an appropriate insulation between the primary and

secondary of the transformer, the converter equipment is located on a platfor~n duly

insulated from the ground [47-481.

Prcef /%ef ) Qrerf (Xref )

Energy - Source

(Optional)

Figure 2.11 Functional representation of the synchronous voltage source based on a

voltage sourced converter.

The transformer ratio is tailored to the most economical converter design. Without

an energy source, SSSC can only inject a variable voltage which is 90' leading or lagging

the line current. The primary of the transformer and hence the secondary as well as the

converter have to carry full line current including the fault current unless the converter is

temporarily bypassed during severe line fault. An energy storage device can be connected

to the series controller to inject voltage with variable phase angle in series with the line.

2.7.1.2 Thyristor-Controlled Series Capacitor (TCSC)

The two basic schemes of thyristor-controlled series capacitors, using thyristor-

switched capacitors and a fixed capacitor in parallel with a thyristor-controlled Reactor, are

shown schematically in Fig. 2.12 and 2.13. In the thyristor-switched capacitor scheme of

Fig. 2.12 increasing or decreasing the number of capacitor banks in series controls the

degree of series compensation. To accomplish this, each capacitor bank is inserted or

bypassed by a thyristor valve (switch). To minimize switching transients and to utilize

"natural" commutation, the operation of the thyristor valves is coordinated with voltage and

current zero crossings. In the fixed-capacitor, thyristor-controlled reactor scheme of

Fig. 2.13, the degree of series compensation in the capacitive operating region (the

admittance of the TCR is kept below that of the parallel connected capacitor) is increased

(or decreased) by increasing (or decreasing) the thyristor conduction period, and thereby,

increasing the current in the TCR. Minimum series compensation is reached when the TCR

is off. The TCR may be designed to have the capability to limit the voltage across the

capacitor during faults and other system contingencies of similar effect. Connecting a

number of TCRs plus a fixed capacitor in series in order to achieve greater control range

and flexibility may combine the two schemes [ I -21.

Figure 2.12 TCSC with thyristor switched capacitance

Figure 2.13 TCSC with fixed capacitor

2.7.2 Shunt controller

This may be variable impedance, variable source or combination of these. All shunt

controllers inject current into the system at the point of connection. Combined series-series

cantrollers can be combination of separate series controllers which are controlled in a

coordinated manner. Combined series and shunt controllers are either controlled in

coordinated manner or a unified power flow controller with series and shunt elements for a

unified controller. There can be a real power exchange between the series and shunt

controllers via dc power link [I 3. Tasks of dynamic shunt compensation:

Steady state and dynamic voltage control

Reactive power control of dynamic loads

Damping of active power oscillations

Improvement of system stability

2 .7 .2 .1 Stat ic V A R Compensator (SVC)

Shunt-connected static VAR compensators (SVCs) are used extensively to control

the AC voltage in transmission networks. Power electronic equipment, such as the thyristar

controlled reactor (TCR) and the thyristor switched capacitor (TSC) have gained a

significant market, primarily because of well-proven robustness to supply dynamic reactive

power with fast response time and with low maintenance. With the advent of high power

gate turn-off thyristors and transistor devices ( 0 , IGBT, ...) a new generation of power

electronic equipment, STATCOM, shows great promise for application in power systems

installation of a large number of SVCs and experience gained from recent STATCOM

projects throughout the world motivate us to clarify certain aspects of these devices.

Fig. 2.14 shows a schematic diagram of a Static Var Compensator. The

compensator normally includes a thyristor-controlled reactor (TCR), thyristor-switched

capacitors (TSCs) and harmonic filters.

H W bus

LkLU n1n

Fi I ters

TSC

F i g u r e 2.14 S t a t i c var compensator

It might also include mechanically switched shunt capacitors (MSCs), and then the

term Static Var System is used. The harmonic filters (for the TCR-produced harmonics) are

capacitive at fundamental frequency. The TCR is typically larger than the TSC blocks, so

that continuous control is realized. Other possibilities are fixed capacitors (FCs), and

thyristor switched reactors (TSRs). Usually a dedicated transformer is used, with the

compensator equipment at medium voltage. The transmission side voltage is controlled,

and the Mvar ratings are referred to the transmission side [9- 101.

Typical configurations in SVC

The SVC typically consists of a TCR (Thyristor Controlled Reactor), a TSC

(Thyristor Switched Capacitor) and fixed capacitors (FC) in a harmonic filter arrangement

as shown in Fig. 2.14. The TCR consists of reactors and thyristor valves. The TCR

continuously controls reactive power by varying the current amplitude flowing through the

reactors. The TSC consists of capacitors, reactors and thyristor valves. The TSC switches

on and off the capacitors. The AC fiiters provide fixed reactive power and absorb the

harmonic current generated by the TCR. The TCRtFC is the most basic configuration of

the SVC. The TCRtTSCtFC, the more advanced configuration, can be tuned to minimize

the losses at the most frequent operation point. [2], [4].

A Inductive I

Cspdtb. ~ ~ ' R r Cmaciiim SVC Rmctkr)mvrr . SVC Rcutivcpcwr

TCR PC TCR TSC FC

Figure 2.15 Typical configuration of SVC

Applications of the SVC systems in transmission systems:

To increase active power transfer capacity and transient stability margin

To damp power oscitlations

To achieve effective voltage control

In addition, SVCs are also used

1. ln transmission systems

To reduce temporary over voltages

To damp sub synchronous resonances

To damp power oscillations in interconnected power systems

2. In traction systems

To balance loads

To improve power factor

'To improve voltage regulation

3. In HVDC systems

To provide reactive power to ac-dc converters

4. In arc furnaces

To reduce voltage variations and associated light flicker

i) TCR (Thyristor Controlled Reactor)

The amplitude of the TCR current can be changed cantinuously by varying the

thyristor firing angle. The firing angle can be varied from 9 0 9 0 180'. The TCR firing

angle can be fully changed within one cycle of the fundamental frequency, thus providing

smooth and fast control of reactive power supply to the system[l-21.

Figure 2 .16 T C R current and f i r ing angle

ii) TSC (Thyristor Switched Capacitor)

The TSC is used to switch on and off the capacitor bank. The TSC does not

generate any harmonic current components. The capacitor switching operation is completed

within one cycle of the fundamental frequency. The TSC provides a faster and more

reliable solution to capacitor switching than conventional mechanical switching devices.

The TSC can operate in coordination with the TCR so that the sum of the reactive power

from the TSC and the TCR becomes linear. Applications with only TSCs are also available,

providing stepwise control of capacitive reactive power.

V r ~ O t W - ~ c d I wn.w

0)

Figure 2 .17 TCS current and f i r ing angle

2.7.3 Static Compensator (STATCOM)

STATCOM systems essentially consist of a DC voltage source behind self

commutated inverters using insulated gate bipolar transistor (IGBT). gate turn-off (GTO),

or gate commutated turn-off (GCT) thyristors and an interconnecting transformer. The

voltage source inverter set connects to the power system, via a multi-winding or two

winding inverter transformer, depending upon the application. The Fig. 2.17 shows the

basic STATCOM configuration. The inverter and DC voltage source can be modeled as a

variable voltage source, as shown in the equivalent circuit (Fig, 2.18). The power system

also can be modeled as a voltage source. An inductor representing the leakage reactance of

the transformer connects the two voltage sources. The output voltage phase of'the thyristor-

based inverter, Vi, is controlled in the same way as the system vohage. Vs [ 1-21.

The compact design of STATCOM systems takes up approximately 113 the area

and 115 the volume of conventional Static Var Compensation (SVC) systems.

Figure 2.18 Static Compensator (STATCOM) and voltage /current characteristic

2.8 UNIFIED POWER FLOW CONTROLLER (UPFC)

Gyugui proposed unified power flow controller (UPFC) concept in 1991 [I] . The

UPFC was devised for real time control and dynamic compensation of AC transmission

system, providing multifunctional flexibility required to solve many problems facing the

delivery industry 111. Within the framework of traditional power concept, the UPFC is able

to control, simultaneously or selectively, all the parameters affecting power flow in

transmission line ( i t . , voltage, impedance , and phase angle ) and this unique capability is

signified by the ad.jective "unified, to its name. Alternatively, it can independently control

both the real and reactive power flow in the line. The col~trol of real power is associated

with similar changes in reactive power i,e., increased real power flow also results in

increased reactive line power 141.

2.8.1 Basic Circuit Arrangement

In the currently used practical implementation, the LJPI:C consists of two switching

converters, which in the implementations are considered as voltage source inverters using

gate turn-off (GTO) thyristor valves, as illustrated in Fig. 2.19. These back to back

converters, labeled "converter 1" and "converter 2" in thc ligure. are operated from a

common DC link provided by a DC storage capacitor. This arrangement functions as an

ideal AC-to-AC power converter in which the real power can freely flow in either direction

between the AC terminal of the two converters, and each converter can independently

generate or absorb reactive power at its own AC output termitlals 1221.

Figure 2.19 Basic circuit arrangement of Ilnified Power Flow Controller

2.8.2 Basic Operating Principle

Converter 2 provides the main function of the UPFC by injecting an AC voltage Vw

with controllable magnitude Vw (05Vps <Vwmd~) and phase angle 1360°, at the power

frequency, in series with the line via an insertion transformer. The injected voltage is

considered essentially as a synchronous voltage source. The transmission line current flows

through this voltage source resulting in real and reactive power exchange between insertion

transformer and the AC system. The real power exchange at the AC terminal (i.e., at the

terminal of insertion transformer) is converted by the inverter into DC power that appears at

the DC link as positive or negative real power demanded. The reactive power exchanged at

the AC terminal is generated internally by the inverter 120-221.

The basic function of converter 1 is to supply or absorb the real power absorbed by

the converter 2 at the common DC link to support the real power exchange resulting from

the series voltage injection. This DC link power demand of converter 2 is converted back

to AC by converter 1 and coupled to the transmission line bus via shunt connected

transformer. In addition to the real power need of converter 2, converter 1 can also generate

or absorb controllable reactive power, if desired, and thereby provide independent shunt

reactive compensation for the line. It is important to note that there is a closed direct path

for the real power negotiated by the action of series voltage injection through converters 1

and 2 back to the line. The corresponding reactive power exchanged is supplied or absorbed

locally by converter 2 and therefore does not have to be transmitted by the line. Thus,

converter I can be operated at a unity power factor or can be controlled to have a reactive

power exchanged by converter 2. Obviously, there can be no reactive power flow through

the UPFC DC link [20].

2.8.3 Transmission Control Capabilities

By viewing the operation of the unified power flow controller, the traditional power

transmission is based on reactive shunt compensation and phase angle regulation. The

UPFC can fulfill all these functions and thereby meet multiple control objectives by adding

the injected voltage Vw with appropriate amplitude and phase angle to the (sending end)

terminal voltage Vs. Using phasor representation, the basic LJPFC power flow control

functions are illustrated in Fig. 2.20[17].

2.8.3.1 Terminal Voltage Regulation

Voltage regulation is done by continuousiy variable in-phase /anti phase injection

for voltage increments Vpq =fV (~4). This is functionally similar to that which is

obtainable with a transformer tap changer having infinitely small steps.

2.8.3.2 Series Capacitor Compensation

Series reactive compensation is shown in Fig. 2.20(b) where Vpl =Vq is i~~jected in

quadrature with the line current. Functionally this is similar to series capacitive and

inductive line compensation attained by the SSSC; the injected series compensating voltage

can be kept constant, if desired, independent of line current variation, or can be varied in

proportion with the line current to imitate the compensation obtained with a series capacitor

or reactor [21].

2.8.3.3 Transmission Angle Regulation

Phase angle regulation is shown in Fig. 2.20(c) where Vpq =VO is in.jected with an

angular relationship with respect to V, that achieves the desired sigma phase shifted

(advance or retard) without any change in magnitude. Thus the UPFC can function as a

perfect phase angle regulator which can also supply the reactive power involved with the

transmission angle control by internal var generation.

2.8.3.4 Multifunctional Power Flow Control

Multifunctional power flow control, executed by simultaneously terminal voltage

regulation, series capacitive line compensation, and phase shifting, is shown in Fig. 2.1 1 (d)

where Vw=V+Vq+V,, This functional capability is unique to the UPFC. No single

conventional equipment has similar multifUnctional capability [16].

(a)

V '.-._ _ _ - v+ V"

(a) Voltage regulation (b) Line impedance cwnpensatton (c) Phase shiing (d) Simultaneous control of voltage, irnpedanoe, and an*

Figure 2.20 Phasor diagrams illustrating the transmission control capabilities of the

Unified Power Flow Controller

2.8.3.5 BASIC PRINCIPLES OF P AND Q CONTROL

Consider Fig. 2.10(a) at (a) a simple two machine (or two bus AC inter tie) system

with sending end voltage Vs, receiving end voltage Vr and the line or tie impedance X

(assumed for simplicity, inducted) is shown. At (b), the voltage of the system in the form of

a phasor diagram is shown with transmission angle 6 and /Vs I=IVrl=V. At (c) the

transmitted P(P=v~/xs~~G) and the reactive power Q=Q,=Q,(Q=V~/X(I-C~S~)) supplied at

the end of the line are shown plotted against angle 6. At (d) the reactive power Q=Q, =Q,

is shown plotted against the transmitted power P corresponding to "stable values of 6 (ie..,

03190)

Figure 2.21 (a)simple two machine system, (b) related voltage phasors, (c)real and

reactive power vs. transmission angle, and (d) sending end and receiving end reactive

power vs. transmission angle.

The basic power of Fig.2.21 with the well known transmission characteristics is

introduced for providing a vehicle to establish the capability of the UPFC. It controls the

transmitted real power P, and the reactive power demands Q, and Q, at the sending end and

the receiving end of the line respectively [34].

Consider Fig. 2.10 the simple power system is expanded to include the UPFC. The

UPFC is represented by a controllable voltage source in series with the line which, as

explained in the previous section, can generate or absorb reactive power that i t negotiates

with the line but the real power it exchanges must be supplied to it, or absorbed from it, by

the sending end generator. The UPFC in series with the line is represented by the phase V,

having magnitude V,, (0<V,5VP,,,,) and the angle p (O<p 5360) measured from the given

phase position of phasor V,, as illustrated in the figure. 'l'lie line current represented by

phasor 1 , flows through the series voltage source, V,, and generally results in both the

reactive and real power exchanges.

In order to represent the UPFC properly, the series voltage source is stipulated to

generate only the reactive power QPq which exchanges with the line. Thus the real power P,,

negotiates with the line assumed to be transferred to the sending end generators if a perfect

coupling for real power flow between it and the sending end generator existed. This is an

arrangement with the UPFC circuit structure in which the DC link between the two

constituent inverters establishes a bi-directional coupling for real power flow between the

injected series voltage source and the sending end bus [24].

As Fig. 2.10 implies, in the present discussion it is further assumed to clarify that

the shunt reactive compensation capability of the UPFC was not utilized. This is the UPFC

shunt inverter assumed to be operated at unity power factor, its sole function being to

transfer the real; power demand of the series inverter to the sending end generator. With

these assumptions the series voltage source together with the real power coupling to the

sending end generator as shown in Fig. 2.10 is an accurate representation of the basic UPFC

It can be readily observed in Fig. 2.10 which shows that the transmission line "sees"

Vs+ V, as the effective sending end voltage. Thus it is clear that the LJPFC effects the

voltage (both its magnitude and angle) across the transmission line and therefore it is

reasonable to expect that it is able to control by varying the magnitude and angle of V,, the

transmittable real power as well as the reactive power demand of the line at any given

transmission angle between the sending end and receiving end voltages [26].

2.9 INTERLINE POWER FLOW CONTROLLER (IPFC):

2.9.1 Basic Principles

The basic family of FACTS controller like SVC, SSSC, STATCOM, UPFC are

devised primarily for the control of single line. But thc objective of Interline Power Flow

Controller (IPFC) is to provide a comprehensive power flow control scheme for a multiline

transmission system, in which two or more lines employ an SSSC for series compensation.

A multiline IPFC comprises a number of n SSSC, one for each line of the transmission

system to be controlled, with a common DC bus, as illustrated schematically by a block

diagram 2.22. The IPFC scheme has the capability to transfer real power between the

compensated lines in addition to executing the independent and controllable reactive

compensation of each line. This capability makes it possible to equalize both real and

reactive power flow between the lines, to transfer power demand from overloaded to under

loaded lines, to compensate against resistive line voltage drops and the corresponding

reactive line power. It also increases the effectiveness of the compensating system for

dynamic disturbances like transient stability and power oscillation damping. The general

schematic diagram of the interline power flow controller is shown in Fig. 2.22. From the

Figure it is evident that by appropriately controlling the real power compensation the

reactive line power flow can be made constant and independent of the real power

transmitted [ 1 11.

Figure 2.22 General schematic of IYFC

2.9.2 Operation

The operation of the IPFC can be described with the two dimensional Q-P plot for

multiline transmission system. For example consider the jkh line of n-line system. This is

similar to that of a UPFC in which the achievable Q and P values lie within a circle drawn

around the Qo and Po points of the uncompensated line obtained at the prevailing phase

angle. The radius of the circle is proportional to the maximum magnitude of the injected

series compensating voltage. The compensation of the J ' ~ line within the IPFC scheme can

be characterized by the parallel shifting of the reactive compensation control line obtained

with purely reactive compensation when no real power supplied to or absorbed from the

line via the jth converter. If the jih converter supplies real power to the jth line, then the

reactive compensation line is shifted in the positive direction i.e., upwards and the reactive

power demand of this line decreases and also changes sign. lf the jth converter absorbs real

power from the jth line, then the compensation control line is shifted in the negative

direction i-e., downwards and the reactive power demand of this line increases. The Q-P

diagram illustrating the operation of IPFC is shown in Fig. 2.24. 'From the figure it is

evident that by appropriately controlling the real power compensation the reactive line

power flow can be made constant and independent of the real power transmitted [ I 2-1 31.

Figure 2.23 IPFC prime converter and corresponding phasor diagram.

P-Q control lines

cornpensatlon control line (qZ = 0)

Figure 2.24 Q-Pdiagramillustrating the operation of IPFC variation of receiving end

real and reactive power as a function of the injected compensating voltage in line I

The operation of a multi line IPFC requires the sum of the real power exchanged by

the total number of converter must be zero. In this the general idea of IPFC is that the

strong or under loaded lines are forced to help the weaker or overloaded lines in order to

optimize the utilization of the whole transmission system. This can be achieved by

connecting a shunt converter to compensate when the real power required by weaker line

exceeds the real power that can be absorbed. [14].

2.9.3 Control Structure

Two unique operating features of the converter based FACTS controllers in

addition to their capability to internally generate reactive power are given below. One is

their capability to maintain maximum compensating current/voltage in face of decreasing

line voltage/current, which results in superior characteristics for shundseries compensation;

the other is their ability to exchange real as well as reactive power with the AC system and

thereby provide independent control of real and reactive power flow in transmission

system.

The basic control scheme for a two line IPFC is shown in Fig. 2.25. In the control

structure each converter is synchronized with its line current by a phase locked loops which

enable to provide independent series compensation and operating under contingency

conditions. The input to the prime converter can be the power or voltage reference derived

from it. This is compared with the actual voltage components V,, and V,, derived from the

measured line current and the series injected voltage vectors 1, and V,,,. Thus the error

signal is obtained after appropriate amplification and possible limitation provided by the

input for the computation of the magnitude V,, and the angle yl of the injected voltage

vector V,,. The prime converter and the other converter controls are different because of

the real and reactive power exchanged [15].

mKlctlv. 1-w. v;

computer -

m-I comp. Vqp computer

K 6 c . Error

amplHkr &m.!l-

Error amplHler

Phase- locked Serles crmvartar 2 l W D A

Ma~nbtudrl Sonar and camrodar 2 - - anole VaP uale patmrn

computer 10gk

Figure 2.25 Basic control scheme for a two converter IPFC

2.9.4 Computer Simulation

The operation of the IPFC has been simulated with the use of the Electromagnetic

Transients Program (EMTP) simulation package. Referring to Fig. 2.25. An elementary

IPFC, composed of Voltage-Sourced Converters 1 and 2 with a common dc link, provides

(real and reactive) series compensation for lines1 and 2.The two-bus lines are assumed

identical; each has a sending-end voltage of 1.0 p.u., a receiving-end voltage of 1.O.p.u.

with a 30' transmission angle, and a line reactance of 5.0 p.u [I].

The converter in line 1 is operated as the "prime" converter which can inject a

voltage in series with line at any angle with respect to the sending-end voltage. Converter

2, on the other hand, is operated as a "support" converter compelled to inject a voltage in

series with line 2 so as to satisfy the real power demand (both supplied and absorbed) of the

converter 1 [ 1 1.

Fig 2.26 Waveforms showing the operation of the IYFC with the "prime" Converter 1

emulating real (resistive), capacitive, and inductive compensations of line 1

Figure 2.27 Waveforms showing the operation of Converter 2providing real power

demand of Converter 1 and capacitive series compensation for Line 2

Consider the case, shown in Fig. 2,28(a), where a voltage phasor VI, with a

magnitude of 0.1 3 p.u. is injected at -30'. In the corresponding simulation, Fig. 2.26 in the

0 to 50 rns interval shows the uncompensated line current i l , and the corresponding real

power P~~andreactive power Qr, at the receiving end of linel. Fig. 2.27 shows the

(identical) uncompensated line current i2, and the corresponding real power P2 and reactive

power Q2 at

1

(b)

6.1

V1p"

1

41

I

3

9J

.l

Figure 2.28 Illustration of the operation of a two-converter IPFC by coordinated phasor diagrams and P-Q plots.

the receiving end bus of line 2. At 50 ms, the voltage phasor V,,, stipulated in Fig. 2.28(a),

is injected to provide only real power compensation. As a result, Converter) supplies Plw

for linel. Converter2 is operated so as to absorb real power Pz, from line2. The sum of

PI,, and P2W for an ideal system must be zero. According to the ideal case illustrated in Fig.

2.28(a), the real and reactive power, PI, and QIr, should change from 11.0, -0.2681 p.u.

According to the illustrations to Fig. 2.28(d), P2, and Q2, should change from [ I .O, -0.2681

p.u. to [0.866, -0.51 p.u. The slight discrepancies in the simulation results are due to the

finite resistance of the transmission line reactance and the leakage reactance of'the coupling

transformer of the converter.

At 150 ms, a voltage phasor VIP, with a magnitude of 0.26 p.u. is injected at +45

degrees, as shown in Fig. 2.28(b), to emulate a capacitive reactance in series with the

transmission line. The line current i l , is at its ~naximum value. The values of PI, and Q I ,

should, with an ideal lossless system, change to [I .5, -0.1341 p.u.

At 250 ms, a voltage phasor VI, with a magnitude of 0.26 p.u. is injected at -75

degrees, as shown in Fig. 2.28(c), to emulate an inductive reactance in series with the

transmission line. The line current il, is at its minimum value. The values of P,, and QI,

should change to 10.634, 0.0981 p.u. In all these cases, the injected voltage phasor, Vlw

moves along the constant Plpq line. Therefore, both Plpq and P2,, should maintain the same

fixed values throughout this time. Also note that Converter2 in these three illustrations i's,

by choice, operated with zero reactive compensation, exchanging no reactive power with

line2 (Q2*=0), and thus the values of P2r and Qzr should stay at [0.866, -0.51 p.u.

At 350 ms, the operation of Converter2 is changed to inject a voltage phasor Vz,

with a magnitude of 0.26 p.u. so as to emulate a capacitive reactance, in addition to the

resistance (i.e., the real power demand of Converterl), in series with the transmission Line.

The real and reactive power, Pzr and Qzr should, in an ideal case, change to [1.366, -0.6341

p.u., without affecting PI, and Qlr.

Time (ms) : ---4 .--4

1 a, 4$0

Fig. 2.29 Real and reactive power in lines 1 and 2 when the IPFC is controlling the real power

in line 1 at unity power factor and maintaining constant real power flow in line2

The real and reactive power plots in Fig. 2.29 show the case in which the lPFC is

controlling the real power flow in (prime) linel, in response to a given reference, while

keeping the corresponding receiving-end reactive power practically at zero and maintaining

a substantially constant real power transmission in line2.This is achieved by appropriately

adjusting the real and reactive compensating voltage components in line2 to meet the real

power demand of line1 and maintain constant real power flow in line2[2].

2.9.5 Practical and Application Considerations

The concept and basic operating principles of the lPFC are explained for clarity

within the framework of two identical (and simple) systems in the previous section. In

practical applications the IPFC would, in general, have to manage the power flow control

of a complex, multiline system in which the length, voltage, and capacity of the individual

lines could widely differ. One of the attractive features of the IPFC is that it is inherently

flexible to accommodate complex systems and diverse operating requirements. A few

considerations relevant to practical applications are listed.

(1) The IPFC is particularly advantageous when controlled series compensation or

other series power flow control (e.g., phase shifting) is contemplated. This is because the

IPFC simply combines the otherwise independent series ctlrnpensators (SSSCs), without

any significant hardware addition, and af'fords some of those a greatly enhanced functional

capability. The increased functional capability can be moved from one line to another, as

system conditions may dictate. In addition, the individual converters of the IPFC can be

decoupled and operated as independent series reactive compensators, without any hardware

change [14].

(2)Although converters with different I)C voltage could be coupled via appropriate

dc-to-dc converters choppers the arrangement would be expensive with relatively high

operating losses. Therefore, it is desirable to establish a common DC operating voltage for

all converter based controller used at one location, which would facilita~e their DC

coupling and thereby an inexpensive extetision of their functional capabilities. Reasonably

defined common DC operating voltage should not impose significant restriction on the

converter design, since at high output power, multiple parallel poles are normally

employed. Apart from the potential for DC coupling, common operating voltage would

also be helpful for the standardization of the converter type equipment used at one location,

as well as for the maintenance of spare parts inventory [15].

(3) The operating regions of the individual converters of the IPFC can differ

significantly, depending on the voltage and power ratings of the individual lines and on the

amount of compensation desired. It is evident that a high voltagelhigh power line may

supply the necessary real power for a low voltagellow power capacity line to optimize its

power transmission, without significantly affecting its own transmission [I 61.

(4)The IPFC is an ideal solution to balance both the real and reactive power flow in

multiline and meshed systems.

(5)The prime converters of the IPFC can be controlled to provide totally different

operating functions, e.g., independent (P) and (Q) control, phase shifting (transmission

angle regulation), transmission impedance control, etc. These functions can be selected

according to prevailing system operating requirements [ I 31.