FACTS

Introduction

Along with the economic development and the social improvement, the electric utilities must run more rapidly to meet the heavily increasing demands of electric power. However, in the procedure of expanding and interconnecting of the power System, accordingly various problems arise:

Ø On account of the irregular distribution of the energy sources, and the vast amount of transfer electric power over long distance between the generation center and the load center, huge power losses in the lines occur.

Ø In the interconnected power system, the power flow from the generator to the consumers is dependent on the location of the generation node, of the consumer nodes and on the transmission paths available, i.e. on the power system topology and the electrical characteristics of the lines involved, the result is transmission bottlenecks and unwanted parallel path or loop flows. To meet the load and electric market demands, new lines should be added to the system, but because of a variety of environmental land use and regulatory pressures, the growth of electric power transmission lines in many parts of the world is restricted.

Ø In the large-scale power system, the stability becomes more critical; several large-area power failures due to damaging of the power system stability resulted in enormous economic losses in the world.

On these backgrounds, there is an urgent demand to realize the rational transfer power allocation, to reduce the power losses and generation costs, and to improve the stability and the reliability of the power system greatly. On the other hand, in recent years the tremendous growth of the modem power electronics, computer control and communication technologies provide the opportunity for the development of FACTS.

Since 1986 Hingorani put forward FACTS, “Flexible AC Transmission System”, and aimed to transport the thyristor-based control technology into the AC system[’]. Firstly FACTS, which aimed to adopt modem power electronics application at the important location of the transmission system in order to control and adjust one or more of the main parameters of the transmission system (voltage, phase angle, and impedance), was restricted in the transmission system and viewed to be irrelevant to the generation and the distribution parts, . Recently, it was considered to contain the technologies of the generation and distribution parts which affected the AC transmission system. This paper regards FACTS as all modem power electronics applications which can make impacts on the AC transmission in the large-scale power system. And the scope of its application is thought to be the whole power system, to contain generation, transmission and distribution parts, in which the transmission system is the key section. By the flexible and rapid control over the AC transmission parameters and network topology,

FACTS can facilitate the power control, enhance the power transfer capacity, decrease line losses and generation costs, and improve the stability arid security of the power system. At the present time, FACTS technology has already been considered to bring a bright prospect for the modem power system.

INTRODUCTION TO FACTS

The reactive power compensation of AC lines using fixed series or shunt capacitors can solve some of the problems associated with AC networks. However the slow nature of control using mechanical switches (circuit breakers) and limits on the frequency of switching imply that faster dynamic controls are required to overcome the problems of AC transmission networks. Recent developments involving deregulation and restructuring of the power industry are aimed at isolating the supply of electrical energy (a product) from the service, involving transmission from generating stations to load centers. This approach is feasible only if the operation of AC transmission is made flexible by introducing fast-acting high-power solid state controllers using thyristor or GTO valves (switches). The advent of high voltage and high power thyristor valves and digital controllers in HVDC transmission has demonstrated the viability of deploying such controllers for power transmission. Thyristor controllers were also utilized in the late seventies to control current in reactors and switch capacitors and this led to the development of Static VAR Compensators (SVC)

Flexible AC Transmission System (FACTS) is a concept proposed by Hingorani (1988. 1991. 1993) that involves the application of high power electronic controllers in AC transmission networks which enable fast and reliable control of power flows and voltages. FACTS do not indicate a particular controller but a host of controllers which the system planner can choose, based on cost bene.fit analysis. The objectives are as below

(1) Regulation of power flows in prescribed transmission routes.

(2) Secure loading of lines nearer their thermal limits.

(3) Prevention of cascading outages by contributing to emergency control.

(4) Damping of oscillations which can threaten security or limit the usable line capacity.

Defenitions

The term “FACTS” (Flexible AC Transmission Systems) covers several power electronics based systems used for AC power transmission and distribution. Given the nature of power electronics equipment, FACTS solutions will be particularly justifiable in applications requiring one or more of the following qualities:

-Rapid dynamic response

-Ability for frequent variations in output

-Smoothly adjustable output.

FACTS is defined by the IEEE as "a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability."

FACTS are a family of devices which can be inserted into power grids in series, in shunt, and in some cases, both in shunt and series. Important applications in power transmission and distribution involve devices such as SVC (Static Var Compensators), Fixed Series Capacitors (SC) as well as Thyristor-Controlled Series Capacitors (TCSC) and STATCOM. SVC and SC have been utilized for a long time.

The first SC installations came on line in the early 1950s. Among the pioneering countries are USA and Sweden. SVCs have been available for commercial purposes since the 1970s. Over the years, more than a thousand SVCs and SCs have been installed all over the world.

IEEE definition of FACTS and FACTS controllers are given as

• Flexible AC Transmission System

(FACTS): Alternating current transmission systems incorporating power electronics based and other static controllers to enhance controllability and increase power transfer capability.

• FACTS Controller: Power electronics based system and other static equipment that provides control of one or more AC transmission system parameters.

FACTS mainly find applications in the following areas:

• Power transmission

• Power quality

• Railway grid connection

• Wind power grid connection

• Cable systems

FACTS Concept

From the general point of view, the FACTS principle is mainly depend on the advanced technologies of power electronic techniques and algorithms into the power system, to make it electronically controllable. Much of the research upon which FACTS rests evolved over a period of many years. Nevertheless, FACTS, an integrated technology, is a novel concept that was brought to fruition during the 1980s at the Electric Power Research Institute (EPRI) for applications of North American army objectives. FACTS can capitalize on the many ideas taking place in the area of high-voltage and high-current power electronics, to improve the control of power flows in networks during both steady-state and transient conditions. The recent reality of making the power network electronically controllable has initiated a change in the way that power plant equipment is designed and built as well as the technology that goes into the planning and operation of transmission and distribution networks. These achievements may also enhance the method energy exchanges are done, as high-speed control of the path of the energy flow is now feasible. FACTS own a lot of promising benefits, technical and economical, which get the benefits of electrical equipment devices, operators, and research groups around the world. FACTS controllers have been installed in various regions of the world. The well known types are: load tap-changers transformer, static VAR compensators, phase-angle regulators, thyristor-controlled series compensators, static compensators, inter-phase power controllers and unified power flow controllers.

This thesis covers in breadth and depth the modelling and simulation methods required for a thorough study of the steady-state and dynamic operation of electrical power systems with FACTS controllers. The characteristics of a given power system evolve with time, as load grows and generation is added. If the transmission grid capacity is not updated sufficiently the power network becomes vulnerable to steady state and transient stability problems, as stability margins will be narrower.

The powerful of the transmission grid to transmit power has constraint by one or more of the following steady-state and dynamic limitations: Angular stability, Voltage stability, Thermal limits, Transient stability, and Dynamic stability. These limits affect the packages of the power to be transferred without blackout to transmission lines and electric apparatuses. Mainly restrictions on power exchange can be controlled by installing new transmission and generation circuits. Also, FACTS controllers can achieve the same tasks to be met with no huge changes to system layout. FACTS controllers save a lot of benefits such as reduction of operation and transmission investment cost, increased system security and system reliability, maximize power transfer capabilities, and an overall enhancement of the quality of the electric energy delivered to customers. From the operational point of view, FACTS technology is concerned with the ability to control, in an adaptive trend, the directions of the power flows throughout the network, where before the advent of FACTS, high-speed control was very limited. The ability to control the line impedance and the buses voltage magnitudes and phase angles at both the sending and the receiving ends of transmission lines, with almost no delay, has significantly increased the transmission capabilities of the network while considerably enhancing the security of the system. In many practical situations, it is desirable to include economical and operational considerations into the power flow formulation, so that optimal solutions, within constrained solution spaces, can be obtained.

BENEFITS FROM FACTS TECHNOLOGY

Power supply industry is undergoing dramatic change as a result of deregulation and political and economical driving forces in many parts of the world. This new market environment puts growing demands for flexibility and power quality into focus. Also, trade of electric power between countries is gaining momentum, to the benefit of all involved. This calls for the right solutions as far as power transmission facilities between countries as well as between regions within countries are concerned.

As indicated by the acronym, FACTS stands for flexibility in AC power systems. Properly utilized, this offers benefits to users of a variety of kinds. Without the need to reinforce the grid by means of additional or upgraded existing lines and/or substations FACTS brings about:

-An increase of synchronous stability of the grid;

-Increased power transmission capability;

-Increased voltage stability in the grid;

-Decreased power wheeling between different power systems;

-Improved load sharing between parallel circuits;

-Decreased overall system transmission losses;

-Improved power quality in grids.

The choice of FACTS device in each given case may not be obvious but may need to be made the subject of system studies, taking all relevant requirements and prerequisites of the system into consideration, so as to arrive at the optimum technical and economical solution. In fact, the best solution may often be a combination of devices.

With FACTS, the following benefits can be attained in AC systems:

• Improved power transmission capability

• Improved system stability and availability

• Improved power quality

• Minimized environmental impact

• Minimized transmission losses

The FACTS technology offers the following advantages:

Ø Increase the amount of power that can be imported over existing transmission lines.

Ø Provide dynamic reactive power support and voltage control.

Ø Reduce the need for construction of new transmission lines, capacitors, reactors, etc which

– Mitigate environmental and regulatory concerns.

– Improve aesthetics by reducing the need for construction of new facilities such as transmission lines.

Ø Improve system stability.

Ø Control real and reactive power flow.

Ø Mitigate potential Sub-Synchronous Resonance problems.

Flexible AC Transmission System Controllers

General Description

The large interconnected transmission networks (made up of predominantly overhead transmission lines) are susceptible to faults caused by lightning discharges and decrease in insulation clearances by undergrowth. The power flow in a transmission line is determined by Kirchhoff’s laws for a specified power injection (both active and reactive) at various nodes. While the loads in a power system vary by the time of the day in general, they are also subject to variations caused by the weather (ambient temperature) and other unpredictable factors. The generation pattern in a deregulated environment also tends to be variable (and hence less predictable). Thus, the power flow in a transmission line can vary even under normal, steady state conditions. The occurrence of a contingency (due to the tripping of a line, generator) can result in a sudden increase/decrease in the power Flow. This can result in overloading of some lines and consequent threat to system security.

A major disturbance can also result in the swinging of generator rotors which contribute to power swings in transmission lines. It is possible that the system is subjected to transient instability and cascading outages as individual components (lines and generators) trip due to the action of protective relays. If the system is operating close to the boundary of the small signal stability region, even a small disturbance can lead to large power swings and blackouts.

The increase in the loading of the transmission lines sometimes can lead to voltage collapse due to the shortage of reactive power delivered at the load centres. This is due to the increased consumption of the reactive power in the transmission network and the characteristics of the load (such as induction motors supplying constant torque).

The factors mentioned in the previous paragraphs point to the problems faced in maintaining economic and secure operation of large interconnected systems. The problems are eased if sufficient margins (in power transfer) can be maintained. This is not feasible due to the difficulties in the expansion of the transmission network caused by economic and environmental reasons. The required safe operating margin can be substantially reduced by the introduction of fast dynamic control over reactive and active power by high power electronic controllers. This can make the AC transmission network `flexible' to adapt to the changing conditions caused by contingencies and load variations. Flexible AC Transmission System (FACTS) is defined as

`Alternating current transmission systems incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability'.

The FACTS controller is defined as

`A power electronic based system and other static equipment that provide control of one or more AC transmission system parameters'.

BASIC TYPES OF FACTS CONTROLLERS

By FACTS, operator governs the phase angle, the voltage profile at certain buses and line impedance. Power flow is controlled and it flows by the control actions using FACTS devices, which include

Ø Static VAR Compensators (SVC)

Ø Thyristor Controlled Series Capacitors (TCSC)

Ø Static Compensators (STATCOM)

Ø Static Series Synchronous Compensators (SSSC)

Ø Unified Power Flow Controllers (UPFC) etc

In general the FACTS controllers can be classified as

Shunt connected controllers

Series connected controllers

Combined series-series controllers

Combined shunt-series controllers

Ø Series controllers such as Thyristor Controlled Series Capacitor (TCSC), Thyristor Controlled Phase Angle Regulators (TCPAR or TCPST), and Static Synchronous Series Compensator (SSSC)

Ø Shunt controllers such as Staic Var Compensator (SVC), and Static Synchronous Compensator (STATCOM)

Ø Combined series-shunt controllers such as Unified Power Flow Controller (UPFC)

Ø Combined series-series controllers such as Interline Power Flow Controller (IPFC)

Depending on the power electronic devices used in the control, the FACTS controllers can be classified as

Ø Variable impedance type(Line Reactance Compensator)

Ø Voltage Source Converter (VSC) based.

The variable impedance type controllers include:

(i) Static Var Compensator (SVC), (shunt connected)

(ii) Thyrister Controlled Series Capacitor or compensator (TCSC), (series connected)

(iii) Thyristor Controlled Phase Shifting Transformer (TCPST) of Static PST (combined shunt and series)

The VSC based FACTS controllers are:

(i) Static synchronous Compensator (STATCOM) (shunt connected)

(ii) Static Synchronous Series Compensator (SSSC) (series connected)

(iii) Interline Power Flow Controller (IPFC) (combined series-series)

(iv) Unified Power Flow Controller (UPFC) (combined shunt-series)

Some of the special purpose FACTS controllers are

(a) Thyristor Controller Braking Resistor (TCBR)

(b) Thyristor Controlled Voltage Limiter (TCVL)

(c) Thyristor Controlled Voltage Regulator (TCVR)

(d) Interphase Power Controller (IPC)

(e) NGH-SSR damping

Objectives of FACTS controllers

The main objectives of FACTS controllers are the following:

1. Regulation of power flows in prescribed transmission routes.

2. Secure loading of transmission lines nearer to their thermal limits.

3. Prevention of cascading outages by contributing to emergency control.

4. Damping of oscillations that can threaten security or limit the usable line capacity.

The implementation of the above objectives requires the development of high power compensators and controllers. The technology needed for this is high power electronics with real-time operating control. The realization of such an overall system optimization control can be considered as an additional objective of FACTS controllers.

Types of FACTS controllers

Series controllers. The series controller could be a variable impedance, such as capacitor, reactor, or a power electronics based variable source of main frequency, subsynchronous and harmonic frequencies (or a combination) to serve the desired load. 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 variable reactive power. Any other phase relationship will involve handling of real power as well. Series controllers include SSSC, IPFC, TCSC, TSSC, TCSR, and TSSR.

Shunt controllers. As in the case of series controllers, the shunt controllers may be variable impedance, variable 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 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 reactive power. Any other phase relationship will involve handling of real power as well. Shunt controllers include STATCOM, TCR, TSR, TSC, and TCBR.

Combined series-series controllers. This could be a combination of separate series controllers, which are controlled in a coordinated manner, in a multiline transmission system. Or it could be a unified controller in which series controllers provide independent series reactive compensation for each line but also transfer real power among the lines via the proper link. The real power transfer capability of the unified series-series controller, referred to as IPFC, makes it possible to balance both real and reactive power flow in the lines and thereby maximize the utilization of the transmission system. The term “unified” here means that the dc terminals of all controller converters are all connected together for real power transfer.

Combined series-shunt controllers. This could be a combination of separate shunt and series controllers, which are controlled in a coordinated manner, or a UPFC with series and shunt elements. In principle, combined shunt and series controllers inject current into the system with the shunt part of the controller and 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 a real power exchange between the series and shunt controllers via the proper link. Combined series-shunt controllers include UPFC, TCPST, and TCPAR.

FACTS controllers

STATCOM. STATCOM is a static synchronous generator operated as a shunt-connected static

var compensator whose capacitive or inductive output current can be controlled independent of the ac system voltage.

SVC. SVC is a shunt-connected static var generator or absorber whose output is adjusted to exchange capacitive or inductive current so as to maintain or control specific parameters of the electrical power system (typically bus voltage). SVC is an important FACTS controller already widely in operation. Ratings range from 60 to 600 MVAR [10]. SVC can be considered as a “first generation” FACTS controller and uses thyristor controllers. It is a shunt reactive compensation controller consisting of a combination of fixed capacitor or thyristor-switched capacitor in conjunction with thyristor-controlled reactor.

TCR. TCR is a shunt-connected thyristor-controlled inductor whose effective reactance is varied

in a continuous manner by partial-conduction control of the thyristor valve. TCR has been used as one of the economical alternatives of FACTS controllers.

TSC. TSC is a shunt-connected thyristor-switched capacitor whose effective reactance is varied

in a stepwise manner by full- or zero-conduction operation of the thyristor valve .

TSR. TSR is a shunt-connected thyristor-switched inductor whose effective reactance is varied in a stepwise manner by full- or zero-conduction operation of the thyristor valve .

TCBR. TCBR is a shunt-connected thyristor-switched resistor, which is controlled to aid stabilization of a power system or to minimize power acceleration of a generating unit during a disturbance .

SSSC. SSSC is a static synchronous generator operated without an external electric energy source as a series compensator whose output voltage is in quadrature with, and controllable independently of, the line current for the purpose of increasing or decreasing the overall reactive voltage drop across the line and thereby controlling the transmitted electric power. The SSSC may include transiently rated energy storage or energy absorbing devices to enhance the dynamic behavior of the power system by additional temporary real power compensation, to increase or decrease momentarily, the overall real (resistive) voltage drop across the line.

TCSC. TCSC is a capacitive reactance compensator, which consists of a series capacitor bank shunted by a thyristor-controlled reactor in order to provide a smoothly variable series capacitive reactance.

TSSC. TSSC is a capacitive reactance compensator, which consists of a series capacitor bank shunted by a thyristor-switched reactor to provide a stepwise control of series capacitive reactance.

TCSR. TCSR is an inductive reactance compensator, which consists of a series reactor shunted by a thyristor-controlled reactor to provide a smoothly variable series inductive reactance.

TSSR. TSSR is an inductive reactance compensator, which consists of a series reactor shunted by a thyristor-controlled reactor to provide a stepwise control of series inductive reactance.

TCPAR/ TCPST. TCPST is a phase-shifting transformer adjusted by thyristor switches to provide a rapidly variable phase angle. This controller is also referred to as TCPAR.

UPFC. UPFC is a combination of STATCOM and a SSSC which are coupled via a common dc link to allow bidirectional flow of real power between the series output terminals of the SSSC and the shunt output terminals of the STATCOM and are controlled to provide concurrent real and reactive series line compensation without an external electric energy source. The UPFC, by means of angularly unconstrained series voltage injection, is able to control, concurrently or selectively, the transmission line voltage, impedance, and angle or, alternatively, the real and reactive power flow in the line. The UPFC may also provide independently controllable shunt reactive compensation.

GUPFC. GUPFC can effectively control the power system parameters such as bus voltage, and real and reactive power flows in the lines .A simple scheme of GUPFC consists of three converters, one connected in shunt and two connected in series with two transmission lines terminating at a common bus in a substation. It can control five quantities, i.e., a bus voltage and independent active and reactive power flows in the two lines. The real power is exchanged among shunt and series converters via a common dc link.

IPC. IPC is a series-connected controller of active and reactive power consisting, in each phase, of inductive and capacitive branches subjected to separately phase-shifted voltages. The active and reactive power can be set independently by adjusting the phase shifts and/or the branch impedances, using mechanical or electronic switches. In the particular case where the inductive and capacitive impedance form a conjugate pair, each terminal of the IPC is a passive current source dependent on the voltage at the other terminal.

The original concept of IPC was first described in and the practical design aspects of a 200

MW prototype for the interconnection of the 120 kV networks.

TCVL. TCVL is a thyristor-switched metal-oxide varistor used to limit the voltage across its terminals during transient conditions .

TCVR. TCVR is a thyristor-controlled transformer that can provide variable in-phase voltage with continuous control .

IPFC. IPFC is a combination of two or more SSSCs that are coupled via a common dc link to facilitate bi-directional flow of real power between the ac terminals of the SSSCs and are controlled to provide independent reactive compensation for the adjustment of real power flow in each line and maintain the desired distribution of reactive power flow among the lines. The IPFC structure may also include a STATCOM, coupled to the IPFC common dc link, to provide shunt reactive compensation and supply or absorb the overall real power deficit of the combined SSSCs.

Benefits of FACTS controllers

FACTS controllers enable the transmission owners to obtain, on a case-by-case basis, one or more of the following benefits:

1. Cost: Due to high capital cost of transmission plant, cost considerations frequently overweigh all other considerations. Compared to alternative methods of solving transmission loading problems, FACTS technology is often the most economic alternative .

2. Convenience: All FACTS controllers can be retrofitted to existing ac transmission plant with varying degrees of ease. Compared to high voltage direct current or six-phase transmission schemes, solutions can be provided without wide scale system disruption and within a reasonable timescale.

3. Environmental impact: In order to provide new transmission routes to supply an ever increasing worldwide demand for electrical power, it is necessary to acquire the right to convey electrical energy over a given route. It is common for environmental opposition to frustrate attempts to establish new transmission routes. FACTS technology, however, allows greater throughput over existing routes, thus meeting consumer demand without the construction of new transmission lines. However, the environmental impact of the FACTS device itself may be considerable. In particular, series compensation units can be visually obtrusive with large items of transmission equipment placed on top of high-voltage insulated platforms.

4. Control of power flow to follow a contract, meet the utilities own needs, ensure optimum power flow, minimize the emergency conditions, or a combination thereof.

5. Contribute to optimal system operation by reducing power losses and improving voltage profile.

6. Increase the loading capability of the lines to their thermal capabilities, including short term and seasonal.

7. Increase the system security by raising the transient stability limit, limiting short-circuit currents and overloads, managing cascading blackouts and damping electromechanical oscillations of power systems and machines.

8. Provide secure tie line connections to neighboring utilities and regions thereby decreasing overall generation reserve requirements on both sides.

9. Provide greater flexibility in sitting new generation.

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

11. Reduce loop flows.

12. Increase utilization of least cost generation.

13. Overcome the problem of voltage fluctuations and in particular, voltage fluctuations.