Santos Jr.

G. Santos Jr.

Eneltec- Energia El ´etrica e Tecnologia, Brazil, South America

This chapter presents the basic operation principles of FACTS

32.1 Introduction

devices. Starting with a brief introduction on the concept and its origin, the text then focuses on the ideal behavior of In 1988, Hingorani [1] published a paper entitled “Power Elec- each basic shunt and series of FACTS device. Guidelines on tronics in Electric Utilities: Role of Power Electronics in Future the synthesis of the first generation of these devices based on Power Systems,” which proposed the extensive use of power thyristors are presented, followed by the newer generations electronics for the control of ac systems, which resulted in the of FACTS devices based on self-commutated semiconductor flexible ac transmission system or the FACTS concepts [2]. The switches.

basic idea was to obtain ac systems with a high level of control

852 E. H. Watanabe et al. flexibility, as in high-voltage direct current (HVDC) systems type [7]. On the other hand, the force-commutated converters

[3] based on the use of the thyristor and the self-commutated using the self-commutated devices are basically of the volt- (controllable turn-on and turn-off) semiconductor devices, age source type. More details about current source and voltage such as gate turn-off thyristors (GTOs), insulated gate bipolar source converters can be found in many power electronics transistors (IGBTs), and integrated gate controlled thyristors books, e.g. [3, 7]. (IGCTs) [4, 5], which were not yet developed at that time.

The switching characteristics of thyristors – controlled turn-on and natural turn-off – are appropriate for using in

32.2 Ideal Shunt Compensator

line-commutated converters, such as in conventional HVDC transmission systems with a current source in the dc side. In

A simple and lossless ac system is composed of two ideal gen- this application, the technology for series connection of thyris- erators, and a short transmission line, as shown in Fig. 32.1, is tors is very important due to the high-voltage characteristics considered as basis to the discussion of the operating principles of the transmission voltage. This is a well-known technology. of a shunt compensator [8]. The transmission line is modeled Maximum breakdown voltage and current conduction capa- by an inductive reactance X L . In the circuit, a continuously bilities are around 8 kV and 4 kA, respectively. These are some controlled voltage source is connected in the middle of the trans- features that make thyristors important for very high-power mission line. It is assumed that the voltage phasors V S and V R applications, although they present some serious drawbacks, have the same magnitude and are phase-shifted by δ. The sub- such as the lack of controlled turn-off capability and low script “S” stands for “Source” and “R” stands for “Receptor.” switching speeds.

Figure 32.2 shows the phasor diagram of the system given in

Self-commutated switches are adequate for use in converters Fig. 32.1 in which the compensation voltage phasor V M has where turn-off capability is necessary. The device with high- also the same magnitude as V S and V R , and its phase is exactly est ratings in this group was, for a long time, the GTO with ( −δ/2) with respect to V S and ( +δ/2) with respect to V R . maximum switching capability of 6 kV and 6 kA. At present,

In this situation, the current I SM flows from the source and there are IGBTs with ratings in the range of 6.5 kV to 3 kA and the current I MR flows into the receptor. The phasor I M is the IGCTs with switching capability of about 6 kV and 4 kA. The resulting current flowing through the ideal shunt compensator; GTOs and IGCTs are devices that normally need small induc- Fig. 32.2 shows that this current I M , in this case, is orthogonal tor for limiting the rate of change of turn-on current (di/dt). to the voltage V M , which means that the ideal shunt compen- Normally, GTOs also need a snubber circuit for limiting the sator voltage source does not has to generate or absorb active rate of change of voltage (dv/dt).

power and have only reactive power in its terminals. The GTOs, IGCTs, and IGBTs are the most used options for

From Fig. 32.2 and knowing that no active power flows to or self-commutated high-power converters. Because the switching from the ideal shunt compensator, it is possible to calculate the time of these devices is in the microsecond range (or below), power transferred from V S to V R that is given by their series connection is more complicated than in the case of thyristors. However, there are examples of series connections of

2V 2

various GTOs or IGCTs and, in the case of IGBTs, the number

sin(δ/2) (32.1)

of series connected devices can go as high as 32 [6].

Because of the commutation nature of the thyristors, the where P S is the active power flowing from the source, V is the converters used in HVDC systems are of the current source magnitude of the voltages V S and V R .

FIGURE 32.1 Ideal shunt compensator connected in the middle of a transmission line.

32 Flexible AC Transmission Systems 853

V S =V e +jδ/2

V S =V e +jδ/2

=V e −jδ/2

V R =V e− jδ/2

FIGURE 32.3 Phasor diagram of the system with shunt reactive and FIGURE 32.2 Phasor diagram of the system with shunt reactive power

active power compensation.

compensation.

If the ideal shunt compensator were not present, the trans- storage element or energy source if active power is to be drained ferred power would be given by

or generated by the shunt compensator.

sin δ

32.3 Ideal Series Compensator

Since 2 sin(δ/2) is always greater than sin δ for δ in the range Similar to the previous section, the ideal series compensator is of [0, 2π ], the ideal shunt compensator does improve the modeled by a voltage source for which the phasor is V C con- power transfer capability of the transmission line. This volt- nected in the middle of a lossless transmission line as shown in age source is in fact operating as an ideal reactive power shunt Fig. 32.4. compensator.

The current flowing through the transmission line is given If the phase angle between V M and V S is different from δ/2 by (as shown in Fig. 32.3), the power flowing through V M has both active and reactive components.

I = (V SR −V C )/jX L (32.3) With the characteristics of the ideal shunt compensator, presented above, it is possible to synthesize power electronics- where V SR =V S −V R . based devices to operate as active or reactive power compen-

If the ideal series compensator voltage is generated in such sators. This is discussed in the following sections. It will be

a way that its phasor V C is in quadrature with line current seen that the requirements of the device synthesis with actual

I, this series compensator neither supplies nor absorbs active semiconductor switches for the situations of reactive or active power. As discussed earlier, power at the series source is only power compensation are different because of the need of energy reactive and the voltage source may, in this particular case,

Ideal series compensator

FIGURE 32.4 Ideal series compensator connected in the middle of a transmission line.

854 E. H. Watanabe et al.

be replaced by capacitive or inductive equivalent impedance. the voltage phasor V L on the line reactance X L and the Then, the equivalent impedance is given by

compensator voltage phasor V C are shown for a given compensation level, assuming that this voltage V C corresponds

X eq =X L (1 + s) (32.4) to a capacitive compensation. In this case, the line current pha- sor leads voltage phasor V C by 90 ◦ , and the total voltage drop in where

the line V Z =V S −V R −V C is larger than the original voltage

X Comp drop V L . The current flowing in the line is larger after com- s =

; (0 ≤ |s| ≤ 1)

(32.5) pensation than before. This situation shows the case where the

series compensator is used to increase power flow. is the compensation factor and X Comp is the series equivalent

The left-hand side of the Fig. 32.5b shows the same non- compensation reactance, negative if capacitive and positive if compensated situation as in the previous case. On the right- inductive. In this case, the compensation voltage is given by

hand side, the case of an equivalent inductive compensation is shown. In this case, the compensation voltage V C is in phase

V C =I L X eq

(32.6) with the line drop voltage V L , producing an equivalent total voltage drop V Z smaller than in the original case. As a result,

and the transmitted power is equal to the current phasor I flowing in the line is smaller than before compensation. This kind of compensation might be interest-

V 2 ing in the case that the power flowing through the line has to P s =

sin δ

be decreased. In either capacitive or inductive compensation

X L (1 − s)

modes, no active power is absorbed or generated by the ideal series compensator.

Equation (32.7) shows that the transmitted power can be Figure 32.6 shows an ac system with an ideal generic series considerably increased by series compensation, choosing a compensation voltage source V C for the general case where it

proper compensation factor s. The reactive power at the series may not be in quadrature with the line current. In this case, source is given by

the compensator can fully control the phase difference between

the two systems, thus controlling the active and reactive power

(32.8) exchanged between them. However, in this case, the compen-

sation voltage source V C may have to absorb or generate active power (P C ), as well as control the reactive power (Q C ). The left-hand side of the Fig. 32.5a shows the phasor

X L (1 − s)

Figure 32.7 shows the phasor diagram of this ideal generic diagram of the system shown in Fig. 32.4 without the ideal series compensator. This figure also shows a dashed-line series compensator. On the right-hand side of the Fig. 32.5a, circle with the locus of all the possible positions that the

V S =V · e +jδ/2

V S =V · e +jδ/2

V R =V · e −jδ/2

V R =V · e −jδ/2

V Z /2

V S =V · e +jδ/2

V C V S =V · e +jδ/2

FIGURE 32.5 Phasor diagram of the series reactive compensator: (a) capacitive and (b) reactive mode.

32 Flexible AC Transmission Systems 855

Ideal generic series compensator

FIGURE 32.6 Ideal generic series compensator.

the terminals of the line can be controlled faster than the con- ventional way, i.e., by controlling the synchronous generator.

In Fig. 32.7, voltage V C may have any phase angle with

V C respect to line current. Therefore, it may have to supply or V S =V·e + j(δ − α ) /2

absorb active power, as well as control reactive power. As in

the case of the shunt device, this feature must also be taken

into account in the synthesis of the actual devices. As a first O

V SI = V.e −j(δ − α ) /2

I approximation, when the goal is to control active power flow-

jX L .I

ing through the medium- or short-length transmission lines, compensator location seems to be a question of convenience.

V R =V·e − j(δ−α ) /2 Figure 32.8 summarizes the active power transfer character- istics in a transmission line as a function of the power angle, δ,

FIGURE 32.7 Phasor diagram of the ac system compensated with an as shown in Figs. 32.1 and 32.2, for the cases of the line with-

ideal generic series compensator. out compensation, line with series or shunt compensation, and

line with phase-shift compensation. These characteristics are drawn on the assumption that the source voltages V S and V R

compensation voltage V C could take, assuming that the mag- (see Fig. 32.2) have the same magnitude. A 50% series compen- nitude shown for this voltage is its maximum. Naturally, if the sation (s = 0.5) as defined in Eq. (32.4) presents a significant sum of the compensation voltage and the source voltage V S is increase in the line power transfer capability. on the circle, the magnitude of V S1 may be smaller or larger

As shown in Fig. 32.8, in general, series compensation is than the magnitude of V S .

the best choice for increasing power transfer capability. The The compensation voltage V C can be added to the V S in a phase-shifter compensator is important to connect two sys- way that the resultant voltage V S1 has the same magnitude of tems with excessive or uncontrollable phase difference. It does

V S but with a phase shifted by an angle α. In this case, the series not increase power transfer capability significantly; however, compensator can be called as a phase-shifter compensator and it may allow the adjustment of large or highly variable phase the power flowing through the transmission line (see Fig. 32.6) differences. The shunt compensator does not increase power is expressed below:

transfer capability in a significant way in its normal operating region, where the angle δ is naturally below 90 ◦ and in general

V 2 around 30 ◦ . The great importance of the shunt compensator is P S =

the increase in the stability margin, as explained in Fig. 32.9. Figure 32.9 shows the power transfer characteristics (P δ × δ) Equation (32.9) shows that transmitted power increases as of a transmission line, which is first assumed to be transmit- the phase difference (δ − α) reaches 90 ◦ . However, its max- ting power P S0 at phase angle δ 0 . If a problem happens in the imum value is the same as in the case of no compensation. line (a fault, for example) the turbine that drives the generator The difference is that with this compensator, the system power cannot change its mechanical power input immediately even if angle, and angle difference between the two voltage sources at there is no power transmission for a short time. This situation

sin (δ − α)

856 E. H. Watanabe et al.

With 50% of series capacitive compensation

With shunt compensation 2

1 With phase shifter compensation

δ (rad) FIGURE 32.8 Power transfer characteristics for the case of shunt, series, and no compensation.

FIGURE 32.10 Stability margin characteristics – unstable situation.

FIGURE 32.9 Stability margin characteristics – stable situation. angle δ increases above its critical value δ c , which is slightly less than 90 ◦ reaching δ 1 . Therefore, the area below the P δ curve

accelerates the generator, increasing its frequency and leading to decelerate the system is not enough leading to an unstable

to an increase of the phase angle δ to δ 1 . If the line restarts system because A 2 is smaller than A 1 .

Looking at Fig. 32.8, it is possible to see that depending on the transmitted power will be P 1 , which is larger than P 0 and the operating point, all three compensation methods increase

operation at the instant corresponding to this phase angle δ 1 ,

decelerates the turbine or generator. The area A 1 corresponds the stability margin as the area under the curve of transmit- to the energy that accelerated the turbine. As the frequency gets ted power P δ versus phase angle δ is increased. The ideal shunt higher than the rated frequency at (P S1 ,δ 1 ), the phase angle will compensator is the one that most increases this area, and thus increase up to δ 2 , where the area A 2 is equal to the area A 1 . If it is said to be the best option to increase the stability margin. the area given by the A 2 plus A 3 is larger than A 1 , the system is

said to be dynamically stable. On the contrary, if it is not pos-

sible to have an area A 2 equal to A 1 , the system is said to be

32.4 Synthesis of FACTS Devices

unstable. An unstable situation is shown in Fig. 32.10 in which the system is the same as in Fig. 32.9 but assuming a longer It has been stated that the synthesis of the compensators pre- interval with no power transmission. In this case, the turbine or sented in Sections 32.2 and 32.3 can be achieved with thyristors generator accelerates more than that in Fig. 32.9 and the phase or self-commutated switches, such as GTO, IGBT, or IGCT.

32 Flexible AC Transmission Systems 857 Each type of switches leads to devices with different operat-

32.4.1.2 Thyristor-Switched Capacitor

ing principles and synthesis concepts, and hence they should Figure 32.13 shows the thyristor-switched capacitor (TSC). The

be discussed separately. Terms and definitions for most of the word “controlled” used in the case of the reactor is substi- FACTS devices are given in [9].

tuted by “switched” because the thyristor is turned on only Thyristor-based FACTS devices use line or natural com- when zero-voltage switching (ZVS) condition is achieved. This

mutation together with large energy storage elements (capac- means that the voltage across the thyristor terminals has to be itors or reactors). On the other hand, devices based on zero at the turn-on instant. In practical cases, it may be slightly self-commutating switches, such as GTOs, IGCTs, or IGBTs, positive because thyristors need positive anode–cathode volt- use gate-controlled commutation. In general, it is said that ages to be triggered (large anode–cathode voltage during turn- the first generation of FACTS devices is based on conven- on); however, it can produce a large current spike that can tional line–commutated thyristors, and the subsequent genera- damage the thyristors. Therefore, due to this switching charac- tions are based on gate-controlled devices or self-commutated teristic, the thyristors can only connect the capacitor to the grid switches. The most important FACTS devices based on thyris- or disconnect it. Consequently, only step-like control is pos- tors and self-commutating devices are presented in the follow- sible, and therefore a continuous control is not possible. The ing sections.

capacitor connection to or disconnection from the grid is nor- mally performed at very low frequencies, and the harmonics, when they appear, are not a serious concern.