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FACTS Conceptand General System

Considerations1.1 TRANSMISSION INTERCONNECTIONS

Most if not all of the world's electric power supply systems are widely interconnected,involving connections inside utilities' own territories which extend to inter-utilityinterconnections and then to inter-regional and international connections. This is donefor economic reasons, to reduce the cost of electricity and to improve reliability ofpower supply.

1.1.1 Why We Need Transmission Interconnections

We need these interconnections because, apart from delivery, the purpose ofthe transmission network is to pool power plants and load centers in order to minimizethe total power generation capacity and fuel cost. Transmission interconnections enabletaking advantage of diversity of loads, availability of sources, and fuel price in orderto supply electricity to the loads at minimum cost with a required reliability. In general,if a power delivery system was made up of radial lines from individual local generatorswithout being part of a grid system, many more generation resources would be neededto serve the load with the same reliability, and the cost of electricity would be muchhigher. With that perspective, transmission is often an alternative to a new generationresource. Less transmission capability means that more generation resources wouldbe required regardless of whether the system is made up of large or small powerplants. In fact small distributed generation becomes more economically viable if thereis a backbone of a transmission grid. One cannot be really sure about what the optimumbalance is between generation and transmission unless the system planners use ad-vanced methods of analysis which integrate transmission planning into an integratedvalue-based transmission/generation planning scenario. The cost of transmission linesand losses, as well as difficulties encountered in building new transmission lines, wouldoften limit the available transmission capacity. It seems that there are many caseswhere economic energy or reserve sharing is constrained by transmission capacity,and the situation is not getting any better. In a deregulated electric service environment,an effective electric grid is vital to the competitive environment of reliable electricservice.

1

Chapter 1 • FACTS Concept and General System Considerations

On the other hand, as power transfers grow, the power system becomes increas-ingly more complex to operate and the system can become less secure for ridingthrough the major outages. It may lead to large power flows with inadequate control,excessive reactive power in various parts of the system, large dynamic swings betweendifferent parts of the system and bottlenecks, and thus the full potential of transmissioninterconnections cannot be utilized.

The power systems of today, by and large, are mechanically controlled. Thereis a widespread use of microelectronics, computers and high-speed communicationsfor control and protection of present transmission systems; however, when operatingsignals are sent to the power circuits, where the final power control action is taken,the switching devices are mechanical and there is little high-speed control. Anotherproblem with mechanical devices is that control cannot be initiated frequently, becausethese mechanical devices tend to wear out very quickly compared to static devices.In effect, from the point of view of both dynamic and steady-state operation, thesystem is really uncontrolled. Power system planners, operators, and engineers havelearned to live with this limitation by using a variety of ingenious techniques to makethe system work effectively, but at a price of providing greater operating margins andredundancies. These represent an asset that can be effectively utilized with prudentuse of FACTS technology on a selective, as needed basis.

In recent years, greater demands have been placed on the transmission network,and these demands will continue to increase because of the increasing number of nonutil-ity generators and heightened competition among utilities themselves. Added to this isthe problem that it is very difficult to acquire new rights of way. Increased demands ontransmission, absence of long-term planning, and the need to provide open access togenerating companies and customers, all together have created tendencies toward lesssecurity and reduced quality of supply. The FACTS technology is essential to alleviatesome but not all of these difficulties by enabling utilities to get the most service fromtheir transmission facilities and enhance grid reliability. It must be stressed, however,that for many of the capacity expansion needs, building of new lines or upgrading currentand voltage capability of existing lines and corridors will be necessary.

1.1.2 Opportunities for FACTS

What is most interesting for transmission planners is that FACTS technologyopens up new opportunities for controlling power and enhancing the usable capacityof present, as well as new and upgraded, lines. The possibility that current through aline can be controlled at a reasonable cost enables a large potential of increasing thecapacity of existing lines with larger conductors, and use of one of the FACTS Control-lers to enable corresponding power to flow through such lines under normal andcontingency conditions.

These opportunities arise through the ability of FACTS Controllers to controlthe interrelated parameters that govern the operation of transmission systems includingseries impedance, shunt impedance, current, voltage, phase angle, and the dampingof oscillations at various frequencies below the rated frequency. These constraintscannot be overcome, while maintaining the required system reliability, by mechanicalmeans without lowering the useable transmission capacity. By providing added flexibil-ity, FACTS Controllers can enable a line to carry power closer to its thermal rating.Mechanical switching needs to be supplemented by rapid-response power electronics.It must be emphasized that FACTS is an enabling technology, and not a one-on-onesubstitute for mechanical switches.

2

Section 1.2 • Flow of Power in an AC System 3

The FACTS technology is not a single high-power Controller, but rather acollection of Controllers, which can be applied individually or in coordination withothers to control one or more of the interrelated system parameters mentioned above.A well-chosen FACTS Controller can overcome the specific limitations of a designatedtransmission line or a corridor. Because all FACTS Controllers represent applicationsof the same basic technology, their production can eventually take advantage oftechnologies of scale. Just as the transistor is the basic element for a whole variety ofmicroelectronic chips and circuits, the thyristor or high-power transistor is the basicelement for a variety of high-power electronic Controllers.

FACTS technology also lends itself to extending usable transmission limits in astep-by-step manner with incremental investment as and when required. A plannercould foresee a progressive scenario of mechanical switching means and enablingFACTS Controllers such that the transmission lines will involve a combination ofmechanical and FACTS Controllers to achieve the objective in an appropriate, stagedinvestment scenario.

Some of the Power Electronics Controllers, now folded into the FACTS conceptpredate the introduction of the FACTS concept by co-author Hingorani to the technicalcommunity. Notable among these is the shunt-connected Static VAR Compensator(SVC) for voltage control which was first demonstrated in Nebraska and commercial-ized by GE in 1974 and by Westinghouse in Minnesota in 1975. The first series-connected Controller, NGH-SSR Damping Scheme, invented by co-author Hingorani,a low power series capacitor impedance control scheme, was demonstrated in Califor-nia by Siemens in 1984. It showed that with an active Controller there is no limitto series capacitor compensation. Even prior to SVCs, there were two versions ofstatic saturable reactors for limiting overvoltages and also powerful gapless metaloxide arresters for limiting dynamic overvoltages. Research had also been undertakenon solid-state tap changers and phase shifters. However, the unique aspect ofFACTS technology is that this umbrella concept revealed the large potentialopportunity for power electronics technology to greatly enhance the value of powersystems, and thereby unleashed an array of new and advanced ideas to make ita reality. Co-author Gyugyi has been at the forefront of such advanced ideas.FACTS technology has also provided an impetus and excitement perceived by theyounger generation of engineers, who will rethink and re-engineer the future powersystems throughout the world.

It is also worth pointing out that, in the implementation of FACTS technology,we are dealing with a base technology, proven through HVDC and high-powerindustrial drives. Nevertheless, as power semiconductor devices continue to improve,particularly the devices with turn-off capability, and as FACTS Controller conceptsadvance, the cost of FACTS Controllers will continue to decrease. Large-scale useof FACTS technology is an assured scenario.

1.2 FLOW OF POWER IN AN AC SYSTEM

At present, many transmission facilities confront one or more limiting network parame-ters plus the inability to direct power flow at will.

In ac power systems, given the insignificant electrical storage, the electricalgeneration and load must balance at all times. To some extent, the electrical systemis self-regulating. If generation is less than load, the voltage and frequency drop, andthereby the load, goes down to equal the generation minus the transmission losses.However, there is only a few percent margin for such a self-regulation. If voltage is


propped up with reactive power support, then the load will go up, and consequentlyfrequency will keep dropping, and the system will collapse. Alternately, if there isinadequate reactive power, the system can have voltage collapse.

When adequate generation is available, active power flows from the surplusgeneration areas to the deficit areas, and it flows through all parallel paths availablewhich frequently involves extra high-voltage and medium-voltage lines. Often, longdistances are involved with loads and generators along the way. An often cited exampleis that much of the power scheduled from Ontario Hydro Canada to the North EastUnited States flows via the PJM system over a long loop, because of the presence ofa large number of powerful low impedance lines along that loop. There are in factsome major and a large number of minor loop flows and uneven power flows in anypower transmission system.

1.2.1 Power Flow in Parallel Paths

Consider a very simple case of power flow [Figure l.l(a)], through two parallelpaths (possibly corridors of several lines) from a surplus generation area, shown asan equivalent generator on the left, to a deficit generation area on the right. Withoutany control, power flow is based on the inverse of the various transmission lineimpedances. Apart from ownership and contractual issues over which lines carry howmuch power, it is likely that the lower impedance line may become overloaded andthereby limit the loading on both paths even though the higher impedance path is notfully loaded. There would not be an incentive to upgrade current capacity of theoverloaded path, because this would further decrease the impedance and the invest-ment would be self-defeating particularly if the higher impedance path already hasenough capacity.

Figure l.l(b) shows the same two paths, but one of these has HVDC transmission.With HVDC, power flows as ordered by the operator, because with HVDC powerelectronics converters power is electronically controlled. Also, because power is elec-tronically controlled, the HVDC line can be used to its full thermal capacity if adequateconverter capacity is provided. Furthermore, an HVDC line, because of its high-speedcontrol, can also help the parallel ac transmission line to maintain stability. However,HVDC is expensive for general use, and is usually considered when long distancesare involved, such as the Pacific DC Intertie on which power flows as ordered bythe operator.

As alternative FACTS Controllers, Figures l.l(c) and 1.1 (d) show one of thetransmission lines with different types of series type FACTS Controllers. By meansof controlling impedance [Figure l.l(c)] or phase angle [Figure l.l(d)], or seriesinjection of appropriate voltage (not shown) a FACTS Controller can control thepower flow as required. Maximum power flow can in fact be limited to its rated limitunder contingency conditions when this line is expected to carry more power due tothe loss of a parallel line.

1.2.2 Power Flow in a Meshed System

To further understand the free flow of power, consider a very simplified case inwhich generators at two different sites are sending power to a load center through anetwork consisting of three lines in a meshed connection (Figure 1.2). Suppose thelines AB, BC, and AC have continuous ratings of 1000 MW, 1250 MW, and 2000MW, respectively, and have emergency ratings of twice those numbers for a sufficient

4

Section 1.2 • Flow of Power in an AC System

SurplusGeneration Area

(d)

Figure 1.1 Power flow in parallel paths: (a) ac power flow with parallel paths; (b)power flow control with HVDC; (c) power flow control with variableimpedance; (d) power flow control with variable phase angle.

5

Impedence = X

Power =2/3

Impedence = 2X

Power = 1/3

Load

(a)Load

HVdc line

Power flow as desired

(b)

Variable impedence


(c)

Variable phase angle



length of time to allow rescheduling of power in case of loss of one of these lines. Ifone of the generators is generating 2000 MW and the other 1000 MW, a total of 3000MW would be delivered to the load center. For the impedances shown, the three lineswould carry 600, 1600, and 1400 MW, respectively, as shown in Figure 1.2(a). Such asituation would overload line BC (loaded at 1600 MW for its continuous rating of1250 MW), and therefore generation would have to be decreased at B, and increasedat A, in order to meet the load without overloading line BC.

Power, in short, flows in accordance with transmission line series impedances(which are 90% inductive) that bear no direct relationship to transmission ownership,contracts, thermal limits, or transmission losses.

If, however, a capacitor whose reactance is - 5 ohms (ft) at the synchronousfrequency is inserted in one line [Figure 1.2(b)], it reduces the line's impedance from10 fl to 5 ft, so that power flow through the lines AB, BC, and AC will be 250,1250,and 1750 MW, respectively. It is clear that if the series capacitor is adjustable, thenother power-flow levels may be realized in accordance with the ownership, contract,thermal limitations, transmission losses, and a wide range of load and generationschedules. Although this capacitor could be modular and mechanically switched, thenumber of operations would be severely limited by wear on the mechanical componentsbecause the line loads vary continuously with load conditions, generation schedules,and line outages.

Other complications may arise if the series capacitor is mechanically controlled.A series capacitor in a line may lead to subsynchronous resonance (typically at 10-50Hz for a 60 Hz system). This resonance occurs when one of the mechanical resonancefrequencies of the shaft of a multiple-turbine generator unit coincides with 60 Hz

1400 MW 1750 MW

2000 MW3000 MWload

2000 MW

1000 MW

3000 MW^ 'oad

1000MW

(a)

2000 MW

(b)

1750 MW 1750 MW

3000 MW^ 'oad

3000 MWload

1000MW

(c) (d)

Figure 1.2 Power flow in a mesh network: (a) system diagram; (b) system diagramwith Thyristor-Controlled Series Capacitor in line AC; (c) system dia-gram with Thyristor-Controlled Series Reactor in line BC; (d) systemdiagram with Thyristor-Controlled Phase Angle Regulator in line AC.

6

A10ft

iC-5 a

A cio a

B

-4.24°iC.A

2000 MWion

B.

^0Q.

B1000 MW

cA

B

Section 1.3 • What Limits the Loading Capability? 7

minus the electrical resonance frequency of the capacitor with the inductive impedanceof the line. If such resonance persists, it will soon damage the shaft. Also while theoutage of one line forces other lines to operate at their emergency ratings and carryhigher loads, power flow oscillations at low frequency (typically 0.3-3 Hz) may causegenerators to lose synchronism, perhaps prompting the system's collapse.

If all or a part of the series capacitor is thyristor-controlled, however, it can bevaried as often as required. It can be modulated to rapidly damp any subsynchronousresonance conditions, as well as damp low frequency oscillations in the power flow.This would allow the transmission system to go from one steady-state condition toanother without the risk of damage to a generator shaft and also help reduce the riskof system collapse. In other words, a thyristor-controlled series capacitor can greatlyenhance the stability of the network. More often than not though, it is practical forpart of the series compensation to be mechanically controlled and part thyristorcontrolled, so as to counter the system constraints at the least cost.

Similar results may be obtained by increasing the impedance of one of the linesin the same meshed configuration by inserting a7f t reactor (inductor) in series withline AB [Figure 1.2(c)]. Again, a series inductor that is partly mechanically and partlythyristor-controlled, it could serve to adjust the steady-state power flows as well asdamp unwanted oscillations.

As another option, a thyristor-controlled phase-angle regulator could be installedinstead of a series capacitor or a series reactor in any of the three lines to serve thesame purpose. In Figure 1.2(d), the regulator is installed in the third line to reducethe total phase-angle difference along the line from 8.5 degrees to 4.26 degrees. Asbefore, a combination of mechanical and thyristor control of the phase-angle regulatormay minimize cost.

The same results could also be achieved by injecting a variable voltage in oneof the lines. Note that balancing of power flow in the above case did not require morethan one FACTS Controller, and indeed there are options of different controllers andin different lines.

If there is only one owner of the transmission grid, then a decision can be madeon consideration of overall economics alone. On the other hand, if multiple ownersare involved, then a decision mechanism is necessary on the investment and ownership.

1.3 WHAT LIMITS THE LOADING CAPABILITY?

Assuming that ownership is not an issue, and the objective is to make the best use ofthe transmission asset, and to maximize the loading capability (taking into accountcontingency conditions), what limits the loading capability, and what can be doneabout it?

Basically, there are three kinds of limitations:

• Thermal• Dielectric• Stability

Thermal Thermal capability of an overhead line is a function of the ambienttemperature, wind conditions, condition of the conductor, and ground clearance. Itvaries perhaps by a factor of 2 to 1 due to the variable environment and the loadinghistory. The nominal rating of a line is generally decided on a conservative basis,envisioning a statistically worst ambient environment case scenario. Yet this scenario


occurs but rarely which means that in reality, most of the time, there is a lot morereal time capacity than assumed. Some utilities assign winter and summer ratings, yetthis still leaves a considerable margin to play with. There are also off-line computerprograms that can calculate a line's loading capability based on available ambientenvironment and recent loading history. Then there are the on-line monitoring devicesthat provide a basis for on-line real-time loading capability. These methods haveevolved over a period of many years, and, given the age of automation (typified byGPS systems and low-cost sophisticated communication services), it surely makessense to consider reasonable, day to day, hour to hour, or even real-time capabilityinformation. Sometimes, the ambient conditions can actually be worse than assumed,and having the means to determine actual rating of the line could be useful.

During planning/design stages, normal loading of the lines is frequently decidedon a loss evaluation basis under assumptions which may have changed for a varietyof reasons; however losses can be taken into account on the real-time value basis ofextra loading capability.

Of course, increasing the rating of a transmission circuit involves considerationof the real-time ratings of the transformers and other equipment as well, some ofwhich may also have to be changed in order to increase the loading on the lines. Real-time loading capability of transformers is also a function of ambient temperature,aging of the transformer and recent loading history. Off-line and on-line loadingcapability monitors can also be used to obtain real time loading capability of transform-ers. Also, the transformer also lends itself to enhanced cooling.

Then there is the possibility of upgrading a line by changing the conductor tothat of a higher current rating, which may in turn require structural upgrading. Finally,there is the possibility of converting a single-circuit to a double-circuit line. Once thehigher current capability is available, then the question arises of how it should beused. Will the extra power actually flow and be controllable? Will the voltage conditionsbe acceptable with sudden load dropping, etc.? The FACTS technology can help inmaking an effective use of this newfound capacity.

Dielectric From an insulation point of view, many lines are designed very conser-vatively. For a given nominal voltage rating, it is often possible to increase normaloperation by +10% voltage (i.e., 500 kV-550 kV) or even higher. Care is then neededto ensure that dynamic and transient overvoltages are within limits. Modern gaplessarresters, or line insulators with internal gapless arresters, or powerful thyristor-con-trolled overvoltage suppressors at the substations can enable significant increase inthe line and substation voltage capability. The FACTS technology could be used toensure acceptable over-voltage and power flow conditions.

Stability There are a number of stability issues that limit the transmission capa-bility. These include:

• Transient stability• Dynamic stability• Steady-state stability• Frequency collapse• Voltage collapse• Subsynchronous resonance

Excellent books are available on this subject. Therefore, discussion on thesetopics in this book will be brief, and limited to what is really essential to the explanation

8

Section 1.4 • Flow and Stability Considerations of a Transmission Interconnection 9

of FACTS Controllers. The FACTS technology can certainly be used to overcome anyof the stability limits, in which case the ultimate limits would be thermal and dielectric.

1.4 POWER FLOW AND DYNAMICSTABILITY CONSIDERATIONSOF A TRANSMISSION INTERCONNECTION

Figure 1.3(a) shows a simplified case of power flow on a transmission line. Locations1 and 2 could be any transmission substations connected by a transmission line.Substations may have loads, generation, or may be interconnecting points on thesystem and for simplicity they are assumed to be stiff busses. Ex and E2 are themagnitudes of the bus voltages with an angle 8 between the two. The line is assumedto have inductive impedance X, and the line resistance and capacitance are ignored.

As shown in the phasor diagram [Figure 1.3(b)] the driving voltage drop in theline is the phasor difference EL between the two line voltage phasors, Ex and E2. Theline current magnitude is given by:

/ = ErfX, and lags EL by 90°

It is important to appreciate that for a typical line, angle 8 and correspondingdriving voltage, or voltage drop along the line, is small compared to the line voltages.Given that a transmission line may have a voltage drop at full load of perhaps 1%/10 km, and assuming that a line between two stiff busbars (substations) is 200 kmlong, the voltage drop along this line would be 20% at full load, and the angle 8 wouldbe small. If we were to assume, for example, that with equal magnitudes of Ex and E2,and X of 0.2 per unit magnitude, the angle 8 would be only 0.2 radians or 11.5 degrees.

The current flow on the line can be controlled by controlling EL or X or 8. Inorder to achieve a high degree of control on the current in this line, the equipmentrequired in series with the line would not have a very high power rating. For example,a 500 kV (approximately 300 kV phase-ground), 2000 A line has a three-phasethroughput power of 1800 MVA, and, for a 200 km length, it would have a voltagedrop of about 60 kV. For variable series compensation of say, 25%, the series equipmentrequired would have a nominal rating of 0.25 X 60 kV X 2000 A = 30 MVA perphase, or 90 MVA for three phases, which is only 5% of the throughput line ratingof 1800 MVA. Voltage across the series equipment would only be 15 kV at fullload, although it would require high-voltage insulation to ground (the latter is not asignificant cost factor). However, any series-connected equipment has to be designedto carry contingency overloads so that the equipment may have to be rated to 100%overload capability.

Nevertheless the point of this very simple example is that generally speakingthe rating of series FACTS Controllers would be a fraction of the throughput ratingof a line.

Figure 1.3(b) shows that the current flow phasor is perpendicular to the drivingvoltage (90° phase lag). If the angle between the two bus voltages is small, the currentflow largely represents the active power. Increasing or decreasing the inductive imped-ance of a line will greatly affect the active power flow. Thus impedance control, whichin reality provides current control, can be the most cost-effective means of controllingthe power flow. With appropriate control loops, it can be used for power flow controland/or angle control for stability.

Figure 1.3(c), corresponding to Figure 1.3(b), shows a phasor diagram of the

10 Chapter 1 • FACTS Concept and General System Considerations

180°

Injected voltage

Figure 1.3 Ac power flow control of a transmission line: (a) simple two-machinesystem; (b) current flow perpendicular to the driving voltage; (c) activeand reactive power flow phasor diagram; (d) power angle curves fordifferent values of X; (e) regulating voltage magnitude mostly changesreactive power; (f) injecting voltage perpendicular to the line currentmostly changes active power; (f) injecting voltage phasor in series withthe line. (Note that for clarity the phasors are identified by their magni-tudes in this figure.)

5

i S

E2£_2

(b)

2

Ez\dz

P&Q

X

frE,\d±

1

(a)

(Ei - E2 cos 3 )

-E1 sin 5 E2cos 6

/P1 = E2 sin S Ix

•*1

-Ei sin 8

E2 sin 6^max

XP = M ^ 2 sina

90°Angle 6

(d)

(£2 - Ei cos <5)^U = (E^-EzcosS)/XE, COS 8

7q2=(E2-E1cos d)/X

(c)

E,

E,-E2

AI

E2

(e)

ts

E,-E2I

£2

(f)

'Injected voltage

E,

E,-E2

E2

(g)

•/

•/

Section 1.4 • Flow and Stability Considerations of a Transmission Interconnection 11

relationship between the active and reactive currents with reference to the voltagesat the two ends.

Active component of the current flow at E1 is:

Ipl = (E2 sin 8)IX

Reactive component of the current flow at Er is:

Iql = (£1 - E2 cos 8)1 X

Thus, active power at the Ex end:

Pi = Ex (E2 sin 8)IX

Reactive power at the Ex end:

Gi = £i {Ex-E2 cos 8)1 X (1.1)

Similarly, active component of the current flow at E2 is:

Ip2 = (Ei sin 8)1 X

Reactive component of the current flow at E2 is:

Iq2 = (£2 - Ex cos S)/X

Thus, active power at the E2 end:

P2 = £2 ( ^ sin 8)IX

Reactive power at the E2 end:

Q2 = E2(E2~ElCos8)/X (1.2)

Naturally Pi and P2 are the same:

P = E1(E2sin8)IX (1.3)

because it is assumed that there are no active power losses in the line. Thus, varying thevalue of Xwill vary P, Qu and Q2 in accordance with (1.1), (1.2), and (1.3), respectively.

Assuming that Ex and E2 are the magnitudes of the internal voltages of the twoequivalent machines representing the two systems, and the impedance X includes theinternal impedance of the two equivalent machines, Figure 1.3(d) shows the half sine-wave curve of active power increasing to a peak with an increase in 8 to 90 degrees.Power then falls with further increase in angle, and finally to zero at 8 = 180°. It iseasy to appreciate that without high-speed control of any of the parameters Eu E2,Ei - E2, X and 8, the transmission line can be utilized only to a level well below thatcorresponding to 90 degrees. This is necessary, in order to maintain an adequatemargin needed for transient and dynamic stability and to ensure that the system doesnot collapse following the outage of the largest generator and/or a line.

Increase and decrease of the value of X will increase and decrease the heightof the curves, respectively, as shown in Figure 1.3(d). For a given power flow, varyingof X will correspondingly vary the angle between the two ends.

Power/current flow can also be controlled by regulating the magnitude of voltagephasor Ex or voltage phasor E2. However, it is seen from Figure 1.3(e) that with changein the magnitude of E\, the magnitude of the driving voltage phasor Ex ~- E2 does not


change by much, but its phase angle does. This also means that regulation of themagnitude of voltage phasor Ex and/or E2 has much more influence over the reactivepower flow than the active power flow, as seen from the two current phasors corre-sponding to the two driving voltage phasors Ex - E2 shown in Figure 1.3(e).

Current flow and hence power flow can also be changed by injecting voltage inseries with the line. It is seen from Figure 1.3(f) that when the injected voltage is inphase quadrature with the current (which is approximately in phase with the drivingvoltage, Figure 1.3(f), it directly influences the magnitude of the current flow, andwith small angle influences substantially the active power flow.

Alternatively, the voltage injected in series can be a phasor with variable magni-tude and phase relationship with the line voltage [Figure 1.3(g)]. It is seen that varyingthe amplitude and phase angle of the voltage injected in series, both the active andreactive current flow can be influenced. Voltage injection methods form the mostimportant portfolio of the FACTS Controllers and will be discussed in detail in subse-quent chapters.

1.5 RELATIVE IMPORTANCE OFCONTROLLABLE PARAMETERS

With reference to the above discussion and Figure 1.3, it is worth noting a few basicpoints regarding the possibilities of power flow control:

• Control of the line impedance X (e.g., with a thyristor-controlled series capaci-tor) can provide a powerful means of current control.

• When the angle is not large, which is often the case, control of X or the anglesubstantially provides the control of active power.

• Control of angle (with a Phase Angle Regulator, for example), which in turncontrols the driving voltage, provides a powerful means of controlling thecurrent flow and hence active power flow when the angle is not large.

• Injecting a voltage in series with the line, and perpendicular to the currentflow, can increase or decrease the magnitude of current flow. Since the currentflow lags the driving voltage by 90 degrees, this means injection of reactivepower in series, (e.g., with static synchronous series compensation) can providea powerful means of controlling the line current, and hence the active powerwhen the angle is not large.

• Injecting voltage in series with the line and with any phase angle with respectto the driving voltage can control the magnitude and the phase of the linecurrent. This means that injecting a voltage phasor with variable phase anglecan provide a powerful means of precisely controlling the active and reactivepower 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 linevoltage, the MVA rating of a series Controller will often be a small fractionof the throughput line MVA.

• When the angle is not large, controlling the magnitude of one or the otherline voltages (e.g., with a thyristor-controlled voltage regulator) can be a verycost-effective means for the control of reactive power flow through the intercon-nection.

• Combination of the line impedance control with a series Controller and voltage

Section 1.6 • Basic Types of FACTS Controllers 13

regulation with a shunt Controller can also provide a cost-effective means tocontrol both the active and reactive power flow between the two systems.

1.6 BASIC TYPES OF FACTS CONTROLLERS

In general, FACTS Controllers can be divided into four categories:

• Series Controllers• Shunt Controllers• Combined series-series Controllers• Combined series-shunt Controllers

Figure 1.4(a) shows the general symbol for a FACTS Controller: a thyristor arrowinside a box.

Series Controllers: [Figure 1.4(b)] The series Controller could be a variableimpedance, such as capacitor, reactor, etc., or a power electronics based variable sourceof main frequency, subsynchronous and harmonic frequencies (or a combination) toserve the desired need. In principle, all series Controllers inject voltage in series withthe line. Even a variable impedance multiplied by the current flow through it, representsan injected series voltage in the line. As long as the voltage is in phase quadraturewith the line current, the series Controller only supplies or consumes variable reactivepower. Any other phase relationship will involve handling of real power as well.

Shunt Controllers: [Figure 1.4(c)] As in the case of series Controllers, the shuntControllers 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 connec-tion. Even a variable shunt impedance connected to the line voltage causes a variablecurrent flow and hence represents injection of current into the line. As long as theinjected current is in phase quadrature with the line voltage, the shunt Controller onlysupplies or consumes variable reactive power. Any other phase relationship will involvehandling of real power as well.

Combined series-series Controllers: [Figure 1.4(d)] This could be a combinationof separate series controllers, which are controlled in a coordinated manner, in amultiline transmission system. Or it could be a unified Controller, Figure 1.4(d), inwhich series Controllers provide independent series reactive compensation for eachline but also transfer real power among the lines via the power link. The real powertransfer capability of the unified series-series Controller, referred to as Interline PowerFlow Controller, makes it possible to balance both the real and reactive power flowin the lines and thereby maximize the utilization of the transmission system. Note thatthe term "unified" here means that the dc terminals of all Controller converters areall connected together for real power transfer.

Combined series-shunt Controllers: [Figures 1.4(e) and 1.4(f)] This could be acombination of separate shunt and series Controllers, which are controlled in a coordi-nated manner [Figure 1.4(e)], or a Unified Power Flow Controller with series andshunt elements [Figure 1.4(f)]. In principle, combined shunt and series Controllersinject current into the system with the shunt part of the Controller and voltage inseries in the line with the series part of the Controller. However, when the shunt andseries Controllers are unified, there can be a real power exchange between the seriesand shunt Controllers via the power link.


1.6.1 Relative Importance of Different Types of Controllers

It is important to appreciate that the series-connected Controller impacts thedriving voltage and hence the current and power flow directly. Therefore, if the purposeof the application is to control the current/power flow and damp oscillations, theseries Controller for a given MVA size is several times more powerful than theshunt Controller.

As mentioned, the shunt Controller, on the other hand, is like a currentsource, which draws from or injects current into the line. The shunt Controller istherefore a good way to control voltage at and around the point of connectionthrough injection of reactive current (leading or lagging), alone or a combinationof active and reactive current for a more effective voltage control and dampingof voltage oscillations.

This is not to say that the series Controller cannot be used to keep the linevoltage within the specified range. After all, the voltage fluctuations are largely aconsequence of the voltage drop in series impedances of lines, transformers, andgenerators. Therefore, adding or subtracting the FACTS Controller voltage in series(main frequency, subsynchronous or harmonic voltage and combination thereof) canbe the most cost-effective way of improving the voltage profile. Nevertheless, a shuntcontroller is much more effective in maintaining a required voltage profile at a substa-tion bus. One important advantage of the shunt Controller is that it serves the busnode independently of the individual lines connected to the bus.

Series Controller solution may require, but not necessarily, a separate seriesController for several lines connected to the substation, particularly if the applicationcalls for contingency outage of any one line. However, this should not be a decisivereason for choosing a shunt-connected Controller, because the required MVA size ofthe series Controller is small compared to the shunt Controller, and, in any case, theshunt Controller does not provide control over the power flow in the lines.

On the other hand, series-connected Controllers have to be designed to ridethrough contingency and dynamic overloads, and ride through or bypass short circuitcurrents. They can be protected by metal-oxide arresters or temporarily bypassed bysolid-state devices when the fault current is too high, but they have to be rated tohandle dynamic and contingency overload.

The above arguments suggest that a combination of the series and shunt Control-lers [Figures 1.4(e) and 1.4(f)] can provide the best of both, i.e., an effective power/current flow and line voltage control.

For the combination of series and shunt Controllers, the shunt Controller canbe a single unit serving in coordination with individual line Controllers [Figure 1.4(g)].This arrangement can provide additional benefits (reactive power flow control) withunified Controllers.

FACTS Controllers may be based on thyristor devices with no gate turn-off(only with gate turn-on), or with power devices with gate turn-off capability. Also, ingeneral, as will be discussed in other chapters, the principal Controllers with gateturn-off devices are based on the dc to ac converters, which can exchange active and/or reactive power with the ac system. When the exchange involves reactive poweronly, they are provided with a minimal storage on the dc side. However, if the generatedac voltage or current is required to deviate from 90 degrees with respect to the linecurrent or voltage, respectively, the converter dc storage can be augmented beyondthe minimum required for the converter operation as a source of reactive power only.This can be done at the converter level to cater to short-term (a few tens of main

Section 1.6 • Basic Types of FACTS Controllers 15

Figure 1.4 Basic types of FACTS Controllers: (a) general symbol for FACTS Con-troller; (b) series Controller; (c) shunt Controller; (d) unified series-series Controller; (e) coordinated series and shunt Controller; (f) unifiedseries-shunt Controller; (g) unified Controller for multiple lines; (h)series Controller with storage; (i) shunt Controller with storage; (j)unified series-shunt Controller with storage.

frequency cycles) storage needs. In addition, another storage source such as a battery,superconducting magnet, or any other source of energy can be added in parallelthrough an electronic interface to replenish the converter's dc storage. Any of theconverter-based, series, shunt, or combined shunt-series Controllers can generallyaccommodate storage, such as capacitors, batteries, and superconducting magnets,which bring an added dimension to FACTS technology [Figures 1.4(h), 1.4(i), and1-4G)]-

(i) G)

StorageStorage

e

Line Line

/

(b)(a)

dc powerlink

(c)

e

Line

/

(e)

Coordinatedcontrol

dc power link

e-

(d)

dc power link

(f)

Line Line

(g)

Line

dc power linkStorage

Ia

CDC

8


The benefit of an added storage system (such as large dc capacitors, storagebatteries, or superconducting magnets) to the Controller is significant. A Controllerwith storage is much more effective for controlling the system dynamics than thecorresponding Controller without the storage. This has to do with dynamic pumpingof real power in or out of the system as against only influencing the transfer of realpower within the system as in the case with Controllers lacking storage. Here also,engineers have to rethink the role of storage, particularly the one that can deliver orabsorb large amounts of real power in short bursts.

A converter-based Controller can also be designed with so-called high pulseorder or with pulse width modulation to reduce the low order harmonic generationto a very low level. A converter can in fact be designed to generate the correctwaveform in order to act as an active filter. It can also be controlled and operated ina way that it balances the unbalance voltages, involving transfer of energy betweenphases. It can do all of these beneficial things simultaneously if the converter isso designed.

Given the overlap of benefits and attributes, it can be said that for a givenproblem one needs to have an open mind during preliminary evaluation of seriesversus shunt and combination Controllers and storage versus no storage.

1.7 BRIEF DESCRIPTION AND DEFINITIONS OFFACTS CONTROLLERS

The purpose of this section is to briefly describe and define various shunt, series, andcombined Controllers, leaving the detailed description of Controllers to their ownspecific chapters.

Before going into a very brief description of a variety of specific FACTS Control-lers, it is worth mentioning here that for the converter-based Controllers there aretwo principal types of converters with gate turn-off devices. These are the so-calledvoltage-sourced converters and the current-sourced converters. As shown in the left-hand side of Figure 1.5(a), the voltage-sourced converter is represented in symbolicform by a box with a gate turn-off device paralleled by a reverse diode, and a dccapacitor as its voltage source. As shown in the right-hand side of Figure 1.5(a), thecurrent-sourced converter is represented by a box with a gate turn-off device with adiode in series, and a dc reactor as its current source.

Details of a variety of voltage-sourced converters suitable for high power applica-tions are discussed in Chapter 3, and current-sourced converters in Chapter 4. It wouldsuffice to say for now that for the voltage-sourced converter, unidirectional dc voltageof a dc capacitor is presented to the ac side as ac voltage through sequential switchingof devices. Through appropriate converter topology, it is possible to vary the ac outputvoltage in magnitude and also in any phase relationship to the ac system voltage. Thepower reversal involves reversal of current, not the voltage. When the storage capacityof the dc capacitor is small, and there is no other power source connected to it, theconverter cannot supply or absorb real power for much more than a cycle. The acoutput voltage is maintained at 90 degrees with reference to the ac current, leadingor lagging, and the converter is used to absorb or supply reactive power only.

For the current-sourced converter, the dc current is presented to the ac sidethrough the sequential switching of devices, as ac current, variable in amplitude andalso in any phase relationship to the ac system voltage. The power reversal involvesreversal of voltage and not current. The current-sourced converter is representedsymbolically by a box with a power device, and a dc inductor as its current source.

Section 1.7 • Brief Description and Definitions of FACTS Controllers 17

From overall cost point of view, the voltage-sourced converters seem to bepreferred, and will be the basis for presentations of most converter-based FACTS Con-trollers.

One of the facts of life is that those involved with FACTS will have to get usedto a large number of new acronyms. There are and will be more acronyms designatedby the manufacturers for their specific products, and by the authors of various papers

Figure 1.5 Shunt-connected Controllers: (a) Static Synchronous Compensator(STATCOM) based on voltage-sourced and current-sourced converters;(b) STATCOM with storage, i.e., Battery Energy Storage System(BESS) Superconducting Magnet Energy Storage and large dc capaci-tor; (c) Static VAR Compensator(SVC), Static VAR Generator (SVG),Static VAR System (SVS), Thyristor-Controlled Reactor (TCR), Thy-ristor-Switched Capacitor (TSC), and Thyristor-Switched Reactor(TSR); (d) Thyristor-Controlled Braking Resistor.

Line Line Line

(a) Interface

Storage

(b)

Line

TCR TSC FilterTSR

(c)

(d)


on new Controllers or variations of known Controllers. The IEEE PES Task Forceof the FACTS Working Group defined Terms and Definitions for FACTS and FACTSControllers. Along with a brief description of FACTS Controllers, appropriate IEEETerms and Definitions are also presented in this section in italic for reference. Gener-ally, this book will use the IEEE terms and definitions.

Flexibility of Electric Power Transmission. The ability to accommodate changes in theelectric transmission system or operating conditions while maintaining sufficient steady-state and transient margins.

Flexible AC Transmission System (FACTS). Alternating current transmission systemsincorporating power electronic-based and other static controllers to enhance controllabilityand increase power transfer capability.

It is worthwhile to note the words "other static Controllers" in this definition ofFACTS implying that there can be other static Controllers which are not based onpower electronics.

FACTS Controller. A power electronic-based system and other static equipment thatprovide control of one or more AC transmission system parameters.

1.7.1. Shunt Connected Controllers

Static Synchronous Compensator (STATCOM): A Static synchronous generator oper-ated as a shunt-connected static var compensator whose capacitive or inductive outputcurrent can be controlled independent of the ac system voltage.

STATCOM is one of the key FACTS Controllers. It can be based on a voltage-sourced or current-sourced converter. Figure 1.5(a) shows a simple one-line diagramof STATCOM based on a voltage-sourced converter and a current-sourced converter.As mentioned before, from an overall cost point of view, the voltage-sourced convertersseem to be preferred, and will be the basis for presentations of most converter-basedFACTS Controllers.

For the voltage-sourced converter, its ac output voltage is controlled such thatit is just right for the required reactive current flow for any ac bus voltage dc capacitorvoltage is automatically adjusted as required to serve as a voltage source for theconverter. STATCOM can be designed to also act as an active filter to absorb sys-tem harmonics.

STATCOM as defined above by IEEE is a subset of the broad based shuntconnected Controller which includes the possibility of an active power source orstorage on the dc side so that the injected current may include active power. Such aController is defined as:

Static Synchronous Generator (SSG): A static self-commutated switching power con-verter supplied from an appropriate electric energy source and operated to produce a setof adjustable multiphase output voltages, which may be coupled to an ac power system forthe purpose of exchanging independently controllable real and reactive power.

Clearly SSG is a combination of STATCOM and any energy source to supplyor absorb power. The term, SSG, generalizes connecting any source of energy includinga battery, flywheel, superconducting magnet, large dc storage capacitor, another


rectifier/inverter, etc. An electronic interface known as a "chopper" is generallyneeded between the energy source and the converter. For a voltage-sourced converter,the energy source serves to appropriately compensate the capacitor charge throughthe electronic interface and maintain the required capacitor voltage.

Within the definition of SSG is also the Battery Energy Storage System (BESS),defined by IEEE as:

Battery Energy Storage System (BESS): A chemical-based energy storage system usingshunt connected, voltage-source converters capable of rapidly adjusting the amount ofenergy which is supplied to or absorbed from an ac system.

Figure 1.5(b) shows a simple one-line diagram in which storage means is con-nected to a STATCOM. For transmission applications, BESS storage unit sizes wouldtend to be small (a few tens of MWHs), and if the short-time converter rating waslarge enough, it could deliver MWs with a high MW/MWH ratio for transient stability.The converter can also simultaneously absorb or deliver reactive power within theconverter's MVA capacity. When not supplying active power to the system, the con-verter is used to charge the battery at an acceptable rate.

Yet another subset of SSG, suitable for transmission applications, is the Supercon-ducting Magnetic Energy Storage (SMES), which is defined by IEEE as:

Superconducting Magnetic Energy Storage (SMES): A Superconducting electromagneticenergy storage device containing electronic converters that rapidly injects and/or absorbsreal and/or reactive power or dynamically controls power flow in an ac system.

Since the dc current in the magnet does not change rapidly, the power input or outputof the magnet is changed by controlling the voltage across the magnet with a suitableelectronics interface for connection to a STATCOM.

Static Var Compensator (SVC): A shunt-connected static var generator or absorberwhose output is adjusted to exchange capacitive or inductive current so as to maintain orcontrol specific parameters of the electrical power system (typically bus voltage).

This is a general term for a thyristor-controlled or thyristor-switched reactor, and/orthyristor-switched capacitor or combination [Figure 1.5(c)]. SVC is based on thyristorswithout the gate turn-off capability. It includes separate equipment for leading andlagging vars; the thyristor-controlled or thyristor-switched reactor for absorbing reac-tive power and thyristor-switched capacitor for supplying the reactive power. SVC isconsidered by some as a lower cost alternative to STATCOM, although this may notbe the case if the comparison is made based on the required performance and notjust the MVA size.

Thyristor Controlled Reactor (TCR): A shunt-connected, thyristor-controlled inductorwhose effective reactance is varied in a continuous manner by partial-conduction controlof the thyristor valve.

TCR is a subset of SVC in which conduction time and hence, current in a shunt reactoris controlled by a thyristor-based ac switch with firing angle control [Figure 1.5(c)].

Thyristor Switched Reactor (TSR): A shunt-connected, thyristor-switched inductor whoseeffective reactance is varied in a stepwise manner by full- or zero-conduction operation ofthe thyristor valve.


TSR [Figure 1.5(c)] is another a subset of SVC. TSR is made up of several shunt-connected inductors which are switched in and out by thyristor switches without anyfiring angle controls in order to achieve the required step changes in the reactivepower consumed from the system. Use of thyristor switches without firing angle controlresults in lower cost and losses, but without a continuous control.

Thyristor Switched Capacitor (TSC): A shunt-connected, thyristor-switched capacitorwhose effective reactance is varied in a stepwise manner by full- or zero-conduction operationof the thyristor valve.

TSC [Figure 1.5(c)] is also a subset of SVC in which thyristor based ac switches areused to switch in and out (without firing angle control) shunt capacitors units, in orderto achieve the required step change in the reactive power supplied to the system.Unlike shunt reactors, shunt capacitors cannot be switched continuously with variablefiring angle control.

Other broadbased definitions of series Controllers by IEEE include:

Static Var Generator or Absorber (SVG): A static electrical device, equipment, or systemthat is capable of drawing controlled capacitive and/or inductive current from an electricalpower system and thereby generating or absorbing reactive power. Generally consideredto consist of shunt-connected, thyristor-controlled reactor(s) and/or thyristor-switched ca-pacitors.

The SVG, as broadly defined by IEEE, is simply a reactive power (var) sourcethat, with appropriate controls, can be converted into any specific- or multipurposereactive shunt compensator. Thus, both the SVC and the STATCOM are static vargenerators equipped with appropriate control loops to vary the var output so as tomeet specific compensation objectives.

Static Var System (SVS): A combination of different static and mechanically-switchedvar compensators whose outputs are coordinated.

Thyristor Controlled Braking Resistor (TCBR): A shunt-connected thyristor-switchedresistor, which is controlled to aid stabilization of a power system or to minimize poweracceleration of a generating unit during a disturbance.

TCBR involves cycle-by-cycle switching of a resistor (usually a linear resistor)with a thyristor-based ac switch with firing angle control [Figure 1.5(d)]. For lowercost, TCBR may be thyristor switched, i.e., without firing angle control. However,with firing control, half-cycle by half-cycle firing control can be utilized to selectivelydamp low-frequency oscillations.

1.7.2 Series Connected Controllers

Static Synchronous Series Compensator (SSSC): A static synchronous generator operatedwithout an external electric energy source as a series compensator whose output voltage isin quadrature with, and controllable independently of, the line current for the purpose ofincreasing or decreasing the overall reactive voltage drop across the line and therebycontrolling the transmitted electric power. The SSSC may include transiently rated energystorage or energy absorbing devices to enhance the dynamic behavior of the power systemby additional temporary real power compensation, to increase or decrease momentarily,the overall real (resistive) voltage drop across the line.


Figure 1.6 (a) Static Synchronous Series Compensator (SSSC); (b) SSSC with stor-age; (c) Thyristor-Controlled Series Capacitor (TCSC) and Thyristor-Switched Series Capacitor (TSSC); (d) Thyristor-Controlled Series Reac-tor (TCSR) and Thyristor-Switched Series Reactor (TSSR).

SSSC is one the most important FACTS Controllers. It is like a STATCOM,except that the output ac voltage is in series with the line. It can be based on a voltage-sourced converter [Figure 1.6(a)] or current-sourced converter. Usually the injectedvoltage in series would be quite small compared to the line voltage, and the insulationto ground would be quite high. With an appropriate insulation between the primaryand the secondary of the transformer, the converter equipment is located at the groundpotential unless the entire converter equipment is located on a platform duly insulatedfrom ground. The transformer ratio is tailored to the most economical converterdesign. Without an extra energy source, SSSC can only inject a variable voltage, whichis 90 degrees leading or lagging the current. The primary of the transformer and hencethe secondary as well as the converter has to carry full line current including the faultcurrent unless the converter is temporarily bypassed during severe line faults.

Battery-storage or superconducting magnetic storage can also be connected toa series Controller [Figure 1.6(b)] to inject a voltage vector of variable angle in serieswith the line.

Interline Power Flow Controller (IPFC): The IPFC is a recently introduced Controllerand thus has no IEEE definition yet. A possible definition is: The combination of two ormore Static Synchronous Series Compensators which are coupled via a common dc linkto facilitate bi-directional flow of real power between the ac terminals of the SSSCs, andare controlled to provide independent reactive compensation for the adjustment of realpower flow in each line and maintain the desired distribution of reactive power flow among

LineLine

(a)

(b)

Storage

Interface

Line Line

(d)(c)


the lines. The IPFC structure may also include a STA TCOM, coupled to the IFFC's commondc link, to provide shunt reactive compensation and supply or absorb the overall real powerdeficit of the combined SSSCs.

Thyristor Controlled Series Capacitor (TCSC): A capacitive reactance compensatorwhich consists of a series capacitor bank shunted by a thyristor-controlled reactor in orderto provide a smoothly variable series capacitive reactance.

The TCSC [Figure 1.6(c)], is based on thyristors without the gate turn-offcapability. It is an alternative to SSSC above and like an SSSC, it is a veryimportant FACTS Controller. A variable reactor such as a Thyristor-ControlledReactor (TCR) is connected across a series capacitor. When the TCR firing angleis 180 degrees, the reactor becomes nonconducting and the series capacitor hasits normal impedance. As the firing angle is advanced from 180 degrees to lessthan 180 degrees, the capacitive impedance increases. At the other end, when theTCR firing angle is 90 degrees, the reactor becomes fully conducting, and the totalimpedance becomes inductive, because the reactor impedance is designed to bemuch lower than the series capacitor impedance. With 90 degrees firing angle, theTCSC helps in limiting fault current. The TCSC may be a single, large unit, ormay consist of several equal or different-sized smaller capacitors in order to achievea superior performance.

Thyristor-SwitchedSeries Capacitor (TSSC): A capacitive reactance compensator whichconsists of a series capacitor bank shunted by a thyristor-switched reactor to provide astepwise control of series capacitive reactance.

Instead of continuous control of capacitive impedance, this approach of switchinginductors at firing angle of 90 degrees or 180 degrees but without firing angle control,could reduce cost and losses of the Controller [Figure 1.6(c)]. It is reasonable to arrangeone of the modules to have thyristor control, while others could be thyristor switched.

Thyristor-Controlled Series Reactor (TCSR): An inductive reactance compensator whichconsists of a series reactor shunted by a thyristor controlled reactor in order to provide asmoothly variable series inductive reactance.

When the firing angle of the thyristor controlled reactor is 180 degrees, it stopsconducting, and the uncontrolled reactor acts as a fault current limiter [Figure 1.6(d)].As the angle decreases below 180 degrees, the net inductance decreases until firingangle of 90 degrees, when the net inductance is the parallel combination of the tworeactors. As for the TCSC, the TCSR may be a single large unit or several smallerseries units.

Thyristor-Switched Series Reactor (TSSR): An inductive reactance compensator whichconsists of a series reactor shunted by a thyristor-controlled switched reactor in order toprovide a stepwise control of series inductive reactance.

This is a complement of TCSR, but with thyristor switches fully on or off(without firing angle control) to achieve a combination of stepped series inductance[Figure 1.6(d)].


1.7.3 Combined Shunt and Series Connected Controllers

Unified Power Flow Controller (UPFC): A combination of static synchronous compensa-tor (STATCOM) and a static series compensator (SSSC) which are coupled via a commondc link, to allow bidirectional flow of real power between the series output terminals of theSSSC and the shunt output terminals of the STATCOM, and are controlled to provideconcurrent real and reactive series line compensation without an external electric energysource. The UPFC, by means of angularly unconstrained series voltage injection, is ableto control, concurrently or selectively, the transmission line voltage, impedance, and angleor, alternatively, the real and reactive power flow in the line. The UPFC may also provideindependently controllable shunt reactive compensation.

In UPFC [Figure 1.7(b)], which combines a STATCOM [Figure 1.5(a)] and anSSSC [Figure 1.6(a)], the active power for the series unit (SSSC) is obtained from theline itself via the shunt unit STATCOM; the latter is also used for voltage controlwith control of its reactive power. This is a complete Controller for controlling activeand reactive power control through the line, as well as line voltage control.

Additional storage such as a superconducting magnet connected to the dc linkvia an electronic interface would provide the means of further enhancing the effective-ness of the UPFC. As mentioned before, the controlled exchange of real power withan external source, such as storage, is much more effective in control of systemdynamics than modulation of the power transfer within a system.

Thyristor-Controlled Phase Shifting Transformer (TCPST): A phase-shifting trans-former adjusted by thyristor switches to provide a rapidly variable phase angle.

In general, phase shifting is obtained by adding a perpendicular voltage vector inseries with a phase. This vector is derived from the other two phases via shunt connectedtransformers [Figure 1.7(a)]. The perpendicular series voltage is made variable witha variety of power electronics topologies. A circuit concept that can handle voltagereversal can provide phase shift in either direction. This Controller is also referred toas Thyristor-Controlled Phase Angle Regulator (TCPAR).

Interphase Power Controller (IPC): A series-connected controller of active and reactivepower consisting, in each phase, of inductive and capacitive branches subjected to separately

Figure 1.7 (a) Thyristor-Controlled Phase-Shifting Transformer (TCPST) or Thyris-tor-Controlled Phase Angle Regulator (TCPR); (b) Unified Power FlowController UPFC).

Line

SSSC

3-phase line

STATCOM

(b)(a)

dc link


phase-shifted voltages. The active and reactive power can be set independently by adjustingthe phase shifts and/or the branch impedances, using mechanical or electronic switches. Inthe 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 theother terminal

This is a broadbased concept of series Controller, which can be designed to providecontrol of active and reactive power.

1.7.4 Other Controllers

Thyristor-Controlled Voltage Limiter (TCVL): A thyristor-switched metal-oxide varistor(MOV) used to limit the voltage across its terminals during transient conditions.

The thyristor switch can be connected in series with a gapless arrester, or [asshown in Figure 1.8(a)] part of the gapless arrester (10-20%) can be bypassed by a

(c)

Figure 1.8 Various other Controllers: (a) Thyristor-Controlled Voltage Limiter(TCVL); (b) Thyristor-Controlled Voltage Regulator (TCVR) basedon tap changing; (c) Thyristor-Controlled Voltage Regulator (TCVR)based on voltage injection.

Line

(a)

Line

(b)

Section 1.8 • Checklist of Possible Benefits from FACTS Technology 25

thyristor switch in order to dynamically lower the voltage limiting level. In general,the MOV would have to be significantly more powerful than the normal gaplessarrester, in order that TCVL can suppress dynamic overvoltages, which can otherwiselast for tens of cycles.

Thyristor-Controlled Voltage Regulator (TCVR): A thyristor-controlled transformerwhich can provide variable in-phase voltage with continuous control.

For practical purposes, this may be a regular transformer with a thyristor-con-trolled tap changer [Figure 1.8(b)] or with a thyristor-controlled ac to ac voltageconverter for injection of variable ac voltage of the same phase in series with the line[Figure 1.8(c)]. Such a relatively low cost Controller can be very effective in controllingthe flow of reactive power between two ac systems.

Another family of Controllers, not defined here, but mentioned in Section 1.6are the ones that link two or more transmission lines, in order to balance the real andreactive power flows. These are discussed in Chapter 8.

1.8 CHECKLIST OF POSSIBLE BENEFITS FROMFACTS TECHNOLOGY

Details of various FACTS Controllers will be discussed throughout the book. It isappropriate to state here that within the basic system security guidelines these Control-lers enable the transmission owners to obtain, on a case-by-case basis, one or moreof the following benefits:

• Control of power flow as ordered. The use of control of the power flow maybe to follow a contract, meet the utilities' own needs, ensure optimum powerflow, ride through emergency conditions, or a combination thereof.

• Increase the loading capability of lines to their thermal capabilities, includingshort term and seasonal. This can be accomplished by overcoming other limita-tions, and sharing of power among lines according to their capability. It is alsoimportant to note that thermal capability of a line varies by a very large marginbased on the environmental conditions and loading history.

• Increase the system security through raising the transient stability limit, limitingshort-circuit currents and overloads, managing cascading blackouts and damp-ing electromechanical oscillations of power systems and machines.

• Provide secure tie line connections to neighboring utilities and regions therebydecreasing 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 loop flows.• Increase utilization of lowest cost generation. One of the principal reasons for

transmission interconnections is to utilize lowest cost generation. When thiscannot be done, it follows that there is not enough cost-effective transmissioncapacity. Cost-effective enhancement of capacity will therefore allow increaseduse of lowest cost generation.

There is a natural overlap among the above-cited benefits, and in reality, any one or


TABLE 1.1 Control Attributes for Various Controllers

FACTS Controller Control Attributes Figure

Static Synchronous Compensator(STATCOM without storage)

Static Synchronous Compensator(STATCOM with storage, BESS,SMES, large dc capacitor)

Static VAR Compensator (SVC,TCR, TCS, TRS)

Thyristor-Controlled Braking Resis-tor (TCBR)

Static Synchronous Series Compen-sator (SSSC without storage)

Static Synchronous Series Compen-sator (SSSC with storage)

Thyristor-Controlled Series Capaci-tor (TCSC, TSSC)

Thyristor-Controlled Series Reac-tor (TCSR, TSSR)

Thyristor-Controlled Phase-ShiftingTransformer (TCPST or TCPR)

Unified Power Flow Controller(UPFC)

Thyristor-Controlled Voltage Lim-iter (TCVL)

Thyristor-Controlled Voltage Regu-lator (TCVR)

Interline Power Flow Controller(IPFC)

Voltage control, VAR compensation, damping oscillations, volt- 1.5(a)age stability

Voltage control, VAR compensation, damping oscillations, tran- 1.5(b)sient and dynamic stability, voltage stability, AGC

Voltage control, VAR compensation, damping oscillations, tran- l-5(c)sient and dynamic stability, voltage stability

Damping oscillations, transient and dynamic stability 1.5(d)

Current control, damping oscillations, transient and dynamic sta- 1.5(a)bility, voltage stability, fault current limiting

Current control, damping oscillations, transient and dynamic sta- 1.6(b)bility, voltage stability

Current control, damping oscillations, transient and dynamic sta- 1.6(c)bility, voltage stability, fault current limiting

Current control, damping oscillations, transient and dynamic sta- 1.6(d)bility, voltage stability, fault current limiting

Active power control, damping oscillations, transient and dy- l-7(a)namic stability, voltage stability

Active and reactive power control, voltage control, VAR compen- 1.7(b)sation, damping oscillations, transient and dynamic stability,voltage stability, fault current limiting

Transient and dynamic voltage limit 1.8(a)

Reactive power control, voltage control, damping oscillations, 1.8(b)transient and dynamic stability, voltage stability 1.8(c)

Reactive power control, voltage control, damping oscillations, l-4(d)transient and dynamic stability, voltage stability

two of these benefits would be a principal justification for the choice of a FACTSController. It is, however, important to check the list of these benefits on a value-added basis.

Because the voltage, current, impedance, real power, and reactive power areinterrelated, each Controller has multiple attributes of what they can do in terms ofcontrolling the voltage, power flow, stability and so on. These controllers can havemultiple open loop and closed loop controls to accomplish multiple benefits. Table1.1 gives a checklist of control attributes for various Controllers. The list is not exhaus-tive, in particular it does not include the Controllers that link multiple lines, discussedin Chapter 8. Of course the degree of controllability, in particular the control ofstability and power flow will vary from Controller to Controller. For understandingthe value of these Controllers, one needs to read detail in various chapters of this book.

1.9 IN PERSPECTIVE: HVDC OR FACTS

It is important to recognize that generally HVDC and FACTS are complementarytechnologies. HVDC is not a grid network in the way that an ac system is, nor is itexpected to be. The role of HVDC, for economic reasons, is to interconnect ac systemswhere a reliable ac interconnection would be too expensive.

There are now over 50 HVDC projects in the world. These can be divided intofour categories:

Section 1.9 • In Perspective: HVDC or FACTS 27

1. Submarine cables. Cables have a large capacitance, and hence ac cables requirea large charging current (reactive power) an order of magnitude larger thanthat of overhead lines. As a result, for over a 30 km or so stretch of acsubmarine cable, the charging current supplied from the shore will fully loadthe cable and leave no room for transmitting real power. The charging currentflowing in the cables can only be reduced by connecting shunt inductors tothe cable at intervals of 15-20 km, thus requiring appropriate land location.With HVDC cable on the other hand, distance is not a technical barrier. Also,the cost of dc cable transmission is much lower than that of ac which worksto HVDC's advantage to cover new markets for long distance submarinetransmission. In this area, FACTS technology (e.g., the UPFC) can providean improvement by controlling the magnitude of one of the end (e.g., thereceiving-end) voltages so as to keep it identical to that of the other one. Inthis way, the effective length of the cable from the standpoint of the chargingcurrent can be halved. This approach may provide an economical solutionfor moderate submarine distances, up to about 100 km, but for long distancetransmission HVDC will remain unchallenged.

2. Long distance overhead transmission. If the overhead transmission is longenough, say 1000 km, the saving in capital costs and losses with a dc transmis-sion line may be enough to pay for two converters (note that HVDC representstotal power electronics rating of 200% of the rated transmission capacity).This distance is known as the break-even distance. This break-even distanceis very subject to many factors including the cost of the line, right-of-way,any need to tap the line along the way, and often most important, the politicsof obtaining permission to build the line. Nevertheless, it is important torecognize that while FACTS can play an important role in an effective useof ac transmission, it probably does not have too much influence on the break-even distance. Thus, the principal role of FACTS is in the vast ac transmissionmarket where HVDC is generally not economically viable.

3. Underground transmission. Because of the high cost of underground cables,the break even distance for HVDC is more like 100 km as against 1000 kmfor overhead lines. Again, FACTS technology probably does not have muchinfluence in this break-even distance. In any case, to date there have been nolong distance underground projects, either ac or dc, because, in an openlandscape, overhead transmission costs so much less than underground trans-mission (about 25% of the costs of underground transmission). Cable transmis-sion, on the other hand, has a significant potential of cost reduction, both inthe cost of cables and construction cost.

4. Connecting ac systems of different or incompatible frequencies. For historicalreasons, the oceans in effect separate the globe's electric systems into 50 Hzand 60 Hz groups. The 60 Hz normal frequency pervades all the countries ofthe Americas, excepting Argentina and Paraguay. Those two countries andall the rest of the world have a 50 Hz frequency except Japan, which is partly50 Hz and partly 60 Hz. In general, the oceans are too huge and deep tojustify interconnections of 50 and 60 Hz systems. Thus there is a limitedmarket for HVDC for connecting 50 and 60 Hz systems.

The related, but larger, HVDC market consists of tying together ac systems thathave incompatible frequency control such that the phase angle between the two systems


frequently straddles the full circle or very large part of a circle. Even once or twiceper day excursion of phase angle in excess of 60 degrees would be often enough torule out any contrived mechanical phase shifter. HVDC can solve this problem. Todate there are many HVDC projects, most of them back-to-back ties, which serve thisneed, and some include some length of dc line as well. Experts agree that some tiesof this kind will drop out of use once the neighbors agree upon and implement acommon frequency control strategy. This is in fact what has happened in Europe. Notso very long ago, members of the former Comecon (East Europe and the Europeanpart of the former USSR) were interconnected as one ac system, but could not beconnected to the West European system, known as UCPTE, because they employeddifferent kinds of frequency control and available reserve margins. Then the desireto exchange power with East Europe prompted the installation of three back-to-backties: the Diirnrohr 550 MW tie connecting Austria to Poland, the Etzenricht 600 MWtie connecting Germany to the Czech Republic, and the Vienna East-West 600 MWtie connecting Austria to Hungary. However, with the subsequent unification of Ger-many and other political developments, East Germany and the East European coun-tries of Poland, the Slovak and Czech Republics, and Hungary formed themselvesinto the Centrel Group and installed and field-calibrated their controls at their powerplants and control centers. Then in September and October of 1995, their systemswere connected to the West European system by ac. Other members of the formerComecon are either operating in isolation or connected to the Integrated Power Systemwith Russia. One side effect is that the three back-to-back HVDC ties, the Diirnrohrand Sud-Ost in Austria and the Etzenricht in Germany, were bypassed and no longerused. This will also happen in India where electricity regions have been connectedwith back-to-back ties.

There are also many locations in the world where frequency control strategiesare the same, but, because of the remoteness of centers of electrical gravity from thedesired points of connection, phase angle drifts and varies over a wide angle withdaily variations in load and even go full circle. The Eastern and Western United Statessystems are an example of this, and therefore they are connected with several back-to-back HVDC ties. Conceptually, FACTS technology can also solve this problem ofwide variations of phase angle. It requires, for example, a phase-angle Controller forone side to chase or lead the other as required through full circle. This is in facttechnically feasible with known concepts of phase angle Controller. However, theeconomics are perhaps in favor of HVDC. After all, HVDC is independent of phaseangle and therefore, in a way, equivalent to a 360 degree phase angle Controller, anda true 360 degree phase angle Controller may be more expensive.

Large market potential for FACTS is within the ac system on a value-addedbasis, where:

• the existing steady-state phase angle between bus nodes is reasonable,

• the cost of a FACTS solution is lower than the HVDC cost, and

• the required FACTS Controller capacity is less than, say, 100% of the transmis-sion throughput rating.

Figure 1.9 conveys the basic attributes of HVDC and FACTS, and also general costcomparison between HVDC and FACTS where FACTS solutions are viable and couldhave scope of applications.

References 29

HVDC:

• Independent frequency and control• Lower line costs• Power control, voltage control,

stability control

FACTS:

• Power control, voltage control,stability control

Installed Costs (millions of dollars)

Throughput MW HVDC 2 Terminals

200 MW500 MW

1000 MW2000 MW

$ 40-50 M75-100

120-170200-300

FACTS

$ 5-10 M10-2020-3030-50

Figure 1.9 HVDC and FACTS: Complementary solutions.

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FACTS Concept and General System Considerationscatalogimages.wiley.com/images/db/pdf/0780334558.excerpt.pdf · Chapter 1 • FACTS Concept and General System Considerations ... involves