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Power Quality Implications Associated with a Series FACTS Controller

Kingdom

RE. Momson

S.B.Tenuakoon

Monash University Clayton, Victoria 3168,Australia

staf€ordshk.university

e an increase in power transfer in related issues such as voltage

S y n c h o w Series Compensator (SSSC), Flexible AC Transmission Systems (FACTS), power flow control, harmonic voltage distortion.

I. INTRODUCTION The need for load flow control to improve utilisation of power systems has long been recognised [l]. At present power flow control may be realised by deploying phase shift and tap changing transformers, which allow steady state control only due to the slow response of the mechanically operated tap changing equipment [2], [3]. The application of dynamically controllable elements in power systems will lead to increased power system utilisation and more reliable operation [4]. Moreover with the deregulation of electricity markets world wide, devices able to control power flow dynamically, may be applied to realise power transport contracts (e.g. contract path arrangements [SI). A new concept, referred to as flexible AC transmission systems (FACTS), was proposed in the early 1990s to enhance dynamic control in power systems and to improve system utilisation [4]. FACTS is based on the application of high power, power electronic shunt and series devices. Within the FACTS concept, series devices are effective dynamic power flow controllers. The first controllable series devices installed in transmission systems were thyristor switched series capacitors (TSSC) and thyristor controlled series capacitors (TCSC) [4], [6], [7], [8]. The TSSC and TCSC rely on passive elements to provide the reactive power. However inverter b h d devices generate the reactive power by switching, thus only a small energy storage element may be needed on the dc side to allow power exchange during transients.

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A series controller using VSI technology may be considered as part of the unified power flow controller (UPFC) [3], [6]. If the shunt inverter of the UPFC is replaced by a dc capacitor, the series device is ]mown as a static synchronous series compensator (SSSC) [6], [A. The work presented considers the application of a SSSC to increase the power transfer capacity of a meshed transmission network. The paper discusses the effect of the SSSC on power quality issues, such as voltage variation and fluctuation, voltage levels and harmonic voltage distortion. It is shown that depending on the SSSC control strategy load voltage levels may be adjusted to some degree and the magnitude of the voltage variation may be changed. It is also shown, that "reflection" of voltage dips across the transmission system is influenced by the way the SSSC is controlled. Another critical aspect considered in this paper is the relationship between the level of injected voltage and the level of harmonic voltage distortion at the load bus bars. The SSSC may need particular control to avoid variation of harmonic voltage distortion.

II. CHARACTERISTICS OF A SSSC The SSSC consists of a power electronic inverter connected to an energy storage element on the dc side. Its ac terminals are connected in series with a transmission line. With the appropriate inverter topology the SSSC may inject a voltage directly into the transmission system. However an injection transformer is likely to be required not only to isolate the inverter fiom the high voltage system, but also to adjust the current and voltage levels to values more convenient for the power electronic converter. For an application in a transmission system a SSSC may require a high power rating (> 50 MVAr). Thus high power GTO thyristors are needed. In many high power, power electronic inverters the major component of the operational losses is caused by switching. Therefore the power electronic devices are usually limited to one on-off and one off-on transition per cycle. Consequently instead of deploying a simple three-phase bridge inverter using pulse width modulation (PWM) to reduce harmonic distortion, a more complex multi-pulse inverter is considered here. Multipulse inverters employ a transfomer arrangement coupling

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the response of protection systems, power quality related issues, e.g. voltage fluctuation, reflection of voltage variation to adjoining systems and harmonic distortion. This paper considers the power quality issues.

the ac terminals of a number of basic six-pulse inverters to cancel harmonic voltage components. Fig. 1 shows an experimental model multi-pulse SSSC, which was used for the investigation presented in this paper. 1:43

Reactancemode

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Algorithm

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Voltage controlled SSSC

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b) Cumnt control mode Fig. 2 Block diagrams of SSSC control systems

m.SYSTEM USED TO INVESTIGATE THE IMPACT OF A SSSC ON POWER QUALITY

Fig. 3 shows the transmission system used to investigate power quality issues. The three phase transmission lines were modelled as n-sections with parameters given in table 1. Transmission lines 2 and 3 have a thermal current limit of 5.0 pu whilst transmission line 1 has a current limit of 8.0 pu. The load data are given in table 2. All pu values correspond to a base voltage of 275 kV and a base power of 100 MVA. The feeding transmission system has a fault level of 50 pu at nominal voltage. It is represented by a constant voltage of 1.085 pu connected to a reactance of 0.02 pu.

Fig. 1: 24@e SSSC

A series controller constructed as fig. 1, can neither provide nor absorb active power during the steady state as the dc terminals are connected to a capacitor. However, by changing the firing angle of the inverter the capacitor voltage can be increased or decreased to control the amplitude and direction of the injected voltage. Thus a SSSC is basically a controllable reactive voltage source.

With an appropriate control algorithm, a SSSC can be forced 1 to have the same voltage versus current characteristic as a capacitor or a reactor. Using the SSSC in so called reactance mode (X-mode SSSC operation) reduces or increases the reactance of the power flow path in series with the SSSC. It I is possible to regulate the current of a transmission line with a SSSC to a constant level (I-mode SSSC operation) in order to redistribute load flow in meshed transmission systems. Fig. 2 shows the block diagrams of possible control systems for a SSSC either operating to represent a reactance (fig. 2a) or to hold constant the current in a transmission line (fig. 2b).

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As a SSSC can be controlled in a very flexible manner, it is important, not only determine a control strategy, but also to investigate how alternative control strategies may affect the interaction with other FACTS controllers, automatic voltage regulators or governor systems,

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Fig. 3: Transmissionsystem used for power quality studies

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The transmission system without the SSSC was selected to supply loads 1 and 3 only (configuration 1, table 3). Under this load condition line 3 operates at 84 % of its thermal current limit. Due to the addition of load 2 the natural load balance is disturbed such that line 3 is over loaded and line 1 and line 2 'are not fully utilised (table 3, configuration 2). Fluctuation of load 2 will disturb all system voltages iqcluding those in adjoining transmissionand distribution systems. In order to prevent line 3 fiom being over loaded current is routed to line 1 by controlling the SSSC to reduce the reactance of the current flow path of transmission line 1. The harmonic distortion to the load is minimised, if the SSSC is located at the sending end bus bar of the transmission line (fig- 3)The load flow, after deployment of the SSSC to raise the current in transmission line 1 to its thermal rating, is given as configuration 3 in table 3. Comparison between 'Cod5guration 2' and '-gumtion 3' in table 3 demonstrates the capability of the SSSC to increase the overall power transfer capacity and improve the utilisation of the power system. Table 3: Pa"-

Loadl Load2 Load3

3.6 2.3 1 1 4 1 ~ ~ 4.2 1.032 Vi. Iuu * V2lpu 0.998 V3lpu 1.005

4.99 0.56 7.62 0.966 ,0.911 0.909

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To allow comparison with the uncontrolled system, the voltage levels have also been analysed for the system without the SSSC. The effect of the SSSC control strategy on the voltage levels at bus-bars 1, 2 and 3 is shown in fig. 4 and fig. 5 for the medium and maximum load condition (c.f. table 4). Fig. 4 and fig. 5 indicate that the both schemes raise the voltage levels and support the load and supply bus bar voltages. Table 4: Load conditions Load2

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Condition Min load Low load

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its thermal limit. Such an operating mode may be of interest in a deregulated electricity market, where power is transferred by contract path [SI.

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Bus Bar 1

Bus Bar 2

Bus Bar 3

Fig. 4 Voltage levels at medium load (c.f. table 4) 1.02

0.160

IV. EVALUATION OF THE VOLTAGE LEVELS AND VAFUATIONS

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In order to evaluate the impact of the control mode on voltage quality, load flow calculations have been carried out. The term voltage quality refers in this section to the effect of the SSSC on voltage levels and on voltage variation following load changes.

0.94 0.92 0.9

Bus Bar 1

Bus Bar 2

Bus Bar 3

Fig. 5: Voltage levels at maximum load (c.6 table 4)

Two modes of operation of the SSSC have been considered. In scheme 1 the SSSC operates in reactance mode @-mode) where the gain between SSSC voltage and SSSC current is 19.78 lo-' pu. This gain causes the current in transmission line 1 to be maintained at the thermal limit with the highest load considered (configuration 3 in table 3).

The second scheme considers the SSSC operating in current regulation mode (I-mode) such that the current in line 1 is at

Variations in load and disturbances due to switching and faults in one part of a transmission system may result in reflected voltage variation in other parts of the transmission network. The ability of the SSSC to reduce voltage variation has been analysed. The results given in fig. 6, fig. 7 and fig. 8 indicate that the distribution systems connected to bus bars 2 and 3 are subjected to lower voltage variation when the SSSC is

present. Comparison between fig. 6, fig. 7 and fig. 8 also shows, that when the magnitude of the load changes increases the I-mode scheme provides a lower voltage variation at bus-bars 2 and 3 than the %-mode scheme. However systems connected upstream to bus bar 1 are subjected to an increased voltage variation if the I-mode control scheme is applied. Moreover fig. 6, fig. 7 and fig. 8 reveal that the reflection of a voltage dip to the supply transmission system upstream of bus bar 1 is minimised, when the SSSC operates in reactance mode. Clearly the SSSC has a role to play in reduction of voltage fluctuation if the dynamic control is arranged to be sufficiently fast. 14

12

no SSSC \

Bus Bar 1

Xc mode I

I mode

Bus Bar 3

Fig. 6 Variation at change 6um max load to medium load (c.f. table 4) 14 12

.no SSSC

Xc mode

Of particular interest is the harmonic voltage distortion introduced by the 24-pulse SSSC; its voltage spectrum is shown in fig. 9. The small components in the vicinity of the 12* and 36" harmonics could be eliminated, by using three phase summing transformers at zig-zag connection at some cost [9].

p I mode

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An SSSC will always generate some harmonic voltage distortion. In order to analyse the distortion a three phase model of the system was implemented on a hardware simulator. This device uses gapped inductors to represent system inductance and capacitors to model line capacitance. The SSSC has been implemented as model 24-pulse inverter (fig. 1) and controlled by a digital signal processor in current regulation mode. Bus bar voltage waveforms have been measured under the load conditions given in table 4 and the magnitude of each harmonic voltage component has been derived.

The results given in fig. 9 to fig. 11 show the harmonic voltage spectra as a percentage of the fundamental frequency component for the SSSC operating in current control mode only. The I-mode system gives a more pessimistic scenario than reactance mode.

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Bus Bar 2

V. ASSESSMENT OF HARMONIC DISTORTION

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0.5 0 10

15

20

25

30

35

40

45

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Fig. 9: Bus Bar 1

Bus Bar 2

Bus Bar 3

Fig. 7: Variation at change 6um max load to low load (c.f. table 4) 14

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12

Bus Bar 1

Bus Bar 2

Bus Bar 3

Fig. 8: Variation at change h m max load to min load (c.f. table 4)

Harmonic spectrum of the SSSC voltage as a percentage of the fundamentalcomponent

Fig. 10 presents the harmonic voltage spectra at bus bar 2 and bus bar 3 with the system operating under medium load and maximum load (table 4). Although the voltage of the SSSC operating in current control mode increases from 0.160 pu to 0.181 pu when load 3 is disconnected, the voltage magnitudes for most of the harmonic components are reduced (fig. 10). Fig. 11 illustrates the harmonic voltage spectra at bus-bars 2 and 3, under low load and maximum load. Compared with fig. 10, the results shown in fig. 11, indicate a significant reduction in the voltage magnitudes of the harmonic components in spite of the increase in the SSSC voltage fiom 0.160 pu for maximum load to 0.239 pu for the low load condition.

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The reason for the reduction in voltage distortion with an apparent injected voltage increase lies in the transfer characteristic of the transmission system, which changes with load. The results given in fig. 10 and fig. 11 reveal that the harmonic voltages of the SSSC may be magnified between the point of injection and the load. It is important to investigate, whether a certain load condition could raise harmonic voltages.

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20

30

40

50

Order N

a) V,, medium load @right) and max. load (dark), c.f. table 4

10

20

30

40

The ratios Vfl,,,, and V,N,,, have been evaluated directly from the model (fig. 12) over a fi-equencyrange covering the iirst 50 harmonic orders and for the load conditions shown in table 4. The load has been modelled by R-L-circuitS (fig. 12). This method of modelling the loads minimises the effect of the reactive power component of the load.

50

Order N

b) V,, medium load (bright) and max.load (dark),c.f. table 4 Fig. 10: Harmonic voltage specr” at bus bar 2 and 3

10

20

30

40

50

Order N

resonant analysis (cf. fig. 3)

Fig. 13 shows the results of the transfer ratio analysis. It can be seen that there are two regions, where harmonic voltage components at the load bus bars exceed the level at the SSSC terminal. Each load condition is marked in fig. 13 by: o = m a x i ” load, + = medium load, x = low load The harmonic voltages in the vicinity of the 24* order are increased, when loads 1,2 and 3 are connected. The second highest magnification of the SSSC harmonic voltages close to the 24* order is caused by the configuration with load 3 switched off and load 1 and 2 switched on. It is clear that amplification due to resonauce occurs and that the resonant frequency is dependent on load.

a) V,, low load (bright) and max. load (dark), c.f. table 4

Standard harmonic penetration studies consider distortion sources to be parallel current loads where the link between harmonic current and system voltage is the self and transfer admittance values. When a system has a resonant characteristic the self and transfer nodal admittance values are dependent on the load but to a lessor degree than the voltage transfer ratios measures here. It is thus important to model the loads correctly when considering distortion from series compensators and the simple representation used here may not be accurate in all circumstances. Order N

b) V,, low load (bright) and max. load (dark),c.f. table 4 Fig. 11: Hamonic voltage spectrum at bus bar 2 and 3

Fig. 13 also reveals, that the SSSC harmonic voltages in the vicinity of the 12th order are magnified by a factor larger than 2. Based on this resonant characteristic the use of a 12

- 181. pulse SSSC inverter is likely to lead to a need for harmonic filters.

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Han, ZX.: ‘Phase shifter and power flow control’,IEEE Transactions on Power Apparatus and Systems, Vol. PAS 101, No. 10, October 1982, pp. 3790-3795. Nelson, RJ.: ‘Transmissionpower flow controk Electronics vs. Elect“gnetic Alternatives for Steady State Operation’,IEEE Transactions on Power Delivery,Vol. PWRD 9, No. 3, July 1994, pp. 1678-1684. Gyugyi, L.: ‘Dynamic compensation of ac transmission lines by solid state synchronous voltage sources’, IEEE Transactions on Power Deliwry, Vol. 9, No.2, April 1994, pp.904-911. Hingorani, N.G.: ‘FACTS - flexible ac tmsmission systems’, Fifth InternationalConference on AC and DC Power Tnurrmission, 1720 September 1991. London, United Kingdom, pp. 1-8. Weedy,B.M., Cory, B.J.: “ElectricPower Systems”, 4* Edition, Chichester, New York,John Wiley, 1998, p. 515. IEEE FACTS Tums & Definitions Task Force: ‘Proposedterms and definitions for flexible ac eansmiSsion system (FACTS)’, IEEE transactions on Power Delivq, Vol. 12, No. 4, October 1997, pp. 1848-1853. Mihalic, R: ‘Power flow control with controllable reactive series elements’, IEE F’roc.-Gener. Transm. Diseib., Vol 145, No. 5, September 1998,493498. Urbanek, J., Piwko, RG., Larsen, E.V., Damsky, B.L., Furumasu, B.C., Mittlestadt, W., Eden, J.D.: ‘Thyristor controlled Series Compensation P r o t o w at the Slatt 500 kV Substation’, IEEE Transactions on Power Delivery, Vol. PWRD 8, No. 3, July 1993, pp. 1460-1469. Pen& F.Z., Lai, J.-S.: ‘Dynamic Performance and Contml of a Static Var Genemtor Using Cascade Multilevel Invertexs’, IEEE T d o n s on Industry Applications, Vol. 33, No. 3, MayIJune 1997, pp. 748-754.

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VII. REFERENCES

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VIII. BIOGRAPHZES

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Karl-Heinz Kuypers was born in Kerkcn, Germany, in 1969. He received the DiplomIngenieur (FH) degree in Eleci~icalEngineering finm the Fachhochschule Koblenr, Germany, in 1996 and his PhD d e w fium Staf€ordsbire University in 2000. He worked at StatTordshire Uni-ty as a Research Student and Research Associate from 1995. In 1999 Karl-Heinz joined AEA Technology in London as a Systems Engineer.

2a

b)V3 1vss.x (bus bar 3) Fig. 1 3 Frequency charactaistic of the power system given in fig. 3

VI. CONCLUSION

Professor Bob Morrison was bom in Stoke on Trent, United Kingdom, in 1951. He received his BSc degree and PhD degree fium Staffordshire University, United Kingdom, in 1973 and 1981 rcspcCtivcly. Professor Morrision worked in ALSTOM (UK) h m 1973 to 1983 and at Staffordshire University fium 1983 to 1997. Professor Morrison joined Monash University, Victoria,Australia, in 1997.

The studies described in this paper demonstrate the SSSC as a device capable of increasing the power transfer capacity of a meshed transmission network by redistributing power flow to paths not fully utilised. Harmonic analyses of voltage waveforms have demonstrated that the harmonic voltage distortion introduced by the SSSC depends not only on the level of the injected voltage, but also on the load connected to the system. As particular load configurations may cause a magnification of some harmonic components, it is concluded, that the voltage transfer characteristic of the transmission system should be analysed carefully and that the loads should be correctly modelled.

S. B. Tennakoon (M 1987) was born in Maho, Sri Lanka, on January 18, 1953. He obtained his

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Bachelor, MSc and PhD degrees fium University of Sri Lanka, University of Aston and Lancashire University respedvely. His research interests are FACTS, power quality, HVM: and circuit breakers. Dr Tennakoon is currently Reader in Electrical Engineering at StaEordshireUniversity, United Kingdom.