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21, rue dArtois, F-75008 PARIS B4-308 CIGRE 2006http : //www.cigre.org

APPLICATION OF A TCSC-SCHEME TO DYNAMICALLY CONTROL THEPOWER TRANSFER ON A 380-KV TIE LINE

A. HAMMAD* D. MOOR S. LAEDERACH

Nordostschweizerische Kraftwerke AG (NOK)

Switzerland

SUMMARY

Since the Blackout incident of September 2003, the southern UCTE region has been in focus. New

transmission lines and means for adding transfer capacities on existing tie lines have, therefore, been

in process. This paper elucidates the different methodologies for increasing and dynamically

controlling the power transfer on an existing 380-kV tie line between the Swiss and the Italian

networks after operation of a newly constructed transmission corridor in parallel. The aim is to ensure

that the total power transfer transaction between the two networks is maintained within its set limits,

also during contingencies, without resorting to curtailment of power production.

The classical application of phase shifting transformers is ruled out due to the drawbacks of

mechanical tap changers and heavy weight. Two FACTS schemes for series compensation using

thyristors (TCSC) and voltage sourced converters (VSC) are proposed. For the TCSC solution, three

alternative schemes that have the same current rating and the total series capacitive impedance but are

different in the split between the fixed and the variable portions are examined in detail. A new

economic technique for extending the inductive range of the TCSC is introduced and compared to

existing techniques. For the VSC solution with PWM, two approaches, namely; Static Synchronous

Series Compensator with booster transformers and Transformerless Reactive Series Compensator are

briefly discussed. A detailed electromagnetic transient model is used for technical comparison of the

proposed schemes and an economic comparison of all schemes is presented.

KEYWORDS

FACTS TCSC - Voltage Sourced Converters SSSC - TRSC

1. INTRODUCTION

The introduction of the open electricity market in Europe has produced a surge of power exchange

opportunities among different entities of the UCTE grid. Many short and long-term power exchange

transactions in the grid have taken place in patterns that were not anticipated by the original grid

planners. In particular, the southern region encompassing southwest Germany, western Austria,

Slovenia, Switzerland, northern Italy and eastern France has been in focus since the Blackout incident

* e-mail: [email protected]

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of September 2003 [1]. New transmission lines and means for adding transfer capacities on existing tie

lines have, therefore, been in process. For example, a new transmission corridor between southeast

Switzerland and northern Italy, consisting of two 380-kV circuits, has been in commercial operation

since January 2005. The realisation of this new 200 km corridor was made in a record time after it was

deblocked by this incident end of 2003 when most of the planning and erection work particularly on

the Swiss side was already carried out since 1995.

The new transmission corridor has indeed raised the wire transfer capacity between Italy and its

neighbouring countries. However, other existing 380-kV tie lines that run almost in parallel to the new

transmission have seen their share of power transfer dwindling even when one of the new circuits is

out of service. One particular tie line between the Swiss and the Italian networks, denoted hereafter as

S-I, that has a power ampacity of 1600 MW, normally carried 1000 MW power flow but now transfers

only 600 MW. Tripping one of the new circuits while carrying 1000 MW, when the total power

transfer from Switzerland to Italy is 3500 MW, causes the net power flow to drop by about 700 MW.

Only 300 MW are picked up by the remaining tie lines, including the S-I line with a share of about

200 MW. The rest of the power flow to Italy is diverted to other interconnections from France, Austria

and Slovania, some of which could be overloaded. A reverse scenario may occur due to power

generation management or switching operations outside the Swiss network that can lead to excessivenatural flows on its tie lines to Italy. The S-I line has also suffered from severe overloads in the past

that could have jeopardised the security of the whole network on several occasions.

Means are, therefore, sought to maintain the total Swiss-Italian power transfer transaction within its set

limits, also during contingencies, without resorting to curtailment of power production. In this regard,

the S-I line can play a major role. By controlling the power carrying capability of the S-I tie line to

dynamically reach its secure full capacity of 1600 MW the power transfer level between the two grids

can be regulated according to the required power transaction values. This will also complement the

objective of the new transmission corridor in effectively raising the power transfer capability of the

whole interconnection network.

The existing S-I line, besides being connected to strong points in the grids, it fulfills the twoconditions of sufficient voltage angle across its terminals and sufficient length, i.e. impedance, to

allow equally effective application of both series impedance compensation and series voltage injection

techniques for the dynamic control of its power flow.

This paper elucidates the different methodologies for increasing and dynamically controlling the

power transfer on the existing 380-kV S-I inter-tie after operation of the newly constructed

transmission corridor in parallel. The application of phase shifting transformers with mechanical tap

changers is ruled out, in this case, due to their well known technical drawbacks [2], intensive

maintenance requirements and heavy weight that is impractical to transport to such an alpine region.

2. THYRISTOR-CONTROLLED SERIES COMPENSATOR

Capacitors inserted in series with transmission lines have been utilised for several decades in electric

networks around the world to increase their power transfer capabilities. This simple and economic

technology has proved its maturity and reliability through its well established track record [3]. The

introduction of the Thyristor-Controlled Series Compensator (TCSC) in the early 1990s has

complemented the role of Fixed Series Capacitors (FSC) in dynamically regulating the power flows

and avoiding voltage collapse [4, 5]. This approach is proposed here for the 380-kV S-I tie line inthree alternative schemes. The three schemes have the same current rating of 2400 A and the total

effective series capacitive impedance of 37.2 Ohm/phase but are different in the split between the

fixed (FSC) and the variable (TCSC) portions. The thyristor valves used in all schemes have the

capability to switch currents up to 3 kA rms but have different voltage ratings according to their

effective impedance control range. With such differences the schemes are not identical in terms ofdynamic current control range, performance, number of components, layout, availability and cost.

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2.1 TCSC Alternative Schemes

Figure 1 shows the schematic diagram of alternative scheme-1. The FSC segment of 12.4 Ohm/phase

is placed on a platform and connected to the northern infeed to the S-I line. The TCSC segment is also

placed on a platform but connected between the existing double busbars (S1 and S2) of substation S.

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-Xc Copmensation [Ohm]

S-ILine

Current[A]

-Xc Compensation [Ohm]

FSC TCSC

Controller

S2 S-I Line I

6 mH260 uF

260 uF

S1

Network380 kV

Figure 1: Schematic of alternative scheme-1 Figure 2: Scheme-1 line current-reactance regulation

MOV with CTs

DampingCircuit

Triggerdspark gap

BSC TCSC

Figure 3: TCSC-BSC segment details of scheme-2 Figure 4: Scheme-2 line current-reactance regulation

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urrent[A

]


The TCSC has a nominal capacitive impedance of 12.4 Ohm/phase, which is the value of its capacitor.

By regulating the TCSC thyristors, as will be shown later, the effective TCSC capacitive impedance

can reach twice the value of its capacitor to 24.8 Ohm. By switching-in the FSC, then switching-in and

regulating the TCSC the current on the S-I line is increased from 900 A to 2400 A as shown in Figure

2. Although the two impedance steps and the TCSC regulated impedance range have the same value,

their effect on the line current is remarkably different. The higher the line current level at which the

capacitive series impedance is inserted the higher the increase in line current flow. Therefore, a good

practical operation of the scheme would be to leave the FSC permanently connected to the line and

perform the current regulation by the TCSC.

Alternative scheme-2 is similar to scheme-1 except the TCSC segment is replaced by a combined unit

consisting of a smaller TCSC of 6.2 Ohm and a breaker switched capacitor (BSC) of 12.4 Ohm. The

combined unit has a master controller that regulates both the TCSC thyristor firing angle (vernier

control) and switching of the BSC (step control). As shown in Figure 3, the complete combined

segment, including housing of water-cooled thyristor valves, all capacitor banks with their associated

MOV arrestors, triggered spark gaps, bypass switches and damping circuits as well as all necessary

CTs, is placed on a single platform [6]. The platform measures approximately 15 m x 30 m per

phase. The BSC breaker, the TCSC air-cored reactor and the segment bypass switch are placed

separately outside the platform. The functions of the spark gap and the MOV could be replaced by

special high power thyristors for protecting the series capacitors [7]. These thyristors can be integrated

with the TCSC thyristor valves and share the same housing. The line current-impedance regulationrelationship for this alternative is given in figure 4.

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Alternative 3 is a further modification of alternative 2 in which the BSC is split into two units each

with 6.2 Ohm. Together with TCSC this provides a larger current dynamic range as shown in figure 5.

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urrent

Figure 5: Scheme-3 line current-reactance regulation

Figure 6: Simulation of TCSC scheme-3

(a) Effective reactance & line current (c) Scheme total and TCSC effective inserted voltages

(b) TCSC thyristor firing angle & internal currents (d) Line flows & terminal voltages

2.2 TCSC Operating Characteristics

In order to illustrate the operating characteristics of the TCSC scheme, a detailed electromagnetic

transient model for the transmission network is used employing alternative scheme-3 as an example.

For illustration purpose, the scheme is assumed to be off line and is slowly switched-in sequentially. In

actual operation the TCSC can be ramped to full rating in less than 50 ms plus the switching delays of

the BSC bypass breakers. As shown in figure 6, at line current of 900 A, the FSC is energised (t=2 s)

followed by the TCSC (t=3 s). After 0.5 s, the controller slowly ramps the line current by reducing the

thyristor firing angle until the TCSC effective impedance matches that of the first BSC (6.2 Ohm) at

which point the BSC is switched-in by opening its bypass breaker and the firing angle is reset to 180

as shown in figures 6-a and 6-b. This process is repeated with the second BSC until the line current

reaches the set value of 2400 A. Note that for this capacitive mode of operation, the TCSC capacitor

carries the sum of thyristor (or TCSC reactor) and line currents. Figure 6-c depicts the corresponding

rms voltage across the complete scheme and across the TCSC segment. These values of currents and

voltages provide the criteria for dimensioning the principal components of the scheme. Figure 6-d

shows the power flow on the S-I line and the voltage at its terminals S1, S2 and I. Note how thescheme is self regulating in terms of reactive power compensation. The more active power flows in the

line, the more reactive power is produced with better voltage support at both terminals of the line.

(a) (b)(d)

(c)-Xc Copmensation [ Ohm]

S-ILine

C


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Figure 7 shows snap shots of the waveforms of phase voltage at terminal S1, voltage across the TCSC,

line current and TCSC thyristor and capacitor currents at different points in time of the TCSC ramp-up

process. When the thyristor firing angle is 180 , the full line current flows in the TCSC capacitor. As

the firing angle is reduced an intermittent current flows through the thyristor and its series reactor.

This current flows in the TCSC capacitor and consequently increases the voltage across this capacitor

and, in turn, its effective impedance. As a result, the line sees more series compensation and the line

current increases. Note how most of the thyristor current flows in the parallel capacitor which serves

as a harmonic bypass. This harmonic distortion appears in the waveform of voltage across the TCSC.

Figure 7: Waveforms at TCSC ramp-up starting and end points

The harmonic content of the inserted voltage across the whole scheme varies with the TCSC operating

point where the highest is for 3rd

harmonic of 5% that appears just before switching the first BSC.

Considering the relatively small TCSC inserted voltage with respect to system phase-ground voltage,

the 'end effect' on the line voltage at the TCSC two terminals (S1 and S2) is less than 1% and 0.5%respectively for 3rd

harmonic. Other harmonic orders (5th, 7

th, etc.) are insignificant. The level of such

'end effect' depends upon the line parameters and the harmonic characteristics and loading conditions

of the interconnected networks. The same argument applies for the line current where the maximum

3rd

harmonic line current is about 0.7%.

2.3 Comparison of TCSC Alternative Schemes

Despite the apparent simplicity of scheme-1, it represents the worst choice mainly due to the large

thyristor valve rating of its TCSC. Scheme-3 offers the best operational flexibility and availability

(redundancy) and the lowest risk since each of the BSC rating equals the TCSC dynamic range. This

scheme, however, requires an additional platform/phase to accommodate the second BSC. This should

be reflected on the larger footprint of the scheme and on its cost as will be shown later under point 5.

Apart from the imperfect operational flexibility of scheme-2, it shares with scheme-3 the advantage of

minimum harmonic generation and no need for installing harmonic filters. In fact, this is a critical

issue considering the sensitivity of this region part of the network to 7th harmonic resonance and

harmonic current injections from neighbouring networks as pointed out in reference [8].

2.4 Operation in the Inductive Range

Conventional TCSC schemes allow inductive mode operation at maximum line current in a range that

is confined to a small region above the point of full thyristors conduction. The reason is that thyristors,

unlike during capacitive mode, conduct the sum of TCSC capacitor and line currents. In the case of

scheme-2 or -3, if the line current jumps to 3.1 kA while the TCSC is opearting with 180 thyristorfiring angle and all BSC's are switched off, an attempt to limit the line current by switching the firing

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angle to 90 would result in a capacitor current of 2 kA and a thyristor current of 4 kA. This is far

beyond the thyristor design value. Therefore, operation of the TCSC in the inductive range would be

very restricted if at all allowed. Moreover, inserting line series reactors using bypass switches or

thyrsitors [9] would be respectively complicated or very expensive since they can be applied only in

discrete steps and, therefore, must consist of several units to allow line current regulation.

2.5 Extending the TCSC Inductive Range

If in scheme-2 or -3 a reactor (Xp) is switched in parallel to the TCSC module, the inductive range of

the scheme can be enormously enlarged and regulated as well. Such an economic air-cored reactor can

be reliably inserted by means of a modern MV vacuum switch in series. Using the scenario described

above to illustrate this technique, the reactor Xp is switched-on and simultaneously the thyristor firing

angle is flipped to 90 as shown in figure 8. As a result, the line current is immediately reduced from

3.1 kA to 2.2 kA. The corresponding capacitor current is reduced from 3.1 kA to 1 kA. The total of

line and capacitor currents is now shared by the thyristor branch (1.9 kA) and the Xp reactor (1.3 kA).

Further regulation of line current is also possible by regulating the thyristor firing as shown in figure 8.

Note that increasing the firing angle reduces the thyrsitor current but increases both capacitor and

reactor Xp currents which sets the limits for this operating mode. Figure 9 shows the waveforms ofTCSC thyristor and capacitor currents as well as the Xp reactor current at maximum inductive point.

Figure 8: Simulation of TCSC for operation in inductive mode with Xp Figure 9: Waveforms of TCSC with Xp

3. VOLTAGE SOURCED CONVERTERS

The rapid development of power semiconductor devices with turn-off capabilities, such as IGBT, has

realised voltage-sourced converter (VSC) valves with high levels of reliable operation. Compact and

modular designs of VSC's with pulse-width-modulation (PWM) are commercially available basically

for industrial applications. In inverter operation, the VSC is supplied by a capacitor on its dc side and

produces an ac voltage of any frequency. The synthesized voltage produced by these inverters can be

inserted in series with a transmission line for power flow control by two methods; through series

(booster) transformers or directly with series filters. The former is known as Static Series Synchronous

Compensator [10] and the latter is known as Transformerless Reactive Series Compensator [11].

These two methods can be used in a scheme similar to the TCSC alternative-2 or -3 by replacing theTCSC module with a 3-level IGBT inverter having 36 MVA/phase rating as given below.

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3.1 Static Series Synchronous Compensator (SSSC) Scheme

As shown in figure 10, two booster transformers with their primary windings connected in series per

phase are used to inject the total voltage of 15 kV. Parameters of the secondary windings are set to

allow utilising available IGBT inverter modules. To compensate the converter internal losses and

inject a component of voltage to counteract the effect of the resistive voltage drop on the transmission

line, the dc bus must be connected to a small auxiliary dc power supply. A thyristor short circuit

switch (Crow Bar) is also required to protect the inverters against overcurrents in case of line faults.

Unlike the TCSC where current harmonics are generated on account of thyristor switchings, the SSSC

inserted voltage with PWM contains high order harmonics. A low rating HP filter tuned to the

switching frequency maybe necessary. To achieve the same level of maximum line current of 2400 A,

an additional BSC of 6.2 Ohm is required. Figure 11 depicts the line current and the injected SSSC

and total scheme voltages in a ramp-up simulation similar to that shown in figure 6. The SSSC

injected voltage can also be reversed to effectively reduce high line current flows by about 1200 A

from full boost to full buck.

IGBT Inverter (3-Level)

DC Capacitors

DC Bus

HP Filter

ACDCAC DC

CBCB

Figure 10: Schematic of SSSC module Figure 11: Simulation of SSSC scheme

3.1 Transformerless Reactive Series Compensator (TRSC)

In order to eliminate the need of costly booster transformers of the SSSC scheme, the IGBT modules

have to be connected in series to form an inverter unit capable of generating the required injected

voltage of 15 kV. Two additional large decoupling reactors/phase must be inserted between the

inverter unit terminals and the transmission line connected in series. These are necessary to protect the

thyristors against internal faults and together with a bypass HP filter provide the necessary filteringand damping functions. Moreover, the crow bar switch of the SSSC must be of higher voltage rating

and the complete assembly, including the dc supply, is placed on a plateform. It should be noted that,

so far, there is no practical experience in connecting IGBT modules directly to EHV lines.

4. ECONOMIC COMPARISON

Using a 400 kV phase shifting transformer of 1650 MVA throughput rating with 23 angle regulation

as a basis, the cost of the various schemes discussed earlier are compared as given in figure 12. It

should be emphasised that the comparison reflects only equipment turnkey costs without accounting

for redundant equipment or transportation costs. Operating and maintenance costs, spare parts and

losses are also not included. The costs of the TCSC alternatives do not include the proposed Xp

switched reactor decribed in section 2.5 earlier and the SSSC and TRSC schemes are based on bothTCSC alternatives 2 and 3 configurations.

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Phase Shifter TCSC-1 TCSC-2 TCSC-3 2 3

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ostin

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Figure 12: Cost comparison of alternative schemes

5. CONCLUSIONS

The liberalisation of the European electricity market and the evolving new patterns of power exchange

in its transmission networks is demanding new solutions to be adopted. The Swiss grid, with its central

location and flexible hydropower storage stations, plays the role of handling the power transit over the

alps. Optimal operation of the grid should ensure the security of the local network and the exchange

level with other networks. A control device that can positively participate in critical network

conditions by redirecting the power flow into desired corridors and by improving voltage stability is,

therefore, highly favoured.

The FACTS devices discussed in the paper for dynamic power flow control enhance the utilisation of

the north-south axis of the Swiss 380-kV transmission network. Such devices, with modular structure

and low maintenance requirements, are not necessarily more expensive than conventional phase

shifting transformers but also have substantial advantages in the control, response time and reactive

power behaviour. The solution with TCSC (either alternative-2 or -3) is established with some

advantages against the SSSC and TRSC techniques in their present state of the art.

BIBLIOGRAPHY

[1] S. Corsi and C. Sabelli General Blackout in Italy Sunday September 28th, 2003 (Proceedings

IEEE PES General Meeting, June 2004).

[2] N. Hingorani, L. Gyugyi Understanding FACTS (John Wiley, 2002, pages 267-279).

[3] IEEE Std 824TM-1994 Standard for Series Capacitors in Power Systems.

[4] N. Christl, R. Hedin, P. Luetzelberger, M. Pereira, K. Sadek, D. Torgerson Advanced Series

Compensation with Thyristor Controlled Impedance (CIGRE 1992-Session, paper 14/37/38-5).

[5] A. Hammad Comparing the Voltage Control Capabilities of Present and Future VAr

Compensating Techniques in Transmission Systems (IEEE Power Delivery, 1996, pages 475-

484).[6] IEEE Std 1534-2002 Recommended Practice for Specifying Thyristor-Controlled Series

Capacitors.

[7] L. Kirschner, J. Bohn, K. Sadek Thyristor Protected Series Capacitor (IEEE T&D Conference

2002)

[8] A. Hammad, G. Koeppl, S. Laederach Harmonic Voltage Amplification in a Distribution

Network due to Resonance of Transfer Impedance to the EHV Transmission (CIRED 2005,

Session 2, paper 2-25).

[9] G. Karady, T. Ortmeyer, B. Pilvelait, D. Maratukulam Continuously Regulated Series

Capacitors (IEEE Power Delivery, 1993, pages 1348-1354)

[10] Task Force 14-27 CIGRE Unified Power Flow Controller (CIGRE 1998)

[11] A. Beer, H. Stemmler, T. Fujii, H. Okayama High Efficiency Single Phase Control Method for

the Transformerless Reactive Series Compensator (IPEC, Tokyo 2002)

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