Thyristor Controlled Series Capacitors

SE 970958

2

Series compensation – a powerful tool for efficiency in power transmission

Series capacitors have been successfully utilized for manyyears in electric power networks. With series compensa-tion, it is possible to increase the transfer capability ofpower transmission systems at a favourable investmentcost and with a short installation time compared to thebuilding of additional lines. This is due to the inherentability of series capacitors to achieve: • Increased dynamic stability of power transmission

systems• Improved voltage regulation and reactive power

balance• Improved load sharing between parallel lines

With the advent of thyristor control, the concept of series compensation has been broadened and its use-fulness has been increased further.

TCSCThyristor-controlled series compensation (TCSC) introduces a number ofimportant new benefits in the application of series capacitors:• Mitigation of subsynchronous resonance risks• Damping of active power oscillations• Post-contingency stability improvement• Dynamic power flow control

Applicable in new aswell as existing systemsThe benefits of thyristor-controlled series compensation are by no meansattainable only for installations starting from scratch. It is fully possibleand practicable also to uprate existing series capacitors by making all orpart of them thyristor-controlled, thereby extending their impact and use-fulness in the grid most considerably. This is being practised in reality already.

Uprating of series capacitor into TCSC(Typical values)

Stage 1

Stage 2

Degree of compensation: k (%)

0,7k-0,9k 0,1k-0,3k

TCSC

SE 950323

Subsynchronous resonance: shaft torsional vibrations sustained due to resonant behaviour of the electric system.

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Mitigation of SSR

SSR mitigation: with TCSC, torsional vibrations die out due to elimination of the resonant behaviour for SSR frequenciesin the electric system.

The phenomenon of subsynchronous resonance (SSR) has caused concern in the past in situationswhere the risk for occurrence of SSR has acted as an impediment to the use of series compensa-tion in cases where the technology would otherwise have offered the best and most economicalsolution. With the advent of TCSC, no such concerns need be entertained any longer, and seriescompensation can be used to its fullest merit.

The SSR risk used to be linked to the utilization of series compensation of transmission lines fedby thermal generation, particularly in cases of high degrees of compensation, where analysis sho-wed that the complementary series resonance frequency of the compensated line(s) co-incidedwith some poorly damped torsional vibration frequency of the turbo-generator shaft, and couldhence induce increased mechanical stresses in the shafts. The TCSC acts to eliminate this risk forco-inciding resonance frequencies by making the series capacitor(s) act inductive in the subsyn-chronous frequency band, thereby rendering the occurrence of series resonance in the transmis-sion system for subsynchronous frequencies altogether impossible.

This inductive character of the TCSC is made possible by the use of a thyristor-controlled inductorin parallel with the series capacitor. The system is governed by an advanced, patented controlscheme called SVR (synchronous voltage reversal).

T

Turbine-Generator set

Stator

Series compensatedtransmission

network

T

Stator

E

Network with TCSC

Power oscillations damped out by means of TCSC.

Power oscillations on a 500 kV intertie excited by a temporary short circuit. Power flow through the intertie:2000 MW.

Time (s)

20

15

10

5

0

-5

-10

-15

0 1 2 3 4 5 6 7 8 9 10

Angulardifference

TCSC(deg.)

0 1 2 3 4 5 6 7 8 9 10

Time (s)

30

25

20

15

10

5

0

-5

-10

-15

-20

Uncontrolled

Angulardifference

(deg.)

4

Damping of power oscillations

Oscillations of active power in power transmission systems may arise incorridors between generating areas as a result of poor damping of theinterconnection, particularly during heavy power transfer. Such oscilla-tions can be excited by a number of reasons such as line faults or asudden change of generator output.

The presence of active power oscillations acts to limit the power trans-mission capacity of interconnections between areas or regions or evencountries. It is often possible to find remedy by building additional linesor upgrading existing lines, but this costs a lot of money and takes a lotof time, if not rendered impossible altogether by lack of the necessarypermits. In some cases, it may also be possible to introduce powersystem stabilizers (PSS) on generators, but this will not always work, par-ticularly not for inter-area power oscillations which tend to be of a lowfrequency (typically 0,2 Hz to 0,7 Hz).

In either case, TCSC will be an attractive alternative to consider. It offersa cost-effective, robust power oscillation damper, insensitive to its loca-tion in the system and non-interacting with local oscillation modes. In anumber of cases, it will turn out to be the best practicable solution.

XL Xc

PU ∠O1

U ∠ψ2

P(t)=U1U2sinΨ

XL-X

C

Power transmission P over a series compensated tieline isgoverned by the expression

By proper control of the TCSC, the overall transfer reactanceis modulated in time in such a way that the power oscillationsare damped out.

Control of power flow between regions.

TCSC for post-contingency stability improvement and loadabilitycontrol.

An important benefit of TCSC is its ability for quickboosting of its degree of compensation, making it veryuseful as a tool for improving the post-contingency behaviour of networks. By means of this quality of theTCSC, the degree of compensation of a series capacitorcan be increased temporarily following upon a networkcontingency, thereby adding to the dynamic stability of thenetwork (voltage and angle) precisely when it is needed. By this means, the series capacitor can be lowerrated for steady-state conditions, thereby keeping trans-mission losses smaller.

For instance, in many applications it is desirable to enable an underlying transmission system to carry a higher amount of power under contingency conditionswhere a circuit is tripped in higher voltage transmissionpaths. Series compensation is not needed during normaloperation as it would only influence overall system losses inan unfavourable way. In order to meet both objectives, i.e.safeguarding power transmission capability during conting-encies and at the same time keeping transmission losses aslow as possible during normal operating conditions, controlled series capacitors can be advantageously utilized.

Power flow controlIn interconnected power systems, the actual transfer of power from one region to another might take unintended routes depending on impedances of transmiss-sion lines connecting the areas. Controlled series compen-sation is a useful means for optimizing power flow bet-ween regions for varying loading and network configura-tions. It becomes possible to control power flows in orderto achieve a number of goals:• Minimizing of system losses• Reduction of loop flows• Elimination of line overloads• Optimizing of load sharing between parallel circuits• Directing of power flows along contractual paths.

5

P

Post-contingency stability improvement and loadability control

In a TCSC, the whole capacitor bank or alterna-tively, a section of it, is provided with a parallelthyristor controlled inductor which circulatescurrent pulses that add in phase with the linecurrent so as to boost the capacitive voltagebeyond the level that would be obtained by theline current alone. Each thyristor is triggeredonce per cycle and has a conduction intervalthat is shorter than half a cycle of the ratedmains frequency. By controlling the additionalvoltage to be proportional to the line current,the TCSC will be seen by the transmissionsystem as having a virtually increased reactancebeyond the physical reactance of the capacitor.

The thyristor valve is integrated in the capacitorovervoltage protection scheme. It replaces thespark gap and allows a reduction of the ratingof the protective parallel varistor.

6

Thyristor control

b

SE 970964

Thyristor-controlled segment of series capacitor.

Ind.

Cap.

Rotor

Fixedcapacitor

IdealSVR Transition

frequencyband

Power flowcontrol

frequency band

Increasingboostlevel

Frequency

Virtualreactance

-fN

0 fN0

Stator

SSRfrequency

band

TCSC in steady-state a) circuit b) waveforms

Apparent reactance characteristic of TCSC

Control scheme, SVR

7

Control scheme

Due to an ingenious control scheme, ABB isable to offer a TCSC which has a characteristicsuitable for mitigation of subsynchronous reso-nance (SSR). A patented control algorithmnamed Synchronous Voltage Reversal, SVR, hasbeen developed by means of which the TCSCexhibits an inductive virtual impedance in thecomplete frequency range of concern for SSR.As a consequence of this property of the seriescapacitor, SSR is efficiently ruled out.

The SVR approach to controlled series compen-sation offers a number of benefits to the user:• No resonance risk throughout the SSR

frequency range.• Robust performance of the TCSC, without

any need for specific tuning to meet varyingnetwork configurations and various rotatingmachine modes.

• Additionally variable voltage boost can beutilized for system related control purposes,such as power oscillation damping.

Power oscillation damping: A Brazilian caseSince spring of 1999, Eletronorte of Brazil hasbeen operating a Thyristor-controlled SeriesCapacitor (TCSC) and five Fixed SeriesCapacitors supplied by ABB in Eletronorte´s500 kV interconnector between its northernand southern power systems. In all, about1.100 Mvar of series capacitors have beeninstalled by ABB. The TCSC is located atImperatriz at the northern end of the powercorridor connecting the two systems whichwere previously not connected.

The series capacitors installed in the North-South Interconnection have the task of raisingthe steady-state ande dynamic stability of theinterie. The TCSC at Imperatriz, the first of itskind to be installed in Latin America, has thetask of damping low-frequency inter-area poweroscillations between the power systems on eitherside of the interconnection. These oscillations

(0,2 Hz) would otherwise have constituted ahazard to power system stability and thereby topower transmission capability. The TCSC efficientlyeliminates this obstacle to power transmission.

SE 990211

Swedish National Grid TCSC at Stöde

Ela

nder

s N

ovum

020

8

k=49% k=21%

System voltage: 400 kVTotal rated power: 493 Mvar

SSR mitigation – a Swedish caseIn Sweden, electric power generation is based chiefly on hydro power inthe north and nuclear power in the middle and south of the country.Power transmission is performed on 400 kV over interconnecting lines ofconsiderable lengths. To improve power transmission capacity of theselines and thereby keep their needed number to a minimum, series com-pensation is used throughout.

The Forsmark nuclear power plant situated in mid Sweden is inter-connected with the north of the country by means of a number of 400 kV lines of varying lengths, all series compensated. However, one ofthe generator units at Forsmark, rated 1300 MW, is subject to SSR risk inconjunction with certain conditions in the power grid. To counteract this,one of the series capacitors, located at Stöde, used to be bypassed uponSSR indication previously, which meant loss of power transmission capa-bility of the system. In case of failure of the series capacitor SSR protec-tion, the generator was tripped, with subsequent loss of generation. Suchmeasures, however, have now become unacceptable, as demands onavailability of power have steadily grown.

Part of the existing series capacitor at Stöde (30 % of its totally installed reactance) has consequently been rebuilt into a TCSC. The series capacitor thus equipped was commissioned in 1997.

Increased availability of power

With the TCSC in operation, the SSR risk is eliminated for all possiblesystem operating conditions. The benefits will be increased availability ofpower from Forsmark as well as a preserved high level of power trans-mission over the system, thanks to series compensation.

Technical data, Stöde TCSC

System voltage: 400 kVRated continuous current: 1500 ARated overall power: 493 MvarDegree of compensation:

Total: 70%Thyristor controlled: 21%

TCSC control mode: SVR

ABB Utilities ABPower SystemsS-721 64 VästeråsSwedenTel: +46 21 32 40 00Fax: +46 21 18 31 43E-mail: info.sepow@se.abb.comwww.abb.com/FACTS

A02

-015

8E

SE 970765