EDITORIAL

Introduction to special section: Static compensationfor AC power systems

The papers included in this special section of IEE ProceedingsPart C stem from contributions presented at two technicalseminars organised by the IEE. The seminars were held inorder to fulfil a need for an opportunity to discuss the latestmethods of reactive compensation in power systems.

The first technical seminar, entitled 'Reactive compensationin power systems' was held, under the chairmanship of K.M.Jones, at Birmingham University on 1 lth and 12th September1979.

The basic principles of compensation operation, physicallayout and application for both industrial and transmissionsystems were covered at this seminar, and during discussionthere was an excellent exchange of views of the varioustechniques and application methods. Naturally, in discussion,the trend was towards the relatively new types of equipment;static VAR systems (SVS) and the modern forms of seriescapacitors. Typical applications of both the saturated reactorand thyristor type SVS equipments, together with seriescapacitors, were discussed.

There was an obvious need for further opportunity toexchange ideas and opinions on the various techniques inapplying compensation. It was, however, appreciated that forthe most difficult applications the method of control was ofsupreme importance, and that in a seminar based on a generalscope it was not possible to explore the implications of controlpossibilities.

Because of the success of this seminar and the apparentneed to have a further opportunity for discussion on controlaspects the second technical seminar, entitled 'Control ofcompensation for AC power systems' was organised and held,under the chairmanship of Prof. C.B. Cooper, at the IEE inLondon on 8th and 9th September 1980.

There are modern methods of both shunt compensation,using the SVS, and series compensation using series capacitorswith control innovations. Each method has its merits, onebeing preferred to the other according to application. Themethods may also have complementary roles, although this hasyet to be fully explored.

It was appreciated, at the Seminars, that the SVS providesthe continuous control of the synchronous compensator withthe high availability and low maintenance experienced withconventional reactor and capacitor equipment. The method ofcontrol, whether it be based on saturated reactor or thyristortechnology, was seen to provide a rapid response to a degreenot possible with synchronous machines, thus making possiblegreater benefits to be realised for many applications of interestto system planners, such as:

(i) increased power transfer limits due to improved systemstability

(ii) suppression of system oscillations and improved systemdamping

(iii) control of system overvoltages due to line switching orloss of load

(iv) maintenance of system voltage limits when suddenlyswitching large loads

(v) suppression of voltage fluctuations caused by dis-turbing loads, e.g. rolling mills and arc furnaces.The suppression of light flicker created by arc furnaceoperation was one of the principal problems discussed at theseminars. UK Electricity Council recommendation P7/2 [1]sets out a method for predicting light flicker levels. The

method retains its validity when the type of compensator tobe used effectively increases the short-circuit level at the pointof common coupling with the arc furnace, without modifyingthe frequency spectrum of the arc furance. The difficulty theprospective user faces, and one which is of considerableinterest, is that some types of compensator modify thefrequency spectrum of the arc furnace and, whereas this maynot disqualify their use for light flicker suppression, there isnevertheless a grey area concerning a suitable criterion onwhich to base a prediction of performance. A dynamiccriterion needs to be established, and it is hoped that formalpapers in future issues of the IEE Proceedings Part C will beable to report on the work of modelling of control methodswhich is currently being undertaken.

In his introductory remarks as chairman of the firstseminar K.M. Jones surveyed the historical background leadingup to the present forms of static compensation equipment.

Static shunt compensator

There are two basic forms of this equipment available today.One takes advantage of the thyristor switching technologymotivated for industrial use and later used for high-voltagedirect-current convertors for HVDC transmission. The second,which is the original, and conceived in the early 1930s toimprove long-distance transmission, stems from the technologyof saturation in iron, being essentially based on an AC reactorwith a closed iron core in continuous saturation [2]. The lateErich Friedlander was a leading investigator in this field.

The first problem in the development of the saturatedreactor was due to the current harmonics caused by ironsaturation. This seemed to have been solved by the early1930s, but further development towards reducing iron losseswas necessary to make saturated reactors of the sizes requireda practical possibility. This development took place just beforethe Second World War, and the first large reactor for operatingcontinuously in the saturated region was installed at the GECworks at Witton UK in 1953. This reactor could absorb460MVAR and has been in regular use, until recently, forartificially loading large turbogenerators.

The next stage in the development was quite unconnectedwith long-distance power transmission, and was concernedwith the problem of suppressing voltage fluctuations due todisturbing loads, and in particular due to arc furnaces. By thistime, there had been much activity in exploring the subjectiveannoyance effect of light flicker caused by voltage fluctuationsfrom arc furnaces. This resulted in standards being established,with which many countries tried to comply. Light flickermeasuring instruments and methods of analysis had come intobeing. There was a need to stabilise voltage to avoid theexpensive alternatives of strengthening systems by feederreinforcement or by the use of rotating plant to raise short-circuit levels.

A major team effort was made with the GEC in the UKco-operating with the British Steel Industry. This resulted inthe development of multiphase compensation of theharmonics created by saturated reactors and the use ofcapacitors connected in series with the saturated reactor inorder to compensate for the leakage reactance and so producea low slope characteristic suitable for voltage control withinvery narrow limits.

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Interest in the AC saturated reactor compensator graduallycaught on, leading to commercial applications, first for voltageflicker control and then for reactive power control ontransmission systems.

The first application of compensators at more than onelocation for the control of a long-distance transmission line,which was the original purpose of the early development workby Friedlander and his colleagues, is on the 764 km 132 kVsystem in North Eastern Nigeria. Merz & McLellan were theconsultants engaged for this application which wascommissioned in 1978 [3] and is giving reliable service.

The development of heavy power thyristor switches pavedthe way for the eventual introduction of static compensatorsusing thyristor switch modules to control conventionalreactive elements. Prototype equipment with a rating of500kVAR was completed in 1969. This led to the com-missioning by ASEA in 1972 of a 60MVAR installation at theDomnarvet Steelworks in Sweden. This equipment employedthyristor switched capacitors. Other developments whichoccurred almost simultaneously in Europe, Sweden and theUSA led to the use of thyristors for the continuous controlof linear reactors for industrial and transmission compensationapplications.

Series capacitors

The original conception of static compensation was motivatedby the need visualised by Friedlander in the early 1930s forsome automatic means of varying the compensation applied tolong-distance lines. Development of shunt compensation wasimpeded, primarily because of the problems of designingsuitable AC saturated reactors. On the other hand thedevelopment of capacitors advanced rapidly to meet demandsfor power-factor correction equipment.

It was appreciated at that time that capacitors in series withtransmission lines would compensate part of the line reactanceand so allow power levels to be increased without expensiveline reinforcement. Trials were carried out in Sweden withseries capacitors in distribution lines and intensive studyeventually resulted in applications for large scale transmission,starting with the commissioning of series capacitors giving 20%compensation of a 500km 220 kV line in January 1950. Sincethen, series capacitors have been installed in rapid successionfor compensating long lines at 400 kV and later at 5.00 and750 kV. These applications were mainly in Sweden and theUSA, and later in Canada and South America.

Series capacitors became regarded as a normal componentfor compensating long distance transmission lines inconjunction with shunt reactors of conventional design. Wenow have modern methods of both shunt and series staticcompensation.

Review of papers

The six papers published in this special section of IEEProceedings Part C are based on a selection of the subjectsintroduced at the two technical seminars. Each of the authorshas been working in the field of system compensation formany years, and quite naturally they draw on their previouswork in reviewing their subjects. It is inevitable that in someinstances repetition from previously published material isnecessarily retained to contribute to a degree of completenessin this series of papers.

After reiterating the features of static shunt compensation,with the authors of the first two papers giving their own view-points, reference is made to the combined thyristor switchedcapacitor (TSC) and thyristor controlled reactor (TCR)scheme with permits maximum flexibility in the use of

compensation required for both reactive power generation andabsorption.

The advantages of 12-pulse operation of the combinedTSC/TCR scheme, notably the reduction of harmonicgeneration and elimination of harmonic filters combined withminimum losses, suggest that this method has good possibilitiesfor applications where large rated equipment is required. Thediscussion on control methods includes a reference to theimportant question of damping of power oscillations insystems to result in an improvement of transient stability-following system disturbances.

The second paper also distinguishes between two roles forcompensation equipment: load compensation which, apartfrom rapid power factor correction, may be required moreimportantly to compensate for the effects of load disturbanceson the system, usually from industrial loads; the biggestoffender being the arc furnace which is a ferociouslyfluctuating unbalance load. Other unbalanced loads are ACelectric railways, for which phase balancing can be the primaryreason for the installation of a phase-by-phase controlledcompensator which also provides or absorbs reactive poweron a three-phase basis for power factor correction, and indeedmay be combined with filtering equipment to absorb both itsown harmonics as well as those created by the load. The otherrole is one of terminal voltage regulation which meets therequirements of the network supplying the load. Clearly thetwo roles in some applications may be met by one installationof compensator equipment.

The SVS operates faster than any other reactive powerproducing equipment on the system, and in the context ofoperation in conjunction with other reactive power sources onthe system it is important to maintain adequate reserve if it isessential to avoid the SVS operating at its limits and havingreduced effect as a system controller.

In the interests of economy of equipment, however, a'degraded' state of control may in certain circumstances bepermitted, as pointed out in the James Bay paper.

The second paper goes on to refer to some novel featureswhich are of particular interest and gives an insight to thefuture forms static compensation equipment might take. Theyconsist of methods stemming from the AC/DC convertor usingforced commutation of the thyristor valves to obtain thenecessary VAR generation property. The most elegant of thevariations stemming from the basic approach is the 'powerdoubling' scheme, and suggests that this is likely to emergeas a low-cost solution meeting reactive compensation require-ments when suitable semiconductor switching devices becomeavailable.

The authors of the third paper are primarily concerned withthe control aspects of the thyristor-controlled types of SVS.To set the scene they briefly describe the operating principlesof the various types of static equipment for which controlcircuitry is required.

The relationship between control performance and systemcharacteristics reveal the most significant properties to beshort-circuit levels and resonance conditions. Situationsleading to a low short-circuit level, a feature of weak systems,and more generally applicable to weak points of connectionwith the SVS, have a predominating influence on the selectionof control settings for stable operation, owing to the rise insystem gain. The most onerous test for the SVS may occurwhen the system is in a weakened state due, for example, to aline outage when transmitting peak load. Short-circuit levelmay not be at a minimum, but maximum control responsemay be essential.

Resonance conditions in the system, either inherent, orbrought about by the effect of the shunt capacitors providedas part of the SVS equipment, have a similar effect and in this

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case filters may have to be incorporated in the control systemin order to make it possible to retrieve a satisfactory controlresponse.

The author refers to optimising control which can lead toadopting a slope characteristic of 3—4%. This indicates apossible limitation of roles for the SVS. An SVS for systemcontrol may not have characteristics ideally suitable for thesuppression of voltage fluctuations, and there are manytransmission system applications where at least a part of theload supplied is of a disturbing nature.

The fourth paper describes the various types of SVSequipment considered for use on the James Bay system anddiscusses the performance specification as determined bysystem operating requirements.

The 735 kV network has been controlled by a combinationof synchronous compensators and switched reactors, but nowstatic compensators are considered because of their increasedeffectiveness in controlling the system and their greaterreliability compared with synchronous compensators.

One of the features attributed to the SVS is its ability todamp system oscillations during severe disturbance threateningsystem stability without there being a significant risk of theSVS becoming disconnected, as could happen withsynchronous compensators suffering angular instability.

It is interesting that the authors have found that the threetypes of SVS studied — the saturated reactor type, thethyristor controlled reactor and thyristor switched capacitortypes — gave similar performances in respect of response time,control of overvoltages and effectiveness in damping systemoscillations. Model details were provided by the manufacturersand, as implied in the paper on analytical methods, this can beessential when the finer details of control method have to betailored to a particular application.

Performance specifications distinguish between thoseconditions for which the SVS is expected to operate withinnormal control limits in an automatic manner and theconditions under which the system operates in a 'degraded'state when a reduced SVS performance with operation outsidenormal limits would be acceptable.

The specifications relate to normal system variations duringswitching operations and during system disturbances, andcover system overvoltage requirements including thosefollowing load rejection. Particular attention is paid toensuring stable control of the SVS under weak systemconditions with minimum short-circuit level. This aspect,which is referred to in the third paper, includes a study of thesystem natural frequencies and the provision of filters in thecontrol circuits of the thyristor-controlled-type SVS equip-ment to avoid a tendency to reinforce the dominant naturalfrequency. The inclusion of filters in the control circuit mustof course have a minimum effect on the response time which is50 ms to achieve 90% of the signalled change with a further50ms for the output to come within 5% of the final value.

It is interesting to note that fixed capacitor banks are notswitched out following load rejection, which is contrary tonormal practice in many systems. The reason is because it isnot possible to distinguish in the first few cycles betweenovervoltages due to fault clearing, and overvoltages due to loadrejection. One would think that the saturated reactorcompensator with its high inherent overload capacity would beparticularly useful in controlling voltage following loadrejection.

In common with authors of previous papers, a change incontrol settings to accommodate short-circuit levels below thenormal minimum to retain stable control is advocated. SVSoperation with open-loop control is also a proposal whenputting the system together after a shunt-down with the SVSbehaving as a fixed impedance. This is one of the advantages

the static compensator has over the synchronous compensator,and enables dead line charging with compensation immediatelyeffective.

The paper on series capacitors reviews the variousprotective requirements of EHV series capacitor installations,emphasising, in particular, the form of protective spark gapsrequired to limit capacitor transient voltages to withineconomical levels.

The paper then refers to methods of obtaining rapidreinsertion of capacitors after system faults have causedprotective gaps to operate. Four different variants of a dual-gap scheme are described which overcome the necessity for thedelay normally required to ensure that protective gaps havehad time to deionise before capacitor reinsertion can takeplace. The dual-gap schemes not only lead to improved systemperformance but they also result in an overall reduction in thecost of capacitor installations.

The paper goes on to describe the dual-gap/bypass-resistorscheme which employs a nonlinear resistor in the capacitorbypassing equipment. This scheme was first used on the ElChocon—Cerros Colorados 500 kV transmission system inArgentina and is giving reliable operation. The nonlinearresistor not only limits transient voltages created by systemfaults but allows a reduced level of compensation to bemaintained during the fault period. This is an importantinnovation since the full amount of compensation is restoredinstantaneously on fault removal as the fault current reducesto normal current. Compared with conventional schemes, thedual-gap/bypass-resistor scheme equivalently increases theamount of compensation effective during transient conditionson the system, and this leads to improved system stability. Insome cases, the additional cost of the bypass equipment isjustified by the reduced cost of capacitors brought about bythe reduction of the voltage transients.

Other features of the new schemes described include theirability to reduce the risk of damage to turbogenerator shaftsystems should subsynchronous resonance (SSR) occur. Thepaper also makes reference to overcoming problems due toSSR by deriving signals from shaft speed oscillating detectorsfor use as input signals for supplementary excitation dampercontrol or for the control of thyristor-controlled shuntreactors designed to damp oscillations. Similar signals areproposed for the switching of series capacitor modules toinhibit resonance, or as inputs to series capacitor thyristorcontrolled bypass damping equipment.

The incidence of shaft damage 10 years ago at the Mohavegenerating station in the USA prompted a reappraisal of seriescapacitors and countermeasures to SSR and those referred tohave helped to restore confidence in the series capacitorcompensation method.

The final paper is on analytical and modelling methods. Theauthors separate the requirements for the steady state, slowtransients, and fast transients.

Steady-state problems such as harmonic penetration studiesare suitably performed mathematically. Although mathematicalmodelling is also common for problems involving slowdynamic transients, such as system dynamic stability, physicalmodelling is more convincing and has the advantage ofenabling the actual control system to be used with only themain primary circuit simulated at reduced current and voltage.An important point raised in the paper is that detailedrepresentation of the control system should be carried outwith the co-operation of the manufacturer of the controlequipment. There may be occasions, however, whenmanufacturers are reluctant to reveal full details of their mostadvanced control methods, leaving the only course to be forthe manufacturer to carry out the studies.

The preferred course for the study of fast transient

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phenomena is to again resort to physical models, such as thetransient network analyser, since the small computational steplength which is necessary to faithfully reproduce oscillationsleads to excessive computational costs. The physical modelmust have high Q-factors and the smallest low-current modelsmay have only a restrictive application. Representation of truesystem losses is of course no problem with the computationalmethods.

Minimum cost may, of course, be achieved without loss ofaccuracy if the physical model is used to identify problemareas before resorting to computational methods to examinecritical conditions in more detail.

The future

There has been a rapid increase in the number of applicationsfor which static-compensation equipment has been proposedor projected over the past five years, and this trend is likely tocontinue as favourable reports on commissioning and fieldtrials become published.

There has been a tendancy to favour the thyristor-controlledequipment for transmission applications, although thesaturated-reactor type remains competitive and continues tobe the solution most favoured for the suppression of voltagefluctuations created by arc furnaces.

New forms of both types are being considered in parallelwith the development of static equipment for transmissionphase-angle control. Complementary roles are likely to befound in new designs of power systems.

Despite the introduction of the SVS, the number of newseries capacitor applications are not expected to diminish,now that countermeasures to the SSR problem are beginningto have a wide understanding. Indeed, there should beadvantages in having both series capacitors and the SVSperforming complementary roles to optimise long-distancetransmission, the SVS having the additional role of dampingSSR.

K.M. JONES

References

1 Electricity Council Engineering Recommendation P7/2, July 19702 AINSWORTH, J.D., FRIEDLANDER, E., and RALLS, K.J.:

Recent developments towards long distance AC transmission usingsaturated reactors'. IEE Conf. Publ. 197, 1973, pp. 242-247

3 JONES, K.M.: 'Application philosophy and operating experienceconcerning three AC saturated reactor type static compensatorinstallations'. To be presented at the IEE international conferenceon thyristor and variable static equipment for AC and DCtransmission, Dec. 1981

K.M. Jones has been concerned withpower transmission system design forover 30 years and is Chief Systems DesignEngineer with Merz & McLellan inNewcastle upon Tyne, whom he joined in1970.

Formerly, he was Chief Engineerof GEC Power Transmission ProjectsDivision in Trafford Park, when hisinterests were primarily the developmentof new equipment for the improvement

of power transmission, which included HVDC equipment,resonance links and the saturated reactor type of static com-pensator.

Mr. Jones is an active member of the IEE having served onmany international conference organising committees. He is aformer NE Centre Power Section Chairman and a formermember of professional group committees P7 - power systemequipment — and P9 — system planning.

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