IEEE Transactions on Power Systems, Vol. PWRS-2, No. 2, May 1987

SURVEY OF THE SECONDARY VOLTAGE CONTROL IN FRANCE:PRESENT REALIZATION AND INVESTIGATIONS

J.P. PAUL J.Y. LEOST J.M. TESSERON, Member IEEE

Electricite de FranceDirection des Etudes et Recherches

Clamart, France

Abstract - The voltage control of the french powersystem is organized in three hierarchical levels, whichconcern distinct geographical areas and time constants. Therapid and random variations are compensated by local prima-ry and automatic actions on the generators A.V.Rs, then theslow variations are regulated by the secondary (at regionallevel) and tertiary (at national level) actions. The paper givesa review of the present regional secondary voltage control,and the prospects for the evolution towards a co-ordinatedsystem, based on a sensitivity matrix model, closed-loop poleassignment and adaptive techniques. Analog and digital simu-lations are presented.

INTRODUCTION

The voltage profile of the french power system is theresult of actions performed on voltage sources (generatingunits and synchronous compensators), and of voltage drops inthe transmission lines and transformers, owing to reactivepower transfers and reactive losses. Although it may seemquite ordinary to manage to maintain at every moment thevoltage of such a large power system within acceptablelimits, it is important to observe that such a situation is onlypossible because suitable decisions were made long before,including design of control systems, investments in localmeans of reactive power compensation, and elaboration ofmethods in order to optimize and co-ordinate the use of allpossible means. Between today and the day when the deci-sions were made, the french power system has deeplychanged, in particular with the commissionning of a largenumber of nuclear power stations and the associated develop-ment of the 400 kV power system. This evolution is still inprogress, and the installed nuclear power capacity will thusincrease from 35 GW in 1985 to 54 GW in 1990.

Furthermore, the constraints imposed on the authori-zed location of large nuclear units (900 MW, then 1 300 MW,and 1 400 MW units in the future) result in an increase of theelectrical distance between generation and loads, in particu-lar when the thermal generating units located close to theloads are shut down owing to their lack of economic interestcompared with nuclear power plants. In these conditions, inorder to cope with power transfers, the length of theinstalled 400 kV circuits should reach 20 000 km in 1990, andcorrelatively a great number of capacitors must be installedat the H.V. bus bars of EHV/HV substations (about 2 000 Mvarhave already been installed during the last years).

Consequently, the purpose of this paper is first toshow what are the means used by EDF in 1985 for voltagecontrol and what are the obtained results, then to present the

86 SM 344-6 A paper recommended and approvedby the IEEE Power System Engineering Committee ofthe IEEE Power Engineering Society for presentationat the IEEE/PES 1986 Summer Meeting, Mexico City,Mexico, Jtuly 20 - 25, 1986. Manuscript submittedFebruary 3, 1986; made available for printingApril 25, 1986.

Printed in the UI.S.A.

actions in progress to cope with the evolutions to come andprovide a convenient voltage control in the 1990's.

Principles of the french EHV voltage control

In order to achieve the regulation of the voltagewithin the allowed range, despite the unavoidable variationsdue to the modifications of demand and generation or to thetopology changes, a continuous control of the generatorsvoltage set-points and of the compensation means has beenimplemented on the french EHV power system. The possibleactions have been organized in three hierarchical levels,which concern distinct geographical sizes and timeconstants:

- the rapid and random voltage variations are compensatedby local "primary" actions, which are automatically perfor-med to ensure a fast adjustment, mostly by action of thegenerators automatic voltage controllers (A.V.R.);

- the slow and large variations, which are likely to affect alarge part of the power system, are dealt with by the co-ordinated "secondary" (at a regional level) and "tertiary"(at national level) actions. These actions co-ordinate theprimary controllers in order to provide an optimized repar-tition of the production and generation of reactive power,taking security and economical aspects into account. Moreprecisely, the secondary voltage system (french acronym:R.S.T.) dynamically manages the reactive power availablein a regional "zone", with a time constant of about3 minutes; the tertiary control allows a global staticcontrol at the national level, and is performed by remotemanual action on the settings of the secondary voltagecontrol.

This report intends to describe the realization and theperformances of the present Secondary Voltage Control(R.S.T.), and an experimental integration of H.V. capacitorsin this system. Then, we will present the prospects for theevolution towards a new co-ordinated secondary control(french acronym: C.C.).

THE PRESENT SECONDARY VOLTAGE CONTROL (R.S.T.)

Review of the control principles [1]

The principle of the R.S.T. is to share the powernetwork into distinct geographical parts, called "zones", andto control the voltage profile separately in each zone byautomatic adjustments of the A.V.Rs of some units (called"controlling generators") located in the zone. These adjust-ments lead to variations of the reactive power supplied bythe controlling units. The size of the adjustments is deter-mined by the difference between a set-point value and thevoltage value of a special node in the zone, called "pilot-node", which must be chosen so as to have voltage variationsrepresentative of the voltage evolutions throughout the zone.This condition is fulfilled if the electric distance between thepilot-node and the other nodes is short. It must be stressedthat two other conditions must be fulfilled to ensure a goodcontrol:- sufficient reactive power must be available in the zone;- the electric distance between the pilot node and the

nearest adiacent zones must be large enough to prevent

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GENERAL PRINCIPLE OF CONTROL LOOPS

* N : oc Vc -Vp dt+p Vc - Vp Electrical system0 Vn VnElcrcls te

Fig. 1 General principle of control loops

- 400 kV lines225 kV lines (not shown)

* 400 kV pilot-node225 kV pilot-node

C Thermal power plantr Hydroelectric power stat

(1 Zone boundary

Fig. 2 Secondary voltage control zones

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undesirable influence between different zones. This condi-tion is generally easily satisfied, with regard to the voltageproblem characteristic, which is to be a local problem.

Figure 1 shows how the control is performed. Thereactive power of the controlling units is adjusted by twocontrol loops, which are superimposed on the A.V.Rs of thesegenerators. A control signal N (the "R.S.T. zone level") iselaborated from the difference between the pilot-node'smeasured voltage Vp and the set-point Vc (which can bedetermined by the tertiary control system), using a proportio-nal integral law. The control signal is elaborated in adedicated microcomputer located in the regional controlcentre; then it is transmitted to each controlling unit, to beused as the input of a second control loop (the reactive powerloop) which modifies the A.V.R.'s set-point value, taking thepossible participation factor Qr of the generator intoaccount, in order to make its reactive power output equal to(N . Qr).

This intermediate control, performed through a reacti-ve power loop, provides a simple co-ordination of the reacti-ve outputs of the different controlling generators. But itmust be mentionned that it can lead, under certain condi-tions, to an unwishable transient behaviour after a suddendisturbance, due to the effect of the time constants of thevarious control loops (mainly caused by the communicationdelays of the signals between the generators and the regionalcentre).

Behaviour of the R.S.T. in operation

After an experimental implementation which concer-ned one zone in 1974, Electricite de France decided in 1977to extend the system to the control of the whole frenchpower system, and the secondary voltage control wascommissionned in the first zones in 1979. In 1985, almost thewhole french power system has been equipped with theR.S.T., resulting in 27 zones with a total of some 100 thermalcontrolling generators and 150 hydroelectric ones (seefigure 2).

The total capacity of the reactive power involved inthe secondary voltage control is approximately 30 000 Mvar.

The operating performances confirm the advantages ofR.S.T., which are namely a better control of the voltageunder normal conditions, and a good co-ordination of thereactive power generation of the controlling units, resultingin a reduction of the stresses on the generators.

Typical results obtained with the present R.S.T. willbe presented in the next part of the document, and comparedwith the performances expected from a new control system,called Co-ordinated Control.

Experimentation of H.V. capacitors control through theR.S.T.

As the capacitors located on the H.V. french powersystem also play an important part in the control of thevoltage and in the compensation of the reactive power, it isinteresting, and sometimes necessary, to be able to switchthese capacitors on and off, and to co-ordinate these opera-tions with the voltage control, in order to avoid any undesira-ble interaction between the controlling units and the capa-citors.

On the french power system, the H.V. capacitors canpresently be switched either manually, by remote control, orautomatically, by using a local criterion such as:

- voltage of the substation where the capacitor is installed;

- reactive power losses on the H.V. system and on the E.H.V.lines feeding the substation.

Another possibility, which is presently experimented,is to consider the capacitors as reactive power producers inthe same way as the generators, and to include them in thesecondary voltage control. To keep a large reactive marginon the generator, it is preferable to switch firstly thecapacitors in a zone before modifying the reactive output ofthe generators, when the pilot-node evolution asks for reacti-ve generation increase; this method provides a supplementa-ry amount of immediately available reactive generation onthe units, thus enabling to face more easily outages, whichcould otherwise lead to a voltage collapse when the genera-tors reach their reactive limits.

An experimentation is presently carried out in a zone(in Normandy), equipped with four capacitors of 30 Mvareach. The control of the capacitors is performed by adedicated microcomputer in the following way:

when the production of reactive power of the generators isnear to its limit (this situation is indicated by a high value ofthe control signal N, with dN/dt positive), a signal is automa-tically sent to switch on the H.V. capacitor located in theE.H.V./H.V. substation characterized by the lowest voltage.After a time delay (3 minutes), other capacitors can beswitched on by the control law if it is required. To counteractany control failure, the capacitors are locally monitored byvoltage relays.

In figure 3, recordings show:

- the demand curve in the considered zone during 24 hours;

- the pilot-node voltage variations (with the set-point Vc indotted line);

- the variations of level N and the capacitors switchingoperations;

- the reactive power supplied by a 600 MW unit belonging tothe zone.

It can be noticed that the pilot-node voltage is wellmaintained around its set-point. The capacitors are respecti-vely switched on and off when the load increases anddecreases, thus preserving a substantial reactive margin onthe generator, as the maximum reactive power supplied bythis unit is only 100 Mvar, to be compared with the290 Mvar maximum possible supply.

Finally, this actual in-situ recording emphasizes thegood behaviour of the present R.S.T., and the ability toenlarge the R.S.T. action by integrating the H.V. capacitorscontrol.

Needs for an improved regulation

In the present R.S.T., the voltage control areas (the"zones") are assumed to be decoupled; if this condition isfulfilled, the multivariable system (i.e. the french powersystem) can be dealt with as an aggregation of separate andindependant monovariable sub-systems (i.e. the zones). Thiscondition has been fulfilled up to now, which resulted in agood R.S.T.'s behaviour, as was shown previously.

However, as the mesh of the french power system isgetting increasingly dense, simulations have shown that itwould become difficult by 1990 to define appropriate zones insome regions, with sufficient homogeneity and independancewith regard to voltage control.

The main principle of the present R.S.T. becomingquestionnable, E.D.F. has carried out new studies in order totake the multivariable aspect into account. First, an automa-tic determination of the control zones is under development,as well as the automatic determination of the best pilot-nodes, by using algorithms based on the concept of electricaldistance and statistical classification.

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Secondly, new voltage control algorithms have beenstudied so as to take the interactions between zones intoaccount, when it is necessary. The result of these secondstudies, referred to as Co-ordinated Control (C.C.), will nowbe examined further on in this document.

Regional demand curve (MW) versus time (h)

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Pilot-node voltage (kV) versus time (h)

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Level variations (p.u.) versus time (h)

Havre 2 reactive power output (Mvar) versus time (h)

Fig. 3 R.S.T. action in-situ recordingsH.V. capacitors switching experiment

THE CO-ORDINATED CONTROL (C.C.)

Functional requirements

Let us consider an area consisting of several zonesstrongly coupled, which must be co-ordinated. Each zoneincludes a pilot-node and a number of controlling units.

With:

Vp : vector of voltages at pilot-nodesVc: vector of set-point values of pilot-nodesQ: vector of reactive power outputsU: vector of generators AVR set-pointsz: number of pilot-nodes (and zones)g: number of unitsgi number of units in zone i

The new Co-ordinated Control must meet the fourfollowing functional requirements R.:Rl : The pilot-nodes voltages Vp must be regulated around

their set values Vc, with controlled dynamics, characte-rized by an absence of offset and by an aperiodicresponse, with a fixed time constant chosen between Iand 3 minutes in order to avoid any interaction with theprimary voltage control. In addition, a decouplingbetween the voltage Vp evolutions must be ensured.

R2 :The C.C. must warrant an equilibrated distribution ofreactive power outputs between the various sets of onezone, with identical dynamics. Namely, the differentcontrolling units of a considered zone must supply anidentical per unit reactive power.

R3: The system must take the generators limits into account.In case of lack of reactive power, requirement R 1 haspriority, trying to keep a minimum distance between thevariables and their set-points.

R4: The system must be able to face the network disturban-ces, including the slow ones (load changes) and thesudden ones (topology changes), without any degradationof the previous specifications respect.

Modelling

The basic necessity of avoiding any interactionbetween the primary and secondary voltage control implies toensure a hierarchical decoupling between A.V.R. control andCo-ordinated Control; this is achieved by a temporal decou-pling between these actions. Consequently, the A.V.R.transients must be ignored by the co-ordinated control. As adigital processing of the C.C. algorithms is realized, thisremark leads to choose a time sampling period To between 5and 10 seconds, in order that the network and units evolutionsmight be expressed in terms of gains only. It must be noticedthat this choice of the To value permits to reduce thecommunication system requirements, and nevertheless tomeet the specifications, regarding the slow dynamics of C.C.

We will now proceed with the description, for onearea, of the power system evolutions around a feasibleworking point, by using the sensitivity matrixes Cv and Cq,which give, for a first order development and for fixed loads,the variations of reactive output Q and of pilot-nodes volta-ges Vp, following a variation A U of the A.V.R. set-points.Therefore, we have:

AVp = Cv AU(1)

AQ= CqAUThis set of equations (1), using the control variable U,

will lead to a system design where the reactive loop can besuppressed. The system can be settled in a dedicated area

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computer, which will directly adjust the A.V.R. set-points ofall the controlling units, to meet the functional requirementsRi.

Let us underline that the previous model (1) simplifiesthe network equations, particulary concerning the effects ofactive power variations or load variations, which appear tothe C.C. system as disturbances, in the same way as topologychanges. The experience obtained with present R.S.T. allowsto think that this does not create deep difficulties.

Nevertheless, to meet the R4 specification (respect oftransient and steady-state specifications R1 and R2 in thelargest domain), adaptive techniques have been chosen, inorder to impose the on-line identification of Cq and Cv.Although the on-line reliability of adaptive algorithms haslong been a critical point, such a solution is now possiblewithout reduction of the C.C. reliability, thanks to the recentprogress in stabilization of adaptive technique algorithms(detailed information can be found in reference [2]).

We will now explain the control law principles, basedon the estimations Cq and Cv of the matrixes Cq and Cv.

Control law principle

We will assume that the area computer receives theunits active and reactive outputs and pilot-node voltagesunder control.

To meet the Rl and R2 requirements, the regulatormust control the following variables:z pilot-node voltages Vpi, i varying from 1 to z.

(g - z) reactive power unbalance variables qilj = Qij - Qilwhere i varies from 1 to z, and j from 2 to gi.

The set-points of (Vpi) are (Vci) and the set-points of(qiI )are0.

The control law for these g variables is a closed-looppole assignment. Assume the estimations Cq and Cv arecorrectly determined, we obtain an aperiodic responsewithout steady-state offset for V and Q variables, withdecoupled outputs and a determined time constant, generallychosen around 80 s for To = 8 s.

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VERGE : pilot-nodes

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Further to this simplified control law, we must in factcope with steady-state and transient group limits to reachthe objective R3. These limits are expressed as:

Qi (Qio, Ui, Pi) S Qi S: Qi (Qio, Ui, Pi), i = I...g

Aui au 'Aui

The coefficients AUi and A Ui, and those involved infunctions Qi and Qi, depend on units characteristics, and onauxiliaries and units transformers taps. All theses terms areavailable in the computer, except Pi and Qi which aretransmitted at each step.

To take these bounds into account, the control lawcomputation becomes a quadratic minimization problemunder these constraints for all units.

Finally, to cope entirely with R3 specification, thereactive equations are no more taken into account when agenerator reaches its reactive bounds. In these conditions,when there are active constraints, we privilege the voltageaspect, allocating the available reactive power in the bestway to minimize IIVc -VP II.

RESULTS OBTAINED WITH DIGITAL ANDANALOG MODELLING OF THE POWER SYSTEM

The above algorithms have been tested in two diffe-rent ways, by using digital and analog modelling of the powersystem area to be controlled.

Digital modelling

The algorithm of the C.C. system has been implemen-ted in the CODYSIL model [3], which is the digital programused by E.D.F. to simulate the long term dynamic response ofthe power system (this program, which can represent thepower system evolution during about one hour, uses a timestep of one second).

The power system network used to perform the simu-lations represents the centre of France, where three stronglycoupled zones will appear in the early 90's (see figure 4).

Fig. 4: Network used for Co-ordinated Control studies

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Fig. 5a: Pilot-node voltages evolutions after lines outageSecondary Voltage Control

The studied network consists mainly of five 900 MWunits belonging to the three zones (Table I):

TABLE IDescription of the studied region

Zones Groups Pilot-nodes

I Dampierre TabarderieBelleville

2 St Laurent Verger

Chinon 2 Distre

Figure 5 shows that the C.C. system meets its firstobjective, which is to maintain the voltages of the pilot-nodes as close as possible to their set-points. For example,let us compare the voltage evolutions of the three pilot-nodesafter a line outage:

PN3 voltage (kV) versus time (s)

Fig. 5b: Pilot-node voltages evolutions after lines outageCo-ordinated Control

- Figure 5a exhibits clearly the coupling between the diffe-rent system outputs which would occur during the transientbehaviour, if the zones remained equipped with the presentR.S.T. The voltages are led back to their set-points after6 minutes;

- on the other hand, the coupling is completely removed byusing the adaptive C.C. (figure 5b), which also enables tomeet the dynamics requirements (output decoupling, timeconstant, dynamic performances).

Analog modelling

The algorithms of the co-ordinated control have alsobeen implemented in a real time minicomputer to assess theircapability of working in a noisy environment, coping withperturbation arising from power system transients. For thisaim, simulations have been performed with the E.D.F.'sMicronetwork (in french, MICRORESEAU), which is an actualsmall scale model of an electric power system, where thegenerating units (up to 14) are represented by 3 phase A.C.mnachingesoiteveral kVADoower, driven bv D C. motors andwnere m iIerenT reguia ors o tur ne an excitatioh are

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represented by analog computers.

In this case, the studied power system consisted ofthree generators (GI, G2, G3), 15 transmission lines, and twopilot-nodes PNI and PN2.

For this example, the general requirements Ri becomethe particular ones:

- voltages at the pilot-nodes are to be equal to their set-points;

- Gl and G2 have to supply the same reactive level, relatingto zone 1.

Experiments similar to the previous ones were carriedout; the obtained results confirm the digital simulations andassess the merits of the new co-ordinated control. For exam-ple, let us compare the voltage evolutions of the pilot-nodesand of the reactive power outputs of the units, following a5 % step applied to PN2 set-point. Figure 6 shows that PN2reaches its new set-point value after 3 minutes, while PNIvoltage is not altered; regarding the reactive power outputs,it is to be noticed that the evolutions of Gl and G2 (whichbelong to the same zone) are identical, with dynamics similarto those of voltage evolutions.

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Fig. 6: Temporal evolution for a 5 % PN2 set-point stepMICRORESEAU simulation

A great number of other digital and analog simulationshave been carried out, and their results confirm the C.C.'sability to meet the functional requirements Ri.

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Furthermore, the new synthetic voltage profile controlprovided at regional level by the Co-ordinated Control wouldconstitute a useful dynamic tool for a future steady-statevoltage optimization of third level. Namely, an automatictertiary control of the Optimum Reactive Power Dispatchtype [4] could periodically determine the set-points of areliable secondary level (which would be the Co-ordinatedControl), in order to optimize economy of the power systemwith respect of security constraints. In such a scheme, thesecondary level, performed by the Co-ordinated Control,would contribute to increase the system security between theoptimizations, facing all the network disturbances by settingup regularly the set-points of the units AVRs.

This complete voltage control organization, brieflydescribed above, is presently under studies.

CONCLUSION

The voltage control plays an important role withregard to the economy and reliability of a power system. Inparticular, the present french Secondary Voltage Control(R.S.T.), which co-ordinates the primary controllers at regio-nal level, gives the dispatchers an easy and reliable means toobserve and adjust the voltage profile in a whole geographiczone. Furthermore, integrating the H.V. capacitors into theSecondary Control action could still improve security, bykeeping a larger reactive margin on the controlling units of agiven zone; such an experiment is presently in progress.

Nevertheless, E.D.F. has engaged investigations insecondary voltage control evolution, in order to cope with theincreasingly dense mesh of the power system in the early90's. The aim of the investigated Co-ordinated Control is totake the interactions between adjacent zones into account.The studied algorithm, which includes use of adaptive techni-ques, provides an efficient means for transient voltage andreactive power control, so as to face normal and outageconditions; this can be performed without needing preciseand reliable information about the power system topology,nor voltage measurements in many substations. Simultationshave shown that the specifications seem to be met. If theseconclusions were confirmed by an experiment in a zone,which might be decided in 1987, the Co-ordinated Controlcould be an interesting evolution of the present R.S.T. in theregions where this system will reach its functional limits.

REFERENCES

[1] J.P. Barret, F. Maury, G. Cotto, "Reglage de la tension",CIGRE - IFAC, Florence 1983.

[2] E. Irving, "Commande adaptative", Ecole Superieured'Electricite 1985.

[3] G. Pioger, G. Testud, "Long term dynamic behaviour ofthe network. Quality of supplied energy sollicitationssupported by power plants", IFAC, Calcutta 1979.

[4] J.L. Carpentier, "Optimal Power flows: uses, methodsand developments", IFAC, Rio de Janeiro 1985.

VOLTAGE CONTROL. EVOLUTION

As it was previously stressed, the voltage control callsfor a co-ordinated use of all the reactive resources availableon the power system. A global strategy is presently studiedby E.D.F. The delayed action of switchable H.V. capacitorscould be probably easily integrated in the Co-ordinatedControl, in a way similar to what was described above forR.S.T. Other means, such as static var compensators, couldalso be included in the global strategy.

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