VOLTAGE REGULATION OF DISTRIBUTION NETWORKS THROUGH REACTIVE POWER CONTROL

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1 Copyight 22 IFAC 5th Tiennial Wold Congess, Bacelona, Spain VOLTAGE REGULATION OF DISTRIBUTION NETWORKS THROUGH REACTIVE POWER CONTROL G. Tapia (*), A. Tapia (*), R. Ciado (**), J.R. Saenz (***) (*) EUITI-D (Univesity of the Basque County), Contol Depatent, Avda. Felipe IV, B, 2 DONOSTIA, SPAIN. Tlf: , Fax: , e-ail: isptaota@sp.ehu.es (**) IBERDROLA, Gadoqui, 8, 488 BIBAO, SPAIN, Tlf: e-ail:.ciado@ibedola.es (***) ESII (Univesity of the Basque County), Electic Engineeing Depatent, Alda. Ukijo s/n, 483 BILBAO, SPAIN. Tlf: , Fax: , e-ail: iepsauj@bi.ehu.es Abstact: Until ecent yeas, the connection of dispesed Independent Powe Poduces to electical netwoks has not been a poble fo utilities, due to the fact that installed powe epesented a sall aount of the total powe connected to the syste. But in the last few yeas this scenaio is changing and especially wind enegy has tun out to be one of the ost ipotant and poising souces of enewable enegy. Consequently, it is easonable to think that wind geneatos should take pat in the contol of electical vaiables of the netwok they ae connected to. Copyight 22 IFAC Keywods: eactive powe contol, voltage contol, distibution netwoks, wind geneation. INTRODUCTION Distibuted enegy geneation is becoing one of the ost ipotant enegy souces along the last few yeas. Even if these geneation units bing ipotant advantages fo utilities and custoes, the pobles associated to the total aount of powe connected to a distibution netwok ae continuously inceasing. As an exaple, pobles elated to the no-egulation of fequency and voltage of those units, pobles elated to the non-dispatchable chaacteistic of ost the dispesed geneation and those elated to assets anageent could be entioned. Seveal types of electic powe geneation ae eeging with diffeent degees of developent and futue pospective (co-geneating units, PV cells, cobustion cells, etc), but today, one type of units wind fas- is inceasing its pesence in a continuous and aazing way. This can be due to seveal easons: wind units ae available in an useful aount of powe, they ae zeo fuel cost, thei poduction is envionentally clean, etc. In ode to avoid the fact that dispesed and not contolled geneation adds isks to exploitation of electical systes and deceases the quality of electical sevice (electical vaiables out of exploitation ange, isk of outages, etc.), Spanish utilities wok tying to popely locate new geneation units and oe ecently, to contol the voltage in new types of wind diven geneatos. The fist stage of the wok pesented in this pape has been tackled in ode to achieve pope locations fo new wind fas and to evaluate thei ipact on the electical netwok. Consequently, an accuate wind fa odel fo the netwok analysis pogas has been developed. Then, the possibility of a voltage contol (though the eactive powe contol) has been exploed fo wind fas endowed with doubly fed induction achines (the new type of geneatos that ae used in the lately built wind fas in Spain).

2. WIND GENERATOR MODELLING The fist step of the wok developed in ode to obtain the wind fa odel was to analyse the double fed induction geneato, the ost widely used geneato in the Spanish wind fas.

2 2. WIND GENERATOR MODELLING The fist step of the wok developed in ode to obtain the wind fa odel was to analyse the double fed induction geneato, the ost widely used geneato in the Spanish wind fas. To odel a double fed induction achine (DFIM) unde diffeent wind speeds, it is ipotant to conside that this kind of wound oto achine has to be fed both fo stato and oto sides, Fig.. deivative opeation, p = d/dt. The oles of cuent and voltage coponents in () need to be intechanged so as to teat voltages as independent vaiables syste inputs and cuents as dependent vaiables syste outputs, as coesponds to a voltage fed doubly fed induction geneato (DFIG). Fig. 2 epesents a typical DFIM contol stuctue that woks with two cascaded contol loops. The inne loop (stato-flux oiented vecto contol) task consists in contolling independently the oto cuent diect - i x - and quadatue i y - coponents expessed accoding to the efeence fae fixed to the stato-flux linkage space phaso. Two identical PI contolles ae typically used to ipleent this inne contol loop. The oute one, govens both the stato side active and eactive powes. Figue. Double fed induction achine Noally, the stato is diectly connected to the gid and the oto is intefaced though a vaiable fequency powe convete. In ode to cove a wide opeation ange fo subsynchonous to supesynchonous speeds, the powe convete placed on the oto side has to be able to opeate with powe flowing in both diections. This is achieved by eans of a back-to-back PWM invete configuation. The opeating pinciple of a DFIM can be analysed using the classical theoy of otating fields and the well-known d-q odel, as well as both thee-to-two and two-to-thee axes tansfoations, Vas (99). In ode to deal with the achine dynaic behaviou in the ost ealistic possible way, both stato and oto vaiables ae efeed to thei coesponding natual efeence faes. In othe wods, the stato side cuent and voltage coponents ae efeed to a stationay efeence fae, while the oto side cuent and voltage coponents ae efeed to a efeence fae otating at oto electical speed, ω. When aiing to expess the induction achine electical odel in the above-entioned efeence faes, it is fist necessay to pefo the Clake s tansfoation, Vas (99) fo the thee-phase to the d-q cuent and voltage syste. In this way, taking the geneal thee-phase odel of the electic achine dynaic pefoance as a stating point, the uadatue-phase Slip-Ring odel [] fo the DFIM ight be expessed though the following atix equation Fig. 2. Oveall contol stuctue fo the DFIM 2. Opeating conditions In ode to develop a coect odel of the wind geneato, soe opeative estictions ust be consideed. Fistly, the eactive powe geneation/absoption capability fo each geneated active powe of the odelled geneato (a 66 kw geneato) ust be consideed, that eans, the typical P/ cuves depending on the abient tepeatue. On the othe hand, the cuents that can be diven by the double-sided PWM ust be analysed to obtain the opeative conditions. Fo these eally pefoed tests, the cuves appeaing in Fig. 3 have been obtained. u u u u sd s α β Rs + pls = pl cosθ pl sinθ R + pl s pl sinθ pl cosθ s pl cosθ pl sinθ R + pl pl sinθ i pl cosθ i i R + pl i sd s α β () whee, L s and L ae the stato and oto side inductances, L is the agnetising inductance, R and R s ae the stato and oto side esistances, θ, is the oto angle (vaiable) and p coesponds to the Fig. 3. P/ cuves fo the geneato + invete

3 It can be seen that the oute cuve, coesponding to the liitations iposed by the invete ae less estictive than those fixed by the geneato itself. Consequently, it ust be ensued that the opeation ange of the geneato is always aintained inside the ost estictive cuve, the one obtained fo the hotte abient tepeatue, Fig i ax i ax Pi ax Fig. 5. Geneato odel pefoance unde step changes in the powe facto Fig. 4. Liitation fo the P/ opeation in the DFIM geneato 2.2 Expeiental validation of the odel Pio odelling the coplete wind fa, the validity of the developed DFIM odel ust be deonstated. Table I pesents the ain paaetes, efeed to a 2º C abient tepeatue, of a typical double fed induction achine of 66kW that is used in a wind fa deployent. Table I. DFIG electic paaetes Paaete Value R s, stato esistance pe phase.67 Ω X sl, stato leakage eactance pe phase.3 Ω n, geneal tuns atio.386 X, utual eactance 2.36 Ω R, oto esistance pe phase.399 Ω X l, oto leakage eactance pe phase.349 Ω The odel pesented in section 2 has been ipleented unde the MATLAB/SIMULINK siulation package. Fig. 5 and Fig. 6 show espectively, the dynaic behaviou of the siulated and the eal 66 kw geneato dynaic pefoance when, geneating 3 kw as a esult of the wind speed with a cosϕ=, a powe facto set-point change takes place. This new set point coesponds to a cosϕ=.857 capacitive powe facto. Geneated active and eactive powes ae consideed to be positive. Since wind conditions eain constant duing the whole tial, it can be seen how the geneated active powe is not changed, while the geneated eactive powe vaies apidly so as to tack the new powe facto set point. It can be seen fo Figs. 5 and 6 that the odel dynaic pefoance coesponds exactly with the eal geneato pefoance and so, it will be used to develop a coplete wind fa odel. Fig. 6. Real geneato pefoance unde step changes in the powe facto 3. WIND FARM MODELLING To odel a wind fa ade up of doubly fed induction achines, n geneatos paallel connected to an infinite bus have been consideed. Even if the wind speed on each geneato can be quite diffeent (soe geneatos can be disconnected while othes ae geneating at thei axiu level), the total active and eactive powe of the fa can be epesented as the su of the active and eactive powe geneated by each windill in the fa, (2). P s total s total n = p i i= n = q i i= (2) whee, P stotal epesents the geneated active powe in the wind fa, stotal epesents the geneated/absobed eactive powe in the wind fa, p i epesents the geneated active powe by each i geneato and q i, the geneated/absobed eactive powe by each i geneato. Figs. 7 a) and b) pesent soe siulation esults of the pefoance of a wind fa, with n=37 geneatos, unde step changes in the powe facto. The siulation has been ade in ode to obseve the wind fa pefoance when the powe facto changes at t=. fo cos ϕ = to cos ϕ =.98 in Fig. 7 a) and fo cos ϕ = to cos ϕ =.9 in Fig. 7 b). As only.2 seconds ae epesented, the wind speed has been consideed constant duing that peiod.

4 contol-law descibed by (3) and epesented in Fig. 8. e sp e 2sp e 3sp e nsp q eisp eax = k q eiax k < e ax( ind cap / ) e ax( ind cap 2 / ) e = ef q eiax eax. Si e ef > eax e ef = eax e ax( ind cap 3 / ) enax ( ind / cap) e ef Fig. 7. Fa pefoance unde step changes in the powe facto set-point Pe Pe 2 Pe 3 Pe n It has to be pointed out that the wind fa pefoance follows a fist ode syste esponse. This dynaic pefoance atches the esponse that a single geneato had, with alost the sae tieconstant, Fig. 5. The powe facto set point in Fig. 7 b) is not eached due to the safety-levels iposed on each geneato. The wind speed hitting the blades of soe geneatos can cause an ipotant level of active powe geneation and consequently, because of the safety-level liitation in the P cuves of those geneatos, the efeence powe facto (eactive powe), can not be eached. 4. REACTIVE POWER CONTROL Once the wind fa ade up of doubly fed induction achines has been copletely odelled, a contol stategy fo the eactive powe contol has been poposed. As it has been entioned above, the opeation liitations of the geneato ae quite ipotant in ode to ensue a coect dynaic pefoance of the geneato and a sufficient secuity-level fo the aintenance point of view. So, the developed contol algoith ties to distibute the eactive powe that the fa has to geneate/absob in a popotional way, taking always into account the instantaneous active powe geneation level of each geneato, (3). q eisp eax = eax n eef = q i= * q eiax eiax (3) whee q eisp is the eactive powe set point calculated fo each geneato, q eiax is the axiu of the eactive powe that each achine can geneate and eef is the eactive powe efeence fo the fa. The popotional distibution algoith has to easue the woking conditions (geneated active powe and P/ liits) of each geneato and then, obtain the eactive powe geneation deanded fo each achine. The used algoith is based on the Fig. 8. Ipleentation of the contol law. 4. Siulation esults Soe siulations have been pefoed unde MATLAB/SIMULINK package, fo a wind fa ade up of 37 geneatos paallel connected. The above entioned tests wee siulated unde diffeent counication conditions, but unde the best ones, siultaneous easueents of P and fo each achine and siultaneous sent of set-points to all geneatos, vey adequate esults have been obtained, Fig. 9. Wind Fa Powe Facto Tie (s) Fig. 9. Dynaic powe facto pefoance It has to be pointed out that fo deceasing powe factos, less than.95, soe oscillations occu due to the fact that soe geneatos can be in thei satuation liits because of the wind speed changes. Anyway, fo the obtained esults it can be concluded that the developed contol algoith can be pefectly ipleented in the eal fa with esults that ae expected to be adequate. 4.2 Expeiental esults Seveal siulation tests have been pefoed unde MATLAB/SIMULINK siulation package in ode to ensue the coect pefoance of the developed

algoith and once the needed ipoveents have been copleted in the eal fa counication netwok, the eactive powe contol-law has been ipleented in the cental copute of the wind fa.

5 algoith and once the needed ipoveents have been copleted in the eal fa counication netwok, the eactive powe contol-law has been ipleented in the cental copute of the wind fa. Soe of the eal esults obtained unde step changes in the powe facto appea in Fig.. Satuation Fig.. Reactive powe contol of the fa In the pefoed test, the eactive powe set point fo the fa is sent in 2 kvar steps. It can be seen that thee is a satuation because the instantaneous wind speed on each geneato, causes soe of the to wok in the eactive powe geneation liits. Anyway, the set-point is coectly tacked along the test even if soe counication faults occu duing the fist pat of the test. It can be seen fo Fig. that the fist esults obtained fo the application of the developed contol law can be used to ensue a good pefoance of the fa unde step changes in the eactive powe geneation level, always aintaining the active powe geneation in its optiu liits. 5. VOLTAGE CONTROL The esults obtained fo the eactive powe egulation ae satisfactoy enough to enable the electic copany to fix the eactive powe contol fo each fa, with the data obtained fo an OPF algoith. The poble that can appea in this stage of the wok is that the fa tacks pefectly the equied eactive powe but because of sudden changes in the electic netwok topology, soe voltage-level violations can occu. Consequently, even if a coplete voltage contol algoith has not yet been developed, soe secuity levels have been iposed in the eactive powe contol and the voltage-level can be always ensued. Fo the secuity level ipleentation, thee voltage levels have been fixed: V noal, V adissible and V secue, Fig.. If V in noal V V ax noal, the developed popotional distibution eactive powe egulation can be applied, that eans, the contol law (3) is diectly applied.

6 V Vax secue Vax adissible Vax noal Secuity Eegency Contol Reactive Powe Contol Additionally, it is ipotant to conside a coplete voltage egulation syste fo the wind fas connected to the electical netwok, in ode to ensue the utility and effectiveness of the developed and pobed eactive powe contol. Reactive Powe Contol Eegency Contol Secuity Vin noal Vin adissible Vin secue e=, V= P.U. e In ode to allow the axiu powe to be connected to electical subsystes, studies fo each paticula situation should be the way that dictate the possibility and constaints fo new wind fa connections. Moeove, to avoid pobles when weake connectivities of a subsyste with its feeding netwoks exist, eote tipping fo selected wind-fas should be set. Fig.. Voltage levels fo a secue eactive powe contol If V ax noal V V ax adissible an eegency contol is developed. In this second aea, the new eactive powe set point (obtained fo the application of the popotional contol law) is accepted if it is less than the pevious one. If it is geate, then the old eactive powe set point will be applied. The sae occus if V i noal V V i adissible, but in the opposite sense. Finally, if V ax adissible V V ax secue, the eactive powe set point is autoatically and continuously deceased in ode to aintain the voltage level unde secuity conditions. That eans, that the developed contol law is not applied and the eactive powe set point is deceased o inceased though continuous steps until the V adissible level is eached. As in the pevious case, the sae occus fo V in adissible V V in secue in the opposite sense. Anyway, if any of the peviously entioned actions is enough and the last voltage level is eached, V= Vinsecue o V= Vaxsecue the powe facto of the fa is autoatically eset to cosϕ =. Though all this egulation levels, the eactive powe contol is ensued but at the sae tie, the voltage level will be aintained inside the peitted liits. 6. CONCLUSIONS Fo the esults obtained in all the siulations and eal expeients that have been pefoed, it can be concluded that wind fas ade up with double fed induction geneatos constitute an ipotant tool fo the voltage egulation point of view. Double fed induction achines have a uch bette dynaic pefoance than squiel cage ones and do not need capacitos. They ae stongly ecoended fo the new wind fas fo the powe syste viewpoint, especially in aeas with a lage wind geneation concentation. REFERENCES Akhatov V., Knudsen H. (999), Modelling of windill induction geneatos in dynaic siulation pogas, PoweTech Budapest 99. Abstact Recods. (Cat. No.99EX376). IEEE Piscataway, NJ, USA; xviii+38pp., p. 8. Bhowik S., Spee R., Enslin J.H.R. (999), Pefoance optiization of doubly-fed wind powe geneation systes, IEEE Tans. on Industy Applications, vol. 35, no.4, pp Dittich A., Hofan W., Stoev A., Thiee A., (997), Design and contol of a wind powe station with double fed induction geneato, EPE th Euopean Conf. on Powe Electonics and Applications, EPE Assoc, Bussels, Belgiu, vol. 2, p Leith D.J., Leithead W.E. (996), Appopiate ealization of gain scheduled contolles with application to wind tubine egulation, Int. J. Contol, vol. 65, no. 2, pp Liaw C-M., Pan C-T., Chen Y-C (988), Design and Ipleentation of an Adaptive Contolle fo Cuent-fed Induction Moto, IEEE Tans. on Industial Electonics, vol. 35, no,3, pp Papantoniou A., Coonick A. (997), Fuzzy logic contol of a unified powe flow contolle fo wind fa applications, IEE Colloquiu on Powe Electonics fo Renewable Enegy (Digest No. 997/7). IEE, London, UK, pp Sioes M.G., Bose B.K., Spiegel R.J (997), Fuzzy-Logic Based Contol of a Vaiable- Speed Cage Machine Wind Geneation Syste, IEEE Tans. on Powe Electonics, vol. 2, no., pp Spee R., Bhowik S., Enslin J.H.R. (995), Novel contol stategies fo vaiable-speed doubly fed wind powe geneation systes, Renewable Enegy, vol. 6, no.8, pp Tapia A., Tapia G., Ostolaza X., Saenz J.R. (2), Modelling and dynaical analysis of a wind fa, IEEE VII Intenational Powe Electonics Congess (CIEP), Acapulco, pp P. Vas (99), Vecto Contol of AC Machines, Oxfod Univesity Pess, New Yok.

Control Limits of Three-Phase AC Voltage Controller under Induction Motor Load A. I. Alolah Ali M. Eltamaly R. M. Hamouda

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