Reactive power compensation and ıts drawbacks

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1995Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 1995Elektrik enerji sistemlerinde işletmeyi kolaylaştırmanın, verimliliği arttırmanın ve enerji tasarrufunu sağlamanın en etkin önlemlerinden biri "Re- aktif güç kompanzasyonu"dur. Tezde, reaktif güç kompanzasyonunun gerekliliği, matematiksel esasları, modern reaktif güç kompanzatörlerinin yapısı, kompanzasyon tesislerinde harmonikler ve rezonans olayları anlatılmıştır. Tezde, yapılan çalışmalar söyle özetlenebilir. Birinci bölümde reaktif güç kompanzasyonun gerekliliği anlatılmış, izlenen yöntem hakkında genel bilgi verilmiştir. İkinci bölümde reaktif güç kompanzasyonu tanımlanmış, matematiksel esaslar verilmiştir. Üçüncü bölümde kompanzasyon tesisleri anlatılmış, kompanzasyon tesislerinde kondansatör gücü hesabı verilmiştir. Dördüncü bölümde aydınlatma tesislerinde kompanzasyon anlatılmış, kompanzasyon için gerekli kondansatörün hesaplanmasına ilişkin program Ek-A'da verilmiştir. Beşinci bölümde modern kompanzatöıierin yapısı üzerinde durul muştur. Altıncı bölümde kompanzasyon tesislerinde harmonikler anlatılmış, bir doğrultucu devresinin meydana getirdiği harmoniklerin hesaplanmasına ilişkin bîr algoritma geliştirilmiş ve EK-B'de verilmiştir. Yedinci bölümde harmoniklerin etkisi ile oluşabilecek rezonans olayları anlatılmış, bir doğrultucu sisteminin bulunduğu bir tesiste buna paralel olarak bağlı bulunan bir kompanzasyon sisteminin ne şekilde etkileneceği anlatılmış ve buna ilişkin program da Ek-C'de sunulmuştur.Reactive power appears in every AC power system. Loads consume not on!y active but also reactive power. There is an equally strong coupling between the reactive power balance of a power system and the voltages. In itself, a reactive power balance will always inherently be present, but with unacceptablevoltages if the balance is not a proper one. Excess of reactive power in an area means high voltage, a deficit means low voltages. The reactive power balance of power system also influences the active losses of the network, the heating of components and in some cases, the power system stability. Contrary of the active power balance, which has to be effected by means of the genarators only a proper reactive power balance has to be effected both by the genarators and by dispersed special reactive devices, shunt compensation, is an straightforward reactive power compensation method. In the 2nd chapter, the load compensation requirements and mathematical equations are given. In an ideal AC power system, the voltage and frequency at every supply point would be constant and free from harmonics and the power factor would be unity. Inparticular these parameters would be independent of the size and characteristics of consumers loads. In an ideal system, each load could be designed for optimum performance at the given supply voltage, rather then for merely edequate performance over an unpredictable range of voltage. Moreover, there colud be no interference between different loads as a result of vaviations in the current taken by each one. Load compensation is the management of reactive power to improve the quality of supply in an AC power systems. The term load compensation is used where reactive power management is effected for the single load (orgroup of loads), the compensating equipment usually being XV installed on the consumer's own premtses near to the load. Th© techniqu©s used, and indeed some of the objectives, differ consîderably from those met in the compensation of buik supply networks (transmission compensation). in load compensation inere are three main objeeîives. 1. Power factor conrection. 2. Improvement of voltage regulation. 3. Load balancing. Power factor correction and ioad balancing are desirable even when the supply voltage is very stiff (constant and independent of the load). Power factor correction usually means the pratice of genarating reactive power as close as possible to the remote power station. Most industrial ioads have lagging power factorsıthat is, they absorb reactive power. The load current to supply the rai power alane. Voltage regulation becomes rmpoîant and sometimes critical issue inthe presence of loads which very their demond for reactive power, altough they differ widely in their range and rate of variation. The third main concern in load compansation is load balancing. Most AÇ power systems are three-phase and are desingned for balanced operation. Unbalanced operation gives rise to componenis of current in the wrong phase-sequence. The three main functions of the ideal compensator are interde- penî. in particular, the power factor correction and phase balancing them- selves tend to improve voltage regulation. Generally, loads whieh are inductive, will absord reactive power, (Figüre 1), so there ts an voltage drop on üne reactance X and resistance R due to the reactive power of the load. fxisinp=AV* ^RIcospsjû^ v - Figüre 1.Voltage drop on line reactance and resistence reactive power of the load. xvi Voltage and reactive power equation can be expressed by the following equations, AV =AVR 4-jAVx AW RP+XQ,.XP-RQ O) ^y = u V V P and Q load active and reactive power, AV is the vollage drop, V is the voltage end of the Üne, A¥R is in phase wtth V and AVx quadrature wtth V. From this expession it can be seen that the voltage drop wili be recuced by varijing reactive power Q. it is thus possible to keep the voltage V constant, in spite of load variations in botj the active and the reactive power, by begin able to control Q. in contrast, if the voltage is stabilize desired level, the system which requıes reactive power can be supplied by compensator. in the 2nd chapter, general compensation problem for both distribution and transmission system are considered by using symmeüical components. A general three phase delta connected load can be considered with compensators show in figüre 2. vâ=v Vir^Vr ' J V^aVf | ,,r(c) V,t0) ,rrW . ,-, . % lo L* % Ao *a yV» v/v» / j 1 \ i l / l | \ rJ°) Y Dab ab compensator load Figüre 2.Three phase un balanced toads represented by delta connected admintances and îhe delte connected compensator. lî the load admintances are composed of real components (conducîance) andreactive components (syseptance); xvfi Xab ~^»k +JBab Ybc =Gbc +jBbc (2) Yca =Gca +JBca The reactive parts can as a fırst step of the compansation, be cancelled by an appropriate compensating suseptances -Bab, --Bbc and -Bca. As a second step of the compensation, real admintances has to be eomplemenîed with a reactive admintances network so as to obtain a resultant balanced !oad on the AÇ supply. These reactive admintances are, R(ab) = Gab R(ab) __:Qab Bbc -J^, B,, -^-^ B(bc)=jG| f B^)=H^ (3) o(ca) _,Gça a(ca) _ ; Gça Bab J ^ğ - öbc -İjş Superscript (ab), (be) and (ca) are used to indicate that the suseptances are needed to compansate phases ab, be and ca.Subscript ab, be and ca are shown reactive suseptances which are'connected to the phases. The îhree suseptances requıred to compansate a general unbalanced load, can be expressed in term of the real and reaetive ports of the ioad admintance, that is Bİb^HB^+KG^-G^Î/Vâ Blc^HB^+KG^-G^/Vs (4) B^^-ÎB^+KG^-G^)/^ This equetion transfomn any iinear unbalanced reactive load into balanced real îoad. it is difficult to measure load admintances therefore it can be used symmmeîrical component anaiysîs to determine cornpensator suseptances. The objectives of load compensation can be stated in terms of the synnmeîrical lîne-current components as fallows: 1. Eliminate the negative~sequence components (balancing). 2. Eliminate (reduce) the reactive part of ihe positive sequence components (power factor correction) The compensation requirennents can be formulated mathema- tically as fallows, xvöi k +IS) =0 a* a, ^ W* <} =° laı and fee are posiîive and negative sequence components of line currents. ^ and 1^ are positive and negatîve sequence components of compensators. The symmetrical components analysis gives the suseptances in terms of complex line currenîs as faliows: BLb} ^-^m^ 43vReI% "âv1^ Bj? ^Imla, -^la, (6) Bra =~^lmIa2 ~^ReIa2 ~3y ^ The results in (6) are the same as (4), but compensator suseptances are defined with respect to line currents and voüages. The transmisston neîwork compensation requîres a different approach than load compensation. it may not be possible ör practical, to measure the quantities that wou!d meaning fuiiy characterize the load. The primary interest at the therminal of a transmission line is generally the voltage. The objectîves of compensation may be stated as fallows: 1. EHminate the negative sequence voltage, 2, Stabilize the positive sequence voltage at the terminal Matihematically this means that, Va2=0 Vat =V=constant (7) in the 3.rd chapter, compensation systems and iîs equipments are explained.' {n the 4.th chapter, compensation types in illimînation systems are given. Chapter 5, introduces and describes in detail the pricciples of modem static reactive power componsetora including the tyristor-controlled reactor, the thyristor-switched capacitor, and the saturated reactor. In 6 in. chapter, harmonics in compansated network are given. Harmonic currents are generated by non-linear loads, such as are furnaces, static converters, rectifiers, inverters, etc. In 7 îh. chapter possibilities of resonance in compensated networks are explained.Yüksek LisansM.Sc