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Negatif bileşen aşırı akım koruma rölesi yazılım tasarımı

Başlık çevirisi mevcut değil.

  1. Tez No: 75580
  2. Yazar: NİHAT AKAT
  3. Danışmanlar: DOÇ. DR. ÖMER USTA
  4. Tez Türü: Yüksek Lisans
  5. Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1998
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Elektrik Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 68

Özet

ÖZET Bir elektrik güç sistemindeki en önemli ve pahalı elemanların bir tanesi generatörlerdir. Sistemdeki oluşabilecek arızalara karşı yeterli ve hızlı korumanın sağlanması çok önemlidir. Sistemdeki dengesiz arızaların sebep olabileceği dengesiz çalışma koşullarına karşı generatörü koruyabilecek koruma sistemlerinden bir tanesi de negatif bileşen aşın akım koruma rölesidir. Dengesiz sistemlerin analizinde Fortescue Simetrili Bileşenler yöntemi ballanılır. 3 faz dengesiz bir sistem simetrili bileşenler yöntemi ile 3 farklı dengeli sisteme ayrılabilir. Bunlar pozitif bileşen sistemi, negatif bileşen sistemi ve sıfir bileşen sistemidir. 3 fazlı dengeli bir sistemde yalnızca pozitif bileşen vardır. Sistemin dengesiz bir hal alması durumunda negatif bileşen ve sıfir bileşenler ortaya çıkar. Elektrik enerji sisteminde dengesiz çalışmaya sebebiyet verebilecek arızalar faz- faz, faz-toprak, faz-faz-toprak, fazlardan birinde ani olarak yük kalkması ve fazların dengesiz yüklenmesi sayılabilir. Tüm bunlar sistemde bir negatif bileşen kaynağının oluşmasına ve negatif bileşen akımın akmasına neden olur. Bu durum generator sargılarının aşın ısınmasına yol açar. Elektrik güç sistemlerindeki generatörlerin dengesiz arızalara karşı korunabilmesi için mevcut bir negatif bileşen aşın akım koruma algoritması çok fonksiyonlu bir koruma rölesine yüklenebilecek şekilde düzenlendi ve C programlama dilinde yazılımı yapıldı. Koruma sistemi uluslarası standartların belirlediği dengesizlik oram (UB) sınırında alarm, generator dengesiz çalışma yeteneği (Ks) sınırında açma olacak şekilde tasarlanmıştır. Koruma sisteminin performans analizi için EMTP'den alman simülasyon sonuçlan kullanılmıştır. Algoritmanın preformans analizleri sonucunda bahsedilen dengesiz arızalarda koruma sisteminin öncelikle alarm verip gerekli uyarıyı yaptığı ve arızanın devam etmesi durumunda generator gurubunu devre dışına çıkardığı görülmüştür. Bu haliyle algoritma çok fonksiyonlu dijital bir koruma rölesine yüklenebilecek şekle koyulmuştur. vu

Özet (Çeviri)

SUMMARY SOFTWARE DESIGN OF NEGATIVE SEQUENCE OVER CURRENT PROTECTION RELAY In a three-phase power system the generated voltages are sinusoidal with the individual phases 120° apart. At the distribution end the unbalanced single phases loads and nonlinear loads cause unequal voltage drops in the transformer and line impedances. The results is an unbalanced supply voltage at the point of common coupling. In the power system, balanced conditions are normally maintained by distributing the load equally among the three phases. The single phase loads on the system now vary continuously with larger hourly variations. The result is a continuously varying unbalanced load that leads to unbalanced voltage and current problems. These effects are explained in terms of the negative sequence component in the unbalanced voltages and currents under normal balanced conditions positive sequence voltages and currents are only presents in a system. In the case of unsymmetrical fault or unbalanced loads, a fictitious source of negative sequence voltage is considered to be present at the point of faults, the direction of negative sequence power is from the machine to the bus. However, for external faults of unbalanced loads, the directions of negative sequence power flow is reserved. In the event of an unsymmetry in the armature currents of a synchronous generator, a negative sequence component of current flows in the stator. This currents produces a second harmonic current can be used to indicate the presence of an abnormality. Based on recent studies conducted indepently by ANSI, NEMA and EEC the proposed limits on unbalance are viii1) continuous voltage unbalance - 5% 2) continuous current unbalance - 10% The percentage unbalance is calculated using the IEC definition, «,,, ". Negative sequence component %unbal(UB)= -rr^n (S.l) Positive sequence component Unsymmetrical faults may produce more severe heating in machines than symmetrical faults or balanced three phase operation. The negative sequence currents which flow during these unbalanced faults 120 Hz rotor currents which tend to flow on the surface of the rotor forging and in the non magnetic rotor wedges and retaining rings. The resulting I2R loss quickly raises the temperature. If the fault persist, the metal will melt, damaging the rotor structure. Industry standards have been established which determine the permissible unbalance to which a generator is designed. The general form of the allowable negative sequence current is stated following equation. (I2br.t = K (S.2) In stated equation, I2b is per-unit negative sequence current; t is the time in seconds. When the such an unbalance occurs, it is not uncommon to apply negative sequence relays, on the generator to alarm first, alerting the operator to the abnormal situation and allowing corrective action to be taken before removing the machine from service. The relay itself consists of an inverse time-delay over current relay operating from the output of a negative sequence filter. On a log-log scale the time characteristic is a straight line of the form (l2b)2t = K and can be set to closely match the machine IXcharacteristic. All faults other than one involving all three phases of the primary system give rise to a system of unbalanced currents which may be resolved into its positive-, negative-, and zero-sequence components. Three-phase short circuit, which is the most severe fault, keeps the system totally balanced, while other kind of faults such as line-ground fault, line-line fault or double- line-ground fault render the system unbalanced, thereby requiring special treatment through the application of symmetrical components. In this concept, the unbalanced system will be broken down into three balanced subsystems: namely, the positive- sequence system, the negative-sequence system and the zero-sequence system. The relations between the phase quantities Fa, Fb and Fc and the symmetrical components F0, Fi and F2 are given by Fo = Fa + Fb + Fc Fı = Fa + aFb + a2Fc (S.3) F2 = Fa+a2Fb + aFc In equation (S.3) a represents a phase-rotating operatör a = exp ( j 2tc/3) = exp (-j 47i/3) (S.4) a2 = exp ( j 4tc/3) = exp (-j 2rc/3) This means that multiplication of a complex phasor with the quantity a corresponds to a rotation of 2tc/3 rad in the positive direction, or 4ti/3 rad in the negative direction of that phasor. A microprocessor has to calculate the symmetrical components from sampled data of the phase quantities in real time. This means that a time representation of the relation between the symmetrical components and the phase quantities must be found.The needed algorithm presented below for protection against asymmetrical faults requires the computation of various quantities that are normally used for negative sequence relaying. fo (t) = 1/3 ( fa(t) + fb(t) + fc(t) ) fi (t) = 1/3 ( fa(t) + fb(t-2T/3) + fc(t-T/3)) (S.5) f2 (t) = 1/3 ( fa(t) + fb(t-T/3) + fc(t-2T/3)) where T=time of one period of fixed frequency w. With equation (S.5) have found time representations of the symmetrical components. The algorithm presented above can be used in microprocessor to calculate the symmetrical component which are related to values measured in the past, as long as time t is considered to represent the present. Hydro generators having damper windings can often sustain high levels of negative sequence current without damage. However modem, direct-cooled turbogenerators and salint pole generators without damper windings, can be damaged and must therefore be tripped if negative sequence currents exceed 5% of the machine rated current for long periods of time or as specified by standards. The negative sequence current relay provides a trip and alarm setting capability corresponding to the machine capability. In addition, the inverse-time circuitry incorporates a memory function that takes into consideration negative sequence currents that have not caused tripping due to their short duration. Repetitive, short duration, negative sequence currents can be accumulated and when the set value is reached, tripping is obtained. For example; some characteristic values belong to any negative sequence over current relay given following. Rated current, In 1,2 or 5 A Setting, IN (0,7-1,1) x In XIRated frequency Operation alarm level Ks Value Operate time for alarm 50 or 60 Hz (l-3)x3 or 7% of IN l-63sinstepsof Is 0,1s or 6s (settable) Operate time for tripping ace. to S. Fig.l.100 S. Fig. 1 Time delay to negative sequence over current relay The required algorithm for using in microcomputers is stated below. This algorithm is used for developing the software program for negative sequence over current relay. Software programs have been written in assembler language to achieve real-time operation. In this lecture C software programs have been used to achieve simulation. Algorithm : Step 1: Current phase quantities sampled with frequency 600 Hz, sampled period l,667ms.,quantity of sample 12. per period Step 2: Calculate negative sequence currents is representation of time by using xiithe equations below i2(t) = 1/3 [ia(t) + ib(t- T/3) + ic(t- 2T/3)] (S.6) Step 3: Calculate negative sequence currents as being rms. h = 11 2 \l\2^h\t,+nto) 0 (S.7) to : time of first sampled At : sampled period (Ts) T : period (2tc) Step 4: Calculate the unbalanced ration on the electrical power system Ub (%) = y- (S.8).'at In= Rated current of generator I2= Negative sequence current Compare the calculated Ub value with the constant UB which is given by standards If : Ub > U& provide alarm Step 5: Calculate the generator K constant which represents the length of time the machine can withstand a negative sequence current equal to rated current. XlllIf calculated K value equal or bigger than the constant Ks given by standards provide a trip. In this thesis, above mentioned algorithm is improved and appropriated to load a multifunction digital protection relay. For testing the performance of algorithm computer simulations were carried out xiv

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