EMPT ile enerji iletim sistemlerinde açma-kapama olayları analizleri
Switching phenomena analyses of power transmission systems with EMTP
- Tez No: 46308
- Danışmanlar: DOÇ.DR. ADNAN KAYPMAZ
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 121
Özet
ÖZET Bu çalışmada enerji iletim sistemlerinde, başta açma-kapama olayları olmak üzere, geçici olayların analizinde kullanılan teoriler üzerinde durulmaktadır. Tüm çalışma boyunca teorilerin bilgisayar benzetişimleriyle uygulanması hakkında bilgiler verilmekte, bu amaçla I.T.Ü. bünyesinde bulunan eiektromagnetik transiyent programından (EMTP) yararlanılmıştır. Gerilimlerin yükselmesi ve şebeke boyutlarının giderek büyümesi, gerçekleştirilen açma-kapama işleminden kaynaklanan geçici olayların önem kazanmasına neden olmaktadır, özellikle 380 kV ve daha yüksek gerilimlerde, bu önem daha da artmaktadır. Bu nedenle, yapılan açma- kapamanın şebekeye olası etkilerinin önceden bilinmesi şarttır. Bu zorunluluk tezin çıkış noktasıdır. Geçici olay analizlerinin doğruluğu, en başta hat parametrelerinin doğru olarak hesaplanmasına bağlıdır. Burada, hat parametrelerinin bilgi¬ sayarlarda uygulanmaya elverişli bir yolla hesaplanmasına ilişkin metodlar anlatılmaktadır. Daha sonra, geçici olaylarda kullanılan hat modelleri hakkında bilgiler verilmektedir. Açma-kapama türlerinden, özellikle bir fazlı açma-kapama üzerinde durulmaktadır. Bu tip bir acma-kapamanın uygulanabilme ihtimalinin yüksekliği ve kararlılık korunumuna imkan vermesi, bu çalışmada ele alınmasının temel nedenidir. Ayrıca, küçük boyutlu bir şebekede faz-toprak kısa devresi ve bir fazlı açma-kapama analizleri, farktı hat modelleri ile yapılmakta; sonuçlar, ilgili veri dosyalarıyla sunulmaktadır. Analizler sonucu, dağılmış parametreli ve frekansa bağımlı hat modellerinin oldukça makul sonuçlar verdiği anlaşılmaktadır. Ayrıca, çok kısa hatlar dışında, açma-kapama olayları için dağılmış parametreli modellerin güvenle kullanılabileceği görülmektedir.
Özet (Çeviri)
SUMMARY SVVITCHING PHENOMENA ANALYSES OF POWER TRANSMISSION SYSTEMS WITH EMTP Electrical power systems consist of three groups which are generating system, transmission system and distributing system. Power transmission systems are established to supply electric energy from the production centers, through long distance power lines into larger centers of population. in recent years, the need of electric energy is expeditiously increasing in view of the technological developments. Hovvever, the generating of elec- trical energy from the basic energy sources is not augmenting at the same rate. Hence, current energy sources must be used more efftcentiy than before and optimum operating effıciency must be maintained during the operating of electrical power systems. in order to maintain maximum operating effıciency, several different power systems are interconnected, moreover some countries share the same large scale power systems. Thus the structure, the quality and proportions of the electrical power systems are to change and become more complicated. in the power transmission systems, öne of the important problem is the transient phenomena. These phenomena can be studied under two groups: -Transient switching surges of relatively short duration. -Lightning surges with a very short duration. in the povver transmission systems whose operating voltages are relatively high the lightning surges are insignificant compared to the switching surges Therefore at the operation voltages of 380 kV and above, switching surges play an important role in determining the insulation level of the po¬ vver transmission systems. Also, in the switching operation of povver trans¬ mission systems, the system parameters are subjected to change with voltages and currents in a wide range of frequency. This frequency range may extend from 50 Hz to 100 kHz and accordingly the values of system parameters and eath path exhibit a large variation. For this reason, in the calculation of transient studies to use a method vvhich accounts into the variations with the frequency of lumped and distributed parameters of system is necessary. This method should also take çare of the effect of corona, magnetic saturation, the operations of circuit-breaker. in practice, to ensure that ali off these factors are taken into consideration, they must to be imbeded in the method. But this may not easy. On the ottıer hand, xiiidifficulties in the calculation of transient phenomena are not peculiar only to the method but also to that of results, a knowledge as to how the power system parameters vary with frequency is necessary. For a number of years, computer simulations have been used for studying switching surges and other eiectrical transients on power transmission lines Many of the techniques employed are based on the use of travelling waves, with the assumptlon of lossless ör distortionless propagation characteristics The general purpose transient program developed at the Bonneville Power Administration (BPA), which is based on Bergeron's method of characteristics, was also atfirst restricted to lossless, distortion¬ less, ör combination with lumped line resistance. Simulation of this type are computationally very effıcent, essentially requiring only time delay and attenutation to represent the propagation on transmission lines. Hovvever, the assumption of lossless ör distortionless transmission, ör even constant line parameters, may produce highly inaccurate results, due to frequency dependence of the line parameters. More recently, methods of transmission line simulation have been intro- duced to account for frequency dependence of line parameters. Bunder's method was fırst incorporated into the BPA transients program (Electro- magnetic Transients Program ör EMTP). This method produced good method results but required large amounts of storage and computation time, since it was based on the time-domain convoluation of traveling wave variables with complicated long-term transmission response functions. Snelson and later Mayer improved this approach for transients programs using Bergeron's method, by greatly simplifying the response functions required for convulation in the time domain. Instead of using the short- circuit admittance approach of Budner, they evaluated the impulse res¬ ponse functions with lines terminated in the surge empedance, thereby greatly reducing the effects of line reflections. This resulted in acom- putationaly more effıcient method, requiring less storage and execution time for the convulation process. A knovvledge of the parameters of multiconductor transmission lines is necessary in analysing a number of problems in power-transmission systems. The parameters may be generally divided into two groups, namely those as power frequency, which are required in order to study load flow, system stability and fault levels, and those at higher frequencies, which are needed for studying the effects of restriking voltages, switching overvoltages, radio interference and the propagation characteristics of power-line carrier signals. Because present transmission lines are elec- trically short at power frequency, it is usually permissible to calculate series and shunt parameters seperately. At higher frequencies, as line lenghts approach and exceed öne quarter wavelength, it is no longer possible to seperate shunt and series parameters, and the problem must be solved in terms of wave propagation parameters. in the case of electrically long iines, a method of calculating the modal parameters of the line is described. This is based on the evaluation of the xiveigenvalues and eigenvectors of a complex matrix derived from the basic matrixes of the system. The present state of art for calculation of electromagnetic transients is based on: (1) Anolog and Hybrid techniques including TNA (Transient Netvvork Analizer), (2) Lattice diagram method and (3) Method of charac- teristics, also known as Bergeron's method. it is knovvn thatthe method of characteristics for distirbuted parameter lines is a suitable technique for graphical ör digital computation of electromagnetic transients when the system contains nonlinear lumped parameters, such as lightning arresters nonlinear resistance and transformer magnetizing inductance. The parameters of overhead transmisson lines are eveniy distributed along the üne, and can, in general, not be treated as lumped elements. Some of them are also functions of frequency; therefore, the term“line constants”is avoided in favour of“line parameters”. For short-circuit and power flow studies, only positive and zero sequence parameters at power frequency needed, which are readily available from tables in handbooks, ör can easily be calculated from simple formulas. For the line models typically needed in EMTP studies, however, these simple formulas are not adecjuate enough. Usually, the line parameters must therefore be computed, with either öne of the two supporting routines line constants ör cabie constants. These supporting routines produce detailed line parameters for following types of applications: -Steady-state problems at power frequency with complicated coupling effects. An example is the calculation of induced voltages and currents in a de-energized three-phase line which runs parallel with an energized three-phase line. Both lines would be represented as six coupled phases in this case. -Steady-state problems at higher frequencies. Examples are the analysis of harmonics, ör the analysis of povver line carrier communication, on untranspozed lines. -Transients problems. Typical examples are switching and lightning surge studies. Line parameters could be measured after the line has been built; this is not easy, however, and has been done only accasionally. Also, lines must often be analyzed in the design stage, and calculations are the only means available for obtaining line parameters in that case. The supporting routine line constants is heavily based on the work done by M. H. Hesse, though some extensions to it were added. On high voltage power lines, bundle conductors are frequenty used, where each phase ör bundle conductor consiste of two ör more subconductors held together by spacers (typically 100 m apart). The bundle is usualy symmetrical, but unsymmetrical bundles have been proposed as well. XVTwo methods can be used for calculating the line parameters of bundle conductors. With the first method, the parameters are originally calculated with each subconductor being represented as an individual conductor. Since the voltages are equal for the subconductors within a bundle, this voltage equality is then used to reduce the order of the matrices to number of equivalent phase conductors. With the second method, the concept of geometric mean distances is used to replace the bundle of subconductors by a single equivalent conductor. Both methods can be used to replace the bundle of subconductors by a single equivalent conductor. Both methods can be used with the supporting routine line constants. The supporting routine cable constants is limited to second method. Method 1 is more general than method 2. For instances, it can easily handle the current distribution in asymmetrical bundles. A balanced transmission line is defined as a line where all diagonal elements of [Z1] and [Y1] are equal among themselves, and off-diagonal elements are equal among themselves. The only line which truly balanced is the symmetric bipolar dc line. Single-circuit three-phase lines become more or less balanced if the line is transposed, provided the length of the barrel (3 sections, or one cycle of the transposition scheme) is much less than the wavelength of frequencies involved in the particular study. A transposed double-circuit three-phase line can be approximated as balan ced lines with high zero sequence coupling. The solution M-phase transmission line equations becomes simpler if the M coupled equations can be transformed to M decoupled equations. These decoupled equations can be solved as if they were single-phase equations. For balanced lines, this transformation can be achieved with symmetrical components. Many lines are transposed, however, or each section of a transposition barrel may no longer be short compared with the wave length of the highest frequencies occuring in a particular study, in which case each section must be represented as an untransposed line. Fortunately, the matrices of untransposed lines can be diagonalized as well, with transformations to modal parameters derived from eigenvalue- eigenvector theory. The transformation matrices for untransposed lines are no longer known a priori, however, and must be calculated for each particular pair of parameter matrices [2'] and [Y'J Historically, the first line models in the EMTP were cascade connections of pi-circuits, partly to proof that computers could match switching surge study results obtained on TNA's at that time. The need for travelling wave solutions first arose in connection with rather simple lightning arresters studies, where lossles single-phase line models seemed to be adequate. This method was already known in the 1920's and 1930's and strongly advocated by Bergeron; it is known as the Bergeron's method. It soon became apparent that travelling wave solutions were much faster and better suited for computers than cascaded pi-circuits. To make the travelling wave solutions useful for switching surge studies, two changes xviwere needed from the simple single-phase iossles line; First, losses had to be included, which could be done with reasonable accurary by simply lumping R in three places. Secondary, the method had to be extended to M-phase lines, which was achieved by transforming phase quantities to modal quantities. Originally, this was limited to double-circuit lines, and finally generalized to untransposed lines. While travelling wave solutions with constant distributed L', C' and cons tant lumped R produced reasonably accurate answer in many cases, there were also causes where dependence, especially of the zero sequence impedance, could not be ignored. Choosing constant line parameters at the dominant resonance frequency sometimes improved the results. Eventually, frequency- dependent line models were developed by Budner, Meyer and Dommel. A careful re-evaluation of frequency-dependence by J. Marti led to a fairly reliable solution method. Selective-pole switching means the tripping and high-speed reclosure of the circuit-breaker poles connected to all the faulted conductors of a line, while the circuit breaker poles connected to the unfaulted conductors remain closed. If selective-pole switching were applied to a three-phse single-circuit line, one pole would be tripped and reclosed for a single line- to-ground fault, two poles for a line-to-iine or two-line-ground fault, and all three poles for a three-phase-fault. Such switching was proposed as long as 1939, but it has been seldom, If ever, used, expect for clearing single- line-to-ground faults. Other types of fault are generally cleared by three- pole switching without automatic reclosure. XVII
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