Switching and Fault Transient Analysis of 765 kv Transmission Systems

Transcription

1 Third International Conference on Power Systems, Kharagpur, INDIA December >Paper #< Switching and Transient Analysis of 6 kv Transmission Systems D Thukaram, SM IEEE, K Ravishankar, Rajendra Kumar A Department of Electrical Engineering Indian Institute of science, Bangalore6, INDIA dtram@ee.iisc.ernet.in, ravishankarkurre@gmail.com, sahilraj@gmail.com S R Kolla, SM IEEE Electronics and Computer Technology, Bowling Green State University Bowling Green, Ohio, USA skolla@bgnet.bgsu.edu voltages above kv. These can readily be limited by surge Abstract The analysis of electromagnetic transients arising in diverters and are therefore not considered here. Of the other EHV/UHV power networks gives necessary information about switching operations, line closing and reclosing generally the possible stresses on the different network components, which produce the larger over voltages and consequently we will determine their proper design, limits of operation as well as concentrate on line energization [] and fault transients in this their pertinent protection strategies. This paper describes the paper. At higher transmission volt ages, over voltages caused transient analysis of 6 kv EHV transmission system which is a by switching may become significant, because arrester typical expansion in Indian power grid system. Considering operating voltages are relatively close to normal system voltage various conditions, switching transient and fault transient studies and lines are usually long so that the energy stored on the lines are carried out. A FORTRAN version of EMTP is developed, to may be large. Over voltages will put the transformers into study a practical example, then a comparison with the results saturation, causing core heating and copious harmonic current available in the literature is made. generation. Circuit breaker called upon to operate during Keywords EMTP, Overvoltages, Transients. periods of high voltage will have reduced interrupting capability. At some voltage even the ability to interrupt linecharging I. INTRODUCTION current will be lost. Thus transient voltage magnitudes must be restricted to a safe value less than or equal to withstand The insulation level of EHV and UHV ac systems is largely capacity of the apparatus. In practical system a number of determined by the magnitude of switching over voltages. The factors affect the over voltages due to energization or reclosing. reliable operation of any electrical power system is determined The influence of various factors on switching over to a great extent by the amplitude, duration and frequency of voltages (SOV) has been discussed in []. Magnitude of SOV the transient voltages appearing in different places in the can be controlled by using any one or combination of the network. Therefore switching over voltages and fault transients following methods: are a focal point in carrying studies for these systems. Switching transients are fast transients that occur in the process Circuit breaker with PreInsertion Resistor (PIR) of energizing transmission line, load capacitances immediately Metal Oxide Arrester (MOA) after a power source is connected to the network. Power transformers, surge arresters and circuit breakers are Point on wave controlled switching equipments which are first affected by over voltages. At the Because of higher power consumption, space adequacy, planning stage of the 6 kv systems, the insulation level of significant addition of transients due to opening and closing of apparatus is to be decided on the basis of peak value of PIR s and greater mechanical complexity, PIR usage is being transient over voltages, so enormous numbers of cases are reduced. On the other hand PIR s are cheaper when compared considered to arrive at the maximum magnitude. Methods are to others like metal oxide arresters. Nevertheless, the resistor proposed for reducing the over voltages, of the several methods equipped circuit breakers are more expensive. These complex to mitigate transient over voltages, preinsertion resistors are breakers show mechanical malfunctions as the most common used. This paper presents switching transient and fault transient cause of circuit breaker failures. Controlled switching is being studies for a typical 6 kv EHV transmission systems. A used which reduces magnitude of switching over voltages FORTRAN version of EMTP [] is developed, to study a considerably. practical example, and then a comparison with the results available in the literature is made. III. FAULT TRANSIENT ANALYSIS II. SWTCHING TRANSIENT ANALYSIS When a short circuit occurs on a transmission line, voltage and current signals develop a transient DC offset component In EHV and UHV systems there are a number of switching and highfrequency transient components in addition to the operations, such as over voltages produced during the power frequency components. The analysis of electromagnetic switching of reactors, capacitors and transformers. Transient transients arising in power networks gives necessary over voltages are usually a significant factor at transmission Indian Institute of Technology Kharagpur, INDIA //$. Â IEEE

2 Third International Conference on Power Systems, Kharagpur, INDIA December >Paper #< information about the possible stresses on the different network components, which will determine their proper design, limits of operation as well as their pertinent protection strategies []. The analysis of electromagnetic fault transients on long power lines is carried out. The results of such studies will give information about the transient stresses on the different system components. The transient wave shapes of voltage and currents observed at the relay locations can be significantly vary for various possible combinations of faults on transmission lines [6]. Four different types of faults i.e. AG (phase A to ground fault), AB (phase A to phase B fault), BCG (phase B to phase C to ground fault) and ΦG (three phase to ground fault) were applied to the transmission line. The study of fault transient is carried out considering, fault position at various distances from the substation and fault inception on the voltage wave. when both PIR and reactor are present (Fig. ) as compared to the other cases. For source strength, PIR ohms, Reactor R, Switching angle degree the maximum peak over voltage is. p.u when both PIR and reactor are present. G P B A A: Auxiliary Circuit Breaker G: Generator R: Reactor B: Main Circuit Breaker P: Preinsertion resistor Figure. Equivalent system for switching transients R IV. SIMULATED STUDIES AND RESULTS A. Switching Transient Performance Analysis: As the interconnecting systems are large networks, for transient analysis the systems with an equivalent source, transmission line and a load are considered. Thus, sample system represented is shown in Fig. with base as and phase voltage of 6* / kv. Studies are carried out by considering the range of various parameters such as, Source Strength: in steps of Switching angle: in steps of PIR: ohms in steps of ohms Reactor:,, 6,,, and R Switching transient analyses are carried out for km. and km. (Fig. ) line 6 kv system for following cases: Without shunt reactor and without PIR With shunt reactor and without PIR Without shunt reactor and with PIR With shunt reactor and with PIR For km. line, 6 kv system [] without PIR and shunt reactor the peak over voltage observed. p.u at receiving end with source strength. For the same system with ohm PIR the peak over voltage observed at receiving end is.. Maximum transient over voltage occurs near switching angle for same line length and source strength. The results of km. line, 6 kv system operating with source strength of, closing angle for different cases are shown in Fig. and Fig.. The results for the same system with source strength of 6, closing angle of and are shown in Fig., Fig. and Fig. 6. From these results it can be observed that maximum peak over voltage is lowest (.6 p.u for source strength 6, switching angle deg.) Figure. Voltage waveform for source strength, swtching angle deg., PIR is ohms, R. =.6, =.6, =. Figure. Voltage waveform for source strength, swtching angle deg., PIR is ohms, R. =., =., =. Figure. Voltage waveform for source strength 6, swtching angle deg., PIR is ohms, R. Peak =., =., =. The switching transient analysis for system with km, 6 kv transmission line has also been carried out. Fig. //$. Â IEEE

3 Third International Conference on Power Systems, Kharagpur, INDIA December >Paper #< shows the effect of source strength varying from to on PIR when rector is placed on the receiving end. Fig. shows effect of source strength varying from to on PIR when the reactor is placed on both ends (sending and receiving end). Referring to Fig. and Fig. one can conclude that the optimum PIR value for a fixed length increases with source strength and peak switching over voltages reduced with increase in source strength. Table I shows the results of switching transient analysis for few cases. B. Transient Performance Analysis: For fault transient analysis, 6 kv transmission system of km transmission line length is considered. The line is divided into five equal sections (Fig. ) of km each for simulating faults at various positions. Initial conditions for the fault transient analysis obtained from AC load flow solution. TABLE I. RESULTS OF SWITCHING TRANSIENT FOR KM. 6 KV SYSTEM Figure. Voltage waveform for source strength 6, swtching angle deg., PIR is ohms, R. V a=., V b=., V c=.6 Figure 6. Voltage waveform for source strength 6, swtching angle deg., PIR is ohms, R. Peak V a=., V b=.6, V c=. Sl. No. Source Strength in Reactor in R PIR in Ω Switchi ng Angle in Degree * results are shown in Figures also Peak Over voltage (p.u) when R is at Receiving end *.* *...6* Peak Over voltage (p.u) when Reactor on Both end G 6 kms Figure. Effect of source strength on PIR (Reactor on receiving end) kv kv 6 kv R R 6 kv kv kv Figure. Equivalent system for fault transients Figure. Effect of source strength on PIR (Reactor on both ends) Types of faults considered for study are single line to ground, double line to ground, three phase to ground and line to line fault. The study of fault trasient carried out considering fault location on the line and fault inception on the voltage wave. The various case results are summarized in Table II. //$. Â IEEE

4 Third International Conference on Power Systems, Kharagpur, INDIA December >Paper #< ) Location on the Line: The system considered for fault analysis is shown in Fig.. Initially the system is assumed to be operating in steady state balanced condition and delivering a load of MW and R. The prefault receiving end quantities are:, 6 kv. The initial voltages at the buses are obtained from AC load flow solution. The generator side the source strength considered to be and load side source strength is considered to be. s are simulated at buses,,,, and. Results for selected few cases are presented in Table II. The faults for these cases assumed to occur after two cycles (at. sec). a) Single Line to Ground : Fig., and gives the fault currents and fault voltages for single linetoground fault on phasea at sending end (bus ). The maximum peak value of the current in the faulty phase A is about per unit. The ratio of transient peak to steady state peak current in line is. when fault occurs very near to the sending end, i.e. at bus whereas it is. when fault is on bus Figure. current in line for AG fault on bus Figure. current in line for AG fault on bus Figure. voltage at bus and when AG fault on bus b) Double Line to Ground : The ratio of transient peak to steady state peak of the fault current in the line is. for BCG fault at bus whereas it is. for fault bus, as observed from Fig., and fault voltages at bus & with fault at bus as in Fig Figure. current in line for BCG fault on bus Figure. current in line for BCG fault on bus Figure. voltage at bus and for BCG fault on bus c) Three Phase to Ground : At t=. sec, a symmetrical fault is simulated at a distance km (bus ) from the sending end (Fig. 6, and ). Phase voltages decay practically to zero. Magnitudes and frequencies will depend on the fault location. The exponentially decreasing direct current offset components are noticeable in the phase currents, especially in i b and i c. d) Line to Line : The fault simulated at various lengths of the transmission line. Fig., and shows the currents and voltages when fault occurs at bus. The current of the unfaulted phase remains unchanged for line to line fault. The exponentially decreasing DC offset components are noticeable in the faulted phase currents Figure 6. current in line for φg fault on bus //$. Â IEEE

5 Third International Conference on Power Systems, Kharagpur, INDIA December >Paper #< Figure. current in line for φg fault on bus Figure. voltage at bus and when φg fault on bus Figure. current in line for AB fault on bus Sl. No..... Type SLG ag LLG bcg φg abcg LL ab TABLE II. bus SUMMURY OF FAULT TRANSIENT ANALYSIS Current for line in pu in the line Current in pu in the line.* *.* * * * * for line *.6 * results are shown in Figures also =ratio of Transient peak to steady state peak current ) Inception on the Voltage Wave: The AG fault time is varied from. sec. to. sec and the results are summarized in Table III. For the fault inception time. sec. at bus (Fig. ) the ratio of transient peak to steady state peak of the fault current in the line is. whereas for fault inception time. sec. at bus (Fig. ) it is.. For the fault inception time. sec. at bus (Fig. ) the ratio of transient peak to steady state peak of the fault current in the line is.6 whereas for fault inception time. sec. at bus (Fig. ) it is Figure. current in line for AB fault on bus Figure. current in line for AG fault on bus at. sec Figure. voltage at bus and when AB fault on bus Figure. current in line for AG fault on bus at. sec //$. Â IEEE

6 Third International Conference on Power Systems, Kharagpur, INDIA December >Paper #< From Table III it can be observed that peak fault current will depends on the fault inception time and as well as distance from source end Figure. current in line for AG fault on bus at. sec TABLE III. bus Inception Time (sec) SUMMARY OF FAULT INCEPTION ON THE VOLTAGE WAVE RESULTS (SLG) Current in line (p.u) for line Current in line (p.u) for line Fig...** Fig. Fig...** ** ** results are shown in Figures also V. CONCLUSIONS Switching transient and fault transients studies are carried out for 6 kv transmission systems. Transient analysis study for line energization case is done for 6 kv systems by varying the factors such as source strength, PIR, closing angle etc. To limit the switching over voltages both the systems required of source strength, with the source strength between and by taking certain precaution the line can be charged but the source strength with less than the line cannot be charged. Analyses are carried for various system parameters and results obtained are compared with the available literature. All types of fault analysis are carried out. It is observed that the wave shape of fault current significantly varying for the fault inception instant and also the peak magnitude of fault currents are significantly large in 6 kv systems as compared with kv systems. This indicates the importance of fault transients in selection of switchgear and protection schemes in 6 kv EHV transmission system. REFERENCES [] D Thukaram, EMTP, Fortran Version Developed at Department of Electrical Engineering, IISc, Bangalore, April. [] M.M. Adibi et al, Overvoltage Control During Restoration, IEEE Transaction on Power Systems, Vol., No., Nov, pp 6. [] D Thukaram, H.P. Khincha, Sulabh Khandelwal, Estimation of Switching Transient Peak Overvoltages During Transmission Line Energization using Artificial Neural Network, EPSR, 6(6), Sept., pp. 6. [] Abdullah S. Almid, Mohamed Mustafa Saied, A Method For The Computation Of Transients in Transmission Lines, IEEE Transactions on Power Delivery, Vol., No., January. [] Greenwood, "Electrical Transients in Power Systems", Wiley Inter Science,. [6] Jeyasurya, T.H. Vu, W.J. Smolinski, "Determination of Transient Apparent Impedances of ed Transmission Lines". IEEE Transactions on Power Apparatus and Systems, Vol. PAS, No., October, pp.. [] D Thukaram, B S Sharma, UPSEB Lucknow, et. al., Overvoltage studies for UPSEB 6kV ANPARAUNNAO line operated at kv, Technical Report, Second Workshop & Conference on EHV Technology, Bangalore, Aug,. //$. Â IEEE