SHORT CIRCUIT STUDY UNTRANSPOSED LINES

Abstract

Short circuit studies on large power system network had confronted Engineers and mathematicians with the necessity of solving number of equilibrium equations representing the performance of the system and the restrictions imposed on it. The availability of medium and large scale digital computers has provided a new facility for solving complex network problems. Digital computer method has the unique advantage in gaining speed, accuracy and cost in computation, as compared to other methods. Power line transposition was in practice primarily to reduce the interference on open wire communication circuits arid to limit the negative sequence currents. Short circuit studies on transposed line had become conventional. The use of modern distance relays and improved cooling system of rotating machines has reduced the significance of negative sequence currents. The increased use of lead covered telephone cables an better communication equipment has minimised interferences. As transmission lines are operated on higher and higher volt-ages and the lines being untransposed from ecnomy standpoint, the conventional short circuit analysis of network on single phase frame of reference would not be accurate and cannot be justified. The analysis must then be done on three phase frame of reference. The conventional short circuit studies hitherto adopted have to be refined on several aspects. These refinements which called for rather laborious long hand calculations can now be assigned to digital computers. The digital methods adopted 2 generally for fault studies can be either nodal iterative method or driving point and transfer impedance method. Tech- niques are available to model a multiwinding transformer with off nominal turn ratios, with minimised number of nodes and branch impedances. While Generator and transformer constants can be obtained by tests line constants have to be computed. In the computation of line constants corrections might have to be applied for the skin effect and earth return effect. In the conventional short circuit studies resistances were neglected and pre-fault current assumed as zero. In order to truly represent the performance of the network on faulted condition the load currents might have to be considered. The effect of mutuals which were hitherto neglected on grounds of theoretical transposition, has to be introduced in the determination of fault currents in untransposed lines. A digital computer programme for the solution of short circuit analysis has been developed and applied to the Mysore Power System. In this study complex quantities of net-work constants have been used. Exhaustive information like the fault current, fault voltage, and fault impedance in complex form for all the major busses in the Myso€e Power System tied to Madras Grid - has been obtained.