Unsymetrical fault calculations of BPDB power network

Abstract

Short-circuit studies provide the values of currents and voltages in a power system network under the fault conditions. On the basis of results of these studies, adequate protective systems containing relays and circuit breakers can be designed; Most of the faults that occur on a power system are unsymmetrical faults and causes unbalanced currents to flow in the system. The ratings of the circuit breakers are to be determined for the severest duty and consequently unsymmetrical fault occupies a place of importance in short-circuit calculations, because this type of fault often result in the highest value of the fault current. In this work all important tJ~es of unsJ~metrical faults, such as, single line-to-ground fault, double line-to-ground fault and line-to-line fault together with three-phase-toground fault calculations have been the subjects of investigation. Both the existing BPDB system and the system planned and proposed to be completed by June, 1990 are studied to obtain sufficient short-circuit data for future use. An efficient and time saving Dlothod of short-circuit study using the ~usmatrix has been used here. A computer program has been developed using the mathematical formulae developed for different types of fault conditions. The program has been used to compute the currents, voltages and the fault MVA at any bus for any type of fault. Provisions for opening any line from the faulted bus is also provided. It is observed from the results of the fault calculations that the fault MVA is the highest at a number of buses for single line-to-ground fault whereas at some other buses the fault level is the highest for three phase -to-ground fault. Also at a few number of buses maximum fault MVA is observed for double line-to-ground fault. The post-fault bus voltages at a number of buses are found to increase above the pre-fault voltage for double line-to-ground fault. The maximum fault level is 4343 MVA at Ashuganj-132KV bus of the existing system and that for the 1990 system is 3560MVA at Ghorasal-230KV bus. The procedure for selection of the MVA ratings of circuit breakers on the basis of the short-circuit data of the system has been presented. It has.been observed that at some location of BPDB Power Network,circuit breakers of 5000MVA rating have been used. The MVA rating can effectively handle the short-circuit current at those location. On the other hand, circuit breakers with lower MVA rating could have been used at some locations. rhis would have cut down the cost of the switchgear. resulting in considrable savings. The results of the short-circuit study can be effectively used for checking the ratings of the circuit breakers and for possible modification of the protective system of the existing power network and will provide the engineers with useful information for planning and design of future extension of the BPDB network.