Computer based short circuit harmonic simulation

Short circuit calculations provide information on currents and voltages on a power system during fault conditions. This information is required to design an adequate protective relaying system and to determine interrupting requirements for circuit breakers at each switching location. Relaying systems must recognize the existence of a fault and initiate circuit breaker operation to disconnect faulted facilities. Relays are designed to operate in a nondistorted environment, and they operate very well in that environment. During a period of four years, thirty microprocessor and electromechanical relays were tested with distorted voltage and current waveforms and their operation was recorded. When distortion was introduced, most of these relays misoperated. Existing short circuit studies consider only fundamental voltages and currents, but in this study harmonic frequencies were introduced to the system. Simulating the harmonic source as a current source with multiple frequencies represents the harmonics in the system. For this purpose a Short Circuit Harmonic Simulation program was developed. With this simulation software the user can vary the fault location, harmonic frequencies and phases. The program outputs the fundamental and the harmonic fault currents in the lines. The harmonics used in the prefault conditions are generated by specific nonlinear loads. Also a comprehensive testing system consisting of hardware and software was developed to run and simulate these faults in the lab.