An alternative approach to analyze un-symmetrical faults in power systems

Abstract

In this paper an alternative method is provided to analyze typical un-symmetrical faults in a power system. It uses equivalent sequence current sources to represent the faulty point currents. Then superposition principle is introduced to decompose the three-phase power system with an un-symmetrical fault into three sequence networks with respective equivalent sequence current source connected. The advantage of this approach over conventional method is to make the analysis of three typical un-symmetrical faults, namely single-line-to-ground fault, line-to-line fault and double-line-to-ground fault more unified. That is, there is only one step different for analyzing these three un-symmetrical faults, which is to calculate three sequence components I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fa</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(1)</sup> , I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fa</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> and I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fa</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(0)</sup> of the phase-a faulty-point-to-ground fault current. Other steps are exactly the same. So it is unnecessary to cumbersomely connect three sequence networks when calculating the fault voltages at each bus and fault currents flowing from one bus to its neighboring bus. This alternative method also makes impedance matrix approach more understandable when used to compute sequence voltages at each bus.