Calculation of unbalances in network shunt capacitances of power distribution systems

In resonant grounded power distribution systems (RGPDSs), the fault detection sensitivity relies on the unbalance current which is the result of unbalance in shunt capacitances of distribution lines. Therefore, it is required to maintain the balance in shunt capacitances where the first step is to calculate the unbalance in shunt capacitances for individual phase. This paper presents two different ways to calculate the unbalance in network shunt capacitances of different phases in power distribution networks. In the first method, the relationship between angles of phase-to-ground voltages and residual current (unbalance current) is used to calculate unbalance parts of the per phase network shunt capacitances. This method is effective for the solidly grounded, neutral earth resistor (NER) grounded and RGPDSs with a lower percentage of resonance. The second method modifies the first method considering the equivalent T-model of the power distribution line. The second method has the capability to calculate unbalances in network shunt capacitances more accurately (than the first method) for RGPDSs with a higher percentage of resonance. The developed methods are tested through simulation in MATLAB/SimpowerSystems. Simulation results indicate that the unbalance currents can be minimised to a small value using a three-phase capacitor bank where the values of the capacitor bank are selected based on the calculated unbalance using developed methods.