Influence of Ground Impedance on Unbalanced Analysis of Grid

Renewable energy utilization and projects are being used as green energy initiatives worldwide. There are smallest possible renewable energy sources of the order of few KW to large wind mill and PV farms of the capacity of several 100 MW in India. Current systems are hybrid systems having combination of AC, DC, single phase, 3 phase, grounded and ungrounded systems with significant converters for interface with AC grid. Under these conditions the assumption that the transmission, distribution and loads are balanced is perhaps not acceptable. In this respect the influence of ground impedance modeling or more specifically the intentional impedance grounding of the neutral of the 3 phase network must be considered in the analysis. A common assumption made is “Ground potential is always at zero (0)” and neutral voltage can be eliminated in the formulation of analysis. IEEE standards however specify that ground resistance is neither zero nor uniform. Mathematical models are needed that consider the ground impedances and non zero neutral potentials of star connected windings. In this article hypothetical grounding impedances are considered and its influence on power system balanced and unbalanced operation are computed for static performance and analyzed. The outcome of the analysis shows that it is not correct to ignore ground impedance for unbalanced analysis, nor it is correct to assume neutral voltage as ‘0’ in star connected systems.