Experimental Validation and Review of Neutral Voltage Modeling for Unbalanced System Analysis

Abstract

Present power networks are hybrid systems with significant converter loads and converter interfaces. The generation and load have both AC and DC power components. Further converter interface can be 3 phase AC/DC or single phase AC/DC, with either balanced or unbalanced AC. Additionally distributed generation penetration is becoming prominent. It is also common practice to use both grounded and ungrounded 3 phase AC systems. In this context, the paper reviews the assumption of ignoring the neutral voltage as zero, in unbalanced three phase system analysis using Kron’s reduction technique. Many published papers in the unbalanced three phase analysis of AC system have typically ignored neutral voltage based on the assumption that the neutral voltage is with multiple grounding of earth wires. However, several IEEE standards state clearly that earth resistivity is not uniform and Ground potential is not uniformly zero. Further, it is common practice to ground the neutral point of electrical equipments through intentional impedance to limit the single line to ground short circuit current through the neutral. Under these conditions, the assumption that the transmission, distribution and loads are balanced with neutral voltage 0 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 three phase network must be considered in the analysis. Mathematical models are needed that consider the ground impedances and non zero neutral potentials of star connected windings [three phase, 4 wire systems]. In this paper hypothetical grounding impedances are considered and its influence on power system balanced and unbalanced operation are computed for static performance and analyzed. The suggestion that neutral voltage must be explicitly modeled in unbalanced calculations is further supported from experimental results, which are described. 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. The paper also indicates a method of converting symmetrical component model of a general 3 phase system to equivalent 3 phase 4 wire system for possible unified solution of 3 phase 3 wire and 3 phase 4 wire systems with or without ground connections.