Voltage unbalance emission assessment in radial power systems

Voltage unbalance (VU) emission assessment is an integral part in the VU management process where loads are allocated a portion of the unbalance absorption capacity of the power system. The International Electrotechnical Commission Report IEC/TR 61000-3-13:2008 prescribes a VU emission allocation methodology establishing the fact that the VU can arise at the point of common connection (PCC) due to both upstream network unbalance and load unbalance. Although this is the case for emission allocation, approaches for post connection emission assessment do not exist except for cases where the load is the only contributor to the VU at the PCC. Such assessment methods require separation of the post connection VU emission level into its constituent parts. In developing suitable methodologies for this purpose, the pre- and post-connection data requirements need to be given due consideration to ensure that such data can be easily established. This paper presents systematic, theoretical bases which can be used to assess the individual VU emission contributions made by the upstream source, asymmetrical line and the load for a radial power system. The methodology covers different load configurations including induction motors. Assessments obtained employing the theoretical bases on the study system were verified by using unbalanced load flow analysis in MATLAB and using DIgSILENT PowerFactory software.