Three-phase unbalance of distribution systems: Complementary analysis and experimental case study

Three-phase unbalance is a familiar issue for power system researchers and engineers. This can introduce additional power losses in distribution network in steady states due to both negative and zero sequence components. It could also limit the loading capability of distribution transformers, well below their nominal ratings. There are many voltage and current unbalance definitions (e.g., IEEE and NEMA) for three-phase three-wire systems, assuming zero sequence currents to be of negligible practical value, for they cannot flow in three-wire systems. However, the zero sequence unbalance has significant current magnitude in three-phase with four-wire distribution networks, particularly in developing countries. Hence, this paper concentrates on the distribution unbalance, completing the available definitions in order to maintain tangible relationships between the level of unbalance and the cited consequences in distribution networks. Furthermore, practical works were performed on 11 selected 20 kV/0.4 kV substations within Tehran North-West Distribution System (TNWDS), where data loggers have been installed for 7 days to measure and record operating conditions of substations. Then, detailed analysis and assessment are suggested on empirical data to substantiate the presented complementary definitions and relationships.

Research highlights
► Unbalance definitions are critically examined both for the voltages and currents.
► Zero sequence components are considered in order to complete available unbalance definitions for applying to distribution systems.
► Power losses and unbalance of distribution systems are related.
► It is also related the unused capacity of distribution transformers to unbalance operation of distribution substations.
► Eleven distribution substations were monitored in Tehran for seven days in order to evaluate the suggested definitions as well as relationships.