Contributions to Analytical Methodologies for the Management of Voltage Unbalance in MV/HV/EHV Power Systems

Voltage unbalance (VU) in power systems is a power quality problem of significant concern where its impact on both utility and end user equipment are well known. In spite of the associated adverse impacts, network operators often find it difficult to completely eliminate VU due to some of its unique attributes. Considering two major contributors to VU, it is difficult to keep the loading levels across the three phases balanced at all times in addition to eliminating the contributions made by the asymmetry associated with transmission and distribution lines. Other than the nature of the VU sources which makes the voltage unbalance management exercise onerous, associated network wide behaviour sophisticates the attribution of the responsibilities of excessive VU levels to the network wide dispersed individual sources. Moreover, the transformation of once passive networks into active networks has added new dimensions to the development of understanding and practices pertaining to VU management. The development of well researched engineering practices to understand, manage and mitigate VU can be found in relevant research and in International Electrotechnical Commission (IEC) treatise. The main objective of the current practice is to utilise the VU absorption capacity of power systems by allocating VU emission limits for the individual installations based on their capacity while maintaining stipulated network planning levels. In doing so, the measurements at the point of evaluation (POE) have been the major inputs which hardly capture the network wide behaviour of VU where the associated complexity is greater in interconnected networks. Moreover, in emission assessment which aims at attributing the responsibilities to the individual contributors, also mainly focuses at the measurements at the POE.