A quantitative analysis of indices to assess voltage quality on an electricity transmission network

M.Phil.Nearly every reference document, national and international standard, text book or web page discussing the topic of Power Quality has an introduction that refers to the demand from customers and regulators for better and more detailed reporting from electrical power utilities with regards power quality. Previously power quality was an internal utility measure with no external input. Today contracts are negotiated with customers and limits are set by regulators with regards Power Quality objectives. Current assessment methods defined in various international guidelines and standards recommend that for a “high percentage” of the assessment period, the measured performance must remain below specified levels (“compatibility levels” or contracted levels). In the case of NRS048-2:2008 the assessment criterion for voltage harmonics and voltage unbalance is based on 95% of the time and 95% of the space for an assessment period of 1 week, and that for voltage magnitude is based on 95% of the time and 95% of the space, with the additional requirement that no two consecutive values exceed the specified levels. A statistical research study was undertaken to analyse the performance of the Eskom transmission system in order to determine the impact of using different assessment methods (100% of the week, 99% of the week, 95% of the day etc). This thesis will present the results of the analysis done on the data in the Quality of Supply database. The analysis will mainly focus on the voltage waveform parameters: harmonic THD; unbalance; and regulation as measured in the Eskom Quality of Supply database. The objective of this thesis is to determine how the current performance of an electrical transmission network is characterised by different assessment methods. This thesis makes a contribution to the current international debate on appropriate assessment criteria and the conditions (“normal” vs. “abnormal”) under which these would apply