Evaluation of assessment methods used to analyse measured power quality performance within the South African context

Various assessment methods are used for analysing power quality data when reporting electrical transmission system performance. There is international debate as to which of these assessment methods is the best. A statistical analysis of measured voltage harmonic and unbalance data for the Eskom transmission network was undertaken to investigate the implication of using the various assessment methods - including the maximum 10-min value, the 95% and 99% weekly methods, and the 95% and 99% daily methods. The 95% weekly assessment method is currently being used within Eskom (as well as standards such as EN 50160). The values obtained from the various assessment methods were compared against the corresponding maximum 10-minute values and against each other. The results show significantly higher 10-minute maximum values than the corresponding 95% or 99% values for a given week.