Quality assessment of harmonic measurement data from a utility-scale PV plant

Abstract

Assessing compliance with harmonic emission limits for utility-scale photovoltaic plants requires accurate measurements of harmonic voltages and currents at the point of connection. This paper evaluates the accuracy, reliability and temporal characteristics of harmonic measurement data obtained from a utility-size photovoltaic plant located in the Northern Cape province of South Africa. Measured harmonic voltages and currents up to the 25th harmonic order, using 3-second and 10-minute averaging intervals, are compared using the standard deviation as reference. The measured harmonic voltage and current amplitudes are interpreted with the view to quantify the impact of quantization measurement error. Quantization errors for harmonic voltages are found to be large for many harmonic orders. The quantization errors for harmonic current measurements are smaller, but the low amplitudes observed for many harmonic orders may result in errors due to reduced accuracy of current transformers operating at such low currents and due to electromagnetic noise in the substation environment. It is shown, for the plant considered in this study, that voltage and current harmonic measurements using the standard measuring transducers and measuring instrumentation may not be sufficiently accurate for compliance evaluation or detailed harmonic studies of harmonics above the 15th order and for most even harmonics. Analysis of the 3-second aggregated harmonic quantities indicates that their fluctuation during each 10-minute period is sufficiently small that the 10-minute average values provide reasonable approximation of both the short-term and long-term behavior of the plant.