Instrument transformers influence on harmonic measurements for grid code compliance

Abstract

With an increased penetration of renewable energy sources in South Africa, especially photovoltaic (PV) and wind farm generating plants, power quality is becoming more important for utilities. With that also the accuracy of the measurements involved. For independent power producers (IPPs), these measurements are used to prove compliance to the national grid codes. Inaccurate results could prove devastating for an IPP and the predicted revenue to be generated to recover capital expenditure. One of the many electro-magnetic compatibility factors to be considered in this process of proving compliance, is the harmonic emissions of these plants. Instrument transformers form a key part of the harmonic measuring circuit. It is therefore important to ensure that the frequency response of these instrument transformers and the errors involved are quantified, in order to reduce the uncertainty in measurements. This paper considers literature of studies conducted on both current transformers and voltage transformers in this field. These studies show that significant errors, both in amplitude and phase angle, are involved when using instrument transformers for harmonic measurements. However, no mention is made on how the measuring errors can be taken into account during the characterization of these transformers. This paper focused on the aspects to be considered when quantifying these errors in measurements. A conclusion is drawn as to the important aspects to consider when developing a method to characterize and compensate for measuring uncertainties when instrument transformers are used for harmonic measurements, especially in renewable energy plant applications.