Portable measurement system for the frequency response of voltage transformers

Conventional inductive voltage transformers (VT) are widely used in MV, HV and EHV networks to measure harmonic voltages. Accuracy of VTs at frequencies other than nominal frequency (50Hz/60Hz) are not defined by standards, but may significant influence the overall accuracy of harmonic measurements. Thus a reliable compliance verification of compatibility levels, planning levels or emission limits (e.g. according to IEC 61000-2-12, 61000-3-6) may become difficult. Due to a lot of influencing factors (like manufacturing tolerances, burden or temperature) the frequency response varies more or less between different VTs. To achieve reliable results an onsite measurement of the frequency response is highly recommended. Nowadays the facilities for an accurate measurement of a VT's frequency response are limited to a few laboratories. The equipment is not portable in most cases. The paper describes the development of a first version of a portable measurement system for frequency responses of VTs for MV- and HV-networks. It is based on a widely used type of test generator together with especially developed software. The paper describes in detail the measurement method and discusses the accuracy limitations compared to the laboratory system.