Implementation of Low-Cost High-Performance Generators for Testing the Harmonic Measurement Accuracy of Instrument Transformers

A proper characterization of instrument transformers requires waveform generators able to apply realistic periodic voltages and currents, resembling those typically found in distribution grids. This article proposes a simple approach for dramatically improving the performance of generators based on the usual, low-cost architecture consisting of a power amplifier and a coupling transformer, which enables reaching the required voltage and current levels. The method is based on iterative frequency-domain error feedback, with feedback gain set according to a preliminary frequency response measurement of the open-loop generation system. The theoretical analysis demonstrates that the asymptotic generation error depends on the adopted reference transducer, on the disturbance level, but not on the characteristics of the generation system. This feature thus enables reaching high generation accuracy without using an overdesigned coupling transformer. The proposed approach has been adopted for the implementation of a high current and a medium voltage generator. The experimental results confirm the effectiveness of the frequency-domain feedback method that in both the cases allows for a remarkable accuracy improvement.