A Novel Insulation Monitoring Technique for Converter Transformers using Common-Mode Characteristic Harmonics of VSCs

Converter transformers demand a high standard of insulation design and monitoring due to high frequency electrical stress. Traditional monitoring techniques for power transformers generally suffer from their invasive nature, and may lack sensitivity when applied to converter transformers. Effective and non-invasive insulation monitoring technique is critical for converter transformers. This paper proposes a novel online insulation monitoring technique for converter transformers, which is based on the inherent common-mode characteristic harmonics (CMCH) generated by the power switches. Two transfer functions (TF) are introduced using the measured voltages and currents at the neutral ends. Insulation ageing condition can be evaluated according to the performances of two TFs at the characteristic frequencies. Theoretical analysis, model simulation and experimental results are presented to validate the proposed technique. The importance of the proposed technique lies in its non-invasiveness and simple implementation.