Influence of voltage harmonics on transformer no-load loss measurements and calculation of magnetization curves

This paper investigates the voltage distortion phenomenon during no-load testing of transformers, its influence on no-load loss calculations, and a novel measurement technique that is unaffected by harmonics for obtaining transformer magnetization curves. Owing to economic reasons, no-load loss correction for distorted test waveforms has been addressed to some extent in existing standards, while not much has been done on the aspect of calculation of magnetization characteristics. This is important for parameter estimation and transformer modeling in power system transient simulations. Results from two test cases are presented, one is based on factory testing with 290-MVA, three-phase, three-legged power transformer and second is a more comprehensive analysis using a laboratory test setup with a 22-kVA single-phase transformer. Application of loss correction equations and limits based on existing testing standards was evaluated and found to result in overcompensation, while calculation of magnetization curves based on existing methods resulted in error up to 20%. Whereas, application of proposed measurement technique based on voltage “Zero-Crossing” detection is shown to result in negligible error. The proposed measurement technique uses the same input signals as in a standard no-load test procedure. Hence, it can be easily implemented in parallel with existing instrumentation.