Calculation and Verification of Harmonic Magnetic Loss of Transformer Core Based on Improved Bertotti Model

In recent years, owing to the development of power electronics and the high-voltage direct current(HVDC)transmission system, harmonic pollution has attracted considerable attention.Simultaneously, the emergence of harmonic pollution also poses a major challenge to the efficient operation of the HVDC transmission system.As one of the core pieces of equipment in the transmission system, the converter transformer has problems of partial overheating and the increasing loss and reduced service life of its structural parts, which also results in a large waste of power.Therefore, it is particularly important to accurately calculate the loss experienced by the converter transformer under harmonic conditions.To calculate the transformer magnetic loss under harmonic excitation, herein, we improve the traditional Bertotti model by adapting it to calculate the loss experienced under complex harmonic conditions.We also design and construct a transformer laminated core model based on the requirements of the actual transformer and build a platform for testing the transformer magnetic loss under harmonic excitation.These changes improve the correctness and accuracy of the traditional Bertotti model.In addition, we investigated the distributions of hysteresis loss and eddy current loss under harmonic excitation.First, based on the simplified Bertotti model, we obtain data on the transformer magnetic loss separation under sinusoidal excitation using core process coefficients.By determining the hysteresis and eddy current losses at different frequencies, we found that hysteresis loss accounts for the majority of the transformer magnetic loss at low frequencies.With increases in frequency, the proportion of hysteresis loss decreases, and the eddy current loss begins to occupy the dominant position.Secondly, the correction factor is introduced.Considering the influences of the harmonic phase, harmonic content, and harmonic order, we correct the hysteresis loss and eddy current loss to realize the calculation of magnetic loss under harmonic excitation.A comparison and analysis of the measurement and calculation results shows that the loss error is within 5%, which meets the engineering accuracy requirement and verifies the correctness of the model.The results and conclusions are helpful to the study of magnetic loss under harmonic excitation and provide the data support necessary for the optimal design of transformers and the improvement of product performance. © 2019, Editorial Board of Journal of Tianjin University(Science and Technology). All right reserved.