Investigation of the no-load performance of a model three-phase three-limb laminated transformer core operating under sinusoidal and PWM voltage excitation

摘要

Transformers subjected to PWM voltage excitation are becoming more and more common in industrial applications and renewable energy supply systems. Therefore, the assessment and improvement in the performance of transformer cores under PWM voltage excitation have become prominent. This project intends to characterise the no-load performance, total power loss and acoustic noise level, of a model three-phase, three-limb laminated transformer core, operating under sinusoidal and PWM voltage excitation. Measurements of total power losses and localised flux density in the joint regions of the core under sinusoidal and PWM voltage excitation for assigned modulation index ma with switching frequency fs varied from 1 kHz to 3 kHz, has been carried out. The analysis highlights the form factor Kf of the secondary induced voltage as a key parameter in controlling the performance of eddy-current component loss in the core. The estimate localised rotational losses due to rotating flux and planar eddy-current losses due to normal flux density in the joints have been analysed, also the analysis of measurement results contributes towards a better understanding of the influence of ma and fs. Moreover, the acoustic noise level of the core and the corresponding vibration at investigated points on the core surface has been measured under sinusoidal and PWM voltage excitation. Measurement results show that the values of acoustic noise and core vibration under PWM voltage excitation were much higher than under corresponding sinusoidal voltage condition. Also, the magneto-mechanical resonant phenomenon of the core under PWM voltage excitation has been observed, which was due to switching frequency fs close to the resonance vibration frequency of the core laminations that is a possible cause of increasing acoustic noise. The measurement results inferred that the resonant phenomenon could possibly occur in cores with different length laminations leading to variability of noise output according to how close the magnetising frequency or predominant harmonics are to the resonant frequency.