Effect Analysis of Unbalanced Input Voltage on Diode Open-Circuit in 12-Pulse Transformer Rectifier Units

Unbalanced input voltage can significantly affect the electrical behavior of transformer rectifier units (TRUs), especially when a diode open-circuit (OC) fault breaks the original diode conduction symmetry. However, the effect of unbalanced input voltage on the diode OC fault has not been sufficiently clarified from the perspective of a conduction mechanism. This paper analyzes the effect of unbalanced input voltage on diode OC faults in TRUs by establishing a conduction-oriented mechanism. Unbalanced input voltage is divided into two forms, namely, unequal magnitude and phase-shift deviation. The effects on diode conduction boundaries, conduction angles, and conduction intervals are first derived theoretically. Then, using a 12-pulse TRU with D11 and D21 OC faults as representative cases, current and voltage responses are investigated in both time and frequency domains. The experimental results show that the two forms change diode conduction intervals in different ways. In particular, an unequal magnitude changes the relative driving voltage dominance near the conduction boundaries, resulting in the stretching or compression of diode conduction intervals; phase-shift deviation shifts the angular positions of the driving voltages and modifies the commutation timing. Two forms further aggravate waveform asymmetry and enhance low-order and non-characteristic harmonics under diode OC fault conditions. This effect analysis provides a more comprehensive basis for understanding diode OC fault responses in 12-pulse TRUs and supports the development of more robust diode OC fault diagnosis methods under non-ideal input voltage conditions.