Influence of diode commutation processes on rectifier transformers operation

To obtain rectified voltage needed for d.c. traction networks, different types of energy transformation circuits may be used. This paper deals with 6- and 24-pulse transformer-rectified set operation under different supply condition. Commutation issues have been investigated for Yd transformer supplied with sinusoidal and distorted voltage. It has been shown that supply voltage THD factor affecting rectified voltage quality is not sufficient to examine voltage transformation extensively; phase angles of the harmonics have been researched as well and their impact on commutation processes presented. For 24-pulse system it has been shown that transformer windings connection scheme giving the desired phase shifts between secondary side voltages is not sufficient to achieve 24 pulses in rectified voltage. Due to commutation processes in uncontrolled diode bridges, this system produces 6 pulse output voltage only. In order to modernise traction substation, simple switchover from 3-phase to 12-phase transformer with presented windings connection is not, therefore, possible.