One or several thyristors may carry no current. See also F517.
Check:
− The values of 01.50 Current ripple and 01.51 Current ripple
filtered1.
− The setting of 31.46 Current ripple function and 31.47 Current
ripple level.
− For too high gain of current controller. See 27.29 M1 current
proportional gain.
− The positive/negative current feedback with an oscilloscope (6
pulses within one cycle visible?).
− The thyristor gate-cathode resistance.
− The thyristor gate connection.
− The current transformers (T51, T52).
− The condition of the mains (voltage, cabling, fuses,
switchgear).