Abstract:
The converter transformers are reported to fail generally on the thyristor-valve side, where complex alternating voltage
waveforms superimposed with dc voltages would occur. The effect
of these waveforms on the failure and endurance of the valve side
winding insulation of the transformers is not yet understood, which
is investigated here. In this paper, the voltage waveforms occurring
in a converter transformer are obtained through PSCAD/EMTDC
for a typical double pole ±500 kV, 1000 MW DC-link HVDC
transmission system based on the CIGRE benchmark system.
The waveforms are then amplified using a High Voltage amplifier
for experimental breakdown investigations. Stepped-stress damage
equalization method has been used for obtaining the lifetime characteristics from failure data. A comparison with life curve under
pure sinusoidal and actual alternating voltages with superimposed
dc and harmonic voltage has been made to understand why the
converter transformers generally failed on the valve side. The
results first time give fundamental reasons for the possible failure
of the converter transformers and put forth important design
considerations
