Test Systems and Models for DC/DC Converters intended for DC Transmission Grid Applications


The paper reports on the development of two 600 MW test DC-DC converters in the CIGRE B4.76 working group. Both test converters are designed to fit in the DC-DC placeholders in the CIGRE DC Grid benchmark. The isolated DC-DC is designed to interconnect ±400 kV and ±200 kV DC systems while the non-isolated DC-DC is integrated in the ±400 kV DC system and would be used for power flow control and DC fault isolation.
It is concluded that the isolated DC-DC can be viewed as two MMC and a transformer for the initial cost/size/weight estimations. The test converter design is presented and all parameters are discussed. For practical reasons the operating frequency is selected at 150 Hz. The simulation results illustrate steady-state operation and a worst-case DC fault. The current on the unfaulted DC system drops to average zero in 10 ms and the peak value is only 2 pu, suggesting that this DC-DC has more favourable DC fault response than a DC circuit breaker.
The non-isolated DC-DC can be regarded as 1.1-1.3 pu for initial cost/size/weight estimations, comparing with a similarly rated HVDC MMC converter. The key design steps are illustrated and the EMT simulations confirm that all variables meet the design requirements in normal operation. A worst-case DC fault is shown, which confirms that all variables are within the tolerance range. The non-isolated DC-DC also blocks fault current and the DC current on the unfaulted DC system shows particularly favourable response. The unfaulted DC system sees rapid reduction of current to zero and there is practically no overshoot.


Presented at CIGRE 2022