The medium voltage direct current (MVDC) technology is emerging in power distribution and collection after the high voltage direct current (HVDC) has been demonstrated and exploited as the economically viable solution for long-distance transmission of bulk power. This article focuses on the DC-DC converter for MVDC electrical networks. A classification of DC-DC converters is proposed according to three criteria: power flow directionality, galvanic isolation and modularity. A 20 MW ±10 kV radial MVDC network is defined. Three unidirectional step up, isolated and monolithic DC-DC converter topologies (phase-shifted full bridge – PSFB, single active bridge – SAB and SAB with output inductor – SAB-L) are discussed focusing on the output filter design (LC, C or CL). They all offer full power controllability and fault blocking capability in case of faults in low voltage direct current (LVDC) and MVDC systems. A simulation model of the case study MVDC network and the DC-DC converters is presented. Simulation results are presented in steady state showing the typical voltage and current waveforms of the selected DC-DC converters. Six fault scenarios are defined including pole-to-pole and pole-to-ground faults. Simulation results are presented focusing on the fault current in the diode rectifier bridge. The most severe fault case is the pole-to-pole fault at the DC-DC converter terminals. The MVDC voltage polarity reversal occurs, forward biasing the rectifier bridge diodes. The pole-to-ground fault results in low fault current due the high value grounding resistance. The PSFB is observed to offer the best performance in steady state and fault conditions. The PSFB output filter inductor fulfils the role of current ripple filtering and fault current limitation.
Piotr DWORAKOWSKI, Pierre LE METAYER, Drazen DUJIC, Cyril BUTTAY
Presented at CIGRE 2022