Research on Hybrid Ultra High Voltage DC Transformer for All-DC Transmission of Renewable Energy Bases

[Objective] To reduce the investment cost of the ultra high voltage DC transformer （UHVDCT） and promote its application in the transmission of large-scale renewable energy bases，a hybrid UHVDCT topology is proposed and a corresponding control strategy is designed. [Methods] The proposed topology is based on the concept of hybrid converters，with improvements made to the classic UHVDCT topology formed by face-to-face connections of the AC side of modular multilevel converters （MMCs）. The ultra high voltage side is modified from full-capacity MMCs to a parallel configuration of high-capacity line commutated converters （LCCs） and low-capacity MMCs，while the high voltage side remains composed of MMCs. [Results] In operations of the proposed topology，LCCs on the ultra high voltage side can undertake all active power transmission on this side，while MMCs on the ultra high voltage side operate in V/f control mode，providing voltage and frequency references for the AC links in the UHVDCT，and absorbing the harmonic currents generated by LCC through active power filter control. MMCs on the high voltage side operate in constant DC voltage control mode，maintaining the DC voltage within the renewable energy base connected to the UHVDCT. They can also enable the dynamic reactive power balance within the UHVDCT. The results of simulation based on PSCAD/EMTDC show that the proposed topology demonstrates a good performance in both steady-state and failure conditions. [Conclusions] The proposed topology fully combines the advantages of LCC and MMC，significantly reducing the capacity requirements for the expensive MMC. Taking a UHVDCT with a rated capacity of 10，000 MW as an example，the manufacturer’s quotation shows that the total investment cost can be reduced by approximately RMB 436 million compared to the classic topology，indicating a significant improvement in the economic efficiency.