A Low-Cost DC Chopper With Coupling Transformer for Offshore Wind VSC-HVdc System
Lei Qi, Sihang Wu, Xiangyu Zhang, Meng Zhang, Chengzhu Liu, Hongbin Wang, Long Wu
Abstract
The dc chopper (DCC) is the key equipment of offshore wind voltage source converter based high-voltage direct current (VSC-HVdc) system. Combining the advantages of the ac chopper and the centralized DCC, a novel dc chopper with coupling transformer (CT-DCC) is proposed. The use of the coupling transformer can reduce the voltage on the concentrated resistor to eliminate the high-voltage wall bushing effectively. Compared with the traditional DCC, the cost of CT-DCC is reduced by 35% for ±400 kV/1100 MW offshore wind VSC-HVdc project. In addition, the application of the coupling transformer provides an opportunity for the introduction of freewheeling inductance, which greatly eliminates the interference of pulse-current generated by the existing DCC on dc-link voltage. Through system simulation, the peak value of the dc-link voltage during the fault ride through is 1.02 p.u., and the dc-link voltage ripple is only 1%–2%, which meets the system requirements. After the detailed analysis of the working principle and characteristics of CT-DCC, the control strategy and design principle are given. Furthermore, a prototype experiment of up to 10 kV/10 MW is conducted to verify the feasibility of CT-DCC.