{'fraction_non_alphanumeric': 0.036237471087124135, 'fraction_numerical': 0.013878180416345412, 'mean_word_length': 4.794642857142857, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
A Low-Cost DC Chopper With Coupling Transformer for Offshore Wind VSC-HVdc System

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.