DC Fault Performance of Diode Rectifier based MTDC without Offshore DC Breaker

Multi-terminal-direct-current (MTDC) and diode rectifier based HVDC (DR-HVDC) are economical and efficient power transmission solutions. The combination of them is promising for the integration of remote offshore wind farm (OWF). Generally, offshore DC breakers are required for DC fault isolation in a MTDC system, which adversely increases the cost significantly. This paper studies the operation of OWF connected to DR based MTDC without offshore DC breakers. A DC fault ride-through scheme is proposed, which uses offshore wind farm AC circuit breaker and DC switch to isolate the DC fault rather than offshore DC breaker. Besides, with the PLL based grid forming control and current limiting control, the offshore system behavior after the fault is studied. The validation of the proposed scheme, together with the control, is carried out through PSCAD/EMTDC simulations. The results show that seamless transition of island grid-forming mode and power transmission mode can be achieved without any control strategy switch.