A Novel Protection Strategy for Internal Faults for HVDC Connected MVDC Collection Grids for Wind Power Plants

One of the main addressed problems within a wind farm with a DC collection grid is to achieve an acceptable protection scheme. This report proposes a novel strategy, which are compared with existing strategies in terms of design and operation. The proposed strategy has only DC breakers at the DC/DC platform. At all other positions AC breakers are used as fast disconnectors. The fault handling strategy is based on a fast de-energizing of the collection grid by the DC/DC converter, followed by fault clearance by the AC breakers. When the fault is clear the system will be energized again. The detection and location algorithms are based on differential measurements and communication between the different terminals. The novel strategy is verified in a test model of a DC collection grid in EMTDC/PSCAD based on an unsymmetrical bipole system and where the turbines are connected in rings. The novel strategy will result in longer interruptions times at cable faults but the total power production will return to its full capacity. The energy loss during a cable fault is estimated to only 15 kWh for a wind farm with an installed power of 320 MW. Since no DC breakers are needed in the turbines all required components are already available which means that the strategy can be implemented and used today.