Effects of HVDC connection for offshore wind turbines on AC grid protection

When a transmission line close to points of common coupling (PCCs) encounters a short circuit (SC), the resulting PCC voltage dip triggers fast reactive power control of the corresponding grid side voltage source converter (GSVSC) to boost the PCC voltage. The control action can cause the fault distance to be overestimated by its backup relay located on the adjacent line. It is possible for a Zone 2 fault to be viewed as a Zone 3 event, resulting in mis-coordination between protective relays. Numerical simulations demonstrate the effect of HVDC offshore wind network on distance protection of an ac grid. On the other hand, HVDC reactive power adjustment can increase the stability margin of onshore ac grids, as shown by contingency simulations. With the addition of HVDC-connected offshore wind turbines, the voltage source converter based HVDC (VSC-HVDC) control can function as an element of the overall power system defense plan to prevent system instability, reducing or avoiding the implementation of the last resort remedial option - load shedding.