Fault Discrimination in HVDC Grids with Reduced Use of HVDC Circuit Breakers

HVDC grids are currently considered to accommodate the integration of renewable energy sources into the power system. Since HVDC circuit breakers might at present incur a large investment cost, fault clearing strategies with limited use of HVDC circuit breakers have been recently proposed. These strategies, here called partially selective, split the grid into a number of protection zones which encompass sub-grids of multiple converters, transmission lines or a combination of both. Although the application of such strategies within the HVDC grid has been studied recently, discrimination of faults between zones is not yet widely investigated. This paper analyzes the application of a communication-less protection algorithm introduced for fully selective protection in a partially selective protection strategy. A sensitivity analysis using a reduced model shows that the reach of these algorithms is determined by, on the one hand, the length of the cables and complexity of the grid within a protection zone and, on the other hand, the inductance of the series inductor associated with the breaker between the protection zones. A test case implemented in EMT-type software is used to demonstrate the fault clearing sequence in a partially selective strategy.