Fault detection, discrimination, and recovery in multi-terminal HVDC transmission systems

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Mohamed Haleem, Naushath
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Multi-terminal high voltage direct current (MT-HVDC) grids enable integration of large-scale renewable energy resources and facilitate flexible bulk power transfer for energy markets extending over political boundaries. Preserving the integrity of MT-HVDC grids during DC faults remains a major challenge, primarily due to the lack of effective DC circuit breakers (DCCBs) capable of interrupting the expected fault currents. These DCCB limitations and stringent reliability requirements mandate identification of faulted transmission lines and the faulty conductors at extreme speeds with highly sensitivity. Two techniques were developed to improve the sensitivity and the reliability of fault discrimination while meeting these speed requirements. The first technique introduces directional properties for line and bus protection algorithms that rely on the rate of change of local voltage measurements to improve the fault discrimination. The second technique uses locally measured conductor currents to quickly identify the fault type and faulted conductors. This algorithm makes the decisions based on the ratios of the rate of change of currents computed considering a pair of conductors at a time, and therefore, independent of the fault resistance. Versatility and reliability of the proposed fault type discrimination algorithm was demonstrated by applying it to different transmission configurations. Fault recovery aspects of a novel class of hybrid LCC-VSC MT-HVDC transmission systems in which a number of VSC inverters and rectifiers are connected to an LCC HVDC link was investigated. Two possible fault clearing schemes were proposed. The first approach avoids DCCBs, employs series-connected high power diodes at VSC inverter terminals to block the fault current contributions, and clears faults by de-energizing VSC rectifiers and applying force retardation to LCCs. The second approach utilizes DCCBs installed on the branch lines. These DCCBs activated by the proposed fault discrimination schemes minimize the disruption of power flow through the LCC HVDC link due to faults on the lines branched out to connect VSCs. The capability, speed, and sensitivity of each fault clearing scheme were evaluated considering practical designs.
HVDC transmission, Multi-terminal HVDC grids, MT-HVDC grid protection, HVDC transmission fault type discrimination, Hybrid LCC-VSC HVDC transmission systems, Mt-HVDC fault recovery