Continuous operation of radial multi-terminal HVDC systems under DC fault

For a large multi-terminal HVDC system, it is important that a DC fault on a single branch does not cause significant disturbance to the operation of the healthy parts of the DC network. Some DC circuit breakers (DCCBs), e.g. mechanical type, have low cost and power loss, but have been considered unsuitable for DC fault protection and isolation in a multi-terminal HVDC system due to their long opening time. This paper proposes the use of additional DC passive components and novel converter control combined with mechanical DCCBs to ensure that the healthy DC network can continue to operate without disruption during a DC fault on one DC branch. Two circuit structures, using an additional DC reactor, and a reactor and capacitor combination, connected to the DC-link node in a radial HVDC system are proposed to ensure over-current risk at the converters connected to the healthy network is minimized before the isolation of the faulty branch by mechanical DCCBs. Active control of DC fault current by dynamically regulating the DC components of the converter arm voltages is proposed to further reduce the fault arm current. Simulation of a radial three-terminal HVDC system demonstrates the effectiveness of the proposed method.