Research On Fault Current-limiting For MMC-HVDC Grid

The flexible DC power grid(MMC-HVDC)based on the Modular Multilevel Converter(MMC)has a huge advantage for solving the large-scale consumption of new energy,and becomes the future development trend of the power grid.The emergence of DC circuit breakers provides a reliable means for fault isolation of flexible DC power grids.Compared with AC systems,DC systems do not have mechanical inertia.Once a fault occurs on the DC side,the current will rise to more than tens of times the rated current,which is extremely susceptible to damage to expensive electrical equipment,especially damage to overcurrent-sensitive power electronic devices.Seriously,it may even cause a long-term outage of the entire power grid.Because the DC current does not have zero-crossings,the DC circuit breaker must break a huge short-circuit current,which depends on the series and parallel connection of a large number of power electronic devices,which makes the cost high,and its economy decreases exponentially with the increase of the breaking capacity.Suppressing the excessively rapid rise of the fault current and reducing the breaking capacity requirements of DC circuit breakers is a serious challenge facing MMC-HVDC.The use of a current limiter to effectively limit the rise of fault current and ensure that the short circuit current of MMC-HVDC is within the allowable breaking capacity of the DC circuit breaker is a feasible solution to the fault isolation of MMC-HVDC.This paper presents two hybrid HVDC fault current limiter topologies,and studies its topology,current limiting and exit principles,coordination and coordination scheme with DC circuit breaker,and the energy consumption process of the arrester when cooperating with current limiting.Contents and achievements are as follows:1)A topology of RLC hybrid HVDC fault current limiter is proposed.This topology uses the same current branch as the hybrid DC circuit breaker.It consists of a low-cost mechanical switch and a small number of insulated gate bipolar transistors(IGBTs).It has the characteristics of low on-state losses.Each branch of the main current limiter realizes the current transfer between the branches through the lower cost thyristor and the commutation capacitor,so that the current limiting reactance and the parallel impedance are finally connected in series in the fault loop.The current limiting action flow and exit plan of the current limiter topology are proposed,and the voltage stress and energy transfer in the current limiting process are theoretically analyzed.2)In the internationally renowned power system transient simulation platform PSCAD/EMTDC,a ±500k V four-terminal flexible DC power grid PSCAD/EMTDC simulation model was established,and an RLC-type hybrid HVDC fault current limiter was created.Through a large number of simulation experiments,the effectiveness of the current limiter topology current limit is verified.3)In order to further speed up the time for the DC circuit breaker to remove the fault line,on the basis of the RLC type hybrid DC fault current limiter,an energy absorption enhanced RLC type hybrid high voltage DC current limiter topology is proposed,the current limiter increases The energy-absorbing branch circuit can absorb the energy stored in the DC reactor during the breaking process of the circuit breaker,thereby reducing the energy consumption of the arrester.The energy-absorbing branch utilizes the idle diodes in the current-limiting branch of the current limiter.Under the premise of only a small cost increase,the performance of the current limiter is greatly improved,and the operating conditions of the lightning arrester in the DC circuit breaker are effectively reduced.The effectiveness of the current limiter topology is verified in the simulation model of ±500k V four-terminal flexible DC power grid.4)A recovery scheme for the current limiter after fault removal,which is suitable for the current limiter mentioned in this paper,and a reclosing scheme for the DC circuit breaker under the current limiter scenario are proposed.The simulation analysis verifies the feasibility of the current limiter recovery scheme and the DC circuit breaker coincidence scheme under both transient and permanent faults.