Research On Fault Analysis And Fault Clearing Technology For MMC-based High-Voltage DC Grids

The high voltage DC(HVDC)grid technology,which is based on the modular multilevel converter(MMC),has broad application prospects in renewable energy consumption,asynchronous grids interconnection and power supply of island grids;thus,it has attracted considerable attentions.However,after a DC short-circuit fault occurs in the HVDC grid,a large number of sub-module capacitors of MMC rapidly discharge,resulting in a rather large fault current.If the fault is not cleared in time,the grid equipments may be damaged by overcurrents,and the HVDC grid has a risk of outage.Therefore,the DC fault clearing technology is very important for the safe and stable operations of HVDC grids.This paper focuses on the fault clearing technology of HVDC grids,and the research is divided into the short-circuit fault calculation of HVDC grids,topology designs of DC circuit breakers and adaptive reclosing strategy of DC circuit breakers(DCCBs).The main contributions of this paper are summarized as following:(1)Accurate analysis and calculation of short-circuit faults in HVDC grids are important for the fault clearing technology.Thus,a fast calculation method of short-circuit faults in HVDC grids is proposed,which considers the frequency-dependent effect of transmission lines and the influence of AC grids on DC faults.Firstly,the MMC equivalent model is established in the frequency domain,and an equivalent current source reflects the influence of AC grids.Secondly,the bus impedance matrix of the HVDC grid is established based on the frequency-dependent model of transmission lines,and nodal voltages and branch currents are calculated in the frequency domain.Finally,the electrical quantities are transferred into the time domain using the numerical inversion of the Laplace transform.By considering the influence of the control mode on the equivalent current source,we can reveal that the fault current increases with the ability of stations to control the DC voltage.By ignoring the influence of the control mode on the equivalent current source,the fault calculation can be further simplied.The proposed fault calculation method is verified using the HVDC grid model established in PSCAD/EMTDC.The simulations show that the proposed method not only achieves high calculation accuracy,but also effectively improves the computational efficiency.The proposed method is not only suitable for the parameter design of DCCBs,but also suitable for the protection research of HVDC grids.(2)DC circuit breaker is the key equipment to clear faults in HVDC grids.Thus,a thyristor full-bridge-based large capacity DCCB is proposed,and the proposed DCCB has the following characteristics: no requirement for an additional pre-charge source,preactivation,quick interruption of small currents and quick recovery of interruption ability.Firstly,the topology and operational principle of the proposed DCCB are introduced.Secondly,the major parameter designs of the proposed DCCB is analyzed to ensure that the DCCB can safely and reliably interrupt the fault currents.Finally,the proposed DCCB is compared with the existing thyristorbased DCCBs to show the performance advantages.The feasibility of the proposed DCCB topology is verified using a small power prototype,and the operational performance of the proposed DCCB in HVDC grids is validated by PSCAD/EMTDC simulations.(3)Integrating multiple DCCBs at a DC bus into a single multiport DCCB(MDCCB)can effectively reduce the construction cost.Thus,this paper proposes a thyristor-based micro-loss MDCCB,and all ports share one main breaker and one energy absorber in the proposed MDCCB.Firstly,the operational principle of the proposed MDCCB is introduced.Even if a mechanical switch failure occurs during the interruption,the proposed MDCCB can isolate the port fault using the conventional or proactive backup protections.Secondly,the number and power rating of devices in the proposed MDCCB are analyzed.Finally,the operational performance of the proposed MDCCB is validated using PSCAD/EMTDC simulations.Compared with multiple DCCBs,the proposed MDCCB effectively reduces the construction cost and achieves better bus-fault tolerance.Compared with the existing micro-loss MDCCBs,the proposed MDCCB effectively improves the interruption reliability at the cost of increasing the construction cost,and the reliability advantage increases with the number of ports.Therefore,the proposed MDCCB achieves a compromise between the construction cost and the interruption reliability and is beneficial for the continuous and safe operation of large-scale HVDC grids.(4)Reclosing can reduce the adverse effect of instantaneous faults and improve the reliability of power transmission.Thus,this paper proposes an adaptive reclosing strategy for the thyristor-based DCCB,which effectively suppresses the fault current impulse and the voltage oscillation.Firstly,equivalent models are established to analyze the transient process that the HVDC grid charges the transmission line through the DCCB capacitor,and the differences of transient processes during the instantaneous fault and permanent fault are analyzed.Then,the fault property is identified using the moving average value of increment of capacitor voltage.After identifying the permanent fault,the DCCB can quickly isolate the fault,and because the capacitor has a large initial voltage,the fault current is rather small.After identifying the instantaneous fault,the HVDC grid charges the line to the system voltage through the closing resistor,which effectively suppresses the system voltage oscillation.Finally,the remote DCCB relcoses after the line voltage is charged to the system voltage,thus the transmission line recovers the power transmission.The proposed reclosing strategy is validated using PSCAD/EMTDC simulations.