Research On The Fault Propagation And Protection Mechanisms Of HVDC Grid

DC grids can integrate various renewable energy sources and load to achieve multiple complementarities.The DC grid is an effective method to realize large-scale renewable energy consumption and long-distance transmission.However,because of the low inertia characteristics,the converter and DC side energy storage elements will discharge rapidly in case of a severe DC short-circuit fault,resulting in a rapid development of the fault with wide range of impact and rapid rise of the fault current.At the same time,power electronic devices in the DC grid are also affected by fault currents.However,the current-carrying capacity of the IGBT in the converter is weak,and the current within a few milliseconds under the fault reaches its upper limit of tolerance,which will damage the switching device in the converter or cause converter blocking,resulting in the outage of the whole DC grid.This will further affect the DC grid and its host AC system stability.For the problems after the fault of the DC grid,this paper focuses on fault propagation and protection mechanisms of HVDC grid.Firstly,the system-level and station-level control strategies of DC grid are elaborated and compared.Then,in order to research the fault current propagation in DC grids after dc fault,based on the single-ended converter fault discharge model,a generic DC grid fault current calculation method is proposed.Using this method,a method for optimizing the current limiting reactor configuration of a DC system is proposed.Secondly,a fast fault detection method using the voltage value of the DC side reactor is proposed.Finally,the influence mechanism of the fault current limiter(FCL)on the fault current is studied.The main research work of this paper is as follows:(1)Fault propagation and fault current calculation in DC gridBased on the single-terminal modular multilevel converter(MMC)fault discharge mechanism and equivalent model,this paper analyzes that in the initial short time after the short-circuit fault of the DC grid.In this period,the feed current from AC side is negligible.The DC fault current is mainly constituted by the capacitor discharge current from the converter.The "real node" and "virtual node" in the flexible DC grid are defined,and the DC grid nodes and branches are numbered.The initial matrix building method of DC grid is proposed.This method is easy to change with the structure of DC grid.The matrix can be expanded according to the change of fault point,increase or decrease of inverter,or network changes such as line increase or decrease.Based on the determined fault point,the method of initial matrix modification method is proposed,including the increasing of the branch and the elimination of "virtual node".Finally,the differential equations to be solved for the fault current calculation are determined.The fault current propagation mechanism of DC system with DC/DC converter is analyzed,and the applicability of the proposed fault current method in DC system with DC/DC converter is discussed.The simulation test included two types of DC grids:bipolar and symmetrical monopole systems.The pole-to-ground short-circuit faults and pole-to-pole short-circuit faults of the bipolar system were tested by simulation,and the pole-to-pole short-circuit faults of the symmetrical monopole system were tested.The test results verify that the proposed method has higher accuracy and computational efficiency.Based on the calculation method of the fault current of the DC grid,the influence of inductance and resistance values on the fault current development is analyzed.The contents of this study can provide a theoretical basis for DC grid fault detection,FCL and DC circuit breaker(DCCB)selection.(2)Current limiting reactor configurationBased on the DC fault current calculation studies,the objective function of the current limiting reactor size in DC grid is established.The DC breaker interrupting capacity and breaking time are considered as the constraint conditions.Considering the fault detection time and the hybrid DCCB mechanical switch opening time,the simulated annealing optimization algorithm is used to solve the problem.As a result,the value of the current limiting reactor of the DC grid is relatively small.Through simulation and verification,the proposed method for the optimal configuration of fault current limiting reactors in the DC grid will reduce the value of total current limiting reactors when meet the requirements of interrupting the DCCBs.Comparing with the traditional configuration method,the DC voltage and active power of DC grid recover faster after a DC fault,which improves the dynamic performances of the DC grid.(3)Fast fault detection method for DC gridBased on the proposed DC fault current calculation method,the mechanism of the boundary effect of the current limiting reactor is analyzed.It is proved that the reactor voltages on the faulty line are higher than that on the healthy lines after a pole-to-pole short-circuit fault.Using the voltage value information of the single-ended current limiting reactor,a fast detection main protection is proposed.The method can quickly identify fault lines and fault poles with severe short-circuit faults with low fault resistance.Then a back-up protection strategy was proposed which uses the voltage value information of the double-ended current limiting reactors.The method can reliably distinguish short-circuit faults under high fault resistance.Through simulation,the proposed fast main protection method can quickly identify faulty lines and determine faulty poles with high reliability.In the test DC grid model,it is possible to quickly detect the pole-to-pole faults with a fault resistance of 500 ? and a pole-to-ground fault with a fault resistance of less than 350? within 2 ms.The simulation also verifies that the proposed main protection for rapid detection is also applicable in a symmetrical monopole DC grid.Simulations verify that the proposed differential backup protection is highly sensitive and able to detect pole-to-ground faults with 1000 ? fault resistance.During the DC line power reversal,the AC system short-circuit fault and DCCB interrupting and other disturbance,the protection will not misoperate.The proposed flexible DC grid fast detection method has high reliability and meets the requirements of flexible DC grid protection.(4)The mechanism of the fault current propagation with FCL on the DC gridAfter fault detection the FCL will operate in DC grid.The process of the fault current dropping after inserting the FCL is analyzed,and the relationship between the changing law of fault current and initial current value after FCL operation is revealed.It is proposed that the DC grid fault current evolution with FCL and DCCB operation is divided into six stages,and the expression of the fault current calculation differential equation at each stage is presented.In the test DC grid model,the results is performed to verify the proposed method for calculating the current with FCL operation.The calculation method provides a preliminary theoretical basis for the future research on coordination control and protection of various fault current limiting devices in DC grid.