Research On Pole-to-pole Short-circuit Fault Location Method For Multi-terminal MMC-HVDC Grid

The flexible DC grid is formed by modular multilevel converters(MMC),which has excellent performance of flexibility and controllability,and is increasingly used for consuming large-scale renewable energy.Transmission lines can well perform in extending long distances while transmitting huge amounts of electricity.Therefore,they are often used as construction materials for DC grids to interconnect converter stations in various places.At the same time,long distance and complex environment can also cause frequent DC faults,which develop rapidly and can spread to a wide range.Among them,permanent faults will cause strong system shocks and long-term power transmission interruptions.Therefore,it is necessary to conduct in-depth research on fast and accurate DC fault location methods to improve maintenance efficiency and accelerate fault recovery.First of all,the analyzation of fault characteristics of the symmetrical bipolar MMC-HVDC system points out that the numerical difference of the reactance voltage drop before and after the fault can be used to construct the fault initiation criterion;The relationship of reactance voltage drop between the faulty pole and the non-faulty pole is the key to formulating the basis for polarity discrimination;The consistency of the direction of the sudden-change current at both ends of the line is an important basis for judging the fault area.It is verified by simulation that the above criteria can reliably distinguish the types and area of faults.Aiming at internal pole-to-pole short-circuit faults,the numerical characteristics of voltage and current measured at both ends of the faulted line are analyzed,and the feasibility of omitting the line resistance parameter item is explained.Based on this,a fault location algorithm using the sudden change of current is proposed,which can avoid the interference of fault resistance in a certain degree and find a relatively accurate location.Secondly,to further reduce the location error,a more accurate line model and parameter acquisition method is studied.By analyzing the fault characteristics of several typical transmission line models,it is pointed out that using the first-order filter for electrical quantity filtering can reduce the current fed by distributed capacitance.In the case of data preprocessing to eliminate capacitance parameters,a simplified R-L model is proposed.The electrical data is obtained by pre-setting DC fault,and then substituted into the formula to solve the unit resistance and inductance of the line.Analyzing the line parameter factors and testing model accuracy,it is learned that the simplified R-L model and line parameters can accurately simulate changes in electrical quantities after a DC fault,especially in terms of fault current simulation,which can approximately replace transmission lines.Finally,in order to avoid the loss of location accuracy caused by elimination of calculation items and the error of the line model,the paper adopts the above-mentioned simplified R-L model of transmission line,and draws the equivalent network of pole-to-pole short-circuit DC fault.Based on this,a detailed formula derivation is carried out and an improved pole-to-pole DC fault location method is proposed.Through the simulation tests of multiple fault locations and various fault conditions carried out on the four-terminal DC grid built in PSCAD,the location accuracy of the proposed method,the ability of withstanding fault resistance,adapting various operating conditions and resisting noise interference have all been verified.