Research On The Line Fault Analysis And Protection Of Multi-terminal Flexible DC Grid

Compared with traditional point-to-point DC transmission system,multi-terminal DC(MTDC)grid has more advantages in the aspects of investment economics,power supply reliability and control flexibility,and it is also a good solution to the integration of large renewable sources and the connection among several asynchronous ac networks.However,MTDC grid is a low inertia system because of containing a large number of power electronic components.Consequently,fault currents with rapid-rising speed and large amplitude will be caused for a MTDC grid,which may lead to the serious overcurrent damage to the power electronic components,even the whole system blocking.In addition,the fault transients of the MTDC grid develop very fast,the faulted line may can’t be isolated because of lacking the ultra-high speed relay protection technology,even when the DC circuit breaker is used,and thus,the non-faulted converters and lines can’t keep continuous and reliable operation.Therefore,researches on the protection of MTDC grid is of great engineering significance.With respect to the critical issues of the MTDC protection,this thesis is carried out from the aspects of overcurrent calculation and suppression method,fault propagation mechanism and operating overvoltage analysis,and ultra-high speed DC line protection algorithm.The main research work is as follows.(1)According to the transient characteristics of DC side pole-to-pole fault,a general fault current calculation method based on "discharging mesh" is proposed.Then,an optimal allocation method of current limiting inductance for the MTDC grid is raised based on the proposed current calculation method.The minimum of the total inductance in the whole system is regarded as the optimal objective,and the constraints of DC circuit breaker reliable operation,the converter anti-blocking and protection requirement are considered comprehensively.Finally,an improved teaching and learning optimization algorithm is used to determine the global optimal solution.The proposed method can ensure the continuous and reliable operation of the non-faulted converters and DC lines,and can be applied to different DC topologies and inductance allocation locations.(2)Through in-depth analysis of fault transients and the law of fault current evolution in a pseudo-bipolar flexible grid,the mechanism of operating overvoltage caused by DC side short-circuit faults is revealed.In addition,the effects of fault resistance,converter blocking and line protection on overvoltage are analyzed as well.Then,the effective configuration of arrester is raised for the overvoltage caused by DC faults.Finally,the overvoltage characteristics of single-pole and pole-to-pole faults are compared,and the effective arrester configuration scheme is given.(3)Time domain characteristics of surge impedance of the inductive DC line terminal is analyzed for different fault zones.By introducing a starting criterion based the voltage increment,a fast DC line protection algorithm based on surge impedance is proposed.The proposed scheme can identify fault zone quickly and accurately only by using local wave impedance value,which meets the requirement of fast-moving protection.What is more,the wave impedance criterion eliminates the influence of fault resistance in principle,and can greatly improves the resistance-tolerance ability of protection.(4)The polarity characteristics of traveling waves for forward and reverse fault in a MTDC grid are analyzed.With respect to the oscillation of traveling wave directional criterion caused by the inductive terminal of DC line,a novel protection scheme based on transient energy of traveling wave is proposed.The proposed protection scheme can identify the faulty zone quickly and accuratelly,and two acceleration methods for forward and reverse faults are also designed to improve the operation speed of protection.Finally,the simulation results show that the proposed scheme can be applied to different fault scenarios,and has strong tolerance of fault resistance and disturbances.(5)To solve the shortage of breaking capacity of DC circuit breaker caused by extreme overcurrent fault or protection delay,a novel active control method of fault current is proposed for MTDC grid.The method just requires a parallel thyristor at the outlet of SM and the arm damping module,which can effectively reduce the fault current from the converter.In addition,the working principle of active current control and cooperation principle of active current limiting control and DC circuit breaker are elaborated,and the parameters calculation method of the damping module is also provided.Compared with traditional DTSS,the proposed method can effectively reduce the impact of the fault,and greatly improve the system recovery speed.