Research On Protection Technology Of Flexible Direct Current Transmission Line Based On DC Circuit Breaker

In the future,the main transmission mode of high voltage direct current(HVDC)system will be overhead line.However,the probability of transient fault occurring in overhead lines is high.When the fault is cleared and isolated by direct current circuit breaker(DCCB),the nonlinear and dynamic characteristics of power electronic devices of DCCB will change the fault characteristics of the system,thus affecting the action performance of traditional protection.At the same time,in order to enhance the reliability of power supply,the fault recovery of system needs the DCCB to quickly reclose.On this basis,in order to further shorten the fault clearance time,fast and accurate fault location is also important.Firstly,a protection algorithm based on the concavity and convexity of current waveform is proposed.From the analysis of action characteristics of DCCB,the fault equivalent circuit which contains internal branch of DCCB is established,reveals the change mechanism of system fault characteristic,deduces the fault current analytic expressions considering DCCB commutates through internal each branch at different stages.Based on the expression,the existing DC protection principle of adaptability is analysed,According to the action of each protection,the problem that the adjacent non-fault line protection may false tripping is solved through the change of current waveform characteristics.The simulation results show that the proposed protection algorithm is correct.Secondly,an adaptive reclosing strategy for DCCB is proposed which can effectively identify the fault nature.The equivalent circuit of DCCB recloses with the different fault nature is established,and the difference of the line electrical quantity variation and the reasons are analyzed.Then,a fault nature identification method based on voltage integral value is put forward.It solves the problem that it is not easy to identify the fault nature because of large transient voltage when DCCB recloses due to the induced voltage generated by the equivalent inductance of the line and the voltage distributed on the transition resistance.The simulation results show that the reclosing method proposed is effective.Finally,a single-ended fault location method based on the standard deviation of resistance value is proposed.By means of telegraph equation of DC transmission line,the paper analyzes the modulus characteristics of line,establishes the physical relationship between the electrical quantities under different fault types,points out the resistance of fault points compared with the other point of line is more stable,by using the transient quantity of reclosing can locate the fault.It solves the problem that the applicable scenario of existing fault location algorithm is not wide,and the simulation verifies the accuracy of the proposed fault location algorithm.