Research On Protection Scheme Of Multi Terminal Flexible DC System With Small Current Grounding

Flexible DC system is convenient for the access of distributed generation,energy storage device and DC load,and has the advantages of large transmission capacity,small line loss and high power quality.However,a large number of power electronic devices are used in the flexible DC system,and the system inertia is small.After a fault occurs,the current rises rapidly,and the power electronic devices have weak over-current resistance.Therefore,it is necessary to design a fast and reliable protection scheme for the flexible DC system.However,the fault characteristics of DC system are complex,and there is no consensus on protection scheme configuration at present.In particular,there are few complete protection schemes for multi terminal flexible DC system with small current grounding.After a single pole grounding fault occurs,the voltage of non fault pole rises to twice of the rated voltage,so it is necessary to determine the fault line as soon as possible and give an alarm or isolation.However,the fault current is very small,so it is difficult to select the line in practical engineering.After a bipolar short circuit fault occurs,the fault current increases rapidly However,the influence of distributed capacitance on bipolar protection scheme should be considered.Therefore,it is urgent to design a fast and reliable protection scheme for multi terminal flexible DC system with small current grounding.Based on the above problems,this paper studies the basic structure of multi terminal flexible DC system,fault characteristics analysis of multi terminal flexible DC system with small current grounding and design of complete protection schemeFirstly,the basic structure and working principle of modular multilevel converter(MMC),the core equipment of multi terminal flexible DC system,are analyzed,and the equivalent model of AC / DC side circuit of MMC is derived,which lays a theoretical foundation for subsequent fault analysis.Then,the effects of different topologies and grounding modes on fault characteristics and power supply reliability are compared.It is pointed out that from the perspective of power supply reliability,multi terminal flexible DC system should adopt ring topology and grounding mode should adopt small current grounding mode.Secondly,a detailed theoretical analysis of fault current is carried out for the multi terminal flexible DC system with small current grounding.This paper analyzes the difference of positive and negative current between fault line and non fault line after monopole grounding fault,and the direction difference of current sudden change between fault line and non fault line after bipolar short circuit fault,and discusses the influence of fault location on the difference of positive and negative current,and the influence of distributed capacitance on the direction difference of current sudden change.The simulation verification is carried out in RTDS / RSCAD,which lays the foundation for the protection scheme design.Finally,a fast and reliable protection scheme for multi terminal flexible DC system with small current grounding is proposed.It includes a monopole grounding line selection scheme based on the ratio of wavelet denoising residuals of positive and negative current and a bipolar short-circuit directional pilot protection scheme based on current burst.The verification of RTDS real-time simulation software shows that the single pole grounding fault line selection scheme can quickly and accurately identify the single pole grounding fault line,and has strong resistance to transition resistance and noise,and is not affected by power flow change and AC side fault;the bipolar short-circuit direction pilot protection scheme can quickly and accurately remove the bipolar short-circuit fault line,and has strong resistance to transition resistance and communication delay It is not affected by the change of tidal current.