| As a new generation of DC transmission technology,modular multilevel converter based high voltage direct current(MMC-HVDC)transmission can control active power and reactive power independently and supply power to passive system without commutation failure.It has broad application prospects in the fields of large-scale integration of renewable energy,power supply of offshore drilling platform and asynchronous interconnection of power grid.DC transmission line protection is one of the core technologies of MMC-HVDC transmission.However,there are few researches specifically for MMC-HVDC transmission line protection currently.Meanwhile,there are some problems to be solved when existing DC transmission line protection principles are used in MMC-HVDC system.In order to ensure the safe and stable operation of MMC-HVDC system,this paper proposes several protection principles for MMC-HVDC transmission line based on the characteristics of MMC-HVDC system and the research status of DC transmission line protection.The main contents and achievements are as follows:(1)The system topology and hierarchical control strategy of MMC-HVDC system are introduced.The digital simulation model of MMC-HVDC system is established in RTDS by designing the key components model and parameter configuration.The test results verify the reliability of the simulation model.(2)In order to detect DC transmission line faults quickly and identify the fault type,this paper presents a protection scheme for MMC-HVDC transmission line using only single-ended transient current.Firstly,the characteristics of fault transient current of MMC-HVDC system are analyzed.The results show that the transient current contains abundant frequency components when DC faults occur,while the frequency components of transient current concentrate in the low frequency band when AC faults occur.Moreover,the trend of transient current is one-to-one corresponding to the type of DC fault.It shows an upward trend in the early stage of pole to pole fault,a steady trend in the early stage of single pole to ground fault,and a downward trend in the early stage of line breakage fault.Based on the above features,the protection scheme uses the intrinsic mode energy entropy and trend of transient current to construct DC fault criterion and DC fault classification criterion respectively.Test results show that the protection scheme can operate reliably under a variety of fault conditions.(3)Aiming at the problem that the pilot current differential protection in MMC-HVDC system can not reliably respond to the single pole to ground fault,this paper presents a novel principle specifically for protection of MMC-HVDC transmission line based on power signals.Firstly,the fault voltage and current characteristics of MMC-HVDC system are analyzed.On the basis of these,the difference between positive pole DC power at one terminal and negative pole DC power at the other terminal is defined as DC power difference.The DC power differences are zero under normal operation and AC faults,and have significantly different positive and negative combinations under different DC fault types.Based on the above features,the amplitude of DC power difference is used to construct the DC fault criterion,and the positive and negative combinations of DC power differences are used to construct DC fault classification criterion.(4)Based on the research on the above proposed pilot power protection,this paper further uses the ratio of DC power difference to DC voltage to construct the custom differential current,which is used to construct the pilot protection for MMC-HVDC transmission line.The protection adopts the amplitude of custom differential current to construct DC fault criterion,and adopts positive and negative combinations of custom differential currents to construct DC fault classification criterion.The simulation results show that the protection can classify and identify single pole to ground fault and pole to pole fault reliably and sensitively,and has good adaptability when the system transmission power and DC line length change.The research results of this paper can provide references for the relay protection design of MMC-HVDC transmission line,and is of great significance to ensure the safe and stable operation of MMC-HVDC system. |
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