Research On Novel Protection Scheme For VSC-based Multi-terminal DC Grid

Multi terminal direct current(MTDC)transmission system is one of the valid approaches to solve the integration problem of increasing penetration of renewable energy sources.Unlike the line commutated converter based high voltage direct cuttent(LCC-HVDC)link,reversal of power flow can be achieved with the DC side voltage polarity fixed in a voltage source converter based high voltage direct cuttent(VSC-HVDC)link.Thus,the VSC technology is the better choice for MTDC grids.According to incomplete statistics,until now,24 VSC-HVDC transmission projects have been put into operation worldwide,most of which are two terminal HVDC projects.MTDC is in the initial stage of development.In a DC grid,the rise rate of fault current is very high and DC circuit breakers are not commercially available for high power ratings currently.Therefore,the protection and fault current interruption in VSC-MTDC grids are key research problems at present.For the protection of VSC-MTDC,the work of this paper is as follows:This paper proposes a VSC-MTDC transmission line protection scheme based on polarity comparison of current travelling wave.Assuming positive current direction is from busbar to the dc cable,when a fault occurs on a DC line with positive polarity,both the polarities of initial current travelling wave detected on two ends of the faulty line are positive,while the polarities of initial current travelling wave detected on healthy line are opposite or both negative.Similarly,for a fault on the negative pole,both the polarities of initial current travelling wave detected on two ends of the faulty line are negative,while the polarities of initial current travelling wave detected on healthy line are opposite or both positive.On the basis of fault characteristic analysis,a VSC-MTDC transmission line protection scheme is designed utilizing modulus maximum of wavelet transform to get the polarity of initial current travelling wave.Due to coupling,for single-pole-to-ground DC line fault,the healthy pole may satisfy the polarity relationship as the faulty pole.By comparing the size of the absolute value of modulus maximum,the protection unit can correctly identify the fault type.This paper proposes a DC busbar protection scheme based on the sampled value differential current.According to Kirchhoff's current law,differential current protection is proposed.As a special form of differential protection,current differential protection based on sampling values has the characteristics of fast operation speed and less computation compared to the conventional current differential protection.To verify the correctness of the protection scheme proposed,a VSC based four-terminal DC grid with resistive superconducting fault current limiters(R-SFCLs)and DC CBs are built on the PSCAD platform and several DC faults scenarios are simulated.R-SFCLs utilize a current-driven transition from the superconducting state to the normal state to limit the short circuit current.Thus,DC circuit breakers can be used to break the fault current in the event of a DC fault.Simulation results show that the R-SFCLs has nice fault current limiting effect and the proposed VSC-MTDC protection scheme can distinguish internal faults from external faults correctly and fast,and it is basically immune to fault type,fault resistance and fault locations.