Research On Fault Identification And Isolation Of Multi-terminal VSC-HVDC

With the need of large-scale renewable energy integration,long-distance high-efficiency electricity transmission and interprovincial grid unsynchronized connection in recent years,multi-terminal VSC-HVDC(voltage source converter based high direct current)has come to its time.Nevertheless,the brand-new fault characteristics have placed stringent demands to the fault-handling of multi-terminal VSC-HVDC.The fault identification and isolation are not only required to complete correctly and reliably but also with fast speed,and thus minimize the influence of fault as possible.In this paper,the fault identification and isolation are studied,and the main effort and contribution are summarized as follows.First,to deal with the fault resistance sensitive problem,fault identification strategy with high-resistance tolerable ability for primary protection is proposed.The similarities and differences between internal fault and external fault are explored,and it is revealed that the amplitude of traveling wave front under internal fault with high fault-resistance could be similar to that of the external fault.Further,the differences are discovered from the traveling wave propagation perspective,and it is discovered that there exists the current reduction phenomenon under internal non-metaillic fault,which is different to other fault conditions.The discovered current reduction phenomenon is utilized as the identification criterion,and thus the fault could be identified with the attribute of low-resistance or high-resistance,avoiding the misidentification of external fault and internal high-resistance fault.The simulation results demonstrate that the proposed strategy is insensitive to fault resistance,and not affected by the size of DC reactors.Second,to deal with the parameters error of transmission line,the transmission line parameter error tolerable current differential identification strategy for backup protection is proposed.The parameter error sensitive problem of existing current differential strategies is investigated from the characteristic impedance and wave velocity,and it is found that the influence to the compensated current mainly comes from the error of traveling wave velocity.In order to offset the error of traveling wave velocity,WTMM(wavelet transform modulus)is adopted to calculate the arrival time of traveling wave in the parameter error tolerable current differential identification strategy.The simulation results validate that the proposed strategy is with up to ±50% transmission line parameter error tolerable ability,while without increasing the setting time and value.Third,to deal with the low re-closing speed of the existing DCCB(direct current circuit breaker),a novel hybrid DCCB is proposed for quick tripping and reclosing scenario in multiterminal VSC-HVDC.Benifits from the variable clamped voltage,the proposed DCCB is with fast operation speed and fast re-closing ability.Moreover,it is with excellent fault current limitation effect and would not cause the extra rise of current under operation.The simulation results show that the fault isolation time is within 3 ms,and the re-closing time is within 11 ms.Fourth,to deal with the rapid raising fault current problem,current limiting device configuration for multi-terminal VSC-HVDC based on iterative current reduction ranking is proposed.In ter ms of SFCL(superconducting fault current limiter),the current limitation effects with different configuration options are discussed,and the idea of determination method with analytic derivation is proposed.The current reduction index is defined to evaluate the current limiting effect,and its expression is derived.Based on the derived current reduction index,the determination method considering the cooperation between location and size is proposed.The current limiting effectiveness and feasibility of the proposed method have been proven in the four-terminal VSC-HVDC model.Fifth,since the AC side isolation approach would increase the AC current,a simplified analytic model for AC current calculation is proposed.The principle of fault analysis is established according to requirement of AC current calculation and DC fault characteristics.Based on the established principle,fault characteristics with different fault-resistance is analyzed,and the simplified analytic model is proposed.The validations show that the fault current error between the proposed model and PSCAD/EMTDC model is within 10%,which meets the requirements of AC current calculation.