| The development of power-electronic and controllable devices in power systems is rapidly expanding the field of applications for voltage source converter(VSC)based high voltage direct current(HVDC)technologies,due to its recognized advantages in comparison with traditional line commutated converter(LCC)based HVDC transmission.Therefore,VSC-HVDC has a bright future in the fields of new energy grid,isolated power supply,urban power grid supply and AC/DC hybrid transmission etc.A multi-terminal VSC-HVDC(VSC-MTDC)grid could provide fast and independent control of bidirectional power flow,which is one of the valid approaches to solve the integration problems of increased penetration of renewable energy sources.These distinguishing features make VSC-MTDC been accepted as a feasible solution to implement efficient grid integration and power transmission for renewable energy over long distancesIn VSC-MTDC systems,the unavoidable nonpermanent DC faults on any transmission line can severely undermine the reliability and availability of the transmission system,which will cause the entire grid breakdown.Nonetheless,in a DC grid,the fault rate of long-distance DC transmission lines is very high and the fault current usually arrives at the peak point within a couple of milliseconds.It is essential that trip signals are issued before the fault current develops beyond the modest interrupt capability of DC breakers and before the fragile converters are damaged.If the protection system does not isolate the faulty section timely,the integrity of the entire grid will be put in jeopardy.Therefore,it is vital to design a protection scheme for VSC-MTDC transmission lines to achieve fast fault clearance,as well as automatic and quick recovery.The work of this paper is arranged,as follows:This paper studies the system structure and the mathematical simulation model of VSC-MTDC system,and the application of a trous algorithm in transient signal frequency component extraction and DC protection are studied.With a two-terminal VSC-HVDC and a four-terminal VSC-MTDC model built in PSCAD,numerous faults are simulated to test the proposed protection methods.Analysis of the reflection characteristics of the fault travelling wave at the cable terminal shows that(i)when an internal fault occurs,the protection device will detect the arrival of the forward and backward travelling wave simultaneously and(ii)when an external fault occurs,the arrival time difference between the forward and backward travelling wave is a nonzero value.This paper uses thea trous algorithm to extract the arrival time of travelling wave and utilizes the propagation time difference between forward and backward travelling wave to judge the faulty line.The algorithm has lower sampling frequency,smaller calculation amount and faster operating speed.Low communication speed can meet the requirement of relay protection,which having certain practical value.With accurate amplitude and polarities of the fault initial current travelling wave being collected from the fault-detection devices located at each terminal,the ratio of amplitudes between forward and backward currents travelling wave is used to judge the faulty segment.The square of ratio value smaller than 1 indicates the occurrence of an internal fault,otherwise,it indicates the occurrence of an external fault.The proposed scheme can correctly determine the faulty segment and the faulty pole.Comparing with the existing protection of VSC-HVDC DC line,the proposed protection is simple,quick and not affected by fault type and fault locationBased on the analysis,this paper concludes the polarity of forward current travelling wave can be used to identify the faulty pole.When a positive pole fault,the polarity of 5-mode current travelling wave is positive,and when a negative pole fault,the polarity of 5-mode current travelling wave is negative,while when a pole-to-pole fault occurs,the polarity of that is around zero.Simulation results show that the faulty pole selection is simple and reliable. |
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