Research On The Protection And Fault Isolation For HVDC Transmission Line

With the advancement of smart grid and energy internet strategy in China,Line Commutated Converter Based High Voltage Direct Current(LCC-HVDC)and Voltage Source Converter Based High Voltage Direct Current(VSC-HVDC)have been widely applied for long-distance power transmission,asynchronous grid connection,and new energy sources incorporation,due to their obvious advantages,such as large transmission capacity and convenient power regulation.The LCC-HVDC and VSC-HVDC are becoming the key technologies to build the HVDC grids and the future energy internet.The DC lines in the HVDC system are the link of large-scale energy transmission and interconnection,and the reliability of their operation not only affects the DC transmission system,but also related to the stable operation of the interconnected AC system.Based the status of HVDC technology,this paper conducts a detailed study on DC line protection principles and schemes,and proposes fault isolation technology and scheme for the DC lines in multi-terminal VSC-HVDC and future HVDC grids.The specific contents of this paper are presented as follows:(1)This paper firstly analyzes the DC line boundary in HVDC system and the characteristic of fault transient component.The transient component within entire frequency range would be generated during DC line fault transient,and it contains numerous fault messages unaffected by controller.The impedance characteristics of DC line boundary in LCC-HVDC and VSC-HVDC system are analyzed.Based on the impedance characteristic,the transient component within entire frequency range,specific frequency and specific frequency band are analyzed.These analyses would provide a foundation for the design of DC line protection principles and schemes.In addition,the stage characteristics of DC line fault process in VSC-HVDC system are analyzed,and the requirement for the DC line fault location and isolation in multi-terminal VSC-HVDC system is proposed.(2)To overcome the problem of low reliability when identifying fault using traditional methods based on transient component,this paper proposes a protection scheme for LCC-HVDC transmission line based on the waveform characteristic of transient frequency current.Based on the impedance characteristic of DC line boundary in LCC-HVDC system,the waveform of magnitude of transient frequency current is analyzed.The ratio of the variance and mean of the current magnitude,which defined as fluctuation coefficient,is used to describe the fluctuation characteristic of the current magnitude.The fluctuation coefficients during internal fault and external fault are quite different.Based on the fluctuation coefficient,a single-end protection criterion for LCC-HVDC transmission lineis proposed.The results of the simulation analysis and the test with fault recording data show that the proposed protection scheme is able to overcome the shortages of traditional protections based on transient harmonic,such as low amplitude of transient harmonic and high sensitivity to interference and fault resistance,and it is able to achieve fast protection of the whole DC line of LCC-HVDC system.(3)The obvious attenuation effect of the long HVDC transmission line would increase the difficulty for fault detection.To overcome this problem,this paper proposes a DC line protection scheme based on transient power-impedance for LCC-UHVDC system.The transient power,which defined as the product of transient voltage and transient current within specific frequency band,is adopted to identify the DC line fault from forward external fault.The transient impedance,which defined as the ratio of transient voltage and transient current within specific frequency band,is adopted as a directional element to construct a pilot protection criterion.The transient power and the transient impedance form a directional single-end fault criterion.The results of the simulation analysis and the test with fault recording data demonstrate that the proposed protection scheme is able to overcome the attenuation effect of long DC line on transient component,and it has strong fault resistance withstand ability.By employing the proposed protection scheme,reliable protection of the long DC line of LCC-UHVDC system can be achieved.(4)To overcome the problem of low reliability when applying the traditional DC line protections of LCC-HVDC system to the VSC-HVDC system,this paper proposes a DC line protection scheme using the correlation of the transient current for two-terminal VSC-HVDC system.There is generally a large capacitor in parallel at the DC side of VSC-HVDC system,and the capacitor would discharge when DC line fault occurs.Since the discharge path during the internal fault and external fault of DC line are different,the characteristic of the currents at DC line end and the capacitor branch would be quite different.The Pearson correlation coefficient is adopted to describe the difference,and a pilot fault criterion based on the correlation coefficients at both ends of DC line is proposed.The results of the simulation analysis show that the performance of the proposed protection is not affected by high frequency component,data synchronization and the system controller.Since the proposed protection scheme is able to eliminate the effect of the feed current from non-fault line,it is suitable for multi-terminal VSC-HVDC system.(5)The absence of DC line boundary in multi-terminal VSC-HVDC system makes it difficult to achieve DC fault section location.To overcome this problem,this paper proposes a DC line protection scheme based on the ratio of frequency-band transient voltage for the multi-terminal VSC-HVDC system.A new boundary of DC line in VSC-HVDC system is constructed by connecting inductor into the DC line.A single-end fault criterion based on the ratio of frequency-band transient voltage with high threshold is proposed through analyzing the transient voltage with frequency-band at both sides of the inductor.Since the directions of the transient component during DC line fault and external fault are different,the ratio of frequency-band transient voltage at both ends of DC line would be different.Based on that,a pilot fault criterion is formed.The results of the simulation analysis and the test with RTDS show that the proposed protection scheme has strong fault resistance tolerance ability,and it is able to achieve fast and reliable faulted line location in multi-terminal VSC-HVDC system.(6)The DC line fault current in multi-terminal VSC-HVDC system increases quickly along with large peak,which make it difficult to isolate the faulted DC line.To overcome this problem,this paper proposes a current limition based DC line fault isolation scheme.To overcome the limitation of breaking capacity and speed of DC circuit breaker(DCCB),a hybrid current limiting circuit for the multi-terminal VSC-HVDC system is proposed,and the parameters of the current limiting inductor and the energy dissipation circuit in the proposed circuit are designed and discussted.Based on the proposed hybrid current limiting circuit,a DC line fault isolation scheme for multi-terminal VSC-HVDC system and future HVDC grids is proposed.The results of the simulation analysis and experiment show that with the coordination of DCCB,the hybrid current limiting circuit is able to limit the fault current effectively and accelerate the fault current interruption during the fault clearing.By employing the proposed fault isolation scheme,the faulted DC line can be isolated effectively and the stable operation of the non-fault part can be achieved,indicating that DC fault handling ability of multi-terminal VSC-HVDC system is enhanced.Finally,the work of the whole paper is summarized,and prospect of further research is proposed.