Protection And Fault Location Method For VSC-HVDC Cable

Voltage-source-converter based high voltage direct current(VSC-HVDC)is a DC transmission technique based on voltage source converter,PWM and full-controlled device.Compared with traditional HVDC,VSC-HVDC has lots of advantages.VSC-HVDC can supply power to passive network,and it can control active and reactive power independently,and it does not need the reactive power compensation.When the power flow is turned,the voltage quality is unaltered while the current quality is altered.Therefore,VSC-HVDC has broad development prospect,and is suitable for interconnection of large-scaled clean energy generation and distribution generation,island power supply,urban power grid supply and interconnection of asynchronous AC grid.The converter stations in VSC-HVDC system are usually connected by under-ground or submarine cable.The inertial.coefficient of VSC-HVDC system is small,and there is no low-voltage current limit function and perfect DC switching device.When a fault occurs in the cable,the fault current will rise to a large peak value within a few milliseconds,and make great damage to the system device.Therefore,the fast and reliable protection is needed to clean the fault in time.The DC cables usually cross a long distance,and are buried underground or under the sea,so it is hard to find the fault location and clean the fault after a permanent fault.To reduce the workload and the outage cost,the fast and accuracy fault location technique is required to locate the fault.In above,the research of protection and fault location technique for DC cable is of great significant to improve the security and stability of VSC-HVDC system.In recent years,many overseas and domestic scholars have done a lot of research works on protection and fault location for VSC-HVDC transmission line.However,most of the works did not consider the frequency-dependent characteristic of cable.The traveling wave method has a high speed,and is usually used as main protection for transmission line.The precision of traveling wave method is high,so it is also widely used in fault location.The protection and fault location method based on traveling wave theory are both depend on the capture of traveling wave head.Now the wave head is commonly captured by means of wavelet transform.The traveling wave is sensitive to fault resistance,and the wavelet transform can overcome this shortcoming.However,the wavelet transform result is influenced by mother wavelet and wavelet scale.S transform is a time-frequency analysis method derived from wavelet transform and Fourier transform,and it can extract the time-magnitude characteristic of different frequency components of a signal.Compared to wavelet transform,the S transform can present more abundant frequency spectrum information of a signal,and its result is more intuitive.Based on the S transform and DC transient current,this paper takes the VSC-HVDC cable fault characteristic and the frequency-dependent characteristic into consideration,and works on the DC cable protection and fault location method.The specific research works are as follows:(1)The electromagnetic transient simulation model of VSC-HVDC system is built with electromagnetic transient simulation software PSCAD/EMTDC.Firstly,according to the basic structure,operational principle and control mode,a two-terminal and a three-terminal two-level VSC-HVDC system simulation models are built in PSCAD.Then some internal faults such as the single-pole-to-ground fault and the double-pole fault and some typical external faults are simulated on basis of the simulation model.The characteristics of the fault transient electrical quantities under different fault conditions are extracted.This part of the work laid the foundation for the rest of the work.(2)A pilot protection method for VSC-HVDC cable based on S transform and transient currents at the two terminals is proposed.Firstly,according to the structure of cable,a phase-mode transform method for bipolar cable is derived.Then the propagation of the mode currents in cable is analyzed,and the proper mode current is selected for cable protection.The proper mode currents at the two terminals of the cable are measured and the time-magnitude characteristic curves of them are extracted by means of S transform.According to the sudden change point of the curves,the fault can be detected,and the initial sudden change time of the currents can be determined.The zero frequency component of the mode current is used to identify the fault and noise disturbance.At last,the sudden change values of the mode currents after their initial sudden change time are calculated,and their polarities are used to identify the internal and external faults.The proposed protection method is verified in the three-terminal VSC-HVDC simulation model.The simulation results show that,this protection method can identify the internal faults reliably,even the fault with a large fault resistance near the far end,and can ignore the external faults and the noise disturbance effectively.(3)A boundary protection for VSC-HVDC cable based on S transform and the transient currents at the both sides of single-terminal capacitor is proposed.There are large capacitors in parallel at the terminals of VSC-HVDC cable.This boundary is a low-impedance branch for high frequency signal.According to this characteristic,a single-terminal protection method is proposed in this paper.Firstly,the currents at both sides of the capacitor are measured and their frequency components are extracted by S transform.The sudden change point of the high frequency component of the current at the DC side of the capacitor is used to detect the fault.Then according to the attenuation of the DC boundary to high frequency signal,the ratio of the magnitude of the high frequency components of the currents at the DC and AC sides of the capacitor is used to identify the internal fault and near-end external fault.Finally,according to the attenuation of the DC boundary and DC cable to the high frequency signal,the ratio of the magnitude of the high and low frequency components of the current at the DC side of the capacitor is employed to identify the internal fault and the far-end external fault.The proposed protection method is verified in the three-terminal VSC-HVDC simulation model.The simulation results show that,this protection method can identify the internal faults reliably,even the fault with a large fault resistance near the far end,and can ignore the external faults under different conditions effectively.(4)A fault location method for VSC-HVDC cable based on S transform and transient currents at the two terminals is proposed.A fault in the cable will generate fault current traveling wave propagating from the fault point to the two terminals.According to the arrival time of the initial traveling wave at the two terminals of the cable,the fault location can be calculated.First,the bipolar cable is decoupling through the phase-mode transform matrix,and the propagation characteristic of the mode currents through the cable is analyzed.Then the frequency components of the proper mode current are extracted by means of S transform,and are employed for fault location.According to the sudden change point of the time-magnitude curves of the frequency components,the initial arrival time of the fault current traveling wave at the terminals of the cable can be determined,and finally the fault distance can be calculated.The proposed fault location method is verified in the two-terminal VSC-HVDC simulation model.The factors such as fault type,fault location,fault resistance,noise disturbance and sampling frequency are taken into consideration.The simulation results show that this fault location method can calculate the fault location accurately under different fault conditions.The work of this paper can provide theoretical support for the research of VSC-HVDC cable line protection and fault location.The research achievement will be the effective supplement of the current VSC-HVDC cable line pilot protection theory,and provide a new method of the single-terminal boundary protection,and can improve the performance of the two-terminals traveling wave fault location method.The research results have great practical significance and high project application worth to improve the reliability and security of power system.