Research On Fault Location Method Of Flexible DC Transmission Line

Chinese power energy and load center present an unbalanced and inverse distribution characteristics,making China have a large demand for power transmission.Compared with AC transmission technology,DC transmission technology is suitable for long-distance and large-capacity scenarios,and its advantages are increasingly obvious,playing an important role in power systems.The rapid development of voltage sourced converters(VSC)and power electronics technology has promoted technological innovation of flexible DC transmission systems.The interconnection of power systems with different frequencies is more convenient,so multi-terminal flexible DC transmission systems are gradually appeared.As the key to reliable and stable operation of power system,fault location is top priority in research of flexible DC transmission systems.The transmission distance of DC lines is long,and probability of failure is high.In order to ensure safe and reliable operation of power system and reduce economic losses caused by power outages,it is of great significance to diagnose location of the fault quickly and accurately.Since fault current of DC power system has no zero crossing point,it is difficult to clear fault.Moreover,DC circuit breakers are expensive and have difficult in manufacture,so not all converter station terminals are equipped with DC circuit breakers.Therefore,in order to realize fault location more quickly and accurately,the thesis researches fault location methods of multi-terminal flexible DC transmission system without DC circuit breaker and with DC circuit breaker respectively.Firstly,basic theories of traveling waves on transmission lines are introduced,including generation process of traveling waves,wave equation of traveling wave and refraction and reflection of traveling wave.The modulus transformation of traveling wave is analyzed.Taking the uniform and lossy single conductor as an example,the attenuation of traveling wave on the line is analyzed,and refraction of traveling wave through series inductance,capacitance to ground and bus branch is deduced.In addition,a multi-terminal flexible DC transmission system simulation model was built.The frequency characteristics of overhead line parameters and the frequency characteristics of wave speed were analyzed,which lays a theoretical foundation for the subsequent research on fault location methods of flexible DC transmission lines.Secondly,a time-domain fault location method of multi-terminal flexible DCtransmission lines based on the attenuation characteristics of traveling waves is proposed.According to the multi-terminal topology,the blind zone problem in the multi-terminal flexible DC transmission system without DC circuit breaker is studied.By comparing the attenuation of traveling wave on the line and through the bus branch,a complete fault location method is determined.Considering influence of fault type,fault location,sampling frequency,grounding resistance and noise on fault location results,simulations were performed on PSCAD software to verify accuracy and applicability of the method.Then,a frequency-domain fault location method of double-ended flexible DC transmission lines based on natural frequency is proposed.For a multi-terminal flexible DC transmission system with DC circuit breaker,after the line fails,fault location of the multi-terminal system is equivalent to fault location problem of the double-ended system.A double-ended flexible DC transmission system model was built.From frequency domain perspective of traveling wave,fast Fourier transform(FFT)and multi-signal classification(MUSIC)algorithm are used to extract main components of traveling wave natural frequency.The fault location is realized according to the relationship between the main frequency component,wave speed and fault distance.Using PSCAD software,fault conditions of different fault types,fault locations,grounding resistances and noises are simulated to verify accuracy and anti-interference ability of the method.The comparison of the method of extracting main frequency by FFT and MUSIC and the method only by FFT or MUSIC proves that the method of combining FFT and MUSIC has advantages.Finally,taking the double-ended flexible DC transmission line model as research object,a fault location method based on stacked denoising autoencoder(SDAE)is proposed.The spectral amplitude of traveling wave is taken as an input layer sample,and the SDAE network model of fault location is established through a lot of training.Use the mapping relationship between traveling wave spectrum and fault location to achieve fault location accurately.Simulations were conducted under different fault locations,grounding resistances,noises and sampling frequencies to verify effectiveness of the method.The fault location method based on SDAE is compared with traditional double-end traveling wave method,which proves advantages of the method.