Study On The Protection Principle Of VSC-HVDC Transmission Lines

Flexible HVDC transmission technology has been in development for more than 20 years since it was proposed in the late 20th century.Its theoretical is continuously improved,and the relevant technology is applied to the engineering practice.Flexible HVDC technology has become an important research and development trend in power field in recent years.With the development of Flexible HVDC technology,the voltage level and short circuit capacity are continuously improved that made higher demands on the performance of transmission line protection.Therefore,it is very important to study the principle of relay protection of VSC-HVDC transmission line to improve the safety and guarantee the reliable operation of the system.However,the current research of VSC-HVDC is mostly attention to the operation and control of the system and the research about VSC-HVDC transmission line protection are less.In addition,the transmission line protection of practical VSC-HVDC engineering is almost by reference to the protection principle of conventional high voltage direct current.The protection is not specific and doesn’t take structural and control characteristics of VSC-HVDC system into account,and doesn’t meet the requirements of VSC-HVDC system.In order to promote the development and popularization of Flexible HVDC technology,this paper takes the protection principle of VSC-HVDC transmission line as research content,and puts forward two novel VSC-HVDC transmission line protection,by the analysis of transient wave process when system faults and structural characteristics of VSC-HVDC system.Referring to existing VSC-HVDC engineering,the paper puts up PSCAD simulation model and verifies the reliability of the proposed protection principle.The concrete research contents can be summarized as follows:1.In this paper,the analysis of the structural characteristics of VSC-HVDC system and the transient wave process when system faults is studied.The paper discusses structure composing of VSC-HVDC system,summarizes the common fault type of VSC-HVDC,and analyzes the generation mechanism,propagation process,reflection and refraction characteristics of the fault generated traveling waves.The transmission line parameters modal transformation is studied.2.This paper takes the traveling wave differential protection as the research object,analyzes the cause of the unbalance differential current and the influence of the unbalance differential current on traveling wave differential protection,and demonstrates that when the external fault occurs,the unbalance differential current caused by fault will probably exceed the threshold and cause protection maloperation.By the analysis of transient characteristics of VSC-HVDC system fault,find the current traveling wave energy will outclass the energy of differential current when external fault occurs,and will not when internal fault occurs.Based on this characteristic,a novel traveling wave differential protection scheme was proposed.This proposed protection can accurately distinguish between the external fault and internal high resistance fault,and can effectively solve the problem of protection misoperation caused by the unbalance differential current.The PSCAD simulation with reference to practical engineering is done to verify the reliability of proposed protection.3.Through analyzing the refraction and reflection characteristic of fault generated traveling waves occurs at both dc-link capacitors of VSC-HVDC transmission system,the conclusion that total reflection of fault traveling wave occur at dc-link capacitor when a internal fault occur is reached.This paper analyzes the characteristic of time difference between the first traveling wave heads of fault generated forward wave and backward wave when the internal fault and external fault occurs respectively,and proposed a novel VSC-HVDC directional pilot protection based on the time difference of first head of the directional travelling wave.The Stationary Wavelet Transform is used for the proposed protection to obtain the key information of the fault traveling wave.The PSCAD simulation is done to verify that the proposed principle can correctly discriminate between internal and external faults with high sensitivity and reliability.