Studies On Novel Relay Protection And Action Mechanism For Transmission Line Of AC And Flexible DC

With the development of smart grid and global energy internet strategy,power can be allocated in a larger area.To solve the increasingly serious environmental problems and massive green energy go online,the national grid is becoming an AC-DC hybrid power grid.It exploiting respective advantages to the full,the integration of a pattern.As a medium of current conduction and channel of energy transmission,transmission line has become one of most important part in grid.Safety of transmission line should be emphasized.Double circuit lines on the same tower and the line of Flexible DC are two important type of transmission line in AC and DC system respectively.With the decreasing of transmission hallway and the increasingly improved of DC transmission technic,the application range of double circuit lines and DC transmission line will be wider.Aiming at the two types of transmission line,the dissertation propose novel transmission line protection and analyses the interaction between the two AC and DC system.The main contributions are as follow:(1)Because of complex effect of self-mutual inductance and multiple kinds of short circuit faults,the protection of double circuit lines on the same tower cannot satisfied the requirement when cross country fault or terminal fault occur.To solve the problems,the dissertation propose a novel monolithic faulty phase selection method and line protection based on transverse differential impedance ratio.Different from conventional faulty phase selection scheme,the novel scheme regard the double lines as a whole.It selects only fault phase but regardless which circuit it located.It can improve the accuracy of selection result.Based on the result of faulty phase selection,a kind of calculation impedance which includes self and mutual inductance is defined.Using the calculation impedance,a kind of transmission line protection based on transverse differential impedance ratio is proposed.The scheme only using measurement signal of single side and do not relay on communication and has a strong tolerance to grounding resistance.The reliability of the scheme has been demonstrated by theory analysis,digital simulation and dynamic physical simulation.(2)Redefine the Quasi-Three-Phase operation and demonstrate its reliability and advantages when faults occur in double circuit lines.Based on the Quasi-Three-Phase operation the dissertation propose an improved tripping and reclosing strategy.For the whole line double circuits,change it into Quasi-Three-Phase operation by tipping and reclosing at least faulty lines as possible.It can decrease the possibility of tripping all lines after fault and reclosing to permanent faults.To realize the integrated protection for double circuit lines on the same tower,the split-type of integrated protection structure is proposed.(3)To satisfy the requirement of speedy of flexible HVDC transmission line,the dissertation propose a novel protection scheme based on transverse differential current.Limited by the technique of DC breaker,most of flexible HVDC transmission line operate without DC breakers.In order to decrease the possibility of short circuit,cable is used as transmission line.In this situation,mainly fault of transmission line is single pole short circuit.Transverse differential current will be different when internal faults and external faults occur.Using this characteristic a novel transmission line protection is proposed.It can distinguish AC,DC bus and DC line faults.The scheme does not relay on communication and its strong tolerance to grounding resistance and speedy is demonstrated by theory analysis and simulation.(4)With the development of DC breaker,overhead line will be widely used in flexible HVDC.To solve the single pole faults and pole-pole faults,a transmission line protection based on the calculation power of current limiting reactor is proposed.Considering the complex evolutionary process after faults,the principle of the protection is explain from energy transfer.The calculation power of current limiting reactor is different when faults occur at different position.Using this principle,faults positions can be distinguished.The scheme does not relay on communication and its reliability is still high even faults occur at terminal of line.(5)With the development of DC and AC grid,the connection between the two become stronger than before.The dissertation research the interaction between AC and DC system when faults occur.For DC faults,the change mechanism and characteristic of voltage and current in AC transmission line is researched.Take several protection which is mainly used for transmission line as example,the possibility of mal-operation is analyzed.For AC faults,the variation law of voltage and current in DC transmission line is researched.Take the wavelet protection which is mainly used in DC transmission line as example,the possibility of its mal-operation is analyzed.