Study On Problems And Counter-measures Of The Relaying Protection Affected By The Electric-parameter Characteristic Of MCST And SC Transmission Lines

With the continuous development and expansion of China's power grid,UHV / EHV large capacity and long-distance transmission line increases rapidly,in order to meet the demand of long distance and large capacity,improve the system stability and transmission efficiency,multiple-circuit transmission lines on the same tower(MCST)and series compensation(SC)transmission lines are widely used in the power grid.Due to the difficulty of using ideal transposition erection in multiple-circuit transmission lines on the same tower,brought the serious problem of asymmetric parameter.The series compensation transmission lines due to the system demand and expansion have brought the problem of higher series compensation degree system.At the same time,the two transmission lines will further cause complex changes of parameters and the intensification of asymmetry when the operation mode changes and complex faults occur.This situation often leads to misoperation and rejection of relaying protection,which has formed new challenges to relay protection.In order to solve the above problems,the paper focuses on the study on problems and counter-measures of the relaying protection affected by the electric-parameter characteristics of the multi-circuit transmission line on the same tower and the series compensation transmission lines.And focuses on solving the following scientific and technological problems: the parameters unbalance of the multi-circuit line on the same tower and its evaluation,the optimization method of reducing the parameter asymmetry,the influence factors of the parameters change of the series compensation transmission line,electric-parameter characteristics influence mechanismthe of the multi-circuit transmission line on the same tower and the series compensation transmission lines,the countermeasures and improvement measures of relay protection.These works have important theoretical significance and application value for guaranteeing the safe and stable operation of power system and improve the power reliability.Due to project engineering implementation,multi-circuit transmission lines on the same tower usually adopt the phase sequence with non-transposition structure erection.Therefore,the evaluation of unbalance degree and optimal selection of the phase sequence arrangement is very important for improving the line parameter symmetry.However,at present,there is no comprehensive evaluation method for the symmetry of the parameters which considered multiple imbalances,and it is difficult to obtain the optimal phase sequence arrangement by the least imbalance.In order to solve this problem,the unbalance degrees of the single-sequence component and composite-sequence component are defined as the basic evaluation index of the symmetry of the parameters and obtained by the calculation of the line sequence parameters.Furthermore,an unbalance degree comprehensive evaluation index based on weight distribution and a method for evaluating the symmetry of multi-circuit transmission lines on the same tower are proposed,which solves the problem of contradictory conclusions of the previous unbalanced degree sequence evaluation and realizes the effective evaluation of the parameter symmetry of multi-circuit transmission lines on the same tower.By using the above evaluation method,the optimal phase sequence arrangement of different tower types for four-circuit transmission lines on the same tower is proposed,which effectively improves the symmetry of transmission line parameters.Because of the complex structure of multi-circuit transmission lines on the same tower,it is difficult to realize complete transposition.the asymmetry of parameters will affect the reliable operation of distance protection which is based on parameter symmetry.In order to solve the problem of distance protection performance degradation caused by asymmetric parameters of multi-circuit transmission lines on the same tower,this paper,focuses on the the study of the influence of different fault types,different operation modes and different line lengths of double-circuit and four-circuit transmission lines on the measured impedance of distance protection when using the optimal phase sequence arrangement through theoretical derivation and digital simulation,and demonstrates the mechanism that causes the distance protection to shrink range or exceed the misoperation.According to the measured error magnitude of phase-to-phase and phase-to-ground impedance relays of multi-circuit transmission lines on the same tower,an optimal setting methods of phase-to-phase distance protection and phase-to-ground distance protection are proposed to improve the distance protection performance.The multi-circuit transmission lines on the same tower have complicated installation structures and complex electrostatic coupling between circuits.This increases the difficulty of compensating distributed capacitive currents and improving the sensitivity of longitudinal differential protection.In ord to solve the problem that the capacitance current changes cause by the multi-circuit lines on the same tower affect the performance of the longitudinal differential protection,this paper analyze the capacitance current change characteristics of the double-circuit lines on the same tower and the four-circuit lines on the same tower,and the influence mechanism of the capacitance current change characteristics on the longitudinal differential protection under the partial shutdown and maintenance modes from the aspects of theoretical analysis and simulation calculation.On this basis,in order to solve the problem of low compensation accuracy of the existing capacitance compensation scheme in the case of four multi-circuits transmission lines on the same tower,this paper puts forward the compensation method for the capacitance current of the phase segregated longitudinal differential protection and zero sequence longitudinal differential protection of multi-circuit transmission lines on the same tower with different structures,and verifies the effectiveness of the compensation method through simulation.Series capacitance compensation is an effective way to improve the AC transmission capacity and stability of power system.The problem of insufficient sensitivity in longitudinal protection of series compensated lines in large power grids has not been given sufficient attention in the past.With the wide application of series compensation devices,the development of power grid and the emergence of large series compensation power transmission,this problem become more prominent.For the problem of line differential protection performance caused by the change of the series compensation due to the development and operation change of power grid in the series compensation line.This paper proposes the concept of system series compensation degree,analyzes the influence of series compensation degree on longitudinal differential protection of series-compensated lines,and expounds the problem of deterioration of the sensitivity of longitudinal differential protection.According to the engineering application and the requirements of fault transient analysis,a large series compensation power grid model is constructed.The influence mechanism of fault type,transition resistance,and system operation mode and power angle change on the sensitivity of longitudinal difference protection is analyzed by system simulation.On the basis of this,several improvements have been proposed to improve the sensitivity of the differential protection of the series compensation lines.Simulation analysis shows that the sensitivity of the protection is effectively improved.In order to solve the problem of longitudinal differential protection of series compensation line on the same tower,this paper establishes the simulation model of the double-circuit series compensation line on the same tower,and studies and analyzes the influence of different operation modes,different fault types and transition resistance of the double-circuit series compensation line on the sensitivity of the traditional longitudinal differential protection.Furthermore,the effectiveness of the proposed method for compensating the capacitance current of double-circuit line on the same-tower in the double-circuit series compensation line on the same tower was simulated and verified,and the effect and advantage of the improved scheme of longitudinal differential protection based on the composite scalar product restraint characteristic and hyperbolic restraint characteristic combining proposed capacitance current compensation method in improving the sensitivity of longitudinal differential protection are verified.Finally,the main research work and innovation results are summarized,and the future work is prospected.