| Presently,plenty of Ultra High Voltage(UHV)projects have been put into service in China,and an AC UHV power grid,which connects the major regional power grids of the entire country,has initially formed so far.The secondary arc is still a key challenge for UHV power transmission.Since most of UHV CBs employ the double-break structure,the grading capacitors installed in parallel with each interrupter can be further extended to suppress the secondary arc by simply increasing the capacitance only,which forms the so-called Extended Grading Capacitor(EGC).Besides the great performance in minimizing the arcing current and time,its structure is simple and does not require special protection;the use of EGC is also free of the installation location of shunt reactor.It provides an alternative to existing methods.On this basis,this paper studies the influence of the original topology and parameters of the shunt capacitor access system on the electromagnetic transient of the power system,mainly from two aspects: out of step fault breaking characteristics and transformer closing inrush current electromagnetic transient characteristics.Firstly,the equivalent circuit of parallel capacitor of double break circuit breaker is analyzed;the basic principle of system out of step fault and the relevant standards of circuit breaker breaking characteristics are introduced;the principle of transformer no-load closing inrush current and the selective correlation strategy of restraining inrush current are introduced.Secondly,this paper analyzes the influence of EGC on out of step fault breaking characteristics through theoretical derivation and simulation calculation.The equivalent circuit model for UHV line is established.The voltages on both power source side and line side of Circuit Breakings in the case of out-of-step fault are derived based on Laplace transform and travelling wave theory.It indicates that the oscillation of power source side voltage and the travelling wave process on line side leas to the high and steep Transient Recovery Voltage(TRV).By means of EGC,the voltage on power source side is often shifted from oscillatory state to over-damped state;the period of wave propagation is delayed.The presence of EGC smooths the TRV greatly especially during the initial few microseconds.The Rate of rise of recovery voltage(RRRV)drops as the capacitance increases despite a small increment in TRV.At the same time,the effects of out of step phase angle,the number of parallel lines,shunt reactor and reclosing on the breaking performance are discussed.he angular impact on TRV is obvious whereas minor on the initial RRRV;the more the number of parallel lines,the lower the TRV and RRRV;the shunt reactor is preferred to be installed on busbar side to minimize the interrupting burden;the failure reclosure would gives rise to the extremely high TRV during the second breaking,and it should be avoided.In addition,the influence of different residual magnetization and different closing phase angle on inrush current is studied.Through theoretical analysis and simulation verification,the influence of EGC on residual flux is studied: larger equalizing capacitor increases the amplitude of micro oscillation and reduces the residual flux.The influence of different time dispersion of circuit breaker closing on inrush current and the influence of EGC on inrush current under specific time dispersion are simulated and analyzed.At the same time,considering the residual magnetism before closing caused by EGC as the periodic oscillation variable,the optimal closing time of synchronous closing is proposed according to the average residual magnetism level.The simulation results show that although this method can not completely suppress the inrush current,it can control the inrush current at a low level. |
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