Influence Of Coincidence Timing On Distance Protection Action In AC / DC Power System

With the implement of optimal allocation of power resources cross strategy that "intergrate national power network、transmit power from the west to the east and transmit power from the north to the south", AC system and DC system have become an organic whole. Compared with traditional AC power grid, that because of DC system and DC system’s characteristics, when fault occurring, the transient process characteristics in this organic whole is different to pure AC system. such as when AC system fault, it may be highly possible to cause DC system commutation failure, and in the transient process, the overlap angle, the advance firing angle and the equivalent impedance electric parameters of DC system will all have mutations, which will affect AC system fault characteristics and communication protection action features, Seriously, even leading to the AC system protection misstrip or maloperation。As an effective measures to improve stability and reliability of the power system parallel operation, automatic reclosing has been widely used. However, by a lot researches of the actual grid found, automatic reclosing device exist the obvious reclosing sequence problems in the process of practical application. This thesis investigates that reclosure sequence influence on distance protection of AC line in AC-DC system,The main research points are as follows: Significant effect of three-phase reclosure sequence on distance protection in AC-DC system has been discovered. AC-DC parallel and series system model was established, expression of measuring impedance on perfect line’s both sides was deduced under different reclosure sequence when three-phase fault occured. Hereby, critical fault distance which will influence distance protection’s action characteristic was analyzed. Variation law of measured impedance along with reactance both on rectifier side and inverter side has been researched.Considering the three control modes of DC system such as the constant voltage control, constant current control and constant extinction angle control, the relation among DC system’s equivalent impedance with fault voltage component and fire angle have been deduced respectively under the commutation failure condition and the normal condition. According to the expression, analysis on the variation trend of the perfect line’s measured impedance along with the fire angle and fault voltage component have been performed, and thus the best reclosure sequence to decrease distance protection’s maloperation can be obtained. Simulation results of 2DC system、2015 Southern Power Grid and two-area AC-DC series system verify the conclusions’correctness and validity.Significant effect of single-phase reclosure sequence on distance protection and distance protection using power frequency fault components have been discovered. AC-DC series and parallel system model was established, by using two-port network theory, superposition principle and asymmetric component method, expression of fault components measuring impedance on both terminals of fault line’s sound phase and perfect line were deduced under different reclosure sequence when single-phase earth fault occured. Hereby, variation law of measured impedance using power frequency fault components along with fault distance, reactance both on rectifier side and inverter side and DC system’s equivalent impedance have been researched.Considering the two control modes of DC system such as the constant voltage control and constant extinction angle control, the expression among DC system’s equivalent impedance with fault voltage component and fire angle have been deduced respectively under the commutation failure condition and the normal condition. According to the expression, analysis on the variation trend of fault line’s sound phase and perfect line’s measured impedance along with the fire angle and fault voltage component have been performed, and thus the best reclosure sequence to decrease power frequency fault components distance protection’s maloperation can be obtained. Simulation results of 2015 Southern Power Grid system verify the conclusions’correctness and validity.