Translated Abstract
The method to distinguish instantaneous and permanent fault is the key of adaptive single-phase reclosure technology. In order to compensate reactive power of EHV/UHV transmission line and limit over-voltage, transmission line is generally equipped with shunt reactors at both ends and it has become the characteristics of modern power grid. When single-phase instantaneous fault occurs in the transmission line, the circuit breakers on both ends of line will trip, then electric current whose frequency is slightly lower than power frequency flows through the shunt reactor, which overlay with the power frequency component produces beat phenomenon. So many scholars put forward method to distinguish the instantaneous and permanent fault based on beat frequency characteristic. At present, the study of beat frequency characteristic is not yet perfect. Researches on adaptive reclosure lack analytic expression of beat frequency current; it’s unknown whether the beat frequency current definitely exists and the range of amplitude of the current. The amplitude may be nonexistent or too small when the single-phase transient fault disappears which will affect the reliability of reclosing determination.
Beat frequency current is affected by sound phase voltage and initial state of line components. The sound phase voltage provides fault phase power flow by inductance and capacitance coupling. The initial energy storage will also generate electric current in shunt reactor in the fault phase through network. Therefore, based on the superposition principle, get zero state and zero input beat frequency current separately, then form complete beat frequency current expression; By analyzing expression, find the least beat frequency current and its conditions, and analyze that the beat frequency current is zero belonging to small probability events; Analyze that beat frequency current is zero only under special condition, and the condition belong to small probability events. So it further discusses the beat frequency current amplitude range in general single-phase instantaneous fault, and analyze whether the beat frequency current on this range can be effectively used in the adaptive single-phase reclosure. Beat frequency current is affected by fault time, duration of the fault, transition resistance, phase angle difference and fault location, making beat frequency current clearly different under different working conditions. If the amplitude is too small, it will affect the reliability of the reclosing judgement. In this paper, using orthogonal experiment to calculate the range of beat frequency current amplitude.
Based on the superposition principle, get the complete expression of beat frequency current in shunt reactor after the single-phase instantaneous fault disappears in high voltage transmission line; Obtain the conclusion that the minimum value of beat frequency current is zero, and give the conditions to make the beat frequency current minimum. At the same time, it is concluded that the minimum value of beat frequency current is zero belonging to small probability events; Put forward to solve beat frequency current amplitude range by orthogonal test in mathematical statistics, and obtain that 500 kV transmission line beat frequency current range is between 0 and 200A, on this basis prove that the beat frequency current can＇t be used as a criterion of adaptive single-phase reclosure alone, only combine with other auxiliary criterion can be used in the adaptive reclosure.
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