CURRENT PROTECTION OF A POWER TRANSMISSION LINE AGAINST PHASE-TO-PHASE SHORT CIRCUITS

摘要

The invention is related to electric engineering and can be used in the field of relay protection of power transmission lines of single-end electric systems. The objective of the invention is improvement of current protection sensitivity to asymmetrical short circuits in power transmission lines. Said objective is attained by provision of current protection of a power transmission line against phase-to-phase short circuits comprising input current transducers having their inputs connected to measuring transformers of the protected transmission line current, and their outputs to inputs of a negative sequence current filter, a maxi-selector and a mini-selector; the mini-selector output is connected to the first input of a short-circuit type determining unit, its second input is connected to the output of the maxi-selector; connected to the maxi-selector are the second input of the second-stage measuring instrument with a setpoint adjustment unit, the input of the first-stage measuring instrument with a setpoint adjustment unit, the third input of the short-circuit zone determining unit with a parameter setting unit, and the fourth input of the time delay computation and control unit with a parameter setting unit. The output of the negative sequence current filter is connected to the third input of the second-stage measuring instrument, to the fourth input of the short-circuit zone determining unit and to the fifth input of the time delay computation and control unit, the output of the latter being connected to the first input of an OR gate; the output of the short-circuit type determining unit is connected to the first input of the second-stage measuring instrument, to the third input of the time delay computation and control unit and to the first input of the short-circuit zone determining unit, the output of the latter being connected to the first input of the time delay computation and control unit and to the first input of an AND gate; the output of the second-stage measuring instrument is connected to the second input of the short-circuit zone determining unit and to the second input of the time delay computation and control unit; the output of the first-stage measuring instrument is connected to the second input of the AND gate having its output connected to the second input of the OR gate, the output of the latter being connected, via an alarm device, to an actuator that disconnects the switch of the protected power transmission line. The second protection stage according to the invention is made as responding to total currents of phases in case of symmetrical short circuits in a power transmission line, and as responding to negative sequence current in case of asymmetrical short circuits. As a result, the second protection stage in case of asymmetrical short circuits does not require shifting out of load modes but shifts out of misbalances of negative sequence filters and of negative sequence currents of admissible continuous asymmetry. This ensures significant reduction of the second protection stage actuation current value and leads to higher sensitivity of the second stage to asymmetrical short circuits.