12KV Single-phase Recloser with DNP3.0 Communication Reclosing, 11KV Pole-mounted High-voltage Recloser

Single-phase reclosing is an automatic reclosing method used for transmission line protection in power engineering. Its working mode is as follows: In case of single-phase fault, only the faulty phase is disconnected and reclosed; in case of inter-phase fault, the three phases are tripped open and whether to perform three-phase reclosing is determined according to the situation; if the reclosing is successful on a permanent fault, the three phases are disconnected. This method is mainly applied to 220kV and above large grounding current systems, which can improve power supply reliability and system transient stability. Since the secondary arc current generated by the coupling of the healthy phases through capacitance and mutual inductance will affect the arc extinction at the fault point, measures such as neutral point grounding of high-voltage reactors or short-time closing of grounding disconnecting switches are required for arc suppression. Its phase selection elements include distance phase selection, phase current difference mutation phase selection, phase current instantaneous trip and other types. Internationally, ultra-high voltage lines above 1000kV mostly adopt single-phase reclosing, and double-circuit lines on the same tower adopt phase-by-phase reclosing.

Power system operation experience proves that most faults on overhead lines are temporary, such as insulator surface flashover caused by lightning overvoltage, short-term line touching caused by strong wind, discharge through bird bodies, and short circuits caused by branches and other objects falling on the conductors. When the faulty line is quickly disconnected, the arc will be extinguished immediately, and the insulation strength of the fault point will be restored. Therefore, reclosing once after the line is disconnected can greatly improve the reliability of power supply. Most 110kV and below lines adopt three-phase one-time reclosing. According to operation experience, more than 70% of short-circuit faults on high-voltage overhead lines in large grounding current systems above 110kV are single-phase ground faults, especially for overhead lines above 220kV, due to the large distance between phases, single-phase ground faults even account for about 90%. In this case, if only the faulty phase is disconnected and then single-phase reclosing is performed, while the non-faulty two phases continue to operate during the reclosing period, the reliability of power supply and the stability of system parallel operation can be greatly improved. Therefore, single-phase reclosing is widely used in large grounding current systems above 220kV. When the single-phase instantaneous ground fault protection trips open one phase, the healthy phases supply capacitive current to the fault point through inter-phase and relative-ground capacitance, and at the same time, the healthy phase load current induces an electromotive force in the faulty phase through the coupling of inter-phase transformers, and supplies inductive current to the fault point through inter-phase and relative-ground capacitance. These two currents jointly constitute the secondary arc current. The magnitude of the secondary arc current is related to the line parameters. Generally speaking, the higher the line voltage and the larger the load current, the larger the secondary arc current. Due to the existence of the secondary arc current, the arc at the fault point is not easy to extinguish, which delays the single-phase reclosing. When fast single-phase reclosing is adopted, the single-phase reclosing may fail and the three phases will be tripped open.

The single-phase reclosing time (the time from disconnecting the power supply at the fault point to reapplying the working voltage) must be greater than the time required for arc extinction at the fault point and line insulation recovery. Different from three-phase reclosing, during the single-phase reclosing process where only the faulty phase is disconnected, before the arc at the fault point is extinguished, the fault point still passes through the inter-phase capacitance, and the current is supplied by the other two phases in normal operation to maintain the arc. After the arc is broken, the fault point mainly bears the distributed voltage formed by the two operating phases through the fault relative-ground capacitance. When the arc elongates and breaks due to the current passing through zero, if the insulation recovery speed of the fault point is lower than the rising speed of the cross-connected voltage wave, the insulation of the fault point will break down again near the peak value of the voltage wave. At this time, in addition to supplying arc current through the non-faulty two phases, the electric energy stored in the fault capacitance will also be released to the fault point, forming a re-ignition current whose peak value is much larger than the previous steady-state arc current. This phenomenon repeats once every power frequency half-cycle, thus affecting the final arc extinction time. The higher the line operating voltage and the longer the line length, the longer the final arc extinction time will be. Artificial short-circuit tests and actual operation records confirm this conclusion. In order to shorten the single-phase reclosing time to a reasonable value, for example, no more than 1s, to meet the requirements of system stability (reclosing time), relay protection coordination, etc., without reducing the reclosing success rate, auxiliary arc suppression measures are widely adopted on medium and long lines with voltage of 500kV and above. There are two main types: ① At the neutral point of the high-voltage reactor used for line capacitance compensation on the line, grounding is carried out through an appropriate reactance value, which is equivalent to a resonant inductance connected in parallel, thereby significantly reducing the follow-up current passing through the fault point and the recovery voltage across the fault point, providing a prerequisite for the rapid and successful final arc extinction of the fault arc. This method has been successfully and widely adopted in power systems. ② During the single-phase reclosing process, the grounding disconnecting switches are short-time closed on both sides of the disconnected faulty phase line, so that the fault point is basically at zero potential, achieving the purpose of rapid arc extinction. For special important heavy-load lines (double-circuit lines on the same tower), if phase-by-phase reclosing is to be successfully realized under the condition of different-named phase faults (such as phase A of line A and phase B of line B), the grounding disconnecting switch is a feasible method.