The judgment of PT three-phase voltage loss (symmetrical disconnection) is fundamentally the same for each manufacturer, and it is judged according to the three-phase no voltage and the current of the line. The same is true for PT asymmetrical wire breakage. After analyzing the characteristics of PT wire breakage, specifically aiming at the criterion of PT asymmetric wire break of different manufacturers, combined with a field practice, this paper points out the possible shortcomings of these criteria in the field application, and gives a useful PT wire break criterion, and the bus system proves the correctness and engineering useful value of the criterion after field application.
Characteristics of PT disconnection in busbar system
It is a common problem for PT to be disconnected in a substation. Once the PT is disconnected and the voltage is lost, the voltage of the maintenance device will be incorrect, and the correct acquisition of the voltage is a prerequisite for the correct operation of interval maintenance, directional lockout, and overcurrent maintenance with low-voltage engine elements. In the neutral ungrounded system, the single-phase grounding has the following characteristics: the grounding voltage of the grounding phase becomes zero, the ground-to-ground voltage of the other two phases increases by 3 times, and the load current and line voltage in the three-phase are still symmetrical. Therefore, in the line maintenance device of the neutral ungrounded system, the criterion of PT disconnection should be able to distinguish the single-phase grounding fault and asymmetric disconnection.
PT disconnection can generally be divided into PT primary side disconnection and secondary side disconnection, no matter which side of the disconnection, it will make the voltage of the PT secondary circuit abnormal. When the PT is disconnected on the primary side, one is to disconnect all the wires, and at this moment there is no voltage on the secondary side, and there is no voltage in the open triangle; The other is asymmetrical disconnection, at this moment there is no phase voltage on the secondary side of the corresponding phase, and the secondary voltage of the continuous linear phase remains unchanged, and there is pressure in the open triangle.
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