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When the system resonates, under the action of resonance voltage and power frequency voltage, the magnetic density of the PT iron core quickly saturates, and the excitation current increases rapidly, which can cause serious overheating and damage to the PT winding (all PTs in the unified system are threatened), and even lead to bus faults causing large-scale power outages. It is crucial to ensure the safe operation of equipment by quickly eliminating resonance when it occurs.
1. Classification of Resonance and Analysis of Resonance Phenomenon
The resonance of a 6kV neutral ungrounded system can be divided into three types: fundamental resonance, high-frequency resonance, and frequency division resonance. Resonance generally occurs through grounding and excitation. Based on operating experience, fundamental resonance is prone to occur when suddenly charging an empty bus with only a voltage transformer; When single-phase grounding occurs, frequency division resonance is prone to occur, especially when single-phase grounding suddenly disappears (such as pulling a circuit), which can easily trigger resonance. When resonance occurs, the phase to phase voltage remains unchanged, and the voltage mutual inductance triangle will generate resonance frequency voltage. The central signal will report the "system single-phase grounding" signal. If the voltage change is not carefully analyzed, it will be mistaken as a system single-phase grounding fault. For substations without arc suppression coils, it is more important to quickly eliminate resonance. Below, we will analyze three resonance phenomena:
1. Fundamental resonance: When fundamental resonance occurs, there are two phenomena of relative ground voltage:
1) One phase voltage drops (not zero), and the two phase voltages rise to a high noble line voltage or voltmeter top meter;
2) Two phase voltage drops (not to zero), one phase voltage rises or the voltmeter is on top of the meter;
The overvoltage relative to ground voltage is less than or equal to 3 times the phase voltage;
2. High frequency resonance: When high-frequency resonance occurs, the overvoltage relative to the ground voltage is less than or equal to 4 times the phase voltage, and the three-phase to ground voltage rises together, far exceeding the line voltage or the top meter of the voltmeter.
3. Frequency division resonance: When frequency division resonance occurs, the three-phase to ground voltage rises alternately or simultaneously in phase sequence, and oscillates at low frequencies between (1.2-1.4) times the phase voltage, approximately once per second.
The resonance phenomenon mentioned above can be summarized as follows:
The phenomenon is judged by the ground signal of the busbar (with zero sequence output from the gear triangle), and the voltage of one phase relative to ground crosses the line voltage, while the voltage of two phases relative to ground crosses the line voltage.
Fundamental resonance: The three-phase relative ground voltage crosses the line voltage.
High frequency resonance: The three-phase to ground voltage rises alternately, but does not cross the line voltage. The three-phase to ground voltage rises simultaneously, but does not cross the line voltage frequency division resonance.
2. Handling of resonance occurrence
For our current 6kV ungrounded system, the main methods are to insert the arc suppression coil and change the operating parameters. Generally, inserting the arc suppression coil can eliminate resonance. For fundamental and high-frequency resonance, as long as the harmonic eliminator can operate reliably, resonance can also be eliminated. However, for frequency division resonance, it has zero sequence properties. Generally, the harmonic eliminator cannot eliminate resonance. Switching three-phase symmetrical loads does not work. For those who have not installed the arc suppression coil, it can be handled according to the following methods based on the actual situation:
1. Handling of fundamental or high-frequency resonance:
1) When there is a running capacitor, cut off the running capacitor; When no capacitors are running, put in a set of capacitors;
2) When the above measures cannot eliminate harmonics, cut off all capacitors of the busbar and apply to the regulator to cut off the departmental feeder, preferably by cutting the long line first.
2. Handling of Frequency Division Resonance:
1) Remove all capacitors from the busbar;
2) When resonance cannot be eliminated, apply to the regulator to cut off the line on the busbar until resonance is eliminated;
3) If the resonance cannot be eliminated even after all the lines are cut off, apply to the regulator to cut off the transformer switch and cut off the power to the busbar;
4) Restore power supply to the busbar and lines.