Why is it necessary to perform an impulse test before commissioning a new power transformer or after major repairs? How many times should the test be performed?
Reasons for the impulse test:
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Verifying Insulation Strength
When disconnecting an unloaded transformer, switching overvoltage may occur. In a power system where the neutral point is not grounded or is grounded through an arc suppression coil, the overvoltage magnitude can reach 4 to 4.5 times the phase voltage; when the neutral point is directly grounded, it can reach up to 3 times the phase voltage. To check whether the transformer insulation can withstand the full voltage or switching overvoltage, an impulse test is required. -
Evaluating Mechanical Strength and Relay Protection
When energizing an unloaded transformer, inrush current can occur, reaching 6 to 8 times the rated current. The inrush current decays rapidly at first, typically decreasing to 0.25 to 0.5 times the rated current after 0.5 to 1 second. However, the total decay time can be quite long, especially for large-capacity transformers, possibly lasting several tens of seconds. The large electromagnetic force generated by the inrush current necessitates an impulse test to assess the transformer’s mechanical strength and to evaluate whether the inrush current during its initial decay phase may cause relay protection to malfunction.
Number of impulse tests:
- For new equipment commissioning: Perform the impulse test 5 times;
- After major repairs: Perform the impulse test 3 times.
Operating Requirements Before Commissioning New Equipment (Including Primary and Secondary Equipment)
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Pre-startup equipment operating mode:
- Before startup, all equipment to be commissioned should be switched to cold standby, and high-impedance devices (high reactors) should be connected to the line. The status of the line PT and bus PT does not require special attention;
- If the busbar needs to be de-energized, it should remain in normal operation until just before switching off during the operating steps, minimizing the de-energized time for the busbar;
- All secondary equipment within the scope of the commissioning must be properly operational, including control, protection, and fault recording devices. Relevant temporary measures for relay protection must be implemented. Protective measures that were temporarily disabled before startup (such as cancellation of line protection self-loop) should still be listed.
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Technical principles during the commissioning process:
- Number of charging operations: 3 times for lines, 5 times for the main transformer, and no mandatory requirements for switches and busbars;
- Charging switch requirements: Use switches equipped with charging protection;
- Disconnector operation requirements: In general, disconnectors should not be used to charge equipment that has not been previously energized. Direct disconnector closure for charging should only be performed after on-site verification;
- Selection of charging side: Charging should typically be performed from the opposite side of the line with high reactors, from a substation far from the converter station, or using a switch that is far from important transformers or critical lines;
- Relay protection requirements: During commissioning, minimize the time the busbar remains de-energized and the time multiple switches are subjected to charging protection on the busbar.
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