Precautions for lightning impulse test:
1. All connecting wires should be as short as possible, and all grounding should be low impedance and connected in a standardized manner to avoid interference.
2. According to national standards and relevant technical clauses, confirm the type of test and the test voltage of each terminal, and determine the transformation ratio of the impulse voltage generator and voltage divider used based on the test voltage of different windings. For three-phase transformers with tapping, the lightning impulse test is generally selected at two limit tapping and rated tapping.
3. The transformer is well assembled and injected with qualified insulation oil to reach the specified oil level. Transformers of different voltage levels should meet the corresponding oil injection and static discharge process requirements, and the parts that may accumulate gas should be deflated.
4. Ground the grounding terminal of the transformer oil tank point by point, and the iron core and clamp should also be reliably grounded.
5. After the wiring is completed, connect the main power supply, turn on the power supply of the measuring and control equipment, and the testing personnel debug the impulse voltage generator and complete all pre test preparations. The power on personnel must obtain the Lian system from the monitoring personnel before removing the grounding rod and applying power.
6. Pay attention to the insulation distance of the bare copper wire connected to prevent external flashover.
7. Generally, in the case of a test sample, reduce the voltage (usually 50% of the test voltage) to correct and adjust the voltage amplitude and waveform parameters applied to the test sample line segment. According to the provisions of GB/T76927.1-1999, the standard waveform used during the test is:
a. Lightning shock full wave: 1.2/50us
b. Lightning shock cutoff: cutoff time of 2-6us, zero crossing coefficient not greater than 0.3.
Note: The above allowable deviations refer to the allowable deviation between the specified value and the measured value, and do not refer to measurement errors.
8. When the neutral point is directly pressurized, the allowable wave head time is ≤ 13S. To ensure the wave tail time, it is allowed for the high-voltage section to be grounded through a resistor. However, attention should be paid to measuring the voltage at the support end and ensuring that it does not exceed 75% of the test voltage.
9. The non test terminals of the tested winding of the transformer with autocoupling connection can be grounded through a resistance less than 400 ohms, but the voltage on the non test terminals should not exceed 75% of their rated withstand voltage (for star connected windings) or 50% (for delta connected windings).
10. The wire terminals of the non test winding can be grounded through resistance, which will significantly increase the effective inductance of the tested winding of the test object; But it must be ensured that the voltage on the terminals of the non test winding does not exceed 75% of the rated withstand voltage of the winding (for star connected windings) or 50% (for delta connected windings).
11. When the tested winding is fully insulated or delta connected, the non terminal of the tested winding is allowed to be grounded through resistance, but the voltage on the non terminal should not exceed 75% of its rated withstand voltage (for star connected winding) or 50% (for delta connected winding).