Why do we need to do the impact test when the new main transformer is put into operation 30

When you say "transformer impulse test", you probably mean the transformer impulse closing test.

The impact closing test of a transformer does not necessarily have to be performed from the high voltage side, depending on the application of the transformer. Generally, this test is combined with the operation of the transformer. Since most of the step-down transformers we use are incoming callers, the incoming side is naturally on the high-voltage side, so it can only impact from the high-voltage side.

If the step-up transformer of the power plant is on the low-voltage side, the caller will be shocked from the low-voltage side.

Before the main transformer is put into operation for the first time, it should be closed five times under the rated voltage, the duration after the first power reception should not be less than 10 minutes, and the main transformer should be impacted three times after overhaul; The gas lower float should be tripped before the main transformer impact closing, the impact closing is normal, and the no-load charge is 24 hours under conditions; When the transformer of 110 kV and above is started, if there is a condition, zero voltage boost should be used; The on-load voltage regulating device of the transformer should be switched normally when the transformer is put into operation before it can be put into use.

The purpose of the impact closing test of the transformer is twofold:

1. When the no-load transformer is opened, there is a possibility of operating overvoltage. When the neutral point of the power system is not grounded or grounded through the arc suppression coil, the overvoltage amplitude can reach 4 times the phase voltage; When directly grounded at the neutral point, up to 3 times the phase voltage can be achieved. In order to check whether the dielectric strength of the transformer can withstand the full voltage or the operating overvoltage, an impact test is required.

2. When the live is put into the no-load transformer, the excitation inrush current will be generated, and its value can reach 6 8 times the rated current. The excitation inrush current begins to decay quickly, generally reducing to double the rated current value in seconds, but the total attenuation time is longer, and the transformer with large capacity can reach tens of seconds. Due to the large electrical power generated by the excitation inrush current, in order to assess the mechanical strength of the transformer, and at the same time to assess whether the initial attenuation of the excitation inrush current can cause the malfunction of the relay protection device, the impact test needs to be done.

1. When operating the no-load transformer, there will be operating overvoltage, and the impact closing test will be done to check the insulation level of the transformer and its accessories;

2. There will be a large excitation inrush current at the moment of transformer closing, the value of which is 6-8 times of the rated current, and this current will only appear on a section of CT of transformer differential protection, and there is no current flow on the CT on the other side. Because the excitation inrush current contains a high second harmonic, several impact closing tests can be done to check whether the setting of the second harmonic braking coefficient of transformer differential protection is appropriate;

3. Also because the transformer closes at the moment, the excitation inrush current is very large, which will produce a large electrodynamic force, and the impact closing test can be done to check whether the mechanical properties of the transformer meet the requirements.

Newly commissioned transformers need to undergo 5 impact tests for two reasons:

1) When the no-load transformer is opened, it may produce operating overvoltage, and the overvoltage amplitude can reach 4 times the phase voltage when the neutral point of the power system is not grounded, or when it is grounded through the arc suppression coil; When the neutral point is directly grounded, it can reach 3 times the phase voltage, and in order to check whether the dielectric strength of the new transformer can withstand the full voltage or the operating overvoltage, an impact test is required.

2) When the live is put into the no-load transformer, there will be an excitation inrush current, the value of which can reach 6 8 times the rated current. The excitation inrush current begins to decay quickly, generally after the rated current value, but the total attenuation time is longer, the large-capacity transformer can reach tens of seconds, because the excitation inrush current produces a large electrodynamic force, in order to assess the mechanical strength of the transformer, and at the same time assess whether the initial stage of excitation inrush current attenuation can cause relay protection malfunction, it is necessary to do an impact test.

When the no-load transformer is removed, there is a possibility that the operating overvoltage will be generated, which can reach more than three times the rated phase voltage, and when the no-load transformer is put into the no-load transformer, the excitation inrush current will be generated, which can reach 6-8 times the rated current, and a large amount of electric power will be generated. The purpose of the impact test of the newly put into operation transformer is to check whether the insulation of the transformer can withstand the operating overvoltage. Assess the mechanical strength of the transformer and verify whether the relay protection will malfunction.

Answer: The reasons why the new transformer or the overhauled transformer should be tested before it is officially put into operation are as follows

1. Check the insulation strength of the transformer.

Whether the DAO is exposed to full voltage or operational overvoltage.

When the no-load transformer is opened, it is to cut off a small excitation current, which may be forced to extinguish before the excitation current reaches the zero point, due to the interception phenomenon of the circuit breaker, the operating overvoltage generated by the transformer with inductive properties, its value is not only related to the performance of the switch, the transformer structure, etc., the grounding mode of the neutral point of the transformer also affects the overvoltage of the no-load transformer. Generally, the overvoltage amplitude of the transformer that is not grounded or grounded through the arc suppression coil can reach times the phase voltage, while the operating overvoltage amplitude of the transformer directly grounded at the neutral point generally does not exceed 3 times the phase voltage. This is also the reason why the neutral point of the transformer that is required to do the impact test is directly grounded.

2. Assess the mechanical strength of the transformer under the action of large excitation inrush current and assess whether the relay protection will malfunction under the action of large excitation inrush current.

Number of impact tests:

The new transformer investment generally needs to be impacted five times, and the overhauled transformer investment generally needs to be impacted three times.