Several issues that should be paid attention to in the withstand voltage test of low voltage transformer

The most important indicator of whether a transformer can work reliably is the insulation structure. According to statistics from relevant departments, about 60% of transformer failures occur in the insulation system, which shows how important it is to conduct quality inspections on the insulation performance of transformers. The national standard GB 19212.1-2003 "Safety of power transformers, power supply devices and similar products Part 1: General requirements and tests" has detailed provisions on the voltage value and test parts of the power frequency withstand voltage test of low-voltage transformers. I have been engaged in the design of low-voltage transformers for a long time and have summarized some issues that should be paid attention to in withstand voltage tests. Here is a brief analysis.

1. Withstand voltage test steps

During the test, the following steps should be strictly followed to ensure the correct judgment of the test results and the safety of the test process.

1. Check whether there are any unsafe factors in the test environment. If not, turn on the withstand voltage equipment and preheat it for 5 minutes.

2. Check whether the test equipment is set to the voltage gear required for the test and whether its set leakage current value meets the requirements.

3. Short-circuit the high-voltage output end of the test equipment and power on to check whether the overcurrent relay is activated or whether a breakdown signal is issued.

4. Connect the test clips of the test equipment to the specified test parts (two or more parts isolated from each other such as transformer windings, shielding, core, frame, etc.).

5. Operate the test equipment to increase the voltage. At the beginning of the voltage increase, slowly increase it to half of the specified value (jumps should be avoided), and then quickly increase it to the specified voltage value (the entire voltage increase process is about 10 s), which lasts for 1 minute. During this period, no continuous arcing and breakdown are allowed to occur. Then the voltage is slowly reduced to zero, the power is cut off, and the test is completed.

Remember, do not use the sudden power-off method to avoid the transformer breakdown caused by the oscillation overvoltage caused by the instantaneous loss of pressure.

2. Judgment method of withstand voltage test results

If the voltage drops or the breakdown signal occurs during the test, do not easily judge that the transformer is broken down. Further testing should be continued for further confirmation:

1. Use a megohmmeter to measure the insulation resistance. If the insulation resistance is zero or close to zero, it is judged as breakdown; or a secondary boost test is performed, and the voltage is gradually applied. If it is a breakdown, when the voltage is added to a certain value, continuous spark discharge or heat and smoke can be observed near the breakdown point, which is judged as breakdown. If the voltage rises and then falls during the second pressure application, and the pointer of the ammeter swings violently, it is judged as arcing failure.

2. If the insulation resistance does not change much, or there is no breakdown action for 1 minute after the secondary boost, it is considered that the first breakdown is an air gap breakdown (caused by dust and other substances), which we usually call arcing. After the external voltage disappears, the breakdown gap immediately recovers by itself, the insulation resistance of the transformer will not change, and the insulation performance of the transformer will not change, so it cannot be judged as unqualified.

3. Calculation and analysis of the capacity of the high-voltage equipment used in the test

The capacity of the step-up transformer in the equipment used in the test should be determined according to the capacity of the transformer being tested. If the capacity is too small, the output voltage of the transformer will be significantly reduced due to the sharp increase in leakage current when the pressure is applied, which will affect the correct judgment of the test results. Step-up transformers generally need to meet two conditions: (1) During normal continuous withstand voltage test, the temperature rise of transformer windings should not exceed the allowable value; (2) When the transformer under test breaks down, the transformer should be able to provide a sufficiently large current to pass through the insulation breakdown point.

According to the first condition, the capacity of the step-up transformer can be estimated by an approximate formula:

(3-1)

Where: ——Test transformer capacity, kVA

——Test frequency, HZ

——Test insulation geometric capacity.

If according to formula (3-1), the capacity of the transformer for the withstand voltage test of low-voltage transformers is generally selected to be above 0.5kVA, which is feasible. However, since the transformer capacity is related to its short-circuit impedance, the transformer short-circuit impedance must be small enough to ensure that the current output is not less than 0.5A when the transformer under test breaks down at the minimum possible test voltage. This current value is specified by IEC. This current is actually the short-circuit current of the high-voltage winding when the transformer is output, and is generally between 5% and 10%. Therefore, the transformer capacity can be expressed by formula (3-2).

% (3-2)

Where: S——transformer capacity, KVA

——transformer output maximum test voltage, KV

——transformer output minimum low test voltage, KV

I——current that the transformer must output when the sample breaks down, regulations

——transformer short-circuit impedance percentage, between 5 10.

According to the low-voltage electrical product standard, take ＝3.5KV, ＝1KV, I=0.5A, ％take 10, substitute into formula (3-2) to get:

(kVA)

From the above calculation results, it can be seen that in general, the test transformer capacity is greater than 0.6 kVA, but considering the margin, take 0.75 kVA.

4. Withstand voltage test equipment operating current

In the low-voltage transformer withstand voltage test, under the action of the test voltage, when the electric field strength in the transformer insulation medium reaches a certain critical value, its insulation performance begins to lose, and the leakage current increases sharply. When the current value pre-specified by the overcurrent relay on the high-voltage side of the withstand voltage equipment is reached, the relay operates and cuts off the high-voltage output. The withstand voltage test generally determines whether the relay is actuated or not to determine whether the breakdown occurs. Therefore, the current setting of the overcurrent relay is directly related to whether the test product can be correctly judged. Generally, the relevant standards for low-voltage electrical appliances clearly stipulate that 100mA is the setting value of the overcurrent relay on the high-voltage side.

The withstand voltage test, also known as the electrical strength test or the dielectric strength test, is to prove the rationality of the design, material selection and manufacturing process. It is also one of the very important test items for assessing the safety performance of the transformer. Its purpose is to verify whether the transformer can work for a long time in actual operation without causing arcing accidents due to the rated voltage applied to the insulation.
4.

Connect the test clamps of the test equipment to the specified test parts (two or more parts isolated from each other such as transformer windings, shielding, core, frame, etc.).

5. Operate the test equipment to increase the voltage. At the beginning of the voltage rise, slowly rise to half of the specified value (jumps should be avoided), and then quickly increase to the specified voltage value (the entire voltage rise process is about 10 s), which lasts for 1 minute. During this period, no continuous arcing and breakdown are allowed to occur. Then the voltage is slowly reduced to zero, the power supply is cut off, and the test is completed.

Remember, do not use the method of sudden power off to avoid the transformer breakdown caused by the oscillation overvoltage caused by the instantaneous voltage loss.

2. Judgment method of withstand voltage test results

If the voltage drops or the breakdown signal occurs during the test, do not easily judge that the transformer is broken down. Further tests should be continued for further confirmation:

1. Use a megohmmeter to measure its insulation resistance. If the insulation resistance is zero or close to zero, it is judged as a breakdown; or a secondary voltage rise test is performed, and the voltage is gradually applied. If it is a breakdown, when the voltage is added to a certain value, continuous spark discharge or heat and smoke can be observed near the breakdown point, which is judged as a breakdown. If the voltage rises and then falls again during the second pressure application, and the pointer of the ammeter swings violently, it is judged as an arc failure.

2. If the insulation resistance does not change much, or there is no breakdown action for 1 minute after the secondary voltage boost, it is considered that the first breakdown is air gap breakdown (caused by dust and other substances), which we usually call arcing. After the external voltage disappears, the breakdown gap will immediately recover by itself, the insulation resistance of the transformer will not change, and the insulation performance of the transformer will not change, so it cannot be judged as unqualified.

3. Calculation and analysis of the capacity of high-voltage equipment used in the test

The capacity of the step-up transformer in the equipment used in the test should be determined according to the capacity of the transformer under test. If the capacity is too small, the transformer output voltage will be significantly reduced due to the sharp increase in leakage current when the pressure is applied, which will affect the correct judgment of the test results. The step-up transformer generally needs to meet two conditions: (1) During the normal continuous withstand voltage test, the temperature rise of the transformer winding should not exceed the allowable value; (2) When the transformer under test breaks down, the transformer should be able to provide a sufficiently large current to pass through the insulation breakdown point.

According to the first condition, the capacity of the step-up transformer can be estimated by an approximate formula:

(3-1)

Where: ——Test transformer capacity, kVA

——Test frequency, HZ

——Test sample insulation geometric capacity,

if according to formula (3-1), the capacity of the transformer for the low-voltage transformer withstand voltage test is generally selected to be above 0.5kVA, but because the transformer capacity is related to its short-circuit impedance, the transformer short-circuit impedance must be small enough to ensure that the current output is not less than 0.5A when the transformer under test breaks down at the minimum possible test voltage. This current value is specified by IEC. This current is actually the short-circuit current of the high-voltage winding when the transformer is outputting, and is generally between 5 and 10. Therefore, the transformer capacity can be expressed by formula (3-2).

% (3-2)

Where: S - transformer capacity, KVA

- transformer output maximum test voltage, KV

- transformer output minimum low test voltage, KV

I - current that the transformer must output when the sample is broken down, regulations

- transformer short-circuit impedance percentage, between 5 and 10.

According to the low-voltage electrical product standard, take =3.5KV, =1KV, I=0.5A, % is taken as 10, and substitute into formula (3-2) to get:

(kVA)

From the above calculation results, it can be seen that in general, the test transformer capacity is greater than 0.6&