Verification method

The appearance of the transformer under inspection should be intact, and the nameplate and marking should comply with the requirements of Article 5.1.

6.3.2 Insulation test

A 2.5kV megohmmeter should be used to measure the insulation resistance.

The power frequency withstand voltage test uses a sinusoidal voltage with a frequency of 50Hz plus or minus 0.5Hz and a distortion of no more than 5%.

The test voltage measurement error is no more than 3%.

During the test, the voltage should rise steadily from near zero and stay at the specified withstand voltage value for 1 minute.

Then it should drop steadily to near zero voltage.

There should be no abnormal sound or smell, no breakdown or surface discharge during the test, the insulation should remain intact, and there should be no detectable change in the error.

[Article Interpretation] On-site verification of transformers is usually carried out in conjunction with equipment acceptance and maintenance.

The implementation of the power industry standard DL/T 727-2013 "Transformer Operation and Maintenance Guidelines" and DL/T 596-1996 "Preventive Test Procedures for Power Equipment", and the national standard GB50150-2006 "Electrical Installation Engineering, Electrical Equipment Handover Test Standard" already includes insulation resistance measurement and power frequency withstand voltage test of transformers.

The items and methods of the insulation test adopt the content of the test after minor repairs in DL/T 727-2000, and the insulation resistance adopts the control value currently used by the manufacturer.

It is recommended to use a transformer tester to check the polarity of the winding.

According to the wiring mark of the transformer, after completing the measurement wiring according to the comparison method line, increase the current and voltage to less than 5% of the rated value for test, and use the polarity indication function or error measurement function of the tester to determine the polarity of the transformer.

[Article Interpretation] Although the wiring terminals of the standard transformer and the transformer under test are marked, marking errors may still exist and wiring errors may also occur.

If high voltage and high current are applied to measure when the polarity wiring is incorrect, it will cause overcurrent or overvoltage damage to the equipment.

Therefore, the polarity should be determined before normal measurement.

The polarity of the transformer is either additive or subtractive.

If the wiring is wrong, the measurement result will be abnormal, mainly manifested in the differential pressure or differential current output of the differential circuit is very large.

Some calibrators are equipped with a polarity test switch to check the input level of the differential pressure or differential current circuit.

If the polarity is reversed, the differential pressure will become twice the secondary voltage, and the differential current will become twice the secondary current.

Such large differential pressure and differential current output signals are sufficient to light up the polarity indicator.

For calibrators without a polarity test switch, polarity tests can be performed through error indications.

Regardless of whether a polar coordinate calibrator or a rectangular coordinate calibrator is used, the error indication will be abnormal in the case of polarity errors.

At this time, you only need to swap the polarity of the measurement wiring.

Of course, the reason for abnormal error readings is not just the wrong polarity connection.

Other reasons, such as the wrong ratio of the standard instrument, the wrong number of load boxes, or the faulty measurement circuit or the wrong grounding wiring, can also cause abnormal error readings.

JJG 1021-2007 recommends using a calibrator to check polarity, which is a common practice at present.

During the inspection, according to the requirements of the regulations, according to the wiring mark of the transformer, after completing the measurement wiring according to the comparison method circuit, slowly increase the current (voltage) to less than 5% of the rated value for testing.

During the process of increasing the current (voltage), if an abnormality is found, there is no need to continue to increase the current (voltage).

Do not use more than 5% of the rated current and voltage value for testing, otherwise once the differential pressure and differential current are too large, the polarity indication circuit or error measurement circuit of the calibrator will be damaged due to overvoltage or overcurrent.