JB/T 12685-2016Stator coils for high-voltage electrical machines - Technical requirements (English PDF)

5.6 Corona starting voltage

The specimen is subjected to a corona starting voltage test in a dark room. The test voltage waveform shall be an actual sine wave, which is subject to visual inspection.

5.7 Dielectric loss tangent

5.7.1 Normal dielectric loss tangent tan delta and its increment delta tan delta

The measurement shall be carried out at room temperature. The length of the measuring electrode (see 5.1) shall be the length of the coil slot; the measuring electrode shall be connected to the guard electrode. The width of the guard electrode shall be no less than 10 mm. The gap between the guard electrode and the measuring electrode shall be no more than 4 mm, generally within 2 mm ~ 4 mm. The measuring voltage starts from 0.2UN and is measured every 0.2UN until 1.0UN. The main test equipment shall meet the requirements of JB/T 7608-2006.

5.7.2 Hot dielectric loss tangent tan delta

The specimen shall be placed in an oven with hot air circulation, and the test temperature shall be controlled at 155 °C ± 2 °C. After keeping it for 1 h, measure it under the test voltage of 0.6UN. The length of the measuring electrode is the length of the coil slot; the measuring electrode shall be connected to the guard electrode. The main test equipment shall meet the requirements of JB/T 7608-2006.

5.8 Voltage durability test

5.8.1 Quality assurance test

Before the voltage durability test, the quality assurance test shall be carried out according to the following requirements.

- Specimen appearance inspection;

- Voltage withstanding test according to GB 755-2008;

- Impact voltage level test according to GB/T 22715-2016.

5.8.2 Voltage durability test

Under room temperature conditions, apply a power frequency AC voltage of 2.17 UN or 2.53 UN to the insulation of the coil slot until the coil insulation breaks down. The test voltage waveform shall be an actual sine wave. The accumulated time of electrical aging is the failure time of the specimen.

The specimen shall be at least 2 frame coils or 4 bar coils. Any slot of the frame coil and its end or the entire bar coil is regarded as one specimen. Anti-corona measures shall be taken for all specimens.

5.8.3 Judgment criteria

The judgment criteria for voltage durability test are as follows.

a) If the failure time of all specimens is not less than 400 h (test voltage is 2.17UN) or 250 h (test voltage is 2.53UN), it can be judged that the product it represents meets the test requirements.

b) If the failure time of the specimen is within 200 h ~ 400 h (test voltage is 2.17UN) or 125 h ~ 250 h (test voltage is 2.53UN), but the number of failed specimens is not more than 26% of the total, then an additional frame coil or 2 bar coils shall be selected for testing. If the number of specimens put into the electrical aging test is not less than 6, no additional specimens are required. All remaining coils (including additionally selected coils) shall pass the test. If there are still specimens with a failure time of less than 400 h (test voltage is 2.17UN) or 250 h (test voltage is 2.53UN) during the repeated test, it shall be judged that the product it represents does not meet the test requirements.

c) If the failure time of the specimen is within 200 h ~ 400 h (test voltage is 2.17UN) or 125 h ~ 250 h (test voltage is 2.53UN), meanwhile the number of failed specimens is greater than 26% of the total, it shall be determined that the product it represents does not meet the test requirements.

d) If the failure time of the specimen is less than 200 h (test voltage is 2.17UN) or 125 h (test voltage is 2.53UN), it shall be determined that the product it represents does not meet the test requirements; the breakdown coil shall be dissembled and analyzed; improvement measures shall be formulated and retested.

6.1 Sampling inspection

Sampling inspection shall be carried out every six months; the number of inspections can be increased if necessary. The sampling inspection items are as follows.

a) Test of insulation withstand power frequency voltage to ground;

b) Withstand impulse voltage level; […]