Stator winding leakage current and DC withstand voltage test

November 27, 2022

The leakage current and DC withstand voltage test of the stator winding of the generator can effectively find the end defects and some concentrated defects that have not been completely penetrated. The leakage current test is a non-destructive test, and the DC withstand voltage test is a destructive test.

First, the measurement wiring and method

The wiring of DC leakage and DC withstand voltage test is shown in Figure 4-3. It is best to connect the micro-ampere to the high voltage end and shield it to avoid the interference of strong electric field stray current. However, it should be noted that the micro-ampere meter must be operated with a lever of sufficient insulation level during the shorting and shifting operations during the test. The test phase winding of the generator is shorted and connected to the high voltage, and the non-test phase winding is shorted and grounded.

The steps of the measurement are as follows:

(1) Select the test voltage value according to the requirements of the Pre-Regulation.

(2) Before the test, it should be checked whether the tested generator has been powered off, grounded and discharged, and all external connections have been disconnected.

(3) For the in-service unit, it should be carried out in the hot state before the sewage is removed after the shutdown; for the standby unit, it can be carried out in the cold state. Hydrogen cooling is strictly prohibited during the replacement process, and the hydrogen content after hydrogen discharge should be less than 3%.

(4) After wiring, it should be reviewed by the second person.

(5) The boosting should be slow. According to the requirements of the “pre-regulation”, each stage of 0.5U _N rises in stages, and each stage stays for 1 min.

(6) During the boosting process, the generator, tester and micro-ampere meter should be monitored, and if there is any abnormality, the pressure should be reduced immediately.

(7) After the test is completed, it should be quickly stepped down to zero and the power supply is cut off to fully discharge (more than 5 minutes). When discharging, it should be carried out by a suitable discharge resistor (dedicated discharge rod).

(8) Organize the records and plot the current-voltage relationship.

It should be pointed out that the water-cooled generator should be tested by low-voltage shielding method to eliminate the influence of the waterway on the test. The wiring diagram is shown in Figure 4-4.

When using the low-voltage shielding method to test the stator winding of water-cooled generators, pay attention to:

(1) Water quality requirements. When testing in the water-passing state, the water quality should maintain the conductivity (2~5μS/cm), pH=7~8.

(2) In order to prevent the micro-ampere meter from swinging due to the small resistance of the water tube, a parallel capacitor ((1-2μF) (capacitor housing to ground insulation) or a water conductivity of less than 5μS/cm (such as 2μS/) can be used. Cm)

(3) The measured leakage current should be corrected according to equation (4-4) (considering the parallel connection of the inlet pipe to the ground insulation resistance)

I _x =I _A (1+R _A /R _dx )-I ₀

Where I _A - microampere meter measured current, μA;

R _A - micro-ampere internal resistance, Ω;

R _dx - micro-ampere parallel water resistance, Ω;

I ₀ -- no-load leakage current (ie tester current), μA.

(4) Before the water is pressurized, adjust the potentiometer, that is, adjust the internal resistance of the micro-ampere meter to make the micro-ampere meter indicate zero to compensate the deflection of the micro-ampere meter caused by the polarization electromotive force of the water conduit.

Second, the analysis and judgment of measurement results

(1) Under the specified test voltage, the difference of leakage current of each phase should not exceed 100% of the minimum value; if the maximum leakage current is below 20μA, the phase difference should not be significantly changed compared with the previous test results.

(2) When the leakage current increases disproportionately with the voltage, attention should be paid to the analysis.

(3) Determine the nature of the fault based on the abnormality of the leakage current. Figure 4-5 shows the typical leakage current curve of the generator, which can be analyzed and judged based on the measured relationship between the measured leakage current and the test voltage.

Author:

Mr. Jerry Zhu

E-mail:

sales@hnyongrong.com

Phone/WhatsApp:

+8615515501510