The AC voltage withstand test is one of the generator insulation tests. The advantages is that the waveform and frequency of test voltage is the same with working voltage. The insulation effect on the voltage distribution and breakdown performance of internal is equivalent to generator working state. From the point of view of deterioration or thermal breakdown, AC withstand voltage test is a reliable test method for generator main insulation.
Due to the above advantages, the AC withstand voltage test of manufacturing and installation, motor maintenance and operation and preventive test in generally adopted.
Then, do AC voltage test affect the insulation of the generator? What will be the impact? This is the general operation of personnel concerned, no doubt the motor insulation body will inevitably gas, such as a strong AC field under the gas free and also generates heat insulating oxide concentration, then free explosion, may make the mica insulation loss. The free chemical process of this electrical property is called "electrical aging".
1.The relationship between the breakdown voltage of insulation and the duration of pressurization
In the special cleaning conditions, we can use test method to find out the long time effect on AC voltage on insulator and bushing insulation asphalt mica insulation breakdown voltage and pressure maintaining time curves, respectively, as shown in Figure 1 and figure 2.
Figure 1: Mica asphalt insulation (13.8kV) breakdown voltage and compression maintenance time curve
1 - average; 2 - breakdown voltage spread out of the assembly line
Figure 2: relationship curves between bushing insulation (6.0kV), breakdown voltage and maintenance time
For mica asphalt insulation (13.8kV), the breakdown voltage of 1min is maintained at 75kV and the breakdown voltage of 1H is maintained at 60kV.
For bushing type insulation (6kV), the breakdown voltage which maintain 1min is 30kV,maintain1H is 25kV and maintain about 100h is 18kV. It can be seen that the test voltage decreases sharply with the increase of holding time. However, the breakdown voltage of the 1min is maintained at 5 times the rated voltage, whether it is impregnated with mica or impregnated or insulated by a village.
2.Influence of pressurization times on insulation
Figure 3 shows the relationship between the initial breakdown voltage and the test voltage to maintain the 1min and the number of times the 1min test is maintained.
Figure 3: the relationship between Ufb／UT1 n which is the times of maintaining 1min
Mathematically expressed as
Ufb／UT1＝f(n)
Moderate Ufb - initial breakdown voltage
UT1 - maintain the 1min test voltage
n -times of maintaining 1min test voltage
The generator operates within 30 years, test each year. According to figure 3
Ufb / UT1 = 1.4 UT1 = Ufb / 1.4
From the above test, the generator breakdown voltage (Un) is at least 5 times of the rated voltage. Therefore, after 30 years, the 1min test voltage shall be maintained as
UT1 = 5Un / 1.4 = 3.75Un
When the preventive test voltage is 1.5Un, within 30 years of service period, it should not cause generator insulation breakdown due to the accumulation of AC voltage test effect.
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