Influence of AC withstand voltage test on generator insulation

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 U_fb／U_T1 n which is the times of maintaining 1min

Mathematically expressed as

                                                    U_fb／U_T1＝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.

Operation Method of AC withstand voltage and series resonant test for Generator

With the development of power system , the capacity of the operating equipment is larger, such as thermal power generating units whose capacity is more than 1000MW, and hydraulic power unit which the largest single capacity has reached 800MW; To do AC power frequency withstand voltage test of these devices, if you use the traditional test equipment (frequency test transformer), due to the need of larger capacity , transformer and voltage regulator are very heavy. The power frequency resonance device has great advantages in the aspects of test power supply capacity, equipment weight, test waveform and investment. The device is mainly designed for the AC voltage withstand test of the generator with a capacity of 250MW or less. Specific methods of operation are as follows:

Over current protection setting, 1.2 times the rated current of high voltage side of transformer. For example: the high voltage side rated current of exciting transformer is 50A, the configuration of the transformer is 50/5, so the current relay should be adjusted to 5A.

Over voltage protection settings, according to the test voltage 1.1 times set.

For example: When the test voltage is 39kV, the display should be adjusted to 43kV.

The divider wire is connected to the high voltage current two terminal of the console.  Note: All wire must be connected, firm and reliable.

The high voltage current on the exciting transformer is connected to the high voltage current two terminal of the console.

Pressure time setting: set the test time on the time relay to the required test time.

According to the above connection that test wiring is correct, close circuit breaker of the console . At this time if the regulator is not zero, the regulator will automatically return to zero. Do not turn on the main power source, adjust the reactor core clearance, observe the lifting and clearance limit protection function to make sure whether it is normal.

Close the main power source, push the "boost" button to boost on the reactor gain hundreds of volts, by changing the air gap to tune the output voltage to reach the maximum. At this time it can be tuned, and boost to the value of the test voltage. When the voltage reaches the set value , the equipment will automatically return to zero.

Disconnect main power source.

During the test, the relevant personnel should strengthen the monitoring of the test equipment. Once the abnormal phenomenon occurs, the voltage regulator should be quickly reduced, and the power source will be cut off.

The article is powered by Shanghai Himalayal Co.,Ltd

http://www.himalayal.com.cn

Himalayal,high voltage test equipment expert; we will always by your side.