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.

FAQ about DC Dielectric and Leakage Test for Electric Cables

Question 1：The paper insulated power cable only adopts the DC withstand voltage test.

(1) The capacity of the cable is large. AC withstand voltage test needs test transformer with large capacity，but there is no such test conditions on-site.

(2) AC voltage withstand test may cause free discharge in the gap of paper insulated cable, which damages the cable. When the voltage is the same, the AC voltage is more harmful to the cable insulation than the DC voltage.

(3) The leakage current can be measured at the same time of  AC voltage withstand test. According to the value of the leakage current and the relationship between the leakage current and the test voltage, the insulation condition of the cable can be determined.

(4) If there are some local voids in the paper insulation, the DC voltage is mostly distributed on the parts that are related to the defects. Therefore, it is easier to expose the local defects of the cable.

Question 2：Polyethylene cable should not be tested by DC high voltage

The distribution of electric field of XLPE cable under AC and DC voltage is different. XLPE cable insulation layer is made of polyethylene by chemical cross-linking. It is an integral type insulation structure. Its dielectric constant is 2.1~2.3, and it is generally not affected by temperature change. Under AC voltage, the electric field distribution in the insulation layer of XLPE cable is determined by the dielectric constant, that is, the electric field strength is inversely proportional to the dielectric constant, and this distribution is relatively stable.

Under the influence of DC voltage, the electric field intensity in the insulation layer is directly allocated according to the insulation resistance factor, and the distribution of the insulation resistance coefficient is uneven. This is because the inevitable by-product into a certain amount in XLPE process, such as methane, acetophenone, polyvinyl alcohol, they have a relatively small coefficient of insulation resistance. The distribution of radial insulation layer is uneven. So that under DC voltage, the electric field in the XLPE cable insulation layer’s distribution is different from the ideal cylinder insulation structure, and related to the material inhomogeneity.

Question 3：The main insulation of oil filled cable doesn't do DC withstand voltage test when it is put into operation

(1) Surveillance by other tests. In operation, the external force may have damaging effects on the self-contained oil filled cable, which can be measured by the outer sheath insulation resistance and to monitor the oil pressure. Insulation aging can be carried out through the oil monitoring, performance changes so unnecessary for DC voltage test.

(2) High voltage and difficult test. The voltage level of self-contained oil filled cable is high, so the test voltage is high, but also in the terminal head around and many other electrical equipment, is generally difficult for high voltage test.

Based on the above reasons, the main insulation of the capacitive oil filled cable will not be subjected to DC voltage withstand test except in special circumstances.

Question 4：When measuring the DC leakage current of the cable, the pointer of the micro meter has a swing

If there is no impact on cable terminal and dirt test power instability and other factors, in the measurement, DC microammeter cyclical swing, possibly because of porosity defects in local cable test in. Porosity defect breakdown occurs at a certain voltage, leakage current increases, the cable capacitance is gap after discharge breakdown. When the electric cable exit charging voltage was gradually increased, the gap is again breakdown; then, the insulating gap again to get recovery. Thus, the periodic oscillation of the micro meter in the measurement is repeated.

Question 5：Precautions for measuring leakage current of cables over 10kV

Measurement of 10kV and power cable leakage current combined with DC voltage. The test voltage is divided into 4~5 grades and rises to 3-6 times the rated voltage. Because of the higher voltage, corona discharge may occur at the end of the lead and the cable when the voltage increases. After the DC test voltage exceeds 30kV, the leakage current of electric cable with good insulation increases, so the leakage current with test voltage rise and rapid growth of the phenomenon, does not necessarily mean that the power cable is defective. At this time, we must use the interelectrode insulating layer, barrier or cover, and bold lead, increasing the lead to the distance and other measures to reduce stray corona discharge leakage current, and then judge real power cable insulation level according to the measurement results. 

Measures to improve synchronous performance of impulse voltage generator

The gap discharge is dispersive, and the atmospheric conditions, dust and spherical states may also increase the dispersion. In order to make impulse voltage generator to be reliably adjusted and well synchronized, it is necessary to make the multiple of the overvoltage exceed the scatter range of the ball gap discharge, and some people believe that the overvoltage should be no less than 1.2 times.

At present, high efficiency impulse voltage generator mostly adopts high efficiency loop. Meet the following two conditions:1. If the main capacitor is large, produce lightning surge discharge resistance is too small 2.When the load capacitance is low, operation wave is too large wavefront resistance, which will enable the emergence of natural overvoltage low, only about 1.1 times. Synchronization is not reliable.

To overcome this difficulty, usually in two ways: One is to use the proper arrangement of the circuit increases natural overvoltage; another is to try to make the middle ball gap also trigger ignition, urges each ball gap and discharge.

Figure 1: method for increasing stray capacitance and improving synchronous performance

Figure 1 is a double voltage loop of bilateral charging. The natural overvoltage of this circuit is not high. When the G1 is ignited, the overvoltage multiples present on the G2 are at most 1.5 times. If the X point potential changes, the excess voltage will decrease. The Cg in the figure connects several thousand euro Rg and several hundred skins X at the point. As the Rg and Cg have fixed X point potential, the overvoltage multiples on the G2 are 1.5 times. Generally speaking, this kind of impact voltage generator only starts the two stage discharge, and the discharge of the lower level is easier, so it is not necessary to take measures to fix the potential at all levels

Irradiation can induce gap discharge. Generally, the ball gap of the impulse voltage generator is placed on a vertical line. The first stage of the gap discharge produces ultraviolet radiation to the first stage of the gap, causing it to discharge, thereby improving synchronization performance. Figure 2 is the use of each ball gap between the needle and the small spark between the ball to illuminate the ball gap to improve synchronization performance. Figure 2 at charging point, 1,2,3 potential is zero, 1 ', 2', 3 'potential is +U. Each ball is connected to the pin by a small resistance R, which is equal to its own needle potential when charging, and the stray capacitances C2a, C3a and C2b, are free of charge on the C3b.

Figure 2: methods to improve synchronization performance using triggers

The external pulse is sent to the ball skin of the first stage ball gap G1, resulting in a small spark between the two ball's skin and needle pole, thus causing a G1 discharge. The point 1'potential drops to zero, and the point 1 potential changes to -U. The T2b needle potential changed to -U, but the Tzb ball potential remained zero as a result of the transient isolation of R and the transient voltage stabilization of C2b, thus the discharge between the ball skin and the needle electrode.

Similarly, the needle potential of T2a is +U, and the ball skin potential fluctuates as the point 1 'potential drops to zero, and the discharge between the ball skin and the needle is very close. This small spark irradiates the gap and causes G2 discharge. The same goes for G3, g4,... Discharge. In this way, the gap gap discharge can be expanded by triggering the gap discharge, and the synchronization performance is improved.

One way to reduce the dispersion of the spark gap discharge voltage and improves the synchronization performance, so generally use the ball gap symmetry, the gap distance is less than 1/2 diameter, spherical ball to clean, to be arranged in a vertical etc.measures. Exposed gaps are susceptible to external conditions such as air, dust, etc..

For outdoor impact voltage generator, these effects are more significant, which will increase the dispersion of the ball gap discharge. In order to obtain an impact test data, often dozens of times to adjust the voltage, accurately adjust small distance by mechanical transmission, not only troublesome and difficult, so the gas pressure adjustable discharge sphere gap. The ball gap is fixed in the sealed container, and compressed air is used to regulate the air pressure, so that the discharge voltage of the coup can be achieved. This not only eliminates the influence of the external conditions on the discharge, but also adjusts the discharge voltage more accurately

Figure 3 :multistage gap circuit

There is also a multipole gap, as shown in figure 3. With this gap, there is no need to adjust distance or air pressure, but the gap can be conducted within 20 kV to 200 kV, and the dispersity is less than 50 ns. The working principle of this gap is as follows:

The 200 kV voltage is applied between the ends of the AB, and the voltage at each gap is due to the uniform distribution of the R

200 kV/15 = 13.3 kV

In fact, the breakdown voltage at each gap distance is 16.9 kV. At rated voltage, the applied voltage on each gap is only 79% of the breakdown voltage. The P2 ~ P15 is connected by C with the ignition electrode I, and the voltage of I is increased by Ui, and the potential of P2 to P15 is increased by U1.

Such as G1 voltage between U>16.9 kV, gap G1 breakdown. The P2 potential is equal to P1, and G2 follows the breakdown until the G14 breakdown. The P15 potential is equal to P1. The charge applied to the P16 is the charge voltage, and G15, of course, follows the breakdown. Such as "U1" 16.9 kV, may also start simultaneous breakdown at the end, to the central all through.

The U1 voltage is generated by charging 10 kV DC voltage from the outside and charging the C1 via R1, Ri, and N1. 5 kV pulse voltage by C1 to Gi pole needle electrode gap, the Gi breakdown, so Ci to N1 discharge attenuation oscillation wave, coupled to the N3 U1 output voltage to the ignition electrode I, coupled to the N2, send a pulse voltage to the next level of multipole clearance. The discharge spark of G1 is irradiated to G1, G2 and so on, which can greatly reduce the dispersion of discharge.

The multi pole gap is installed on the ignition gap of the impulse generator and the position of the spherical gap in the middle of each pole. The ignition of the first stage is formed by the pulse voltage through the R1C1, and the ignition pulses at the second level are transmitted via the R2C2 path (see Figure 3).

The voltage drop of the multipole gap arc is large, which makes the generator's output voltage and the charge voltage show a nonlinear relationship. The gap is more prone to arc extinction, and the output voltage waveform may jump discontinuously. Some experts think it is not suitable to use it in the impulse voltage generator of transformer factory.