Impulse voltage generator is a high voltage generator that generates pulse waves.
Originally, it was used only to study the insulation performance of power equipment which subjected to atmospheric over-voltage (lighting）, and now it is used to study the insulation performance of power equipment subjected to over-voltage operation. Therefore, for impulse voltage generator, we require it not only can produce the lightning waveform but also the operating overvoltage waveform . The destructive of impulse voltage not only depends on the amplitude,it’s also associated with the wavefront steepness. Some of the equipment is also tested by the chopping wave.
In addition, impulse generator can also be used as an important component of nanosecond pulsed power device. It can be used as power supply in high power resistance beam and ion beam generator and CO₂ laser.
According to the actual measurement, the lightning wave is a non periodic pulse, and its parameters are statistical. The front time (the time required is about from zero up to the peak) is 0.5μs~10μs, half peak time (about from zero up to a peak and peak time reduced to 1/2) is 20μs~90μs, the cumulative frequency is 50% for the wavefront and the half peak time is about1.0μs~1.5μs and 40μs~50μs.
The duration of impulse wave is much more than the operating voltage of lightning impulse voltage wavelength, with more complex shapes, and his shape and duration, in line with the specific parameters and length vary, but the current international tend to use a few hundred microseconds long wavelength wavefront and thousands of microsecond pulse representing it. Lightning wave can be divided into two kinds of full wave and chopped wave. The chopped wave is used to cut off the impulse wave produced by the impulse voltage generator and cut off the voltage sharply. The truncation time can be adjusted or taken place at the wave front or at the wave tail.
In order to ensure the repeatability of the test results and the comparability of the test results between each test, the definition of waveform and waveform should be clear. To this end, the IEC and the national standard set the standard lightning impulse, full wave, chopping wave waveforms and standard operating impulse voltage waveforms, as shown in Figures 1 through 4.
Figure 1: Lightning impulse voltage full wave
In Figure 1, 0 is the origin. Sometimes the waveform taken by an oscilloscope tends to be blurred near or near 0, or there is an initial oscillation. When the generator of the impulse voltage is big inductance, the beginning of the waveform may also be a little more flat. At this point, the origin of the waveform (the real starting point) is not easily determined on the time axis. The peak point of the voltage wave, due to its flatness, is not easily determined in time. The IEC and the national standard employ the method shown in Figure 1 to obtain the origin O1, and then calculate the wavefront time Tf and the half peak time Tf from O1. The prescribed wave front time Tf is T/0.6. The standard lightning impulse test, the wave front time Tf of voltage wave is 1.2μs±30%, and the half peak time Tt is 50μs±20%.
(a) recording and testing voltage waveforms;(b)After the basic waveform superposition filter, the residual waveform becomes the test voltage waveform
If the wavefront peak overshoot or oscillation with wave (Figure 2a), according to new rules of IEC60060 1 , processing steps at this time should be specified by the standard, the recorded waveform is converted into test voltage waveform (Fig. 2b), the amplitude of latter is
Ut＝Ump＋k（f）（Ue－Ump）
K（f）＝1/（1＋2.2f2）
In the formula, f-the overshoot oscillation frequency, MHz;
K (f) - test voltage factor, it is a function of frequency, and is a variety of insulation, applied the superposition of different frequency overshoot impulse voltage wave of amplitude frequency relationship and insulation breakdown strength.
The standard stipulates that each end of the record waveform will be discarded a little bit, and the good waveform should be fitted by double exponential waveform fitting.
Although the final test waveform contains the components of the filtered residual waveform, the standard specifies the wave front time, the half peak time and the relative overshoot amplitude beta from the test voltage waveform
β’ ＝[（Ut－Ump）/Ut]×100%
The new IEC standard provides that the maximum allowable value of the overshoot β’ of impulse test voltage is 10%, but the maximum allowable value of β’ is 20% when the test voltage amplitude is higher than 2MV.
According to the calculation results of impulse voltage generator with higher order lumped parameter, the frequency of the overshoot wave is unlikely to be higher than 0.5MHz when the standard lightning impulse voltage wave is generated. However, considering the spurious oscillations and multiple reflections of flow waves in the test loop, the impulse wave front may be superimposed with higher frequency oscillation waves. Its maximum oscillation frequency fmax, can be estimated by the press type:
fmax＝c/[4（Hg＋Hc）]
In the formula, C - the propagation velocity of electromagnetic waves in the air,C＝300m/μs,
Hg - the height of the impulse voltage generator, m,
Hc - the height of the load capacitor, m.
The chopping wave of lightning impulse voltage is shown in figure 3. The lightning impulse chopping time Tc is the time interval between the origin and the truncation moment. The apparent characteristics of the voltage period during truncation are defined by breaking the 70% of the instantaneous voltage value and the C and D points of 10%. The voltage drop duration is 1.67 times the time interval between point c and point D. The steepness of the voltage drop I cut off the ratio of the instantaneous voltage to the voltage drop duration.
(a) the lightning impulse truncated at the wave tail (b) the lightning impulse at the wave front truncation
The operation shock waveform defined by the IEC60060 1 is shown in figure 4. Its half peak time Tt is the time interval from the actual origin to the half peak of the wave tail. The wavefront time Tm is the time interval from the actual origin to the peak.
Since the amplitude is flat and the peak point is not easily determined, the new standard specifies the following formula to determine the wavefront time
Tm=KTAB
K＝2.42－3.08×10-3×tab＋1.51×10－4Tt
Tt,TAB, shown in Figure 4, are all in units of μs.
Figure 4: Operating impulse voltage waveform
The IEC 60060 -1 also specifies a time limit of 90% peak over the standard operating impulse voltage, Td which refers to the duration of the impulse voltage exceeding the peak value of 90% Td. The standard wave front time of operation impulse voltage Tm is2 50us±20%, and half peak time Tt is 2500us±60%. IEC 60076-3: 2000 test more than 220 kV rated voltage transformer and reactor internal insulation switching impulse voltage waveform as shown in figure 5. As the front time Tf of at least 1 00μs, usually not more than 250μs, 90% above the peak time of Td is more than 200μs, from the first zero time Tz more than 500μs at the origin. The impulse voltage wave specified by the relevant national standards (GB, 1094., 3--2003) is the same as that of the above IEC documents.
Figure 5: Operating impulse voltage waveform of internal insulation in test transformer
According to the test standard and taking into account the sufficient margin, the relation between the nominal voltage of the impulse generator and the voltage of the device under test is shown in the following table. The lower and lower values of the table meet the need of type test, and the upper limit is used for research and test.
Rated voltage of test sample/KV
|
35
|
110
|
220
|
330
|
500
|
Nominal voltage of impulse voltage generator/MV
|
0.4~0.6
|
0.8~1.5
|
1.8~2.7
|
2.4~3.6
|
2.7~4.2
|
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