The series resonance under UHV power can help many power workers conduct various power tests more conveniently.
It is important to select an appropriate testing frequency range based on relevant technical data from both domestic and international sources. There are some different perspectives and formulations in this regard. Based on current domestic and international terminology, we can summarize it into three categories: the first category covers a wide frequency range of 30-300Hz, 20-300Hz, and 1-300Hz; The second category is the power frequency range, 45-65Hz, 45-55Hz; The third category is close to the power frequency, 35-75Hz.
1. Wide frequency range
The "Test Guidelines" released by the 21st and 9th working groups of the International Conference on Power Grids recommend a frequency range of 30-300Hz. But in reality, even lower frequencies have better equivalence. The draft standards IEC60840 and IEC62067 (2001 and 2000) stipulate that 20-300Hz can be used.
Some foreign manufacturers design series harmonic reactors, and in special circumstances, they also use reactors with a minimum frequency of 25Hz or 20Hz. Of course, the lower the frequency, the longer the length (capacitance) of the tested cable can increase. However, the iron core of the reactor is enlarged, resulting in an increase in weight. According to individual data, The AC test between 1-300Hz also has equivalence with the power frequency AC test, indicating that in practical applications, it is possible to achieve a lower frequency limit, such as less than 20Hz or even up to 0.1Hz, which is feasible. This further indicates that within such a frequency range, the voltage distribution and dielectric characteristics of each medium inside the insulation are still basically the same.
Is it appropriate for the working frequency to exceed 300Hz? There are reports that as the frequency increases, the losses of series harmonic reactors and excitation transformers decrease. However, the polarization heating problem of the tested capacitor medium needs to be considered, so frequencies higher than 300Hz are not advisable.
2. Power frequency range
Internationally, industrial frequency mainly refers to two types: 50Hz and 60Hz. Therefore, the IEC standard stipulates that the industrial test frequency range for high-voltage insulation is 45-65Hz, and in China, the rated power frequency is 50Hz. GB/T16927.1-1997 stipulates that the frequency range for power frequency testing is 45-55Hz.
The view that the test voltage of power cables must also be at the power frequency tends to be more conservative. Regarding this issue, it should be emphasized that the purpose of handover and preventive testing is to determine the ability to detect insulation defects. As long as the voltage distribution of the internal dielectric of the insulation is the same and there is a basic ability to detect insulation faults at different frequencies, the purpose of the test can be achieved. Therefore, even if a frequency wider than the power frequency range is chosen, it is still acceptable.
In the mid-1990s, extensive and meticulous basic research was conducted to select the appropriate frequency range for AC withstand voltage testing. There is no significant difference in the breakdown characteristics of several typical insulation defects inside rubber cables within the frequency range of 30-300Hz. This should be credible and widely adopted. The main reason for the formation of this good breakdown characteristic at different frequencies is the excellent coaxial insulation structure, single insulation medium, relatively pure material, reasonable and regular electric field distribution. Therefore, the internal voltage distribution of the structure is the same at different frequencies, forming the conditions for a wide frequency range experiment.
Oil paper insulated cables have always been subjected to withstand voltage tests using a DC voltage with a frequency equal to zero, and its actual effect is very good, which has not been questioned for decades.
3. Approaching the power frequency range
Foreign countries have conducted breakdown tests on samples of normal XLPE (cross-linked polyethylene) insulated cables at different frequencies. The results indicate that the breakdown voltage falls within a 95% confidence level at frequencies of 35-75Hz. Therefore, some opinions support that the optimal test voltage frequency should be selected between 35-75Hz, which is also closer to the operating voltage frequency of 50Hz. It is worth noting that the above test results are a breakdown test conducted on normal insulation. The test voltage values used for handover and preventive testing are relatively low, and they can only penetrate the weak points of defective insulation (mechanical damage, water branches, incorrect construction or material of stress iron cones in terminal heads or joint boxes, etc.), which is completely insufficient to penetrate the normal insulation of the cable body. It can be seen that the purpose and working mechanism of the two experiments are different. It seems unnecessary to extend the normal insulation breakdown characteristics of 35-75Hz to detect insulation defects.
