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How to do AC withstand voltage test for cables?
- Categories:Experience sharing
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- Time of issue:2019-11-20 14:11
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(Summary description) Due to the large capacity of the cable, the use of traditional power frequency test transformers is bulky, bulky, and high-current working power is not easy to obtain on site. Therefore, series resonance AC withstand voltage test '>AC withstand voltage test equipment is generally used. The input power capacity can be significantly reduced, the weight is reduced, and it is easy to use and transport. In the early stage, the modulated series resonance equipment （50Hz） was mostly used, but there were disadvantages such as poor automation and high noise. Therefore, most of them now use frequency modulation （30-300Hz） series resonance test equipment, which can obtain higher quality （Q value）, and has automatic tuning, multiple protections, and low-noise, flexible combination methods （single piece weight is large Down） and other advantages. Based on the relevant technical data at home and abroad, choosing the appropriate test frequency range is a relatively important issue. In this regard, there are some different views and formulations. In terms of the current domestic and foreign formulations, we can conclude that it can be divided into 3 categories: the first category is a wide frequency range of 30-300Hz, 20-300Hz, 1-300Hz; the second category is the power frequency range, 45-65Hz, 45-55Hz; The third category is close to power frequency, 35-75Hz.
How to do AC withstand voltage test for cables?
(Summary description) Due to the large capacity of the cable, the use of traditional power frequency test transformers is bulky, bulky, and high-current working power is not easy to obtain on site. Therefore, series resonance AC withstand voltage test '>AC withstand voltage test equipment is generally used. The input power capacity can be significantly reduced, the weight is reduced, and it is easy to use and transport. In the early stage, the modulated series resonance equipment （50Hz） was mostly used, but there were disadvantages such as poor automation and high noise. Therefore, most of them now use frequency modulation （30-300Hz） series resonance test equipment, which can obtain higher quality （Q value）, and has automatic tuning, multiple protections, and low-noise, flexible combination methods （single piece weight is large Down） and other advantages. Based on the relevant technical data at home and abroad, choosing the appropriate test frequency range is a relatively important issue. In this regard, there are some different views and formulations. In terms of the current domestic and foreign formulations, we can conclude that it can be divided into 3 categories: the first category is a wide frequency range of 30-300Hz, 20-300Hz, 1-300Hz; the second category is the power frequency range, 45-65Hz, 45-55Hz; The third category is close to power frequency, 35-75Hz.
- Categories:Experience sharing
- Author:
- Origin:
- Time of issue:2019-11-20 14:11
- Views:
Information
Due to the large capacity of the cable, the use of traditional power frequency test transformers is bulky, bulky, and high-current working power is not easy to obtain on site. Therefore, series resonance AC withstand voltage test '>AC withstand voltage test equipment is generally used. The input power capacity can be significantly reduced, the weight is reduced, and it is easy to use and transport. In the early stage, the modulated series resonance equipment （50Hz） was mostly used, but there were disadvantages such as poor automation and high noise. Therefore, most of them now use frequency modulation （30-300Hz） series resonance test equipment, which can obtain higher quality （Q value）, and has automatic tuning, multiple protections, and low-noise, flexible combination methods （single piece weight is large Down） and other advantages. Based on the relevant technical data at home and abroad, choosing the appropriate test frequency range is a relatively important issue. In this regard, there are some different views and formulations. In terms of the current domestic and foreign formulations, we can conclude that it can be divided into 3 categories: the first category is a wide frequency range of 30-300Hz, 20-300Hz, 1-300Hz; the second category is the power frequency range, 45-65Hz, 45-55Hz; The third category is close to power frequency, 35-75Hz.
（1） Type 1 wide frequency range
The “Test Guidelines” issued by Working Groups 21 and 09 of the International Conference on Large Power Grids suggests that the frequency range is 30-300 Hz. But in fact, lower frequencies also have better equivalence. IEC60840 and IEC62067 draft standards （2001 and 2000） stipulate that 20-300 Hz can be used.
Some foreign manufacturers design series harmonic reactors, and under special circumstances, some have the lowest frequency of 25Hz or 20Hz. Of course, the lower the frequency, the greater the length （capacitance） of the cable under test. However, the reactor core is enlarged, increasing the weight. Individual data show that the 1-300Hz AC test is also equivalent to the power frequency AC test, which shows that the lower frequency limit may be lower in practical applications, for example, less than 20Hz or even 0.1Hz is also feasible. It further shows that in this frequency range, the voltage distribution and dielectric characteristics of the various media 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 loss of the series resonance reactor and the excitation transformer decreases, but the polarization heating of the capacitor medium of the test product must be considered, so the frequency higher than 300Hz is not advisable.
（2） The second category is the power frequency range
The international industrial frequency mainly refers to 50Hz and 60Hz. Therefore, the IEC standard stipulates that the industrial test frequency range for high-voltage insulation is 45-65Hz, and the rated power frequency in my country is 50Hz. GB/T16927.1-1997 stipulates that the power frequency test frequency range is 45-55Hz.
believes that the test voltage of a power frequency power cable must also be a power frequency, which is a conservative view. In view of this problem, it should be emphasized that the purpose of handover and preventive testing is to determine the ability to find insulation defects. As long as the dielectric voltage distribution inside the insulation is the same at different frequencies, and the ability to detect insulation failures is basically the same, the test can be achieved. Therefore, even if the frequency is wider than the power frequency range, it is acceptable. In the mid-1990s, in order to select the appropriate frequency range of the AC withstand voltage test, a lot of and careful basic research work was done. It is concluded that there is no significant difference in the breakdown characteristics of several typical insulation defects inside the rubber-plastic cable within the range of 30-300 Hz. This should be credible and has been widely adopted. The analysis and formation of this kind of good breakdown characteristics at different frequencies are mainly due to the excellent coaxial insulation structure, the single insulating medium, the relatively pure material, and the reasonable and regular electric field distribution. Therefore, the internal voltage distribution of the structure is the same at different frequencies, forming conditions for a wide frequency range test. Oil-paper insulated cables have been tested with a DC voltage with a frequency equal to zero. The actual effect is very good and has not been questioned for decades.
（3） The third category is close to the power frequency, 35-75Hz
Breakdown tests were performed on normal XLPE （cross-linked polyethylene） insulated cable samples abroad at different frequencies. The results show that the breakdown voltage is within 95% of the confidence level when the frequency is 35-75Hz. Therefore, some opinions agree that the test voltage frequency is best selected at 35-75Hz, which is also closer to the operating voltage frequency of 50Hz. It is worth noting that the above test result is a breakdown test for normal insulation. The test voltage value used in the handover and preventive tests is low, and it can only penetrate defective insulation weaknesses （mechanical damage, water branches, terminal head or joint box stress iron cone construction or material errors, etc. ）, it is not enough to penetrate the normal insulation of the cable body. It can be seen that the purpose and working mechanism of the two tests are different. It seems unnecessary to “extend” the 35-75Hz breakdown characteristics of normal insulation to detect insulation defects.
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