At present, there are more and more XLPE-insulated power cables in the world and domestically to replace the original oil-filled paper-insulated power cables.
However, due to the large capacity of the test and the test equipment, the test method of DC withstand voltage has been followed for a long time.
In recent years, the research results of many international and domestic research institutions have shown that the DC test has different degrees of damage to XLPE cable.
Some researchers believe that the XLPE structure has the ability to store accumulated unipolar residual charge.
When the DC residual charge cannot be effectively released after the DC test, the DC residual charge plus the AC voltage peak after commissioning may cause the cable.
A breakdown has occurred. Some research institutions in China believe that in the DC withstand voltage test of XLPE cable, the actual electric field strength in the insulation can be up to 11 times higher than that of the cable insulation due to the space charge effect.
XLPE insulated cables can cause serious damage to the insulation even if they do not break through the DC test.
Second, since the applied DC voltage field strength distribution is different from the operating AC voltage field strength distribution.
The DC test can not truly simulate the overvoltage of the cable under running conditions, and effectively find the defects of the cable and cable joint itself and the construction process.
Therefore, the use of non-DC methods for the pressure test of cross-linked cables has received more and more attention.
At present, the ultra-low frequency power supply (VLF) has been used in the medium and low voltage cables for the withstand voltage test.
However, due to the low voltage level of such VLF, it is not applicable to high-voltage cable tests of 110kV and above.
In China, this method has also been used for low-voltage cables, but due to the test equipment, it has not been widely promoted.
In recent years, due to the transformation of urban and rural power grids, there are more and more XLPE cross-linked cables.
The cables are put into operation only after the DC withstands the voltage test, and the cable or cable head breakdown occurs under the operating voltage.
It also happens from time to time. Therefore, everyone is exploring new test methods.
Due to the large capacitance of the cable, the conventional power frequency test transformer is very bulky and bulky, and the high current working power is not easy to obtain on-site.
Therefore, the series resonant AC withstand voltage test > AC withstand voltage test equipment is generally used.
The capacity of the input power supply can be significantly reduced, weight reduced, and easy to use, and transport.
In the beginning, the inductive series resonant device (50 Hz) was used, but there were disadvantages such as poor automation and high noise.
Therefore, most of the current frequency modulation (30-300Hz) series resonance test equipment can obtain higher quality (Q value), with automatic tuning, multiple protection, low noise, flexible combination (single piece weight is large) Drop) and other advantages.
It is an important issue to select the appropriate test frequency range by integrating relevant technical data at home and abroad.
In this regard, there are some different points of view and references.
As far as the current domestic and international formulation is concerned, our summary can be divided into three categories:
The first category is a wide frequency range of 30-300 Hz, 20-300 Hz, and 1-300 Hz.
The second category is a power frequency range, 45-65 Hz. 45-55Hz.
The third category is close to the power frequency, 35-75Hz.
(1) Category 1 wide frequency range
The "Guidelines for Testing" issued by the Working Groups 21 and 09 of the International Conference on Large Power Grids, the recommended frequency range is 30-300 Hz. But in fact, lower frequencies also have better equivalence.
The draft IEC 60840 and IEC 62067 standards (2001 and 2000) stipulate that 20-300 Hz can be used.
Some foreign manufacturers design series harmonic reactors, and in special cases, the lowest frequency is 25Hz or 20Hz.
Of course, the lower the frequency, the longer the cable length (capacitance) can be. However, the core of the reactor is thus enlarged to increase its weight.
Individual data show that the 1-300Hz AC test also has the equivalence with the power frequency AC test, which indicates that the lower frequency limit may be lower in practical applications, such as less than 20Hz or even 0.1Hz.
It is further shown that in such a frequency range, the voltage distribution and dielectric characteristics of the respective dielectrics inside the insulation are still substantially the same.
Is it appropriate to operate at frequencies above 300 Hz? It has been reported that as the frequency increases, the losses of the series harmonic reactor and the excitation transformer are reduced, but the polarization heating problem of the capacitor medium of the test object is considered, so a frequency higher than 300 Hz is not preferable.
(2) Class 2 is the power frequency range
The international industrial frequency mainly refers to 50Hz and 60Hz.
Therefore, the IEC standard specifies that the industrial test frequency range for high-voltage insulation is 45-65Hz, and the rated power frequency in China is 50Hz. GB/T16927.1-1997 specifies the power frequency test frequency range as 45-55Hz.
It is considered that the test voltage of the power frequency power cable must also be the power frequency, which is a trend that tends to be conservative.
Aiming at this problem, it should be emphasized that the purpose of the handover and preventive tests is to determine the ability to insulate defects.
At different frequencies, as long as the internal dielectric voltage distribution of the insulation is the same and there is basically the same ability to detect insulation faults, the purpose of the test can be achieved.
Therefore, even a wider frequency than the power frequency range is acceptable.
In the mid-1990s, in order to select the appropriate AC to withstand voltage test > AC withstand voltage test frequency range, a lot of careful basic research work was done.
It is concluded that the frequency is in the range of 30-300 Hz, and there is no significant difference in the breakdown characteristics of several typical insulation defects inside the rubber-plastic cable.
This should be credible and widely adopted.
Analysis of the formation of such good breakdown characteristics at different frequencies, the main reason is the excellent coaxial insulation structure, a single insulating medium, the material is relatively pure, and the electric field distribution is reasonable and regular.
Therefore, the internal voltage distribution of the structure is the same at different frequencies, forming conditions for the wide frequency range test.
Oil-paper insulated cables have been subjected to withstand voltage tests using a DC voltage with a frequency equal to zero.
The actual effect is very good and has not been questioned for decades.
(3) Class 3 is close to the power frequency, 35-75Hz
Foreign countries have conducted breakdown tests on normal XLPE (cross-linked polyethylene) insulated cable samples at different frequencies.
The results show that the breakdown voltage falls within 95% of the confidence level at a frequency of 35-75 Hz.
Therefore, there is a view that the test voltage frequency is preferably selected at 35-75 Hz, and is also closer to the operating voltage frequency of 50 Hz.
It is worth noting that the above test results are breakdown tests for normal insulation.
The test voltage used in the handover and preventive tests is low, it can only penetrate the defective insulation weakness (mechanical damage, water branches, terminal head or joint box stress iron cone construction or material error, etc. ), completely insufficient to break the normal insulation of the cable body.
It can be seen that the purpose and working mechanism of the two tests are different.
It does not seem necessary to apply the "extension" of the normal insulation 35-75 Hz breakdown characteristics to the detection of insulation defects.
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