Series resonant withstand voltage test? Wuhan UHV specializes in the production of series resonance, with a wide range of product selection and professional electrical testing. To find series resonance, choose Wuhan UHV.
At present, more and more cross-linked polyethylene insulated power cables are replacing the original oil filled paper insulated power cables both internationally and domestically. However, due to the large capacity of the test subjects and the limitations of the testing equipment, the direct current withstand voltage test method has been used for a long time in the testing methods before the operation of cross-linked cables. In recent years, many research institutions both domestically and internationally have shown that direct current testing can cause varying degrees of damage to cross-linked polyethylene cables. Some research perspectives suggest that cross-linked polyethylene cable structures have the ability to store and accumulate unipolar residual charges. If the DC residual charges cannot be effectively released after DC testing, the addition of AC voltage peaks to the DC residual charges after operation may cause cable breakdown.
Some domestic research institutions believe that in the DC withstand voltage test of cross-linked polyethylene cables, due to the space charge effect, the actual electric field strength in insulation can be up to 11 times higher than the working electric field strength of cable insulation. Cross linked polyethylene insulated cables can cause serious insulation damage even if they pass the DC test without breakdown. Secondly, due to the difference in field strength distribution between the applied DC voltage and the operating AC voltage. DC testing cannot truly simulate the overvoltage that cables can withstand under operating conditions, and effectively detect defects in the cables and cable joints themselves and construction processes. Therefore, the use of non DC methods for conducting voltage withstand tests on cross-linked cables is increasingly receiving attention. At present, ultra-low frequency power supplies (XCF) have been used abroad for voltage withstand tests on medium and low voltage cables. However, due to the low voltage level of this type of XCF, it is not yet suitable for testing high-voltage cables of 110kV and above. In China, this method has also been used for low-voltage cables, but due to testing equipment limitations, it has not been widely promoted. In recent years, due to the construction and renovation of urban and rural power grids, there have been more and more cross-linked cables. The cables are put into operation only after DC withstand voltage testing, and incidents of cable or cable head breakdown under operating voltage have also occurred from time to time. So, everyone is exploring new experimental methods.
Test frequency
Due to the large capacitance of cables, traditional power frequency test transformers are bulky and bulky, and high current working power sources are not easily available on site. Therefore, series resonant AC withstand voltage testing equipment is generally used. Its input power capacity can be significantly reduced, weight can be reduced, and it is easy to use and transport. In the initial stage, induction type series resonant devices (50Hz) were commonly used, but they have disadvantages such as poor automation and high noise. Therefore, frequency modulation (30-300Hz) series resonance test equipment is mostly used nowadays, which can obtain higher quality numbers (Q values), and has advantages such as automatic tuning, multiple protections, low noise, and flexible combination methods (greatly reducing the weight of a single piece).
standard question
Due to issues with equipment capacity and volume, there are currently no corresponding standards in the country for conducting AC withstand voltage tests on high-voltage power cables after installation on site. However, due to the reasons mentioned earlier, people have also raised some questions about the standards for DC withstand voltage testing. The "Guidelines for Completion Acceptance Testing of High Voltage Squeezed Insulated Cables" proposed by Working Group 21 of the CIGRI International Conference on Large Power Grids raises doubts about the current DC withstand voltage test method and recommends the use of AC test methods at power frequency and approximate power frequency (30-300Hz). In the IEC 60840 standard, in addition to the original DC test standard, a 1.7U0 5-minute or 1U0 24-hour AC test standard has been added to the cable test standard after 45-150kV installation. In the 220kV level, the IEC 62067/CD draft cancels the standard for DC testing in cable laying tests, and only requires AC testing, which is 20-300Hz 1.4U0 for 60 minutes. In order to conduct more effective handover tests on cross-linked cables after construction, the revised content of the "Code for Handover and Preventive Testing of Power Equipment" has added the AC withstand voltage test standard for cables in the main insulation withstand voltage test section. Afterwards, many regions in China also introduced corresponding local testing standards, with testing frequencies mostly ranging from 30-300Hz. The testing voltage for medium and low voltage cables is 1.6-2.0 times the phase voltage, while the testing voltage for high voltage cables is generally 1.4-1.7 times the phase voltage. Depending on the specific location, it is recommended that the testing frequency for medium and low voltage cables be 45-65 Hz, and for high voltage cables be 35-75 Hz.
Equipment selection
Cables with voltage levels of 35kV and below are widely used and require a large amount of experimental work, so this type of voltage withstand test device should be small in size and light in weight. Thus, the portable cable voltage withstand test device produced by Huatian Power was born.
In this system, it is required that the weight of components that can be moved on site by a single person should not exceed 30kg, and the weight of components that can be moved on site by two people should not exceed 60kg, which is suitable for on-site movement. The reactor part is made of dry epoxy casting, which is beautiful and reliable, and suitable for the requirements of various types of cables. This system has the following characteristics:
(1) Easy to operate with automatic/manual tuning function; The large screen displays parameters such as voltage, current, and frequency; Equipped with overvoltage and overcurrent setting and protection functions; Equipped with detuning protection function; Equipped with automatic timing and time to turn off function; Voltage regulation has a zero start function to ensure the safety of tuning and zero start boost of high-voltage circuits;
(2) Multiple sets of flexible reactors are used for configuration, and the tested equipment can be used in series or in parallel. The number of reactors to be brought to the site area can be determined based on the tested equipment;
(3) Taking a 3kM long 10kV cable (high voltage current 3A) as an example, the weight of a single reactor is 35kg (3 units), the weight of the variable frequency control box is 12kg, and the weight of the excitation transformer is 15kg.