1. Application in cable testing
The extensive use of cables in urban and rural power grids often leads to faults. To ensure the safe operation of cross-linked cables, State Grid Corporation of China has made new regulations on cable handover and preventive testing, replacing the original DC withstand voltage test with AC withstand voltage test to avoid the cumulative effect of DC test causing damage to cables.
Many cables have successfully undergone DC withstand voltage tests according to the GB50150-2006 standard during the handover test, but insulation breakdown accidents occurred shortly after being put into operation, and normal operating cables have also been damaged by DC withstand voltage tests. The AC withstand voltage test is effective for cables as its electric field distribution conforms to the actual operating conditions.
Usually, the capacitance of AC power cables is large, and the test current is also large. The volume of inductive devices will be very large, and inductance adjustment will be difficult. On the other hand, frequency modulation devices are more flexible and easier to implement. Therefore, on-site AC withstand voltage testing of cables often utilizes frequency conversion resonance testing equipment.
2. Application in GIS equipment
After the overall assembly of gas insulated switchgear in the factory is completed or adjusted and tested in units, it is transported to the site for installation in units after passing the test. Vibration, impact, and other factors during transportation may cause loosening or displacement of the internal fasteners of GIS components or assemblies; During the installation process, there may be errors in connection, sealing, and other process treatments, resulting in scratches or misalignment of the electrode surface, leading to electrode surface defects; The installation site may introduce suspended dust from the air. Conductive particles and impurities, which are difficult to detect through routine tests at the installation site, pose a great threat to the safe operation of GIS.
Due to limitations in testing equipment and conditions, most early GIS products did not undergo strict on-site voltage withstand tests. Accident statistics indicate that most GIS systems that did not undergo on-site voltage withstand tests have experienced accidents. Therefore, GIS must undergo on-site voltage withstand tests.
The on-site withstand voltage of GIS mainly includes three testing methods: AC voltage, oscillation operation impulse voltage, and oscillation lightning impulse voltage. The AC withstand voltage test is a common method for GIS on-site withstand voltage testing, which can effectively detect abnormal electric field structures (such as electrode damage).
At present, due to limitations in testing equipment and conditions, only AC voltage withstand tests are generally conducted on site. IEC517 and GB7674 recognize that the voltage frequency of SF6 gas insulation test is basically equivalent to the power frequency voltage test in the range of 10-300Hz. Both domestically and internationally, frequency modulation series resonance withstand voltage testing devices are mostly used for GIS on-site AC withstand voltage testing.
3. Application in AC withstand voltage test of generators (motors)
The stator winding of a generator has a large capacitance to ground or between phases, for example, the stator winding of a 300MW hydroelectric generator has a capacitance to ground of up to 1.7-2.5 μ F. When subjected to power frequency withstand voltage, the capacitance current reaches 25-35A, and the test equipment has a capacity of thousands of kVA. When using conventional test equipment, the equipment is cumbersome. More seriously, when using conventional high-capacity testing equipment, the short-circuit current at the fault point when the insulation of the generator stator winding is broken down is high, which can burn out the iron core and cause significant economic losses. According to the provisions of section 6.2.1.1 of the national standard GB/T16927.2-1997 "High voltage testing techniques", the test voltage should generally be an AC voltage with a frequency of 45-65Hz, commonly referred to as the power frequency test voltage.
In order to meet the requirement that the voltage frequency of the AC withstand voltage test of the generator is the power frequency, the series resonance device of the generator is usually of the induction type. By adjusting the air gap of the iron core to change the inductance, the purpose of power frequency resonance is achieved.
