When conducting AC withstand voltage tests on high-voltage and high-capacity equipment, the capacity requirements of the test transformer are very high, and the test equipment is bulky. However, the application of series resonance principle can utilize equipment with much smaller voltage and capacity to generate the required test voltage, meeting the test requirements. Below, we will introduce the applications of series resonance testing devices in various fields.
1. Application in cable testing
The widespread 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 the cables.
Many cables undergo DC withstand voltage tests in accordance with the GB50150-2006 standard during the handover test, but insulation breakdown accidents occur shortly after being put into operation, and normal operating cables are also occasionally damaged by DC withstand voltage tests. The AC withstand voltage test is effective for cables because its electric field distribution conforms to the actual operation situation.
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, while 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, etc. 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, resulting in electrode surface defects; Suspended dust may enter the installation site from the air. Conductive particles and impurities, which are difficult to detect through routine tests on 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 test methods: AC voltage, oscillation operation impulse voltage, and oscillation lightning impulse voltage. Among them, AC withstand voltage test is a common method for GIS on-site withstand voltage test, which can effectively detect abnormal electric field structures (such as electrode damage).
Currently, due to limitations in testing equipment and conditions, only AC voltage withstand tests are generally conducted on site. IEC517 and GB7674 confirm that the voltage frequency of SF6 gas insulation test is basically equivalent to the power frequency voltage test in the range of 10-300Hz. At home and abroad, most of the GIS on-site AC withstand voltage tests are conducted using frequency modulation series resonance withstand voltage testing devices.
3. Application in AC withstand voltage test of generator (motor)
The stator winding of a generator has a large capacitance to ground or between phases, such as a 300MW hydroelectric generator with a stator winding to ground capacity of up to 1.7-2.5 μ F. During 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 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" for voltage waveform, "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 generator AC withstand voltage test is the power frequency, the generator series resonance device is usually of the induction type, which achieves the purpose of power frequency resonance by adjusting the inductance of the iron core air gap.
The resonant device has flexible combination methods and minimal damage to the test object. At the same time, the volume, weight, and required power capacity of the series resonant test device are much lower than those of traditional test transformers, greatly reducing the workload of on-site testing. Therefore, the series resonance test device will be increasingly widely used in the AC voltage withstand test of power equipment.
The ZCVF-A series variable frequency series resonance testing complete device is suitable for AC withstand voltage testing of transformers, circuit breakers, switches, GIS systems, cables, bushings, insulators, and other equipment. Equipped with multiple protection functions such as high-voltage overvoltage protection, low-voltage overcurrent protection, as well as detuning protection, zero position, flashover protection, emergency shutdown, undervoltage protection, etc. It can meet the requirements of high voltage and low current equipment testing conditions, as well as low voltage and high current equipment testing conditions, and has a wide range of application.
Kvtester Electronics Technology Co.,Ltd. is a high-tech enterprise specializing in power testing, testing, research and development, production, and sales of testing equipment. It has been engaged in the electrical testing industry for many years, and its products are of high quality. We welcome customers to come and purchase.
