The experimental research on frequency conversion series resonance structure can mainly be used for on-site AC voltage withstand performance testing of high-capacity capacitive power system equipment, such as cross-linked density polyethylene (XLPE) cables for medium and low voltage, high voltage, and ultra-high voltage, fully enclosed space combination use electrical appliances (GIS), generators through stators, large transformers (pressure controlled variables), and power as capacitors.
It is best to conduct AC voltage withstand tests using power frequency for the handover or preventive testing of power equipment. However, due to the large capacitance of the test sample, the required equipment power (referring to the amount of work done by the object per unit time) is large and cannot be tested on site. The inductive series resonance test, which can replace one substance for another (often the strong replacing the weak), is only used for voltage withstand tests and experiments on the generator stator (composed of stator core, stator winding, and frame) due to its large and complex equipment. In the late 1970s, foreign power departments discovered that the destructive effect of rubber insulation in DC withstand voltage testing was ineffective. Therefore, currently, China has not even suggested banning the testing method for DC XLPE cables. One fatal weakness of XLPE cables is the tendency for water branches to form inside the insulation, which can rapidly develop into electrical branches under the control of DC system voltage and form a discharge, accelerating insulation degradation; And a simple water tree branch can still maintain a social equivalent withstand voltage under the voltage of communication, learning, and work, and can hold itself for a period of time. Domestic and international investigations and practices have shown that DC withstand voltage tests cannot effectively detect some defects, such as mechanical damage in cable accessories, under the action of AC voltage.
The variable frequency series resonance tester adopts the ultra-low frequency (0.1Hz) AC test method. Although the equipment volume has been reduced, it also has its limitations. Firstly, domestic electronic products are semi mechanical (power (high power) switches (referring to the number of objects working per unit time), which produce a frequency of 0.1Hz (frequency) mode, and the output waveform cannot reach a good equivalent sine wave square wave; Secondly, the testing time is usually 1 hour in the claims (simultaneously for three 1 hour periods, and then for each one hour period if defects are found in the phase), and the efficiency (efficiency) is relatively low; At the same time, the ultra-low frequency (0.1Hz) pressure test is only applicable to low voltage. The "Guidelines for Quality Acceptance Testing of High Voltage Extruded Insulation" published by the 21-09 working group of the International Conference on Large Voltage Grids (CIGRE) in 1997 pointed out that the 30-300Hz AC has good equivalence between withstand voltage performance testing and power frequency withstand voltage testing. It is recommended to use this method to analyze cable AC and testing, and to develop technical standards for test system voltage.
Based on the theories and simulations of some university experiment rooms (defined as virtual experiments on real things or processes), as well as the experience of using frequency conversion experiments in foreign countries for more than a decade, it has good equivalence, high efficiency, lightweight equipment, and almost unlimited sample length. After adding the communication test standard of 1.7/5 minutes or 1/24 hour to the IEC60840 standard for laying cables with a voltage level of 45-150kV, it is recommended; The proposed draft of IEC62067/CD for the 220 kV level only requires AC testing at a frequency of 20-300Hz, with some provinces and regions requiring a 60 minute handover test.