Due to the large capacitance of the cables, the power capacity required for the power frequency test scheme is extremely high, and the equipment is relatively bulky, making it difficult to implement in on-site test scenarios. Therefore, the frequency-modulated resonant withstand voltage test equipment has become the mainstream choice in the industry. Its core principle is to form a resonance with the test sample’s capacitance using a reactor, and a small-capacity power supply can be used to generate high voltage and large current that meet the test requirements.
I. Main Equipment Composition
The variable frequency power control box serves as the core control unit of the system, capable of converting the power frequency power supply into a frequency-adjustable power supply, with a conventional adjustable frequency range of 30-300Hz. It also undertakes the system control and protection functions to ensure the stability and safety of the test process.
The excitation transformer is used to boost the output voltage of the variable frequency power supply, providing energy support for the resonant circuit, and is one of the key links to achieve high-voltage output.
The resonant reactor is a core component of the system, with adjustable inductance parameters, typically adjusted by tap or adjustable core. It can form a series resonant circuit with the test sample’s cable capacitance. This equipment is often designed in multiple sections and can be combined in series or parallel to meet the test requirements of different voltage levels and capacities.
The capacitor divider is mainly used for measuring the test voltage and has the characteristic of high impedance, having a minimal impact on the operation state of the resonant circuit and ensuring the accuracy of voltage measurement.
The compensation capacitor (optional) can be connected when the test sample’s cable capacitance is small to increase the equivalent capacitance of the system, making it easier to achieve tuning of the resonant circuit and ensuring the smooth progress of the test.
High-voltage connection lines and accessories are auxiliary components of the system, used for connecting various devices to ensure the integrity of the test circuit.
II. Key Parameters for Equipment Selection
When selecting equipment, the three core parameters of larger output voltage, rated current, and frequency range should be given priority. For example, in the test of 35kV cables, the test voltage is usually 2U₀ (i.e., line voltage), and the test device should meet the requirement of a higher output voltage of ≥80kV. Common specifications in the industry are 90kV or 108kV.
III. Main Test Standards and Regulations
GB/T 12706.4-2020 “Rated Voltage 1kV (Um=1.2kV) to 35kV (Um=40.5kV) Extruded Insulation Power Cables and Accessories – Part 4: Test Requirements for Power Cable Accessories with Rated Voltage 6kV (Um=7.2kV) to 35kV (Um=40.5kV)”
DL/T 474.4-2018 “Guidelines for On-Site Insulation Tests – Part 4: AC Withstand Voltage Tests”
GB 50150-2016 “Code for Handover Tests of Electrical Installations”, which is the core execution standard for on-site handover tests
DL/T 596-2021 “Regulations for Preventive Tests of Electrical Equipment”, which is the core standard for preventive tests of equipment in operation
Q/GDW 11316-2014 “Test Regulations for Power Cable Lines”
IV. Test Operation Precautions
Safety Protection: The test area should be surrounded by fences and warning signs, and a dedicated person should be assigned for full supervision. High-voltage leads should maintain a sufficient safety distance. Before and after the test, the test sample and related equipment must be fully discharged and grounded to eliminate the risk of electric shock.
Reliable Grounding: The shells of all equipment in the test system, the low-voltage end of the capacitor divider, and the non-tested parts of the cable must be reliably grounded to ensure the safety and data accuracy of the test process.
Frequency Selection: The resonant frequency should be selected to avoid the power frequency and its multiples of the power grid, typically controlled within the range of 30-300Hz. AC withstand voltage tests within this frequency range are recognized as equivalent to power frequency withstand voltage tests at the standard level. Environmental conditions prohibit conducting tests in rainy, foggy weather or when the air relative humidity is greater than 80%. Before the test, it is necessary to ensure that the surface of the cable terminal is dry and clean to avoid the influence of environmental factors on the test results. When conducting preventive tests on old cables with a long operating history, the voltage increase process should be particularly slow, and the state changes of the test sample during the test should be closely monitored to prevent test failures caused by the decline in cable insulation performance.
Post time: Jan-04-2026