According to DL/T 596 "Preventive Test Code for Electric Power Equipment" and the GB/T 3048 series standards, the 0.1Hz very low frequency withstand voltage test has better equivalence for XLPE cables than DC withstand voltage, and the equipment capacity is only about 1/500 of that of power frequency testing. Taking a 35kV cable as an example, the capacitance per kilometer is about 0.11μF. If power frequency withstand voltage is used, the test transformer capacity needs to be tens of kVA, making on-site transportation difficult; while the output capacity of a 0.1Hz very low frequency device can be controlled within several hundred VA, making it suitable for on-site use in substations and cable trenches. Design and selection personnel need to pay attention to three core parameters: output voltage, load capacitance range, and waveform distortion rate. The output waveform distortion rate of this device is <5%, meeting the standard's requirements for test voltage waveform.
The load of the very low frequency withstand voltage test is the equivalent capacitance of the cable, and the capacitance varies greatly among cables of different cross-sections and lengths. The table above lists reference values of capacitance per kilometer for 10kV to 35kV cables with different cross-sections, for example, 10kV/240mm² cable is about 0.38μF/km, and 35kV/185mm² is about 0.19μF/km. During selection, the total capacitance should be calculated based on the total length of the cable under test to ensure that the rated load capacitance of the device covers this value. The rated load capacitance range of HVD-45 is 0.05μF～5μF, which can adapt to the handover and preventive test needs of most medium and low voltage cables. If the cable is long or multiple cables are connected in parallel, it is necessary to check whether the total capacitance exceeds the upper limit, and if necessary, conduct the test in sections.
The device should be stored in a dry and ventilated environment to avoid moisture causing insulation degradation of high-voltage components. Before each test, check the high-voltage output cable and connectors for damage or dirt, and clean the surface with anhydrous ethanol. After the test, it is necessary to wait for the internal discharge of the device to complete (usually more than 5 minutes) before performing grounding operations. When re-enabling after long-term shutdown, it is recommended to first raise the voltage to the rated value with no load and maintain it for 1 minute to observe whether there is abnormal discharge sound. If the ambient humidity is greater than 80%, it is recommended to increase the preheating time or suspend the test.
The test area must be fenced off and warning signs hung, and operators should maintain a safe distance. During the voltage rise process, it is forbidden to touch the high-voltage lead wires and the ends of the cable under test. The grounding terminal of the device must be reliably connected to the substation grounding grid, and the cross-section of the grounding wire shall not be less than 25mm². When overcurrent or flashover protection operates, the power supply should be cut off first, and after sufficient discharge, the cause should be investigated. Do not force reset and continue to apply voltage.
