The VLF Cable Tester under Wuhan UHV can help many power workers conduct various power tests more conveniently.
For most medium-voltage cables and large capacitive equipment, Very Low Frequency (VLF) testing can replace DC withstand testing and is the recommended method. However, in certain specific applications and for high-voltage equipment, DC withstand testing still holds irreplaceable value.
Basic Principles of the Two Tests
1.DC Withstand Test
Principle: Apply a DC high voltage, significantly higher than the operating voltage, to the equipment under test for a specified duration to check if the insulation can withstand it without breaking down.
Characteristics: Since it's DC, the current is determined only by the leakage current of the insulation material. This requires test equipment with smaller capacity, making it lighter and more portable. However, the electric field distribution depends on the resistivity of the materials, which differs from the actual AC operating condition (where the field distribution depends on permittivity).
2.Very Low Frequency (VLF) Test (Typically 0.1 Hz)
Principle: Apply a very low frequency (0.1 Hz or 0.05 Hz) AC high voltage to the equipment. The peak voltage is equivalent to the AC withstand voltage value at power frequency (50/60 Hz).
Characteristics: It simulates the AC voltage condition, so the electric field distribution is closer to the actual operating state. Meanwhile, due to the very low frequency, the required test equipment capacity is much smaller than power frequency test sets, while still maintaining good portability.
Why VLF is Often Superior and Replaces DC Testing?
The biggest issue with DC withstand testing is that it can potentially damage solid insulation materials like Cross-Linked Polyethylene (XLPE) and may fail to effectively detect certain defects:
1.Space Charge Effect: Under DC high voltage, space charges can accumulate within the insulation material. After the test, these charges do not dissipate immediately. When the equipment is re-energized onto the AC network, the residual DC field can superimpose with the power frequency AC field, potentially creating extremely high stress at insulation weak points. This can cause "hidden damage" to the insulation, sometimes leading to failure shortly after returning to service.
2.Reversed Electric Field Distribution: For multi-layer or non-uniform insulation, the electric field distribution under DC voltage is determined by resistivity, whereas under AC voltage, it's determined by permittivity. These distributions can be completely different, meaning the DC test may not effectively verify the insulation's performance under real operating conditions.
3.Difficulty Detecting Certain Defects: DC testing is less effective at detecting typical aging phenomena in XLPE cables like "water trees" and "electrical trees." The alternating nature of VLF testing more effectively stresses these defects, helping to reveal them.
Therefore, for modern medium-voltage (e.g., 10kV, 35kV) XLPE cables, international standards (e.g., IEEE 400.2) and national standards clearly recommend VLF as the preferred method for withstand testing and diagnostic testing.
Situations Where DC Withstand Testing is Still Irreplaceable
Despite the advantages of VLF, DC withstand testing remains relevant in the following areas:
1.High and Extra-High Voltage Equipment: For equipment like transformers and high-voltage circuit breakers, which primarily withstand AC voltage but have complex internal insulation structures, DC withstand testing (often called DC leakage current test) remains a crucial method for checking insulation strength. It effectively detects concentrated defects.
2.Oil-Impregnated Paper-Insulated Cables: For traditional oil-filled or impregnated paper-insulated cables, DC withstand testing has historically been the standard method and is considered less harmful to this type of insulation.
3.Pure Insulation Resistance Measurement: DC testing provides a very stable method for measuring insulation resistance, essentially an extension of the megohmmeter function.
4.Requirements of Certain Standards: Some older equipment specifications or specific industry standards may still mandate the use of DC withstand testing.