The table below summarises the four decisive differences between the two methods, followed by the test-parameter differences that follow from them.
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VLF Testing (0.1 Hz) |
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AC-like. Capacitive and resistive grading both active; stress distribution reproduces service conditions at 0.1 Hz. |
Purely resistive. Field is governed by conductivity, not permittivity, so grading layers and joint geometries are stressed differently from service. |
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Low. Polarity reverses every 5 s; injected charge recombines, and the cable is field-neutral after the test. |
High. Charge trapped at voids, impurities, and interfaces; residual field superimposes on AC on re-energisation and can cause post-test failure. |
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Yes. Repeated field reversals break down tree-weakened insulation at or below test level; tan delta tip-up quantifies the degradation. |
No. Water trees are effectively invisible under DC; degraded cable passes the test and fails in service. |
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Compact. Charging current is 500× lower than at 50 Hz, so a 30-80 kV set weighs about 30-90 kg and runs from a 230 V socket. |
Compact set, but no diagnostic capability and no length advantage; requires a separate discharge and earthing procedure after the test. |
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2U₀ to 3U₀, 15-60 min (IEEE 400.2); 3U₀ for 15 min (HD 620 / VDE 0276-620). |
Historically 2U₀ to 4U₀ for 5-15 min; now withdrawn for extruded cable by IEEE 400.2. |
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Tan delta, PD measurement, and leakage current monitoring on the same connection. |
Leakage current only; no PD or dielectric-loss information. |
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