T&D World Magazine

Hipot AC Field-Tests Cables

The High Voltage, Inc. VLF series of AC Hipots enables utilities to economically AC field-test cables and electrical apparatus instead of DC testing.

A VLF unit is an AC hipot with a 0.1 Hz or lower output frequency, used for AC testing high capacitance loads. At 0.1 Hz, it requires 600 times less power to test a cable, generator, etc., than at 60 Hz and 500 times less power than at 50 Hz. A VLF test is a go/no-go stress test. The load either holds the test voltage or breaks down. It is the surest and easiest way to expose defects in cables and generators.

A VLF test can detect cable defects at a much lower voltage than a DC hipot/proof test. Also, voltage transients are much smaller with a VLF AC hipot test during cable failure or flashover than with a DC hipot/proof test. There are no travelling waves generated during VLF testing.

The VLF AC hipot also burns, breaks down and conditions a cable fault much quicker than conventional DC burners. Therefore, fault locating time is reduced with existing thumpers and radar devices.

The VLF-90CM is a continuously adjustable 0-90 kV continuous duty AC hipot with a 0.1/0.05/0.02 Hz sinusoidal output. The waveshape is independent of capacitive loading and can test cables up to 25,000 feet in length. The two piece design is suitable for test van or trolley mounting and operates from a 230 V electrical service.

Controls are elementary: Power ON and Raise/Lower switches, HV ON and OFF, Fault Reset, and Start Timer push buttons, single range 100 kV peak kilovoltmeter and 100 mA peak charging current meter. In addition, a capacitance measurement circuit for pre-determining cable/load capacitance is provided. For waveform viewing, a BNC output jack for oscilloscope connection is included.

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