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Very Low Frequency (VLF) AC Hipots

A VLF hipot is just an AC output tester but with an output frequency of 0.1 Hz or lower rather than 50/60Hz. Although the frequency is very low, it is still an alternating current with polarity reversals every half cycle. At 0.1 Hz output, rather than 60Hz, it takes 600 times less current and power to apply an AC voltage to a capacitive load, like a long cable. There is no better way to verify the AC integrity of a cable than to hipot it with AC voltage. VLF hipots are used to provide a go/no-go, or pass/fail, voltage withstand test.

VLF test sets are used to field test high capacitance loads like cables and motors/generators. The lower the frequency of an AC source, the lower the current and power required to apply a voltage to a capacitive load like a cable. At 0.1Hz, it requires 600 times less power to test a cable than at 60Hz. The HVI VLF instruments permit users to field test long cables and large generators with a portable and affordable test set. A 100 lb VLF instrument can do the job of a multi-ton 60Hz AC test set. Cables should be tested with AC voltage. With the HVI VLF products, it can be down with a practical, economical, and easy to use package.

The principal use of VLF is testing medium and high voltage shielded power cables. A long cable may have many microfarads of capacitance. To AC high voltage test this cable requires the use of VLF technology. An AC voltage test is the best way to verify the AC integrity of a cable. If a cable can’t hold 2 – 3 times normal voltage, it is not healthy and an in-service failure is likely. Use the VLF to cause defects to fail during the test. Find the fault, make the repair or replacement, and be left with a better cable. It is especially valuable for verifying a cable after installation or repair: far better than using a DC hipot, 5kV megohmmeter, hot stick adaptor, or soak test, none of which provide meaningful information about a cables ability to withstand several times normal AC voltage. IEEE 400, IEEE 400.2, VDE 0276, CENELEC HD 620/621, and IEC 60060-3 standards define VLF cable testing. VLF is also very useful for testing large rotating machinery, since it  provides a portable and affordable method of field testing coils and is sanctioned by the IEEE 433 standard.

The only way to field test high capacitance loads, like cables and motors/generators, with AC voltage, is to use a VLF AC hipot. The lower the frequency, the less current and power needed to test high capacitance loads. Xc (capacitive reactance) = 1/2πfC. A 10,000’ 15 kV cable has approximately 1uF of capacitance. The capacitive reactance at 60 Hz is 2650 ohms. To apply the IEEE recommended 22kV test voltage, it would require a power supply rated for 8.3 amps, or 183kVA. Obviously not practical for field use. At 0.1 Hz, the capacitive reactance is 1.6 megohms. The same 22kV would draw only 14mA, or only .302kVA, or 600 times less than at 60 Hz. At 0.01 Hz, a cable 6000 times longer can be tested than at 60 Hz. Put another way, at 60Hz a cable must be charged to its test voltage every 4.2 milliseconds, 0 – 90 degrees of the waveform. It takes a lot of power to charge a cable that fast. At 0.1Hz, 2.5 seconds are available to charge the cable. It takes 600 times less power than 60Hz. 500 times less than at 50 Hz.

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