4 Advice to Choose a vlf ac hipot tester

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GE Burlington continues to strongly recommend the use of an AC high potential machine for vacuum interrupter integrity tests. DC testing of vacuum interrupters should only be utilized if an AC tester is not available, and should be used for quick field checks only.

Our experience with DC testers over many years indicates they frequently yield false negative test results, due partially to the capacitive component of the vacuum interrupter during DC testing, and to the fact that most lightweight DC testers have a very low leakage current trip setting. They will however, reliably indicate a truly failed bottle if the voltage output is set at 50kV DC.

If using a DC tester, and a test indicates a bad interrupter, retest with the polarity of the DC test voltage reversed. If this results again in a failure, we would recommend a final AC test prior to contacting GE Post Sales Service or discarding the interrupter.

No attempt should be made to try and compare the condition of one vacuum interrupter with another, or to correlate the condition of any interrupter to low values of DC leakage current. There is no significant correlation.
After the high potential voltage is removed, discharge any electrical charge that may be retained.

CAUTION: MANY OLDER DC HIGH POTENTIAL MACHINES ARE HALFWAVE RECTIFIERS. THIS TYPE OF HI-POT TESTER MUST NOT BE USED TO TEST VACUUM INTERRUPTERS. THE CAPACITANCE OF THE POWER/VAC BOTTLE IS VERY LOW AND THE LEAKAGE IN THE RECTIFIER AND ITS DC VOLTAGE MEASURING EQUIPMENT IS SUCH THAT THE PULSE FROM THE HALF-WAVE RECTIFIER MAY ACTUALLY BE IN THE NEIGHBORHOOD OF 120KV, WHEN THE METER IS ONLY READING 40KV.

IN THIS CASE, SOME PERFECTLY GOOD BOTTLES CAN SHOW A RELATIVELY HIGH LEAKAGE CURRENT SINCE IT IS THE PEAK VOLTAGE OF 120KV THAT IS PRODUCING ERRONEOUS BOTTLE LEAKAGE CURRENT. IN ADDITION, THE X - RADIATION WILL BE OF CONCERN.

GE Burlington continues to strongly recommend the use of an AC high potential machine for vacuum interrupter integrity tests. DC testing of vacuum interrupters should only be utilized if an AC tester is not available, and should be used for quick field checks only.Our experience with DC testers over many years indicates they frequently yield false negative test results, due partially to the capacitive component of the vacuum interrupter during DC testing, and to the fact that most lightweight DC testers have a very low leakage current trip setting. They will however, reliably indicate a truly failed bottle if the voltage output is set at 50kV DC.If using a DC tester, and a test indicates a bad interrupter, retest with the polarity of the DC test voltage reversed. If this results again in a failure, we would recommend a final AC test prior to contacting GE Post Sales Service or discarding the interrupter.No attempt should be made to try and compare the condition of one vacuum interrupter with another, or to correlate the condition of any interrupter to low values of DC leakage current. There is no significant correlation.After the high potential voltage is removed, discharge any electrical charge that may be retained.CAUTION: MANY OLDER DC HIGH POTENTIAL MACHINES ARE HALFWAVE RECTIFIERS. THIS TYPE OF HI-POT TESTER MUST NOT BE USED TO TEST VACUUM INTERRUPTERS. THE CAPACITANCE OF THE POWER/VAC BOTTLE IS VERY LOW AND THE LEAKAGE IN THE RECTIFIER AND ITS DC VOLTAGE MEASURING EQUIPMENT IS SUCH THAT THE PULSE FROM THE HALF-WAVE RECTIFIER MAY ACTUALLY BE IN THE NEIGHBORHOOD OF 120KV, WHEN THE METER IS ONLY READING 40KV.IN THIS CASE, SOME PERFECTLY GOOD BOTTLES CAN SHOW A RELATIVELY HIGH LEAKAGE CURRENT SINCE IT IS THE PEAK VOLTAGE OF 120KV THAT IS PRODUCING ERRONEOUS BOTTLE LEAKAGE CURRENT. IN ADDITION, THE X - RADIATION WILL BE OF CONCERN.

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.

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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 , CENELEC HD 620/621, and IEC -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 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 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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