Portable AC hipots are ideal for testing many types of substation apparatus, motors/generators, isophase bus, and other loads requiring an AC withstand test. But did you know that portable AC hipots are offered as either power frequency (50/60 Hz) or Very Low Frequency (VLF) models? Why two styles?
The answer has to do with the capacitance of the load being tested. Capacitive Reactance (Xc) changes as a function of frequency as seen in formula Xc = 1/2πfC. Therefore, if we are testing a 15 kV cable of approximately 10,000 feet the capacitance would be around 1uF. Based on the formula the capacitive reactance at 60 Hz would be 2650 ohms. To apply the IEEE recommended 22 kV test voltage, it would require a power supply rated for 8.3 amps, or 183 kVA. Not very practical as a portable field unit.
If the frequency, however, is dropped to 0.1 Hz, the capacitive reactance becomes 1.6 megohms. The same 22 kV would only draw 14 mA or 0.303 kVA. Therefore, the size, weight and portability of the power supply become convenient for field use.
AC hipots of the power frequency variety are used worldwide for testing vacuum bottles, switchgear, reclosures, circuit breakers, etc. These units will typically offer ratings of 3 to 7 kVA. VLF hipots are the preferred for testing high capacitance loads such as cable, large rotating machinery and occasionally large insulators, arrestors and such.
At Protec Equipment Resources we carry a vast inventory of both power frequency AC hipots and VLF style AC hipots from leading manufactures like HV Diagnostics and High Voltage Incorporated. Still unsure on which style best meets your needs? Consult with our expert staff and be sure you have the right tool for your job!