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VLF vs AC HiPot

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  1. davidantkowiak is offline Junior Member Pro Subscriber
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    Vlf

    Quote Originally Posted by ElectricalTestTech View Post
    So as a brief answer, yes, I have used a VLF to hi pot a breaker in a bind. Just like you, it was all I had on site. In my eyes, a VLF was better than just a 5kV megger and DLRO. What I will speak on is "stress"... Think about why we use VLF on cables. We use it because using an AC hi pot at 60Hz is very stressful and damaging to cables (it's more technical than that explanation, but being brief). In my mind, I want to stress this breaker and make sure it won't fail on me when I rack it back into an energized cubicle. I want to stress it to make sure it wont fail when I close it and energize a circuit. That is the way I think of this topic. Its one of those statements that you make, "use the proper tool for the job" kind of deals. Use what's available to you, because something is always better than nothing

    Rc=1/6.28*F*C the only variable in cable reactance is f. The reasoning behind vlf is that the equipment would be less portable at higher frequency due to Rc. When testing a vacuum bottle the bottle would only be exposed to test voltage at .1 hz so you would have to test for a longer period to obtain sustained voltage at time period.

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  3. BrickSalad is offline Junior Member Pro Subscriber
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    Quote Originally Posted by davidantkowiak View Post
    Rc=1/6.28*F*C the only variable in cable reactance is f. The reasoning behind vlf is that the equipment would be less portable at higher frequency due to Rc. When testing a vacuum bottle the bottle would only be exposed to test voltage at .1 hz so you would have to test for a longer period to obtain sustained voltage at time period.
    Like you, I have also heard that the main reason behind VLF over AC hipot is equipment portability. However, it's also true that frequency does effect stress. Specifically, higher frequencies correspond to lower breakdown voltages.

    I'm not sure if this applies to a vacuum bottle though. The reasons I've read for why high voltage causes more stress on insulation is heat and partial discharge, which wouldn't really effect empty space.

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  5. young_dad is offline Junior Member Pro Subscriber
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    "AC Hipot damaging to cables..."

    Quote Originally Posted by ElectricalTestTech View Post
    So as a brief answer, yes, I have used a VLF to hi pot a breaker in a bind. Just like you, it was all I had on site. In my eyes, a VLF was better than just a 5kV megger and DLRO. What I will speak on is "stress"... Think about why we use VLF on cables. We use it because using an AC hi pot at 60Hz is very stressful and damaging to cables (it's more technical than that explanation, but being brief). In my mind, I want to stress this breaker and make sure it won't fail on me when I rack it back into an energized cubicle. I want to stress it to make sure it wont fail when I close it and energize a circuit. That is the way I think of this topic. Its one of those statements that you make, "use the proper tool for the job" kind of deals. Use what's available to you, because something is always better than nothing
    Just wanted to touch on your reasoning behind not using a power freq test set (AC Hipot) vs using a VLF.

    60Hz will not damage insulation. AC testing is by its nature not destructive to cable insulation. DC testing can reduce cable life of service aged cables by causing/propagating water trees that may not have otherwise been an issue. A 30-60 minute VLF will find damaged insulation and potentially cause a flash over, but generally only in cable that was already defective. Its lifespan is. Ot reduced by the VLF like it is in a DC test.

    However, due to the capacitive reactance of MV cable, 60hz test sets are just not feasible in the field. Increases in frequency. Since the dielectric (insulation) is not fully charged with each cycle, leakage and charging current would be pretty high at 60hz. The power output of a test set that can reach 60kV along with these high levels of current would prohibit field use.

    Since DC will charge and stabilize, current requirements and VA will be lower. Since DC is destructive, we use a VLF which allows us to test very capacitive devices (cable) without needing a trailer sized test set. VLF gives us the best of both worlds.

    In an ideal world we would test cables at power frequency.

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