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Extended (?) CT ratio

Jam

Hi all,

I'm trying to calculate the Accuracy Limit Factor for some protection CTs but the ratio is specified as, for example, “5P20 extd. 1000%”… What does the “extd. 1000%” mean?

My first guess would be extending the 5% accuracy to 20x1000% = 200xIn under some condition, perhaps a specified burden.

The switchgear suppler is trying to help but is not desparately quick.

Thanks

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    Thanks Mike.

    Not at liberty to post a photo of the CT nameplate, sorry, but as you say they are protection CT supplied with an assoicated relay (with the resulting odd winding ratio).

    The CTs are marked “CL: 5P20 extd. 1000%”. Rated burden matches that of the relay. The rest of the markings are pretty standard (low burden to match the relay), but Rct and Ukp are not noted so we're not able to calculate ALF without further information.

    Withstand thermal effects are covered and as it happens the relay is backed up at this fault level by multiple levels of protection upstream, while protection settings are such that we aren't worried about the specific magnitiude seen by the device. We're trying to determine whether there's enough of a sinewave left that the closest relay to the fault operates reliably, i.e. the backup stays as backup.

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  • 0

    Thanks Mike.

    Not at liberty to post a photo of the CT nameplate, sorry, but as you say they are protection CT supplied with an assoicated relay (with the resulting odd winding ratio).

    The CTs are marked “CL: 5P20 extd. 1000%”. Rated burden matches that of the relay. The rest of the markings are pretty standard (low burden to match the relay), but Rct and Ukp are not noted so we're not able to calculate ALF without further information.

    Withstand thermal effects are covered and as it happens the relay is backed up at this fault level by multiple levels of protection upstream, while protection settings are such that we aren't worried about the specific magnitiude seen by the device. We're trying to determine whether there's enough of a sinewave left that the closest relay to the fault operates reliably, i.e. the backup stays as backup.

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