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Extended (?) CT ratio
Jam
94 Posts
Question

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

4 Replies
mapj1
4166 Posts

Can you link to or attach a datasheet of the form causing the confusion.

I'm not familiar with this notation exactly, but it sounds like if this is a protection CT (from the P in the designation) I suspect you are not that worried about linearity in amps per amp in overload, just that nothing is damaged in the time it takes things to trip off.

Normally

I'd expect to see perhaps  20 5P10  - where 20 is the VA of the burden at full load, 5 is the 5% accuracy at full load (a 100A reading may be really anywhere in the range 95-105) and the 10 is the accuracy limit factor - which is a measure of how close we are to saturation of the magnetics and in this case ‘10’ says that the output is still a respectable sinewave at ten times the rated current, I suppose that might be seen as 1000%, but I'd  like to see where it comes from. 

(nice refernce in Cahier Technique)

 

This accuracy limit factor is of course only true when measured  into the reference load/burden resistance, which we deduce from the nominal VA, the nominal full scale current and the turns ratio, and the burden may be very low, and affected by its wiring resistances.

As a rule of thumb, burden resistances from 25% to 100% of nominal VA at full load are not likely to affect accuracy, but much larger loads risk magnetic saturation and unreasonaable secondary side voltage while trying to use much smaller loads  gets upset by the  resistance of the secondary winding itself - which can be allowed for but is a faff.

 

Caveat again 

normally  some situations like UPS outputs that may not be properly sinewave, or systems with bespoke CTs may not behave like this.

Mike.

 

 

Jam
94 Posts

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.

mapj1
4166 Posts

Hmm. Usually magnetic saturation is your friend and prevents the the secondary side thing having its socks blown off in the event of a primary side "bolted fault", and I suspect this is another way of expressing this, as it is about the only thing that should be happening at that multiplier. But I'd expect to  see “Idyn” or something.

Can you say who makes the CTs?

Mike.

Jam
94 Posts
No Idyn but Ith is 20kA/3s and the CT secondary current is low hence not so worried about sock removal (likewise at the relay).
CTs made by TLEP (Chinese firm, not responding to email) on behalf of a reputable switchgear manufacturer.

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