Using relaying CT's for measuring loads. 1

bacon4life · Jan 7, 2008

What it the typical accuracy of relaying CT's at typical load values of 0.1 to 1.0? I am asking because I just tried to add up the MW & MVAR at one our buses with c400 & c800 CT's feeding panel meters. The net MVA at the bus was about 7% of the total flowing through all the lines. What suprised me was variation in the total for each phase was much less than the unbalanced currents between the phases. I guess it is really two questions.

1. For a set of identical CT's, will they normally read the same value within say, 1%.
2. In comparing two different sets of relaying CT's, how much error should I expect?

I know IEEE C57.13 defines worst case accuracy as +/-10% between 1.0 and 20 times full load, but it doesn't address less than 1.0 PU loading, nor would it address typical nominal values.

Thanks

dpc · Jan 7, 2008

At normal load current I'd expect a C400 CTs accuracy to be better than 1% at a reasonable burden, at least in terms of ratio error. I just looked in an old Flex-core catalog and they have 600 V toroidal CTs that are C400, 1200:5 with a metering accuracy of 0.3 at B1.8.

High voltage bushing CTs may not be quite as good as that.

Maybe Scottf will be along to enlighten us both.

scottf · Jan 7, 2008

C800 and C400 CTs should be very accurate at rated current...at a minimum in the 0.3B1.8 range. At 10% Irated, I would expect them to normally be around 0.6%.

However, they aren't tested at those levels for metering accuracies.

The above is assuming the full winding is being used. If one of the taps is being used then it will be worse and potentially much worse, depending on the construction of the CT (i.e. if each winding segment is equally distributed around the core).

In general, you shouldn't see too much variation between like CTs of the same vintage and manufacturer. If they're different types of CTs, then you may see some noticeable difference, but I would think within .5% at rated current. Unless you're using taps, then all bets are off.

waross · Jan 7, 2008

See:
Exchange of current transformers of defferent class
thread238-206308
For other issues.
respectfully

bacon4life · Jan 9, 2008

Thanks Scott. At the particular substation I measured, we used taps on all of the CT's. That is pretty interesting that they could affect accuracy that much.

dpc · Jan 9, 2008

Tapping down a multi-ratio relaying CT also impacts the relaying accuracy. A C400 1200/5 CT tapped for 600/5 effectively becomes a C200 CT.

scottf · Jan 9, 2008

bacon4life-

For a given core size, the fewer the secondary turns, the worse the accuracy.

The example dbc gives is valid for CTs with equally distributed secondary windings, meaning each winding segment is equally distributed around the core. This is not a requirement of the standard and can't be assumed to be true unless specified that way. With that said, most manufactures try to supply distributed windings when possible, i.e. if it doesn't add too much cost.

oldfieldguy · Jan 13, 2008

What you're talking about doing, using relaying CT's for measurement, is a common practice. The fact that most of the new generation of microprocessor-based relays include comprehensive metering functions is testament to the reliability of this means of obtaining signals.

I wouldn't get too concerned unless my end user was pursuing the mythological "revenue accuracy".

old field guy

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Using relaying CT's for measuring loads. 1

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