Electrical Engineering - CT Rating vs overcurrent

Author

CT Rating vs overcurrent

rzoet

2007-08-13, 9:25 am

To all,

At the moment we are using a 5P20 / 500 / 1 A current transformer. Now
we are calculating a primary current of 527.1A. The question is: Can
we still use the 5P20 / 500 / 1 A CT or should we go for the 5P20 /
600 / 1 A CT because of continuous overcurrent.

When 5P20 / 500 / 1 A can still be used but not continuously what
would be the maximum alowable operation time for this CT.

Data:
voltage level =34.5kV
I= 527.1A continuous.
frequency = 60Hz

Does someone know were we can also find the solution according IEC,
because in IEC 60044-1 it is not clear stated how to handle continuous
primary overcurrents vs CT rated current.

Any advise is welcome.

Charles Perry

2007-08-13, 8:25 pm

"rzoet" <r_zoetmulder@hotmail.com> wrote in message
news:1187008015.641370.92870@q75g2000hsh.googlegroups.com...
> To all,
>
> At the moment we are using a 5P20 / 500 / 1 A current transformer. Now
> we are calculating a primary current of 527.1A. The question is: Can
> we still use the 5P20 / 500 / 1 A CT or should we go for the 5P20 /
> 600 / 1 A CT because of continuous overcurrent.
>
> When 5P20 / 500 / 1 A can still be used but not continuously what
> would be the maximum alowable operation time for this CT.
>
> Data:
> voltage level =34.5kV
> I= 527.1A continuous.
> frequency = 60Hz
>
> Does someone know were we can also find the solution according IEC,
> because in IEC 60044-1 it is not clear stated how to handle continuous
> primary overcurrents vs CT rated current.
>
> Any advise is welcome.
>
Not familiar with that particular CT, but shouldn't the manufacturers data
answer this? In the US, CTs have nameplate ratings, such as 500:5 but also
an overload rating. The utility where I worked purchased metering CTs with
an overload factor of 3x, meaning that the CT could carry, and still be
accurate for, a current 3 times the nameplate current.

Charles Perry P.E.

Janos

2007-08-14, 3:25 am

Charles Perry a écrit :
> "rzoet" <r_zoetmulder@hotmail.com> wrote in message
> news:1187008015.641370.92870@q75g2000hsh.googlegroups.com...
> Not familiar with that particular CT, but shouldn't the manufacturers data
> answer this? In the US, CTs have nameplate ratings, such as 500:5 but also
> an overload rating. The utility where I worked purchased metering CTs with
> an overload factor of 3x, meaning that the CT could carry, and still be
> accurate for, a current 3 times the nameplate current.
>
> Charles Perry P.E.
>
>
yes but in this case it is a continuous overload that is asked, this
must be avoided 'cause the transformer will be continuously saturated
what make the output signal no more linear.
regards

Carlton Stedman

2007-08-14, 9:25 am

On Aug 14, 2:04 am, Janos <ja...@netik.name> wrote:
> Charles Perry a =E9crit :
>
>
>
>
>
>
>
ata[color=darkred]
also[color=darkred]
with[color=darkred]
>
>
> yes but in this case it is a continuous overload that is asked, this
> must be avoided 'cause the transformer will be continuously saturated
> what make the output signal no more linear.
> regards

I agree, because of continuous overload, the transformer is going to
saturate, causing nonlinearities in measurement. I would opt for a
higher rated CT, but, as previous posters mentioned, see if you can
contact the manufacturer or get a data sheet. It may be that the
saturation current is higher than 500A, but you'll also have to
consider that this high continuous current will heat up the CT,
causing further nonlinearities. It'll heat up quite a bit, so you may
really need a higher rated/bigger CT. Some things to consider.

Charles Perry

2007-08-14, 1:25 pm

"Janos" <janos@netik.name> wrote in message
news:46c145b2$0$24918$5f6aeac3@news.scarlet.nl...
> Charles Perry a écrit :
> yes but in this case it is a continuous overload that is asked, this must
> be avoided 'cause the transformer will be continuously saturated what make
> the output signal no more linear.
> regards

Depends on the CT. The CTs I refer to were meant to run a continuous
overload of 3 times rated current.

The overload rating indicated the range over which the CT would still meet
meter accuracy requirements, so no appreciable distortion. The CTs with
such a rating are much more expensive, but much cheaper than sending a meter
tech to change them out every time a customer's load increases.

May IEC spec CTs are made to much lower standards.

Charles Perry P.E.

ng2007@saXuna-aXho.com

2007-08-14, 8:25 pm

On Tue, 14 Aug 2007 08:04:20 +0200, Janos
<janos@netik.name> wrote:

>Charles Perry a écrit :
[color=darkred]
>yes but in this case it is a continuous overload that is asked, this
>must be avoided 'cause the transformer will be continuously saturated
>what make the output signal no more linear.
>regards

Continuous current of 1.05 times the rated current
is ok for a 5P20 CT with less than rated secondary
burden.
For a short time this CT can handle 20 times the
rated current , which is 10 kA, with an amplitude
error of 5 % with the rated burden.

More than rated overload will heat up the CT. No
saturation.
Short circuit fault currents about an above 20
times the rated current with rated burden will
cause saturation.

If the burden is less than rated, this CT can
handle event higher currents than 10 kA.

(Remove the two Roentgen-ray characters to reply.)

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