Sizing CT's Primary
Sizing CT's Primary
(OP)
Does anybody know what are the IEC requirements for sizing primary of CT? We are upsizing an 11kV / 3.3kV transformer from 10MVA to 15MVA, but the load is same as before. Do we also need to upsize the CT’s on the load side of the transformer, or can we use the same CT’s because the load is exactly the same as before?






RE: Sizing CT's Primary
RE: Sizing CT's Primary
RE: Sizing CT's Primary
RE: Sizing CT's Primary
We had an industrial customer that needed a second substation that would initially be lightly loaded. To meter accurately, we put 200A CTs on a 1100A transformer secondary. We then had an agreement that they would notify us before adding additional load. A few years latter, they temporary switched all theirs loads to the new substation to perform maintenance on the original substation. The CTs exploded soon there after.
RE: Sizing CT's Primary
RE: Sizing CT's Primary
The protection scheme should be re-evaluated for the new transformer. Higher fault currents may push the CTs into saturation.
Revenue metering:
The existing CTs will meter the existing load accurately.
But, in agreement with bacon4life, I have also had issues when a customer agreed not to increase the load and installed loads that he had specifically been warned about and promised would never be installed.
I will never increase the load and the check's in the mail.
Of the two statements, the check's in the mail may be the most believable statement.
Bill
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"Why not the best?"
Jimmy Carter
RE: Sizing CT's Primary
RE: Sizing CT's Primary
RE: Sizing CT's Primary
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Sizing CT's Primary
RE: Sizing CT's Primary
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Sizing CT's Primary
No protection scheme that relies on words on a paper can be considered effective.
At some point in the future, some plant dude is going to put more load on his system if the system, at a glance, can accommodate it.
It is more often a 'dance of a thousand cuts' where a little bit of load is added for this process here, that project there, until the 'artificial' load limit is exceeded.
Here in Ontario, the local safety authority (ESA) will often require that the primary trip device (typically the main load-side breaker) be 're-plugged' to trip at a lower current limit to protect whatever is the weak link, the breaker MARKED with this lower limit, and that there be a one-line POSTED in the main electrical room that clearly indicates what the limit is and why. Even then, I have seen overloaded systems.
The CTs, buses, etc should be upgraded, particularly if you are leaving them in place to save time / money rather than for a technical reason.
The newer CTs that are accurate down to 1% of nominal rather than the old-school ones that needed 10% of nominal can solve a lot of problems.
RE: Sizing CT's Primary
RE: Sizing CT's Primary