Sizing CT's Primary

[timm33333]

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?

 

[prc]

Since the load is same, measuring CTs need not require any change. But in case you are uprating transformer wthout changing the transformer,say by adding coolers,(ie %impedance 10% become 15% after uprtaing)then also protection CT need not be changed.Otherwise protection CT rating is to be checked depending on short circuit current.

 

[timm33333]

Thanks prc for information. We are actually putting a totally new transformer of higher rating. The IEEE standard says that the primary of CT should be rated to carry the full load current of power transformer. This means that as per IEEE, CT's need to be up-sized even if the load is the same. Do you know whether IEC also has this kind of requirement?

 

[scottf]

What IEEE std are you referring to?

 

[bacon4life]

If the CT rating does not meet or exceed the the transformer rating be sure to address how eventual load increases will be dealt with.

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.

 

[timm33333]

The IEEE std I was referring to was IEEE 242. What kind of agreement (of not putting extra load in future) should be with the customer, should there be something written on single line?

 

[waross]

Protection:
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
--------------------
"Why not the best?"
Jimmy Carter

 

[Mila15]

I agree with waross's comment about saturation. With regards to continuous operation, I don't think it's required to upgrade the CTs with larger transformers, since the load didn't change. IEEE's recommendation is consistent with good practice though. From my experience, loads do increase with time, and customers look at transformer rating as a value they can increase their load to, so it's a good practice to have CTs that are consistent with transformer rating.

 

[timm33333]

How to make agreement with customer, should it be put on single line that no new loads will be added on transformer?

 

[waross]

Have your lawyer prepare a disclaimer holding you free and clear of any future claims should the customer increase his load without having the increase vetted by a competent engineer and that he will also make said engineer aware that the CTs are selected for most accurate metering and that an increased load must be checked by a competent engineer and the CTs may have to be changed depending on the magnitude of the proposed increase.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bacon4life]

The CT's are not typically the limiting component, so if you pursue an agreement to leave lower rated CTs, the load limit should be on every document that lists the rating of transformer or buswork. In addition to the station oneline and the disclaimer Bill mentioned, the limit should be posted with engraved labels on the transformer, the CT cabinet, the metering cabinet and the relaying cabinet. Additionally it should be included with the meter/relay settings files, the utility system model data, the transformer maintenance records and the utility system wide oneline. Not only do customers forget to tell the utility that the loads has increased, the utility has to make sure that if the customer does call, that the utility realized the danger increasing the load poses.

 

[waross]

It's not a bad idea to have accounting set up a check system to flag climbing demand charges and increased monthly consumption. Doable in some plants but not in others.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[tinfoil]

Having worked in the utility industry for 20 years, I can tell you the following:

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.

 

[davidbeach]

Oh, come on. What is the thermal rating factor of the CT? What is the continuous current rating of the relay? Will either be exceeded with the larger transformer? If the existing CTs have a 2000A rating and the new transformer runs at 15MVA the CT will see 131% of rated current. With a thermal rating factor of at least 2 (often 4), the CT could run at 2625A forever without damage. The relays I'm familiar with will take 3 times nominal rating continuously and would also not be damaged. Saturation is the only possible issue and if the CTs are decent sized the increased fault current won't create significant saturation issues.

 

[scottf]

I agree with Davidbeach. And if the CTs are being used for metering, they are probably mis-sized/over-sized to begin with.