Paralleling of CTs

1) why would you parallel CTs? Anyway transformer paralleling requires:

- same primary voltage

-same secondary voltage

- obviously same ratio

(if possible same magnetizing characteristics)

-same idle voltage

If you have larger current than the CT can handle, the solution is to change the CT with a larger one.

2) Why would you parallel CTs from different feeders? But if you have two CTs from different feeders in parallel, the deenergized feeder will be energized through the CT from the other feeder. silly thing to do.

If you have a current that is too large for a Current Transformer, and you add a second CT, you now have two overloaded CTs. If you connect them in parallel to the current meter, you now have a current meter that is carrying more than twice its rated current, until it blows out.

Get an appropriately sized Current Transformer, and spell out the words at least once in your text, so we know you are not referring to 'Center Tapped', or some other meaning of 'CT'.

I am sorry that I have used the acronyms CT instead of current transformers, but one question, who would say "paralleling of CENTER TAPPED"?.... just a thought.. in you try to read standards of current transformers, the letters CT denotes current transformer.. i have not known any other standard that uses CT other that current transformer... if there is such.. please provide me..

sorry that i have not told the whole scenario... in 138kv substation, there are 2 138kv/69kv power transformer that are connected in parallel.

it has four feeders, two feeders per transformer. the buses are interconnected via tie power circuit breaker.. the feeders on the other hand, the only rated at 580 Amps..(power cable) and we want to interconnected it to a 900 Amp-795ACSR overhead transmission line..

to fully use the capacity of the 900Amp overhead line, we plan to interconnect the 2 feeders (2power cables).

In the protection point of view, each feeder has already been connected with its corrected current transformer, our concern is the feeders protection, so we plan to interconnect the current transformer in parallel for the two feeders, on the other hand we can also calculate on the protection settings of the feeders to be coordinated in such a way that even at parallel conditions it is ok..

im just having curiosity on paralleling CTs.. that's all..

anyways, thanks for the help guys..

I can easily imagine someone having a Center Tapped transformer that did not provide enough current, so adding a second in parallel. In theory, with two correctly connected identical transformers, it should work. In practice, it would be dangerous. (I'm thinking of transformers that one can lift with one or two hands, or at most a small forklift.)

I have no experience in substations, nor in your kind of voltages, so I'll bow out...

What exactly is your concern with the feeder protection?

If each feeder has its own protection, or at least it should, why do you want to change the configuration for the protection? What is the whole idea with the protection?

There are summing transformers that allow you to connect CTs from different feeders together. This provides a high impedance path between the CT preventing them from feeding each others.

Of course, without summing transformers, when two CTs try to feed each others, they quickly exceed their VA rating. This leads to overloading and heating while injecting some current in the OFF feeder depending on its protections and characteristics.

Dear Hkian,

I do not understand your idea for paralleling CT, but I have some example installation to connect the CT in parallel. i.e. bus differential relay, restrictive earth fault relay and partial bus differential protection scheme.

For this schceme CT ratio should the same and CT polarity wired correctly. The aux CT is necessary when the the ratio of the feeder is not the same (see protection scheme beloW).

It does not matter the feeder either off or energize.

Sorry if this information not meet your idea. See attachced a one line protection scheme.

Regards,

I would want to know why you want to put your CTs in parallel. Is it because u have ur transformers in parallel? Normally Breakers & RMU are used for paralleling if the two transformers have passed the paralleling condition points as follows

1. The primary and secondry voltage must be the same
2. The same tap ratio
3. Equal percentage impedance
4. The same vector group.

Feeders are used for switching on and off the bulk load. In most u may find more than one feeder and a summation unit used to calculate the total load.

Now for CTs to be connected in parallel, they must meet the following conditions

(1) Same burden

(2) Same curren tratio

(3) same polarity must be observed

If you want to meter the transformer, use a summation CT unit for that purpose.

Dickson