S&C Trans-Rupters

[hidalgoe]

Our Main Substation transformers (15MVA, 69kV/12.47kV)have S&C Trans-Rupters with overcurrent protection on the 69kV side. What would be the best placement for the CTs used with the overcurrent relays: on the line side of the Trans-rupters or on the load side of the Transrupters.

 

[pashwuee]

The best location is load side of transrupter . Because in future for any maintenance purpose you can easily open the trans rupter and work on CT's , If on line side of transrupter CT's are place you have to shut the line down for CT maintenance.

 

[davidbeach]

What are the overcurrent relay protecting? If they are protecting the transformer the CTs should be on the line side of the Trans-Rupter. If they are protecting the line the CTs should be on the transformer side of the Trans-Rupter.

That said, it is all too common that transformer protection is done using just the CTs on the transformer bushing because there is no place to put CTs on the line side of the interrupting device. But where it is at all practical to do so, CTs should always be placed such that the interrupting device is in the zone of protection.

 

[pashwuee]

If CT's are placed on line side then how will it be taken for maintenance . CT,s are mounted on transformer where it is required by customer. CT is are also in the line as well as transformer and busduct.

CT just defines the zone you want to protect. As it is in case of Transrupter self powered relay is used the Ct is used as supply source and will be after breaker

 

[davidbeach]

Do what you want, but in general it is desirable to have the interrupting device in the zone of protection. Check any good relaying text.

 

[cranky108]

Two good points, but puting the CT on the line side of the transrupter means it is difficult to maintain if needed. The exception is if you already have a switch before the transrupter.

However, if you use the transformer BCT's, the zone of protection will not include the high side transformer bushings. But I would presume the high side (line) is protected by a distance, or over current type of protection.

Consiter this: The cost of a breaker with BCT's isen't that much more than a transrupter with an added free standing CT. So if your goal is to cut cost, use the transformer BCT's with the transrupter. However, if your goal is to reduce outages, install breakers.

 

[hidalgoe]

The CTs are protecting the transformer. The 69kV lines feeding the transformers have load break switches up stream of the Trans-Rupter. The lines can be de-energized using these switches. We also have a tie switch at our main substation to parallel the transformers in case we want to shut down either line for maintenance or repairs.

The only protection that the Trans-Rupters now have are the protective relays at the Utility substation where our two 69kV lines are fed from; that is why I think Mr. Beach is strictly correct: The interrupting device should be in the zone of protection. Why subject the interrupting device to fault currents waiting for the Utility protection to kick in?

 

[jghrist]

I agree with David that it is ideal to have the CTs in the zone of protection, and that the Trans-Rupters are protecting the transformer. Normally, this ideal is used to ensure that there are overlapping zones of protection and that the interrupting device is in both zones. In this case, however, there is normally no line protection at the station; the line protection is provided at the source and will extend into the transformer. The only thing gained by the expense of the free-standing CT is a very small decrease in permanent line trips. Without the separate CTs, a fault between the Trans-Rupter and the transformer bushing CTs will result in a line trip, a reclose, then a permanent outage unless the fault is temporary. With the CTs, the fault would result in a line trip, a trip of the Trans-Rupter, and a reclose of the line.

If the fault is temporary, having the separate CTs will result in an unnecessary trip of the Trans-Rupter. If the fault is permanent, not having the CTs will result in an unnecessary permanent trip of the line.