Can I use the same CT for Main & BK Protection? 2

[Champion]

Could any online expert tell me if I can use the same CT for Main & Backup protection for Transformer?

As I think the Main & Backup protection should use different CT. But I have not enough sets of CT in current Transformer. I don't know if there is a standard shows Main & Backup protection shouldn't use the same CT?

Thanks for your help.

 

[buzzp]

It would make sense if you used a different CT. Otherwise you have a place for single point failure and your back-up system is useless if the CT fails (rare but it happens). Standard or not, use another CT.

 

[scottf]

It really depends on your situation, as to use a different CT or not. Tell us a little about the equipment you are using and what kind of CTs (i.e. internal in some apparatus or free-standing) and voltage class.

 

[jwerthman]

It's always best to use different CTs for redundant protection systems. But if it's an existing installation, sometimes you have to compromise and do the best you can with what you have. It's usually not too hard to install slipover bushing CTs on a transformer for your situation. I'm just now working on a similar situation and the slipover CTs would be a great solution, but the customer just doesn't want to spend the extra money. In my opinion, using a common set of CTs doesn't sacrifice too much because CTs are pretty reliable devices.

 

[busbar]

Depending upon availability, security and reliability standards, using a single set for two similarly applied relays should be carefully scrutinized. The relatively small cost compared to the protection investment does not sound like a very economic decision.

The incremental cost of a set of CTs seems minor and a relatively small portion of project cost. One forgotten shorting screw could make a real mess of everything.

Modern multifunction relays are wonderful and can give a lot of bang for the buck, but please don't shoot yourself in the foot.

 

[Champion]

Thanks everyone!

 

[stevenal]

The separate CT has not eliminated common failure points. Do you use a separate battery/charger? Separate trip coils? How about separate interruptors (breaker)?

I know of a simple, low cost item that will provide backup for the entire protection scheme from CT to interruptor without relying on DC power. They are available in a variety TOC curves that can be coordinated with your primary protection. Fuses still have their place.

 

[electricpete]

good point, Steve.

 

[busbar]

Boy Howdy. 10-4 stevenal.

 

[Bung]

It depends also on what your "backup" protection is backing up. If it is on a differential relay, any CT failure is likely to be seen by the relay as a differential fault, and trip anyway, negating any advantage of the second protection!

Bung

 

[electricpete]

good point Bung!