Transformers without CTs

[Mbrooke]

Where protecting transformers through a dead tank circuit breaker instead of a CS or isolator, does anyone skip transformer CTs or see an issue in doing so? Just looking for reassurance.

 

[davidbeach]

What happens when that transformer gets moved to another location? Over the life of the transformer it may live at more than one location, will the lack of bushing CTs cause a problem at the next location?

 

[Mbrooke]

Unlikely to be moved, unless the unit is small most transformers over 20MVA will have a 40 year minimum in service life. However, that is not to say your point is not valid. It could happen.

 

[cranky108]

What about H0 and X0 CT's? It's hard to include them on the circuit breaker.

Besides look at your cost savings for not including CT's. Is there that much savings as a percentage of the transformer cost?

 

[Mbrooke]

There may be an external neutral CT if needed.

But I see no reason in including them if the breaker CTs will do all the work.

 

[dpc]

Would never even consider buying a new 20 MVA or larger xfmr without bushing CTs. I've run across some cheapskate industrial/IPP types who insisted on leaving them out, but the differential cost is trivia and could solve a major problem in the future. Cheap insurance against our inability to predict the future.

 

[Mbrooke]

But doesn't your design/practice drive it?

 

[111R]

If anything, it seems like most are going in the opposite direction and including more CTs in the transformer design on new units after dealing with an insufficient quantity on older units. I don't think there's a solid answer other than to get a quote on a unit with/without CTs and evaluate based on your application.

 

[Mbrooke]

I will. In terms of protection it both the same, right?

 

[DTR2011]

I've seen older units that did not have internal CT's and at some point slip on CT's were installed. I've seen the issues with slip on CT's.

I've often seen HV CB and XFMR use separate CT's for primary and backup 87T. Additionally, in my neck of the woods, the TransCo and DisCo have an agreement where the TransCo uses XFMR internal CT's (HV Side) for 87B.

It seems more and more dedicated CT's are preferred, rather than relays in series- again in my neck of the woods.

 

[111R]

Mbrooke - If you're using transformer diff protection, using CTs on the transformer makes it easier to differentiate between an internal transformer fault and a nearby external fault that would still be within the zone of protection if external CTs are used.

DTR2011 - what issues have you encountered with the slip-on CTs? Just curious in case I ever have to consider this in the future. Do these affect insulation characteristics since they're installed above the bushing flange?

 

[Mbrooke]

Mbrooke - If you're using transformer diff protection, using CTs on the transformer makes it easier to differentiate between an internal transformer fault and a nearby external fault that would still be within the zone of protection if external CTs are used.

Good point, had not considered that. But then how do I relay for an external fault?

 

[Mbrooke]

As is, if I have an event data recorder, can't I tell a turn to turn fault from a bushing flash over?

 

[cranky108]

Internal transformer CTs for differential, while true enough would not respond the external faults. But what is protecting the area between the transformer and it's protective device (breaker or switcher)?
If the goal is to only outage as few customers as possible, put the differential on the protective device.
If you want a verification that the fault was internal, then add a second differential on the transformer CTs.

 

[Mbrooke]

Dead tank circuit breaker which comes with CTs already on the bushings.

Thus, having multiple CTs just makes it more complicated imo. I understand where a transformer is fed directly from a bus bar through a MOD makes internal CTs look practical while achieving knowledge of where the fault took place, but where a breaker is involved be it live or dead tank the space between the breaker and transformer needs protection.

 

[FPelec]

We do not use internal CTs on our transformers (rated power ranging from 10 to 600 MVA). We use instead dedicated external external CTs.
Using internal CTs can jeopardize protections selectivity in case of bushing failure, leading to busbar differential protection trip.
We use internal CTs only in some two-core Phase Shifting Transformer, as some winding are buried and it is not possible to measure their currents otherwise.

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