Transformers without CTs
Transformers without CTs
(OP)
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.
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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RE: Transformers without CTs
RE: Transformers without CTs
RE: Transformers without CTs
Besides look at your cost savings for not including CT's. Is there that much savings as a percentage of the transformer cost?
RE: Transformers without CTs
But I see no reason in including them if the breaker CTs will do all the work.
RE: Transformers without CTs
RE: Transformers without CTs
RE: Transformers without CTs
RE: Transformers without CTs
RE: Transformers without CTs
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.
RE: Transformers without CTs
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?
RE: Transformers without CTs
Good point, had not considered that. But then how do I relay for an external fault?
RE: Transformers without CTs
RE: Transformers without CTs
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.
RE: Transformers without CTs
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.
RE: Transformers without 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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