CTs Wired in Delta for Differential Protection
CTs Wired in Delta for Differential Protection
(OP)
In a system with nearly all CTs wired in wye (excluding those used to account for the phase shift and zero sequence current in Delta-Wye transformer protection), there are three sets wired in delta for the differential protection of an outgoing line.
The outgoing line comes out of a ring bus. The two breaker positions on either side of the outgoing line have CTs wired in delta. The outgoing line connects to an autotransformer (Wye-Wye) and then to a breaker. The CTs on the far side of the breaker are part of the differential circuit and are also wired in delta.
Why would this be the case? I've heard that it could be due to a delta tertiary on the autotransformer. If so, do the CTs on the delta tertiary need to be wired in wye while the CTs on the wye windings need to be wired in delta to account for phase shift and elimination of zero sequence currents?
The tertiary does not seem to be involved in the differential circuit at all, but could the other CTs be wired in delta just in case this was added later?
Thank you for any clarification you can provide.
The outgoing line comes out of a ring bus. The two breaker positions on either side of the outgoing line have CTs wired in delta. The outgoing line connects to an autotransformer (Wye-Wye) and then to a breaker. The CTs on the far side of the breaker are part of the differential circuit and are also wired in delta.
Why would this be the case? I've heard that it could be due to a delta tertiary on the autotransformer. If so, do the CTs on the delta tertiary need to be wired in wye while the CTs on the wye windings need to be wired in delta to account for phase shift and elimination of zero sequence currents?
The tertiary does not seem to be involved in the differential circuit at all, but could the other CTs be wired in delta just in case this was added later?
Thank you for any clarification you can provide.






RE: CTs Wired in Delta for Differential Protection
The tertiary may not be connected to the differential if it is not used, or no other protection is available for the circuit tied to the tertiary. Some times the differential may not be sensitive enough to detect tertiary faults, so the point is moot.
RE: CTs Wired in Delta for Differential Protection
RE: CTs Wired in Delta for Differential Protection
Delta connections are a little more complicated than a standard wye and a little more difficult to design, install and commission correctly.
Where do you put the ground (earth) connection on the CT secondary circuit? It is intuitively obvious the wye point is grounded on a wye, but which corner is grounded on a delta?
With modern relays being able to do the zero sequence filtering digitally, it makes it much easier to keep everything in wye. The same CT can feed multiple protection or metering devices (assuming your reliability procedures allow that).
RE: CTs Wired in Delta for Differential Protection
RE: CTs Wired in Delta for Differential Protection