NUMBER OF CTS REQUIRED IN A BUSHING OF POWER TRANSFORMER

[subash148]

I would like to know the number of CTS that are generally required for a bushing of a delta-wye grounded Power transformer with the following rating:

54/72/90 MVA
13.8 kV/ 242 kV- Grounded Wye
Impedance=8.1%
Single phase but used in three phase application with three similar transformers.

The primary( 13.8 kV) side is delta connected. The secondary side(242 kV)is grounded wye. This makes two bushing on the primary side (delta side) and two bushing on the secondary side ( wye grounded side). One of the bushings on the secondary side will be for the neutral grounding. The neutral bushing does not require any CTS.

Please let me know, how many CTS are required in each of the two bushings on the delta side (13.8 kV) and one bushing on wye-grounded side(242 kV).

SG

 

[cuky2000]

The numbers of BCT’s is determine by the protection and control scheme. Usually you need primary and backup CT or metering. Bushing could accommodate in most case up to 3 CT's. If a CT is not used, this could be shorted and be available as spare for future use.

 

[Marmite]

As Cuky stated, it depends on your protection philosophy and design. Some utilities are moving away from bushing CT's in favour of separate CT's to make it easier to replace a faulty transformer without a complete redesign of the protection system.
Regards
Marmite

 

[davidbeach]

From my perspective:
[ul]
[li]The CTs inside the delta will be trouble, now and for ever; I wouldn't bother unless the transformer thermal model protection requires them.[/li]
[li]I prefer to protect transformers with CTs on the breakers rather than CTs on the transformer, CTs on the wye side bushings may be unnecessary for protection purposes.[/li]
[li]I absolutely would not omit the neutral CTs. These may be a single CT away from the transformer after the three neutrals are tied together or it may be three neutral CTs with the secondaries paralleled.[/li]
[li]There will be some need for a limited number of bushing CTs for the various transformer devices.[/li]
[/ul]

Beyond that, install as many spare CTs as you're comfortable buying.

 

[krisys]

From the OP's description, it appears to be a typical Generator unit transformer. The 13.8 kV side busing is expected to be quite heavy.

If my assumption happened to be true, then the 13.8 kV system earth fault will be part of generator protection scheme. Thus I am not finding any reason to have phase CTs for the delta winding. OP may elaborate little more on the background.