Why do you need dual ratio current transformers?

[electrical429]

I often specify dual ratio CTs based on standards, however when producing multicore wiring diagrams I always choose the ratio which is closest to required load. I never really thought about it, but why do you actually need dual ratio CTs? Even if you have ONAN/ONAF/OFAF transformer where OFAF rating is 2 x ONAN rating, surely field personnel don't recommission CTs to the higher ratio once transformer goes into OFAF rating. So why would you need dual ratio then?

 

[thermionic1]

Back in the days of electromechanical relays, the ranges of settings were somewhat limited. If the system changed and new relay settings were required, it was sometimes easier to change a CT tap to match the setting range of the relay. The same could be true for metering applications. Changing a CT tap is certainly less expensive than changing a CT, especially if it's in the ANSI world and it's a transformer.

 

[cranky108]

Old habits die hard. With the exception of differentials, and metering, most of our CT's end up on the highest ratio.

 

[RRaghunath]

I find the CTs need to be replaced in spare panels when someone decides to use that panel and finds the load intended to be connected is much more than the existing CT primary rating. This is one case where a dual ratio (better even multi-ratio) CTs are helpful.

 

[mvcjr]

For distribution utilities, particularly for revenue metering applications, one reason is to reduce CT inventories.

 

[retriever]

For relay protection it is better to use higher CT ratio than for metering.
Protection CT core (TPX, TPY, TPZ) has to work properly in case of short-circuit conditions.
Metering core has to work properly only in case of normal load condition.

In addition, if you want to choose CT to prevent saturation because of DC decaying component, CT ratio have to be taken as high as possible.

 

[cranky108]

However... the minimum current setting is usually about 0.5 amps for a relay. Below that is noise.