Multi-Ratio CTs

[MJK]

A simple question with many different opinions. What is the proper method of terminating the unused terminals / taps of a multi-ratio CT? It's obvious that the taps which are internal to the selected ratio series should be isolated and insulated from ground. But what about the extended windings, possibly on both sides of the selected ratio? It seems right to short these to the closest used tap. I would expect high voltages to be developed on the these terminals if they remained open. Any info would be appreciated. Thanks in advance for your feedback.

 

[BEPC13]

As long as one tap of a muliratio CT is used the other taps can be left open. It is transformer theory that all the flux will go to the used terminals and the unused terminals will have no voltage on them.

 

[busbar]

One secondary 'loop' will keep the CT core out of saturation.

 

[PWR]

It has been my experience that you cannot short some of the windings and expect to get an accurate ratio on the taps you are using. Therefore, you want to leave the unused taps "floating"

 

[jbartos]

Suggestion: Some CTs have conductors connected to all terminals and brought to terminal box, where they are terminated at terminal boards. One CT secondary winding terminal set is connected to the CT secondary loads.
The secondary multiwinding CTs have unused individual secondary windings shorted.

 

[scottf]

I'm not sure I followed jbartos' comment.

But to be clear, when using multi-ratio CTs, you connect the burden to the 2 terminals needed for the ratio desired. The other terminals are to be left open and not shorted!

 

[jbartos]

Clarification/Suggestion:
The multi-winding secondary CT are accomplished on multicore CTs.
Visit

https://www.kuhlman.com/clientdata/ReplacementGeneratorCurrentTransformer.pdf

for a multi core current transformer, HW-945

 

[busbar]

A typical single-winding tapped-CT style is Table 8 Current Transformer Ratings, Multi-Ratio Type in IEEE Std C57.13-1993 …Instrument Transformers

 

[jghrist]

Overvoltage can occur if a partial winding is connected to a high impedanced differential relay such as a GE PVD. For this reason, you should use the full winding when connecting to these relays. See page 4 of

http://www.geindustrial.com/products/manuals/pvd/gek45405.pdf

 

[scottf]

jghrist-

Thank for the link to the GE manual. I have to admit, I disagree with their comments on high-voltages developing on MR CT where the full winding is not used. They give quite a bit of justification, but I believe some of their assumptions are incorrect. Particularly, when they say that a completed saturated core will not have any voltage across its secondary. This is incorrect...just look at a typical excitation curve...the CT will maintain a voltage close to its knee-point voltage.

In fact, in some applications, intentional saturated CTs are used for power sources of electronics/lights, etc... at HV, when the power requirements are low. This way a power source of relatively constant voltage can be used without having to provide isolation to the HV level.

 

[stevenal]

See IEEE C37.110-1996 for more info on the high impedance differential application (and others). I think the zero voltage assumption is a worst case approximation.