CT accuracy at very low load

[croudk]

Howdy all,

Does anyone know how an ITI model 780 600:5 CT will perform at very low load - such as 3% of rating? A customer has this CT installed in a metering and overcurrent relaying application and they will not take a bus shutdown to change it to a lower ratio CT. They plan on installing a new Cutler-Hammer IQ Dataplus 4000 meter and they want to know if the meter will read within reasonable accuracy down to about 3% of the CT rating. The ANSI metering accuracy of the CT is 0.3% at 100% of rating and 0.6% at 10% of rating, but what happens at 3% of rating??? I'm trying to tell them that this is not a good idea.

 

[croudk]

New info...ITI technical support says that they can extrapolate the performance at 3% of rating to be similar to the performance of 10% of rating, or +/- 0.6%. They can only test down to 5%. Not bad for having a load that is down in the dirt.

 

[dpc]

I think it depends on what they are using the CT for. If it for the IQ meter and is used just for internal power usage, power quality, etc. I suspect it will be accurate enough.

If this was being used for billing or revenue metering, that's another story.

For relaying, we'd be more concerned about how the CT performed at 500% or 2000% current.

The thing that helps you is that the newer digital meters have very low burdens compared with old electro-mechanical meters.

 

[croudk]

The meter is only for internal plant use, so dead-nuts accuracy is not required. I just wanted to make sure it would give readings that aren't nonsense. The relaying performance will have to be evaluated at a later date when we actually get a P.O. for coordination. The stuff I just did was a freebie, but it was a good learning experience.