Install CT on the secondary leads of a 5A CT?

[tcleghorn]

My company wants to standardize on installing a specific 333mV input power meter wherever we are retrofitting power meters to existing services. The primary reason is to use rogowski coil CT's.

Occasionally, we add power meters to services with existing solid-core revenue-grade 5A secondary CT's connected to existing non-functional meters or analog current devices. I would like to keep these existing CT's in-place wherever possible and re-use them but the 333mV input power meter will not accept them.

Looking at a couple options, is it possible to add a split-core or solid-core 5A-to-333mV CT with a 0 degree phase shift on the secondary leads of an existing 5A CT? I know this will add a burden to the existing CT's but are there any other issues I need to be aware of?

 

[coop3339]

I think that it could decrease the accuracy. If both CTs had an accuracy of 99% the resulting accuracy would be the product of the 2 or about 98% in this example. If you had to maintain a certain accuracy like Ansi C12 this could be an issue. Maybe there is a converter of some type?

 

[David Castor]

I'm sure it's possible. If this is being used for revenue metering, you need to carefully look at the accuracy of this setup. Possible sources of error would be CT ratio error (times 2), phase angle error, and CT burden. Standard metering CTs generally have minimum accuracy of 0.3%. I'm skeptical that a Rogowski coil will provide an equivalent accuracy, but maybe it's possible. And its error would be added to the existing CT error. If this is just for internal plant power monitoring, it shouldn't be a problem.

 

[winnie]

Isn't this conversion simply done by having a 0.066 ohm resistor, or is there more stuff going on.

This device appears to be a DIN rail mounted resistor with appropriate terminals and ratings, but based on power dissipation I think it might simply be a resistor:

5 AMP & 1 AMP CURRENT TRANSFORMER SECONDARY, TO 333mVAC CONVERTER - Foxtam Controls (en-GB)

5 AMP & 1 AMP CURRENT TRANSFORMER SECONDARY, TO 333mVAC CONVERTER by Foxtam Controls (en-GB) a British Designer and Manufacturer of Low Voltage Control and Instrumentation products for the electrical industry.

www.foxtamcontrols.co.uk

 

[tcleghorn]

The metering accuracy is not required to be revenue-grade. We intend to compare actual consumption to recorded projections from energy upgrades. The existing CT's are generally 0.3% accurate and the CT's we would use are 0.3% accurate. Total accuracy not including the metering device at 0.6%.

The Rogowski coils are up to 1.2% accuracy. Not as good but adequate for general plant metering.

There are the ohm resistors I looked into but I'd like to keep the CT's shorted when the gear is under load and keep other devices connected to the CT's normally connected such as relays or other Owner meters if they're in use.