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Designmen
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Join Date: Jun 2020
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Multiple current transformers

02/09/2023 2:45 PM

Hello everyone ,

a lot of times I saw multiple current transformers on busbars in electrical panel. There are two per phases. One is for Analyzer, second for kWh meter. My question is, why we need to use multiple CT, why we can't wire it with two wires from one transformer. For exemple one CT, one wires goes to Analyzer and second on kWh meter. Why use two same transformer, insted of one?
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Pathfinder Tags: current mettering transformers
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Brave Sir Robin
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#1

Re: Multiple current transformers

02/09/2023 3:18 PM

The burden that the analyzer places on the CT would potentially impact the reading of the kWh meter and the accountants at the electric utility would be concerned that they were not being paid for all the electrons that get pushed through.
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SolarEagle
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#2

Re: Multiple current transformers

02/09/2023 4:31 PM

You probably could get an analyzer and wattmeter combo machine to do this, but with constant monitoring that would require both circuits energized constantly...from here it gets complicated...Your power provider probably has specified every detail about what is allowed...

https://electrical-engineering-portal.com/three-phase-power-measurement

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SolarEagle
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#6
In reply to #2

Re: Multiple current transformers

02/13/2023 12:50 AM

https://www.linquip.com/blog/what-is-the-current-transformer/

https://www.linquip.com/blog/current-transformer-types-applications/

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Vulcan
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#3

Re: Multiple current transformers

02/10/2023 2:43 AM

It might have something to do with the input circuitry of the analyzer and meter. Maybe connecting everything in series might create an issue with measurement, safety, or both. If I had them, I would study the instruction manuals of the two instruments to learn why.

It might also be that the designer didn't trust the concept of connecting two instruments to one CT. I know some people who are like that.

regards,

Vulcan

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Stef
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#4

Re: Multiple current transformers

02/10/2023 3:47 AM

Most likely one of the devices was added later and it was just bought with its own CT.

I have worked in distribution substations where CTs with one primary winding but up to 6 secondary windings of various ratings and P values to suit the end devices.

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Just an Engineer
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#5

Re: Multiple current transformers

02/10/2023 5:29 PM

Your answer seems within your question.

The CTs are owned by different people for different purposes. The biller CT is for their revenue while the user CT is for whatever they wish to do. In Aus (at least) the biller would not want an independant organisation having any level of control over the CT they are using.

Thus while it might be technically possible, it would seem unlikely in that situation.

If this is on a sub-board internal to the organisation, then such a set-up might be feasible and attractive.

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PWSlack
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#7

Re: Multiple current transformers

02/13/2023 5:38 AM

Ask the <...we...>. After all, the thing will be on (a) drawing(s) with the <...we...>'s name on it.

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rwilliams
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#8

Re: Multiple current transformers

02/15/2023 12:15 AM

Current transformer circuits are series, so if you have 2 loads, the CT must be capable of operating both loads in series. If you wire loads using 2 wires, as you suggest, you split the current to each device, according to the burden of each device. If you have a lot of devices, it is important to stay within the rating of your current transformer.

CTs can be designed to maintain a certain accuracy over a range of current. A relaying class CT will deliver a current at very high currents seen during power system upsets without saturating its output. A metering class CT will deliver currents seen during normal operation with a higher degree of accuracy that relay class CTs, for example.
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Vulcan
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#9
In reply to #8

Re: Multiple current transformers

02/15/2023 8:55 PM

Good answer!
A CT, in addition to the current specs and ratio, has a power spec as well. When you connect a load on the CT, a voltage will appear across the CT. That voltage and the current that the CT is putting out results in a power consumption.

So, adding another load in series will result in a higher voltage across the CT (while maintaining the current). That also results in higher power consumption, making the CT run hotter. If you exceed the power rating, your CT will burn up.

regards,

Vulcan

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Brave Sir Robin (1); Just an Engineer (1); PWSlack (1); rwilliams (1); SolarEagle (2); Stef (1); Vulcan (2)

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