Special Application using Energy meters with External CTs for Power Modification Project

Summary:

● What are external CTs for an energy meter?

● Why we need energy meters with external CTs?

● How to use energy meters with external CTs?

● Other Energy Meters with external CTs.

What are external CTs for an energy meter?

Normally for most energy meters, there are CTs (Current Transformers) built inside the energy meters. So, most energy meters are with internal CTs like the picture on the right.

External CTs for energy meters on other hand, are just CTs which were originally inside the energy meters and were taken to the outside and served as a external CTs. Pretty literally, right?

Why we need energy meters with external CTs?

So, naturally there comes a question that why we need such energy meters with external CTs? It must be done for some practical purposes.

The very first reason is because we can use this type of energy meters to realized a kind of special application. And this so-call special application is to install external CTs of energy meters on the secondary current output cables of other CTs which were already on site. 

For this kinds of application, the rated current of the main circuits that we need to measure the electric data like kWh, current, voltage and etc are normally large (take 1000A AC for example) and didn’t allow to shut down the power to install the CTs them. Obviously, it won’t be possible to install the CTs on the main circuits with 1000A AC while not shutting down the power.

So, there comes a thought, why not to acquire the current signal from the output cable of CTs already work-site? (Since this application are commonly used in power modification project, so there will always be CTs to be paired with energy meters on the previous work-site for measuring the main circuits of large current)

And what’s the current ratio of the previous work-site CTs? Since we have a main circuit with rated current of 1000A AC, then normally we will use 1000A/5A CTs and the ratio is 1000/5=200. So, if the real-time current of main circuits is 800A AC (real-time current not rated current), then the secondary current output of CTs already on work site will be 800/200=4A AC. And we need to get current signal from the secondary current output cable of the work-site CTs, then what’s the current ratio of the external CTs paired with extra energy meter for power modification? It would naturally be 5A/-mA right? (take 5A/1.25mA for example) The picture below is a Acrel energy meter with external CTs of 5A/1.25mA current ratio

How to use Energy Meters with external CTs

For now, this question will be really simple, we just need to install the external CTs of energy meter for modification on the secondary current output cable of the CTs already on previous work site. This will solve two problem, although the current of main circuit is really large, but the secondary output current of CTs on site was low enough (not more that 5A AC) to directly install our so-called external CTs. And since this type of external CTs are usually split core type, we just need to clamp it on without changing any wiring of the work site.