Is monitoring of a CT with another CT viable?

I have a site to install monitoring to that is having several large PV arrays installed, the generation will all be monitored at the individual locations across the site, as will many of the buildings. However part of the plan is monitor the whole site supply to keep an eye on the overall nett import/export and more specifically the possibility of exporting on one phase whilst importing on another.

The main incomers are huge and have several heavy cores per phase. The existing meter looks less archaic than the rest of the plant room and it has 3 very large CT's that provide 3 x 0 to 10amp current circuits at the meter. The meter also has LED pulse output for both VAr and kWh.(Elster A1700 with a serial number that points to a 0-10a CT configuration)

I wonder if there is any reason why I cannot put our CT's on the 0-10a circuits for a reasonably accurate representation of the spread of the power across the phases and this combined with the accurate pulse count(s) should provide an acceptable solution without meddling with the plant or it's wiring as the CT circuits can be accessed close to the meter away from the heavy stuff. 

This sounds easier, cheaper and safer than sourcing huge CT's and getting involved in the big stuff, but I know not if there will be any phasing issues or other accuracy concerns not easily remedied with a change of burden resistor.

Is this feasible or am I barking up the wrong tree?

If it is, would a single AC-AC emonTx do the job adequately? (there are power outlets in the plant room but only on one phase and there is no readily available 3ph, can be done if absolutely needed but would prefer not to)


Robert Wall's picture

Re: Is monitoring of a CT with another CT viable?

What you're saying is you want to use the existing CTs with interposing CTs.  It is done, it is an established technique (and it has a name!), the only thing wrong with it is you have two lots of errors instead of one. (And at 10 A max, you'll struggle for low-end accuracy but that can be circumvented by changing burden resistors.) If the 3-phase sketch gives you adequate accuracy (read the caveats in the comments) then you can use an emonTx with a single voltage input.

Whatever you do, DO NOT OPEN-CIRCUIT THE EXISTING CTs. Even if they are yours and not the supply company's.

pb66's picture

Re: Is monitoring of a CT with another CT viable?

Thanks Robert, having now done a (very) brief search and read on "interposing CT's" I am now wondering if my additional CT would have any noticeable effect on the existing CT circuit? The examples of interposing CT's I've found do not seem to be non-invasive split ring but actual transformers that the primary winding is in series with the existing CT.

Apparently used for phase correction as well as level shifting. I certainly wouldn't use any invasive connections for safety and most probably legal reasons since this is metering circuitry, But would it be ok to assume that using a non-invasive split ring would have "negligible" effect on the existing circuit? I wouldn't want to alter the metering in any way, a reduction would be theft and and any increase would not be welcome, especially when incurred trying to help reduce energy use.

The 10a range isn't a concern as I will change the burdens to suit and I'm only looking for an indication of balance across the phases, approximate current and direction of flow per phase plus the LED pulses should be adequate to determine what we  need, but it will be a good exercise to see how accurate the data is from the CT'd CT's for use where the LED's aren't available.

I see calibrating out the phase shift of the existing CT's as being the tricky bit, I can easily calibrate out the additional CT phase shift using the conventional means before attaching, but unless the meter is able to display PF or kVA and kWh for individual phases, trying to add the 3 additional phase corrections to align with the combined values could well be nigh on impossible. Maybe it's time to shell out on a good meter with an inductive "amp clamp" that can provide real power and power factor etc.



Robert Wall's picture

Re: Is monitoring of a CT with another CT viable?

I doubt very much that adding a YHDC CT would materially affect the existing metering circuit. Certainly, you're drawing power out of it - 5 mA at around 1.1 V if you change the burden, so 5.5 mW.  By adding your CT, you're increasing the voltage that the original CT needs to deliver, by around 0.55 mV (that's what the primary voltage of your CT is) so that power will actually come out of the supply. If your supplier is worried about 5.5 mW on a 3-phase 400 A  infeed, I'd like to see justification. That's some way below the accuracy of the meter itself.

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