Connecting EmonTx in Parallel with existing Export Meter Current Transformers

At my offices I have a solar PV 21.84kwp system across two roofs feeding two three phase inverters.  We have a 'self-imposed' three phase export meter.  I say 'self-imposed' as there is no requirement in the UK for an export meter for PV systems below 30kWp.  We had it installed at modest extra cost for business monitoring purposes.

The export meter is an Iskra 174 with three Hobut current transformers model No. CTSCM40 100/5 so similar I think, if not the same spec, as those in the shop albeit for larger diameter cables.

http://www.hobut.co.uk/images/files/SplitCore%20Clip%20Together%20range%20-%20new.pdf 

I wondered if I could drop the EmonTx v2.2 or 3 CT inputs across the Hobut CTs?

FYI just looking now typical currents by phase measured on an existing Autometers information centre are: 30 - 50Amps per phase.

I did ask the supplier of the meter and CTs about the value of burden resistors in the Iskra 174 meter.  He came back to tell me that instead of burden resistors the meter has air-cored Rogowski coils which have a very low burden of <0.16VA

I have seen Robert mention Rogowski coils in these forums and wondered if he or another learned member could comment on my idea please.

When I first thought about this idea I assumed that the EmonTx without an burden resistor would be presenting a (very) high impedance but when I looked at the circuit diagram of the EmonTx I'm not so sure.  Could I introduce a series resistor?

I appreciate that I might be introducing the potential for inaccuracies of measurement in an otherwise calibrated system but assume, if I get it right, we're talking only a small impact?

I would appreciate any opinions on this possible approach.

Thank you,

Steve

MartinR's picture

Re: Connecting EmonTx in Parallel with existing Export Meter Current Transformers

Something doesn't sound right there. Rogowski coils are an alternative to CTs not burden resistors. Maybe your supplier is trying to baffle you with science?

According to the manual the Iskra 174 should be used with CTs with a 5A secondary so the Hobut CTs sound right. They wouldn't be suitable for emonTx though as their VA rating would limit the output voltage below that required. Perhaps more importantly the CT secondaries are probably not isolated from the mains. Probably not worth the hassle for the cost of 3 more CTs.

edited to say: I suppose it's possible that the CT secondary is simply looped in the meter and a Rogowski coiled is used around this conductor. If so the burden would be close to 0

Robert Wall's picture

Re: Connecting EmonTx in Parallel with existing Export Meter Current Transformers

I don't buy the idea of a Rogowski coil acting on the secondary - it's more likely that it is a Hall Effect device.

I've found a manual for what I suspect is a similar meter, the MT171 (from here: http://www.universalmeterservices.co.uk/) and it really says it is a Rogowski Coil.

You won't have enough voltage available across your Iskra meter. As it's a 5 A meter and 0.5 VA,  it will be very low and not nearly enough to get a sensible voltage to feed into the emonTx, always assuming of course that it is isolated anyway (which it needs to be).

I think the safest (= most likely to succeed as well as least risk of damage or personal injury) is to pop another 3 c.t's onto the main cables.

But be aware that the meter also has a data output, so it's possible that could be used to extract the readings directly from the meter, if you know the protocol:

Communication: Optical port (IEC 62056 – 21) for local meter programming and data downloading, RS485 serial interface.

[Edit - the manual also says:

5. Communication interface
5.1. IR optical port
The meter is provided with an optical port that enables meter parameter setting and data downloading.
The optical port complyes with the IEC 62056-21(IEC61107) standard; mode C protocol is employed;
data transmission rate is 9600 bit/sec.]

MartinR's picture

Re: Connecting EmonTx in Parallel with existing Export Meter Current Transformers

There's a copy of the MT174 manual here and it does indeed state on page 9  that Rogowski coils are used...

"The metering elements consist of a current and a
voltage sensor. The current sensor is a Rogowski
coil (a current transformer with an air core), while
the voltage sensor is a resistive voltage divider."

stevenma's picture

Re: Connecting EmonTx in Parallel with existing Export Meter Current Transformers

Thank you Robert, Martin,

Robert reminded me about the data output and it will be useful to be able to present readings from this export meter and the separate generation meter.  The export meter cycles through instantaneous and cumulative import and export.  We also have an AMR import meter provided by our electricity supplier so the two should track which will be a useful indicator of accuracy.

I am planning PV diversion which will mainly occur at weekends and other holidays when the building is back to base load hence the requirement for CTs in a Type 2 arrangement.  I think to avoid interfering with the export meter and for ease I will do as suggested and introduce additional CTs for the EmonTx.

Regards,

Steve

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