Devices to meter kWh

[Harry Bloomfield]

Reality is otherwise. I wouldn't have suggested Shelly EM if it couldn't distinguish which direction.
I don't think ultimate accuracy is the goal here, simply sense-checking what the inverter is saying.

I don't see how it possibly can determine direction of flow, via monitoring a ct, but reading the spec. it makes no mention that I could see, of being able to do that.

 

[JohnW2]

Common sense, and knowing how AC works, would suggest otherwise.

It would indeed. However, it seemingly is possible measure the direction of AC current flow. In the distant past when I was building VHF/UHK SWR meters, I struggled to understand how they could possibly work (and I still struggle) but they seemingly did work

 

[Why Not Indeed]

I don't see how it possibly can determine direction of flow, via monitoring a ct, but reading the spec. it makes no mention that I could see, of being able to do that.

It definitely can and fortunately it works without me being able to understand or explain it The one it replaced could not so could not distinguish import/export.

 

[Harry Bloomfield]

It definitely can and fortunately it works without me being able to understand or explain it The one it replaced could not so could not distinguish import/export.

Then it must be measuring import, versus export, on two different cables. A CT can only measure current flow, in a cable, they cannot determine direction of AC, because the direction of flow reverses 50 times per second.

 

[Munroast]

Then it must be measuring import, versus export, on two different cables. A CT can only measure current flow, in a cable, they cannot determine direction of AC, because the direction of flow reverses 50 times per second.

up unto a few minutes ago I would have agreed with you - but apparently they can, change of phase by 180° shift of phase shows direction of flow!
they have a direction arrow

L & K ?
according to my mate GPT

• L typically stands for Load
• K typically stands for Konnektor (or sometimes Kraftquelle, German for “power source”) — in English, think of it as the supply or source side.

 

[JohnW2]

up unto a few minutes ago I would have agreed with you

...as I've said, that's also what my intuition tells me (although SWR meters manage to work somehow ).

- but apparently they can, change of phase by 180° shift of phase shows direction of flow! they have a direction arrow

'Phase' is only meaningful in terms of the relationship between two waveforms. The sensor only knows about the current in one conductor - so "180° shift of phase" between what and what??

 

[Munroast]

...as I've said, that's also what my intuition tells me (although SWR meters manage to work somehow ).

'Phase' is only meaningful in terms of the relationship between two waveforms. The sensor only knows about the current in one conductor - so "180° shift of phase" between what and what??

I always struggle a bit getting my head round this, but, it will be the relationship between the current measured and signal from the fluctuating voltage.

 

[Why Not Indeed]

Then it must be measuring import, versus export, on two different cables.

If that were true then it would have been pretty misleading of me not to mention that I have different clamps on my "import" and "export" cables [whatever those are], wouldn't it? For the avoidance of doubt, I have a [one] clamp around the L after the meter and that's what the graph is showing.

A CT can only measure current flow, in a cable, they cannot determine direction of AC, because the direction of flow reverses 50 times per second.

The Shelly EM is powered by the same AC source it's measuring.

There is a hint here about real and apparent power but I cannot find a full explanation for you:

Current Transformer Installation — OpenEnergyMonitor 0.0.1 documentation

 

[JohnW2]

I always struggle a bit getting my head round this, but, it will be the relationship between the current measured and signal from the fluctuating voltage.

That makes absolutely no sense (to me). What on earth is "signal from the fluctuating voltage"?

Apart from anything else, 'voltage' only has a meaning in terms of potential difference between two things - and one of thoise clip-on sensors does not know about one potential, let alone two.

 

[Munroast]

Anyway - back to metering my usage. Sill me I found a new meter amongst all the other new stuff in the cupboard under the stairs

and this I believe ells me what I want to know, the display scrolls through these displays

EXPORTED
Nett Export
HOW MUCH I HAVE USED

Have been watching the bottom figure this past few days and comparing it against the nonsense from the GROWATT dashboard

Main Meter is the Electric Boards meter
Generation meter being the one shown Above
Growatt Meter being the misleading figure

I am very happy the generation meter is showing my true usage

 

[JohnW2]

There is a hint here about real and apparent power but I cannot find a full explanation for you:

Current Transformer Installation — OpenEnergyMonitor 0.0.1 documentation

Yes, it uses 'real power' to relate to direction of current flow as well as magnitude, and 'apparent power' when you don't care which direction it's travelling.
However, whilst it is obviously saying that the CT can sense the direction of current flow, it gives absolutely no insight into how it achieves that!

 

[Munroast]

That makes absolutely no sense (to me). What on earth is "signal from the fluctuating voltage"?

Apart from anything else, 'voltage' only has a meaning in terms of potential difference between two things - and one of thoise clip-on sensors does not know about one potential, let alone two.

AC voltage resonates between abot +350v to -350v, this will create a magnetic signal, clearly they are able to measure the change in angle (how, I don't know)
I guess it is whether or not the signal form the current matches or lags behind the signal form the voltage (but not sure)(they have clever stuff now)

 

[JohnW2]

AC voltage resonates between abot +350v to -350v, this will create a magnetic signal, clearly they are able to measure the change in angle (how, I don't know) .... I guess it is whether or not the signal form the current matches or lags behind the signal form the voltage (but not sure)(they have clever stuff now)

That again all sounds like total nonsense to me.

The magnetic field (which is what the sensor sees) is created by current flow, regardless of any voltages. As I implied in my previous post, there will be no 'signal from the voltage' - nor,, in fact, is it meaningful to talk of the conductor passing through the sensor as 'having a voltage' other than in terms of it's potential difference from something else.

 

[Why Not Indeed]

An Introduction to AC Power — OpenEnergyMonitor 0.0.1 documentation

"Determining the direction of power flow".

 

[plugwash]

I don't see how it possibly can determine direction of flow, via monitoring a ct, but reading the spec. it makes no mention that I could see, of being able to do that.

You can't do it with a CT alone.

But you *can* do it with a CT in combination with a voltage sensor. You multiply the instantaneous voltage by the instantaneous current and integrate. Most commonly the same connection that is used to power the metering device is also used to provide the voltage reference.

I suspect it could also be done with a non-contact voltage sensor, but at a severe loss in accuracy.