How to detect/measure reverse energy flows

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jamesee
Posts: 9
Joined: Mon Apr 21, 2014 11:04 pm

How to detect/measure reverse energy flows

Post by jamesee » Tue Apr 22, 2014 12:50 am

I have purchased two Omnimeter I v.3s to use in a home PV solar electricity system for load and generation management on a three-phase system.

I have just set up one of the meters on a test bed to make sure I can get it working and interrogate and read the meter over my home LAN. That part all went well.

However when I tested the reverse flow measurement capability of the meter on a single phase (by reversing the current transformer), I was surprised and disappointed that I cannot distinguish the direction of energy flow from the meter readings either on the screen or in the meter read over the IP network. I would have expected to see the power factor (cos theta) to be minus 1 in the reverse flow case because the current and voltage are 180 degrees out of phase but it just shows the same value as for the forward flow.

Am I missing something simple here?

Jameson
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Joined: Fri Nov 04, 2011 7:42 pm
Location: Santa Cruz, CA
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Re: How to detect/measure reverse energy flows

Post by Jameson » Tue Apr 22, 2014 9:32 am

Hello jamesee,

Welcome to the forum.

Here is what the v.3 meter will give you:

Total kWh (this is Forward kWh + Reverse kWh)
Reverse kWh

You can use these to calculate Net kWh and Forward kWh.

It will not give you:

Real-time Direction of Current (for this you would need the Omnimeter Pulse v.4). The v.4 meter is the easiest way to determine what you are looking for.

Here is a comparison chart for our Omnimeters: http://documents.ekmmetering.com/Omnime ... -Chart.pdf

If you have any questions, please let us know.
Jameson
EKM METERING
http://www.ekmmetering.com
831.425.7371

jamesee
Posts: 9
Joined: Mon Apr 21, 2014 11:04 pm

Re: How to detect/measure reverse energy flows

Post by jamesee » Wed Apr 23, 2014 9:00 pm

Thanks Jameson

That's very helpful. I obviously should have studied the spec for the v.4 meter. :o I was put off from doing so on account of the "Pulse" part of the name - which made me think that this was only for pulse metering systems, which don't really interest me.

My bench testing is going well. I have a zigbee link working today, and I was delighted to find how easy that was. I've also realised that I am going to need a 3rd meter, probably another v.3, to cover the full metering system for the solar power setup.

Just another thought on the v.3 and the reverse current flows. I wonder if it might an easy matter (for you :D ) to revise the meter's firmware so that a negative power flow shows a power factor of minus 1, because showing plus 1 is theoretically not correct when there is a phase difference of 180 degrees between voltage and current. It really should be minus 1 because cos 180 deg is minus 1 and not plus 1 as the meter currently shows. This might also give the v.3 meter a slightly increased versatility, although it's not now important for my project. 8-)

All the best

jamesee

Jameson
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Re: How to detect/measure reverse energy flows

Post by Jameson » Thu Apr 24, 2014 11:48 am

Thanks for the suggestions and good to hear you have it working over wireless mesh with your 485Bees :D

Yes with solar it can take 2 submeters to know exactly what is going on. Most customers get away with 1 submeter to net meter their solar, but some need 2 or 3 submeters if they really want to get more detail.

I should probably find a graphic to represent this (Im thinking of a triangle, with the house, the utility, and the solar inverter on the points of the triangle), but here goes:
  • 1. If you have a submeter at the output of your inverter, you will know how much power your solar inverter is producing, but you will not know for sure how much your house is using vs how much power you are sending to the grid.

    2. If you have the submeter on your house, you will know how much your house is using, but will not know how much the solar is producing or how much power you are getting from the grid.

    3. If you have a submeter right next to your utility meter, you will know how much energy you are sending and receiving from the grid, but will not know exactly how much your solar is producing and how much your home is consuming from your solar. In this case the submeter could be reading 0 but your solar is producing 3,000 watts and your house is consuming exactly 3,000 watts.
If you just have 2 submeters you could put one on the output of your inverter, and one on your house, this would tell you (with some math) everything you need to know( how much the solar is producing, how much the house is using, and how much is being sent to or received from the grid).
to revise the meter's firmware so that a negative power flow shows a power factor of minus 1,
Yes, I would go with the v.4 meter if I wanted to know more about the direction of my power in real-time. The v.4 meter is just a better option for those that have distributed generation.

Hopefully we will make an image that we can post here to explain what I mean ;)
Jameson
EKM METERING
http://www.ekmmetering.com
831.425.7371

jamesee
Posts: 9
Joined: Mon Apr 21, 2014 11:04 pm

Re: How to detect/measure reverse energy flows

Post by jamesee » Thu Apr 24, 2014 2:54 pm

Thanks Jameson

I follow you quite clearly thanks. My situation is a bit more complex as I am on a rural property with separate sub boards in the house and a barn. The utility meter is about 200m from the house and barn. Envisage a T-shape for the sub-main which links the utility meter board (base of the T) to the two sub-boards (at each end of the bar of the T). The solar system will go on the roof of the barn and have two meters (one for the solar generation and one for the barn load) and the house will have a load meter only. This should show me everything that's going on and allow me to calculate my position relative to the utility by deducting the solar power generated (both instantaneously and over time) from the two load profiles.

You have a great range of products and I wish you well. Today's project is sorting out the CRC16!! :D

BlueHornet
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Location: Victoria, Australia

Re: How to detect/measure reverse energy flows

Post by BlueHornet » Thu Apr 24, 2014 5:41 pm

jamesee wrote: Just another thought on the v.3 and the reverse current flows. I wonder if it might an easy matter (for you :D ) to revise the meter's firmware so that a negative power flow shows a power factor of minus 1, because showing plus 1 is theoretically not correct when there is a phase difference of 180 degrees between voltage and current. It really should be minus 1 because cos 180 deg is minus 1 and not plus 1 as the meter currently shows.
Power Factor is a dimensionless number between 0 and 1. I have always known it to be a positive as it is the ratio of (relationship between) real power and apparent power, both positive numbers. It is not a relationship of which way the the current is flowing from two individual supplies for example: supply from "grid power" as opposed to supply from solar power or any other co-generation supply.

Power factor = cosine of the angle (theta) between Voltage and Current (on a single supply) and is a measure of the voltage and current relationship of the load characteristics for that supply line .

Your installation may have any combination of the following types of loads connected to the supply R - Resistive, L - Inductive & C - Capacitive. The meter (pf reading) simply determines the (total) voltage & current relationship for all loads on the supply line and gives a reading. It goes something like this: if the current waveform lags the voltage waveform then the total of all connected loads are inductive and the meter will measure as L0.XX, if the current waveform leads the voltage waveform then the total of all connected loads is capacitive and the meter will measure C0.XX, but if the current waveform is "in sync" with the voltage waveform then the meter will read 1.00, this will occur if the loads are purely resistive or the sum of inductive, resistive and capacitive loads are "balanced".

Theta`s range is from less (negative) than -90 to less than +90 degrees . The cosine of any angle within the range of theta is positive, therefore the pf is always positive.
This might also give the v.3 meter a slightly increased versatility,
I think the method you are suggesting would only add confusion as it does not follow electrical theory convention!
I think what you are suggesting would only require a "directional" arrow symbol to shown if the current is flowing "from the grid" lets call this Forward (->) or "to the grid" lets call this Reverse (<-)

jamesee
Posts: 9
Joined: Mon Apr 21, 2014 11:04 pm

Re: How to detect/measure reverse energy flows

Post by jamesee » Mon Apr 28, 2014 10:27 pm

Thank BlueHornet

I agree with you, at least on pragmatic grounds. If EKM Metering were to modify the v3 meter as I am suggesting, they would want to leave the existing parsing arrangement in place, in order to achieve backward compatibility with existing applications and software. Thus the only place where the additional information could be encoded would be in the 17 bytes that are currently spare. I would like to see a directional indication for each of the 3 phases, so this could be done by using 3 of the 17 bytes (or 1 byte only if the directions were coded as single bits).

Jameson
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Joined: Fri Nov 04, 2011 7:42 pm
Location: Santa Cruz, CA
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Re: How to detect/measure reverse energy flows

Post by Jameson » Tue Apr 29, 2014 9:25 am

Thanks all,

Jamsee, v.4 meter does provide exactly what you need, it provides direction of current in real time with just 1 byte. Here is the map, it is from this document: http://documents.ekmmetering.com/Omnime ... arsing.pdf

(I know you are probably aware of this, but I wanted to post it here for others as well, so that they can see it)

? Direction of Current
Line1/Line2/Line3
Forward/Forward/Forward = 1 (31)
Forward/Forward/Reverse = 2 (32)
Forward/Reverse/Forward = 3 (33)
Reverse/Forward/Forward = 4 (34)
Forward/Reverse/Reverse = 5 (35)
Reverse/Forward/Reverse = 6 (36)
Reverse/Reverse/Forward = 7 (37)
Reverse/Reverse/Reverse = 8 (38)


This feature does make the v.4 meter quite a bit more useful for customers with solar or wind generation.

Hope this helps.
Jameson
EKM METERING
http://www.ekmmetering.com
831.425.7371

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