Off Grid Power Metering and Monitoring Panel

David and Laura · July 2012

We finished the basic panel construction for the monitoring infrastructure for our off-grid power system, and wanted to share our progress. The mechanical construction consists of two DIN rails and Panduit secured to a Hoffman mounting panel, as shown below:

Attachment not found.

We have mounted on the top rail:

* A pQube Power Quality Monitor
* A EKM kWh meter

And on the bottom rail:

* Three distribution blocks (power, neutral and ground)
* AC breakers for our home, water pump controller, and other local circuits
* A 24 volt power supply
* DC breakers for 24 volt distribution
* Terminal blocks for 24 volt distribution
* A 5-port embedded Ethernet switch.

Not shown is a RS-422 to Ethernet converter that connects to the EKM meter, which we will mount to the side of the enclosure.

The next stage is to complete the panel wiring, mount it into the enclosure, and wire it into the inverter. Ethernet cabling will consist of a home run back to the house, a connection from the switch to the pQube, to the RS-422 converter, and two Ethernet runs outside the cabinet to the MagWeb Inverter Monitor and the TriStar MPPT charge controller.

It's going to be good to start collecting some real data.

BB. · July 2012

Re: Off Grid Power Metering and Monitoring Panel

Looks like a good start to a nice looking installation.

-Bill

David and Laura · August 2012

Re: Off Grid Power Metering and Monitoring Panel

More progress:

attachment.php?attachmentid=3734&d=1345866133

Most of the wiring is now completed and powered up for testing.

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First measurements!

The power quality monitor and Ethernet switch consumes 0.09 amps of power (Approximately 10 Watts), so that won't be too much of a drain on the batteries.

attachment.php?attachmentid=3735&d=1345866147

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Installed into the power shed.

Now, with most of the work done, all that remains to do is:

1. Re-wire the main AC power feed from the inverter through the power meter and power quality analyzer
2. Re-wire the feed to the cabin, the feed to the pump, and the feed to the power shed outlets through the new breakers
3. Energize the system
4. Terminate and connect the Ethernet cable run to the cabin to the switch
5. Install status indicators on the front panel.
6. Wire up the cabinet fan
7. Install the RS-485 to Ethernet converter, and wire it up to the power meter
8. Run Ethernet cable to the Tristar MPPT and Magnum inverter monitor

Lots of fun!

stephendv · August 2012

Re: Off Grid Power Metering and Monitoring Panel

Very neat! Thanks for posting the link to that PQube box, it seems to offer a ton of features. It also seems to measure kWh, so why do you have a another separate meter for that?

David and Laura · August 2012

Re: Off Grid Power Metering and Monitoring Panel

stephendv wrote: »

Very neat! Thanks for posting the link to that PQube box, it seems to offer a ton of features. It also seems to measure kWh, so why do you have a another separate meter for that?

The pQube is a very nice unit — we're very happy with it, especially given that it can log our power quality and consumption data (to a SD card) even during the times of the year when we have to keep our Ethernet network and computer turned off. (During the late winter and late spring we have long periods of overcast sky, and we only have enough water for our waterwheel in the winter)

The reason for two meters is simple: the pQube wasn't part of the original plan, and we discovered it after we had already purchased the EKM meter. So, we figured that we might as well install both, as we weren't sure how much power the pQube was going to consume (and we wanted to be able to turn it off to minimize consumption).

David and Laura · September 2012

Re: Off Grid Power Metering and Monitoring Panel

The panel is all wired in and is now operational full-time. Once of the nice things about the pQube is that it saves the power quality information to a SD card in addition to making it available over Ethernet, so we'll be able to take a look at data despite not having the network wired up and operational.

For low-power off-grid applications, this on-board data storage is critical, as we don't have enough power to run a full network and data collection system 7/24 in all weather conditions.

attachment.php?attachmentid=3846

Later today I'm planning to get the status indicators installed onto the front of the panel.

David and Laura · September 2012

Re: Off Grid Power Metering and Monitoring Panel

For unknown reasons, the photo from our last post won't display, nor can we edit the post to correct this error.

So, here is the missing photo:

attachment.php?attachmentid=3868

Attachment not found.

BB. · September 2012

Re: Off Grid Power Metering and Monitoring Panel

We where having problems with Spammers updating old posts with spam links (and worse)--So, at this point, people can only edit their posts for ~1-2 days.

If needed, just ask one of us moderators to edit an old post to fix any issues.

-Bill

unicornio · September 2012

Re: Off Grid Power Metering and Monitoring Panel

David & Laura wrote: »

For unknown reasons, the photo from our last post won't display, nor can we edit the post to correct this error.
So, here is the missing photo:

Attachment not found.

hello to both, David & Laura,
I have known the PQube thanks to your post, and I am amazed at their ability and precision, so I'm interested in buying one, but even I have not looked much, but I can not find it yet in Spain ... I am very interested to know if it can be affordable, but do not know the price ...

I would love to see the photos you have posted in this thread and continue to provide information about the PQube ... thank you very much! ...

stephendv · September 2012

Re: Off Grid Power Metering and Monitoring Panel

Unicornio, I was equally impressed with the specs. The local reseller is Ocean Winds Valencia, I received a quote for roughly 1500 Euros PVP. Hope you didn't hurt yourself falling off the chair.

unicornio · September 2012

Re: Off Grid Power Metering and Monitoring Panel

stephendv wrote: »

Unicornio, I was equally impressed with the specs. The local reseller is Ocean Winds Valencia, I received a quote for roughly 1500 Euros PVP. Hope you didn't hurt yourself falling off the chair.

yeah!...many thanks Stephen, well, I did not fall off my chair because I know that good things cost money, and it seems to be a well made product and seems to works well, but I will not give up on me one ... :-(
I've found another seller here closer, in Madrid, but I had not yet reached their response to prices ...but for a while I do not excited with their answer! .... hahahaha

David and Laura · September 2012

Re: Off Grid Power Metering and Monitoring Panel

Re #8 — BB.: My second attempt to fix the image seems to have also become broken after a few days. I sent you a private message, so hopefully we'll be able to get this fixed.

Re #9 —*We'll keep posting... The next step will be posting some of the reports the pQube generates, as we now have a couple of weeks of data recorded. And since it supports ModBus, we'll soon be able to dump raw data for doing our own data logging.

The pQube is the least expensive Ethernet-enabled power quality monitor that we could find that also supported built-in data logging. If you have three-phase power, the Satec PM-174 is a pretty nice product (and can be found for quite a bit cheaper), but you can't configure it to run with just a single phase active.

BB. · September 2012

Re: Off Grid Power Metering and Monitoring Panel

I have forwarded your problem to our Admin from NAWS, "Windsun".

-Bill

TheBackRoads · September 2012

Re: Off Grid Power Metering and Monitoring Panel

How do all these sensors interface to the "power" do you have CT's? Or is just power the device enough?

westbranch · September 2012

Re: Off Grid Power Metering and Monitoring Panel

Dave, what did they cost before HST?

David and Laura · September 2012

Re: Off Grid Power Metering and Monitoring Panel

Here is attempt #3 at uploading the last picture:

Attachment not found.

Re #14 — The above photo shows the four current taps (of which two are being used). The neutral and AC power is routed through the taps.

Re #15 — The entire setup was around 2.5K, including tax. That's for the core monitor and data logger, current taps, Ethernet interfaces, power supply and temperature/humidity monitor. That's around 1,900 Euros.

David and Laura · September 2012

Re: Off Grid Power Metering and Monitoring Panel

We have some reports to share:

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David and Laura · September 2012

Re: Off Grid Power Metering and Monitoring Panel

Part 2, due to image restrictions:

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Some interesting observations:

With just LED lights powered up in the cabin, we have a power factor close to 75%, but when we put a significant load on the inverter (our induction cooker hot plate), the power factor jumps up to 100%.

Correlated with loading the inverter, the voltage also jumps from 116 to 122 volts, which was unexpected.

Frequency is pretty stable - you can see a slight variation over a two week period.

David and Laura · September 2012

Re: Off Grid Power Metering and Monitoring Panel

Part 3, due to image restrictions:

attachment.php?attachmentid=3921

looking at the voltage waveform, it is pretty amazing how much better inverters have gotten over the last decade, and looking at the current waveform, how truly ugly the current draw is from switching power supplies.

inetdog · September 2012

Re: Off Grid Power Metering and Monitoring Panel

David & Laura wrote: »

... looking at the current waveform, how truly ugly the current draw is from switching power supplies.

Is that really a zero baseline of the current waveforms, or is there a DC offset which is not shown? If the current really is going negative for part of the cycle it is one more very good reason that you cannot operate an off-grid inverter like this directly from panels without batteries!

David and Laura · September 2012

Re: Off Grid Power Metering and Monitoring Panel

Re #20 - That's the actual waveform — I don't think there is a DC offset, that would show up on the L-N voltage graph.

It's normal for there to be "negative current". That just means that current is being drawn when there is a negative voltage, not that the load is feeding power back into the inverter. AC power is very zen.

Here's a good reference showing what non-linear loads typically look like: http://www.astrodyne.com/power_quality.asp

inetdog · September 2012

Re: Off Grid Power Metering and Monitoring Panel

David & Laura wrote: »

Re #20 - That's the actual waveform — I don't think there is a DC offset, that would show up on the L-N voltage graph.

It's normal for there to be "negative current". That just means that current is being drawn when there is a negative voltage, not that the load is feeding power back into the inverter. AC power is very zen.

Here's a good reference showing what non-linear loads typically look like: http://www.astrodyne.com/power_quality.asp

Actually, if the current is positive when the voltage is negative or vice-versa, the result has to be power flow in the opposite direction. However when looking at the power flow on the AC side of the inverter, flow into the inverter does not necessarily mean power flow back into the batteries. It is more likely to represent flow back into the internal power storage system of the inverter than all the way back into the batteries. But it still makes it hard to satisfy the AC power demand using a constant power solar input with no batteries for buffering.

When you mentioned the current draw from switching power supplies, I was thinking about the inverter itself being a switching power supply rather than the loads on the inverter during the test being mainly switching power supplies. I would very much like to see the current waveform on the DC input side of the inverter!

David and Laura · October 2012

Re: Off Grid Power Metering and Monitoring Panel

Re #22 - If we zoom into a single cycle, we see on the positive voltage half-cycle, there is a positive current flow through L1, and a negative current flow returning over the neutral (which technically is a positive current in the opposite direction).

On the negative voltage half-cycle, there is a negative current flow through L1, and a positive current flow over the neutral, so the current flow is still flowing from the inverter to the load.

If the current polarities were reversed, as you note, that would indicate current flow back into the inverter.

Regarding the DC input side of the inverter, we do plan to monitor that as well, using a current tap that we will be connecting to a PLC for data acquisition, but starting that project is a fair way off. I suspect that the DC waveform is actually pretty steady, due to the amount of power buffering (capacitors) inside the inverter.

It will be interesting to find out...

Another option to monitor this is PSL's DC adapter, which allows you to monitor DC voltage and current, all synchronized with the AC snapshots, is quite useful.

http://www.powerstandards.com/ATT2-600VOption.php

inetdog · October 2012

Re: Off Grid Power Metering and Monitoring Panel

Silly me. I attached too much significance to the fact that your current graphs are labelled + and - rather than L1 and N. :-)

Off Grid Power Metering and Monitoring Panel

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