How does ESS handle AC voltage drop through long wiring?

[phnix]

I'm wondering how ESS batteries with embedded inverters handle AC voltage drop. In the following diagram:

          Solar
            || 0kW
     0kW +------+             130ft AWG 8 Wire
Grid ====| 240V |==================================== Batteries(embedded inverters)
         +------+        7V voltage drop  <- 10kW      247V?
            ||  10kW
            ||   |
            ||  \|/
           Load

Suppose the grid voltage is 240V, the load is 10kW, and the system wants to supply the load fully from the batteries. To compensate for the 7V voltage drop, should the battery inverter output 247V to ensure 240V at the end of the wiring where it meets the grid, solar and the load? If such compensation is needed, is there an upper limit of voltage drop that particular batteries (e.g. Tesla PW 2 AC, Enphase IO 5P) can handle? Will too much voltage drop cause malfunction of the battery?

 

[lodar]

This is why CT placement is important. When the CTs are placed on the left side of your diagram, the batteries will output whatever it takes to balance a net zero current to the grid.

If the CT measures power leaking to the grid, then the battery voltage is too high. If there is current coming in from the grid, then the Powerwall’s voltage is too low to push against the grid to power the load.

Is there a limit to the max output voltage of the Powerwall? Of course! Otherwise, loads closer to the battery are going to get fried so that loads on the other end of that long run get the right voltage.

 

[charlesj]

One can always use larger wires, #6 or #4 for ess voltage drop. Yes, costs more but less power is consumed by the wire. Over time, 20 years, how much?

 

[phnix]

Thanks for the explanation. Now I understand how the system controls the battery output.
However, currently my batteries (Enphase IQ 5P) seem to over-react the demand feedback, causing oscillation of the output, as shown in this video. It causes a lot of unnecessary grid imports when the battery is above the reserve level and the load is much smaller than the maximum power capacity of the batteries. I'm wondering if my 130ft AWG 8 wire is causing problems. I don't see maximum voltage data in either Tesla PW or Enphase IQ datasheets. Is the voltage drop in my diagram too much? Is it worth testing with temporary parallel wires to see if the voltage drop is the problem?

 

[lodar]

It causes a lot of unnecessary grid imports when the battery is above the reserve level and the load is much smaller than the maximum power capacity of the batteries. I'm wondering if my 130ft AWG 8 wire is causing problems. I don't see maximum voltage data in either Tesla PW or Enphase IQ datasheets. Is the voltage drop in my diagram too much? Is it worth testing with temporary parallel wires to see if the voltage drop is the problem?

Caveat: I am not an electrician!

A 7V drop out of 240V is about 2.9%, right at the limit (3%) of acceptable loss for a branch circuit. Overall end-to-end drop should not exceed 5%, so it doesn’t leave much room to work with.

The total distance between the grid meter and the batteries is also a concern. How does the grid CT connect back to the batteries?

I am surprised you had these batteries installed so far away from the grid service? The batteries are the guardians of the service power and should be closer to the main meter.

I often describe this zero current game your batteries are doing like balancing a broomstick on the palm of your hand. With such distance, it is more like balancing a broomstick on the end of a long stick while looking at it through binoculars!

Are the CT readings being wirelessly transmitted back to the batteries? Is there interference affecting it?

Also, what does Enphase batteries to do with Tesla Energy? You should definitely show that flapping to your installer and Enphase support.

 

[wwhitney]

Suppose the grid voltage is 240V, the load is 10kW, and the system wants to supply the load fully from the batteries. To compensate for the 7V voltage drop, should the battery inverter output 247V to ensure 240V at the end of the wiring where it meets the grid, solar and the load?

Yes, and this happens seamlessly. In that the system is likely not measuring the voltage at the left where it's 240V, and if it is doesn't use that as part of its control algorithm; the ESS just sees that its terminal voltage rises as it tries to push out more current, and the ESS always matches the terminal voltage it sees, within limits.

If such compensation is needed, is there an upper limit of voltage drop that particular batteries (e.g. Tesla PW 2 AC, Enphase IO 5P) can handle?

Yes, there's a programmable (by the manufacturer, or perhaps installer) window of acceptable grid voltages (the grid profile). I think it's often the nominal voltage +/- 5%, so for 240V that would be 228-252V. For PV inverters, if voltage rise causes the inverter's terminal voltage to rise outside its voltage window, that would knock the inverter off-line for at least 5 minutes. The PV inverters can start yo-yo-ing where they briefly sync with the grid voltage, start putting out power, that power causes the PV inverter terminal voltage to rise outside the window, the inverter shuts off for 5 minutes, etc. I'm not sure how ESS inverters would typically behave in that scenario, if they will just back off the current output if the voltage gets too high without going offline.

As to your video, if excessive voltage rise is a contributing factor, the ESS should have log files that would indicate when the voltage it sees has risen outside the voltage window defined by the grid profile.

Cheers, Wayne

 

[wwhitney]

This is why CT placement is important.

Seems not directly related to me, the CTs just have to placed to see the appropriate current for the system to be able to offset the desired load.

If the CT measures power leaking to the grid, then the battery voltage is too high. If there is current coming in from the grid, then the Powerwall’s voltage is too low to push against the grid to power the load.

While the above is true, it's a side effect of the resistance between the CT location and the ESS. In that if the CT sees net current to the grid, the ESS's current output is too high, which will also cause the terminal voltage to be a little higher than in the desired state of zero grid current, due to the excess current through that resistance.

Cheers, Wayne

 

[phnix]

Thanks everyone for the replies. Sorry for mentioning Enphase-specific things in this Tesla forum. My purpose is to understand how battery inverter power is controlled in general (perhaps as well as Tesla and Enphase specific technologies), and I know there are enthusiastic experts like you on this forum. Troubleshooting the specific issue is definitely a job of Enhpase, but they are unlikely to tell me most of what you have told me .

I don't like the long distance, but the installer put 2 of the 4 batteries (5kWh each) far away to meet the 3' distance rule (though I heard batteries certified with UL 9540A are exempt from that rule). The wall close to the load center doesn't have enough space for all the 4 batteries. I should have asked the installer to confirm with the city about the 3' rule before the installation. This would not be a problem if I used 2 Tesla PWs but Tesla didn't want to do my project due to the main panel to gas riser distance issue.

In my system, the CT readings are transmitted through wires (about 10') to the Combiner. The Combiner and the Controller (probably equivalent to Tesla's Gateway) control all other devices through a shielded communication cable which has a length limit of 250'.