Assume I have access to my solar panel inverter's voltage, current, and reactive power measurements, and these are taken every 5 minutes.

  1. By inspecting this trace, how can I tell when my panel is feeding energy back to the grid? Is there any other information I need to know?
  2. Can I also calculate how much energy is being pumped into the grid? (Even a rough estimate will suffice.)

I do not have a background in power engineering, so I'd appreciate any responses that assume minimal knowledge of the domain. Thank you!

  • \$\begingroup\$ Isn't this a trick question: it's always feeding power to the grid, provided the current is positive? \$\endgroup\$ – pjc50 Jan 13 '17 at 22:44

If you only know the inverter's voltage and current, then you can't deduce whether it's feeding power to the grid, or drawing power. For this, you need their relative phase. When they're in phase, the power flow is one way, in the opposite phase and the power flows the other way.

An inverter that's designed for grid tie injection will usually have a facility to read power flow. A metering system designed to allow you to claim payment for the power will always be able to read power flow.

  • \$\begingroup\$ Assume that I have access to the a power quality meter installed on the feeder. The metrics that I have access to on the meter are: voltage, current, power, reactive power, power factor, frequency, voltage flicker, voltage THD, current THD, sequence currents, phase imbalance, total energy. The metrics that I have access to on the inverter are: voltage, current, reactive power. Can you clarify how/if I am able to identify the current phase using a combination of these metrics? Thank you. \$\endgroup\$ – Kostas Jan 13 '17 at 21:56
  • \$\begingroup\$ No, I don't see power flow there. The 'total energy' one is interesting. Do you know where that's measuring from/to? Perhaps we need a drawing of the connections of the panel, inverter, grid terminals and the position of any meters. Then we can stop guessing \$\endgroup\$ – Neil_UK Jan 13 '17 at 22:01
  • \$\begingroup\$ I have no idea unfortunately, and I don't have access to a schematic either. Thank you though. \$\endgroup\$ – Kostas Jan 13 '17 at 22:03
  • \$\begingroup\$ You have access to a schematic editor, it's the resistor / capacitor / diode button on the edit bar when you edit your post. If you don't have access to the details of your system you're asking a question about, then ... (<- words have failed me) \$\endgroup\$ – Neil_UK Jan 13 '17 at 22:06

Since the inverters AC output is connected directly to the incoming grid AC mains, and to the house power consuming items via the switchboard..... We can say the inverter output voltage will always be equal the incoming grid line voltage and the invertor output phase cannot be measurably different from the line phase (there might be very small voltage variations because of wire resistance, but it will be minimal; under a few hundred millivots at worst I'd suggest).

This is a great project that might help you understand how to measure power direction based on a comparison of voltage and current polarities in a single line.

Since the Inverter only ever produce power, the current flow from this device is always in the same direction. The same applies to the house side, it's always consuming energy so the current flow is always in one direction.

But with a Current Transformer (CT) on the grid incoming mains you can readily measure the direction and magnitude of current flow using the project above. It's open source so you could choose to build or buy.

Most utilities change the Power Meter when there are large PV arrays on site. The bi-directional meter measures the direction of current flow so the meter consumption numbers can be negative (supplied power to grid).

There are chips such as this that do the work for bi-directional meters but they are complex beasts looking at high resolution voltage and current polarity relationships on a single grid line.

  • \$\begingroup\$ Thank you for that link, I am looking into it. I have upvoted both answers so far, but since my reputation is < 15, this is not shown on the website. I appreciate the pointers and the explanation. (And may come back with follow-up questions.) \$\endgroup\$ – Kostas Jan 14 '17 at 19:26

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