# How is the amount of outgoing power controlled in a grid tie inverter?

I'm trying to figure out how grid tie inverters work on a basic level. From what I understand, the inverter would have to

1. monitor the grid wave
2. create its own wave exactly like the grid
3. make it a little bit higher voltage
4. connect the two and then the power would flow into the grid

What I don't understand though is how it controls the amount of current going into the grid. Current is determined by voltage / resistance right, so, does the grid have a set resistance and does it just raise the voltage until desired amount of power flow is reached?

The grid having a set resistance makes no sense though, maybe it measures the resistance very quickly and then creates a tiny bit of charge at the appropriate voltage that gets dumped into the grid at that instance?

That option seems like it would make the grid wave messy though since there would be little discrete bumps on an otherwise nice sine wave.

• ”Current is determined by voltage / resistance right,” No. In an AC system, phase angle is what primarily determines power transfer. Jul 15, 2018 at 8:02
• Grid tie inverters are effectively CURRENT sources, not voltage sources. Jul 15, 2018 at 9:52

## 2 Answers

Your understanding has some of the elements, but it is more like this:

1. Monitor the grid wave.
2. Create an inverter wave exactly like the grid matching frequency and phase angle.
3. Connect the two and then no power would flow into the grid.
4. Monitor the current output of the inverter very carefully.
5. Increase the voltage very slightly and dynamically control it to deliver the desired current.

The grid appears to the inverter to be an ideal voltage source behind a complex impedance. The inverter voltage must be raised above the grid voltage just enough to drive current through the source impedance. The ideal voltage source is the voltage measured just before the grid tie is closed. Once the grid tie is closed, it can no longer be measured. The inverter voltage increase is performed by a control system that monitors the inverter voltage, frequency and phase angle. Those parameters need to be controlled and adjusted continuously to keep the inverter synchronized with the grid and deliver the desired power. The grid voltage, phase angle, frequency and impedance can all fluctuate.

The inverter control system essentially makes the inverter act as a current source.

This is a simplified explanation of a grid-tie inverter control system that is actually more complex. The system also must include some protective features. While some small inverters may be designed to be used with any type of input system, others may be designed specifically as solar-PV or wind turbine inverters. "Consumer" inverter designs are different from "utility" inverter designs.

• ah, that makes sense. Followup question though, how would the inverter keep monitoring the grid while feeding power into it? Since its voltage needs to be a little higher, I imagine that's all it would see, itself, unless it goes out of phase at which point it would already be bad. Jul 15, 2018 at 11:16
• The grid should be monitored for a power outage, if this happens disconnect the inverter. If grid power is restored and frequency is stable, wait some time and then connect again.
– Uwe
Jul 15, 2018 at 11:23
• I think you'll find that phase angle rather than voltage is the main power control parameter. Precise combination will vary with implementation. Jul 15, 2018 at 12:31
• -1 You don't monitor the output current carefully and then alter the voltage, you synthesise a negative load by creating a current source that tracks the grid voltage in a specific ratio. Jul 15, 2018 at 12:54
• I revised my answer and deleted my previous comments.
– user80875
Jul 16, 2018 at 12:23

essentialy grid tie inverters are designed to be current sources they allow the grid to regulate the frequency and set the voltage and just push energy into the grid (unless the conditions on the grid are such that they should not feed it)

• Have my up-vote Sir, you nailed it in very simple words. Jun 17, 2020 at 19:45