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I have a grid-connected solar inverter. In the swtich board the incoming wire from the street, the incoming wire from the inverter and the outgoing wire to the loads in my house are all connected together (wired in parallel).

It appears that the loads in the house will consume all available solar power (if needed) before drawing power from the grid.

How does this work? Why doesn't the load draw power equally from the solar inverter and from the grid?

I suspect that the inverter monitors the grid voltage and produces an output voltage that is just a few volts higher. Is that all that is needed for the loads in the house to use all the solar power before using grid power?

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4 Answers 4

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I suspect that the inverter monitors the grid voltage and produces an output voltage that is just a few volts higher. Is that all that is needed for the loads in the house to use all the solar power before using grid power?

Yes.

This becomes more obvious if you have a meter between the two: meters measure current, and if the current is flowing out (indicating higher generation than usage), that implies the voltage on the house side is higher than the grid side.

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Kirchhoff's current law works for you.

At some point in your installation, the mains supply, the inverter output and the load representing the appliances in your home must all meet. Kirchhoff's law says that the total current going into that point must equal the total current going out.

If your appliances are using more power than the inverter is generating, then it must be true that...

(power from inverter) + (power from grid) = (power to appliances)

If the inverter is generating more than you need, then...

(power from inverter) = (power exported to grid) + (power to appliances)

Either way, as much power as possible is coming from the inverter to the appliances.

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schematic

simulate this circuit – Schematic created using CircuitLab

Grid tie inverters monitor the circuit voltage and if it is in an acceptable range produce a current into the supply. they don't try to match the voltage they just produce (phase matched) current.

Since a current can only flow one direction in a piece of wire the inverter's current flows to your appliances first, and if there is any excess it flows out onto the grid, if there is a shortfall it is made up by the grid.

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When you place a generator in parallel with the grid generator but apply no torque to the armature, the current from the grid generator will spin it up in the which will in turn produce a counter EMF voltage that counters the voltage of the grid until almost no current passes, and if there were zero friction in the armature then zero current would pass in an ideal situation, and you end up with a deadlock. Current only flows when there's another path for current to flow, such as when a separate load is placed in parallel with the two generators. Now both generators will be powering the load and torque will now have to be applied to the generator in the same direction it is spinning to keep it spinning at the same speed because current will now be flowing in the other direction and trying to slow down the armature (when it was flowing in the other direction it was maintaining the speed of the armature due to back torque), whereas previously the current flowing was in the opposite direction.

enter image description here

Now, the voltage of the 2nd generator (inverter) is set to a voltage where current stops flowing to this parallel load, which is the voltage it measures and not the source voltage, which in the above case would be 237.623V. This clearly shows that the inverter does not match the grid generator voltage but actually the voltage that it measures itself, which has dropped due to voltage loss over the resistance of the transmission line. Only now can it control how much current flows, namely the current will be (V-237.623)/R, where R is the resistance of the whole circuit over which the voltage is being placed, in this case is 1.990099 µΩ, so the current that flows if V=238 is 189kA. Given that 2.19MA is flowing in the other direction when you perform the same calculation from the other side using how 238V presents, which is 235.64V and subtracting that from 240V and dividing by R, the current of 189kA is clearly not flowing in that direction, so it must be flowing over the parallel load. The voltage of the inverter have to be raised to 242.4V in order for current to flow over the grid generator internal resistance and be wasted, which is 11.4GW of power, which of course the inverter gets absolutely nowhere near to producing, so you can be confident that the current being produced by the inverter is actually flowing over useful loads. The inverter inserts 500W of power into the grid just by raising the voltage 1mV above the detected voltage. The current flowing from the inverter will be forced over and fully power the closest load first due to path of least resistance which will be in your own house, and the 2nd and 3rd loads, until the current runs out and doesnt flow any further and the flow from the grid is stronger

enter image description here

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