How does a residential grid-tied solar system work exactly?

Question

Ok.. so I know how solar panels are connected to a combiner box & tied into a charge controller/inverter. I also know that the AC from the inverter must be in phase with the line/mains power.

What I would like to know is the following:

1. Exactly how much voltage (above the line voltage) must the inverter output be in order to cause current to flow out to the grid?

2. On a 100A mains line, how much current could be output to the grid (assuming your solar array had slightly more net capacity than your line is capable of carrying & what would happen to the excess power from your solar system if it could not be sent out to the grid, due to it being higher than the mains line is rated to carry?

3. What would be the difference in an oscilloscope trace if I viewed the output from the solar system when it is grid-tied vs the normal mains trace on the oscilloscope if the grid-tie was not connected?

Answer 1

(1) The inverter output will be exactly the line voltage, having raised the (local) line voltage according to the source impedance of the supply line and the current developed As the inverter is connected to the line, its output is the (locally measured) line voltage by definition. The "meter spins backwards" when generation exceeds use, because it senses the current direction. Often there would be separate meters for consumption and generation.

^{simulate this circuit – Schematic created using CircuitLab}

This is a residential distribution network, your house is No.2, the third from the pole transformer and the only one with solar power. Currently it's in shadow, so the line voltage reduces slightly further from the pole, and the last house (No.3) sees 109V.

When the sun comes out, the line voltage at the pole remains 115V, but your local line voltage also becomes 115V.

No. 1 is now fed from both sides and it's line will rise from 113V to 114V, and No.3 is still 2V below your line voltage, now seeing 113V.

Weather Line No.1 No.2 No.3
Cloudy 115V 113V 111V 109V
Sunny 115V 114V 115V 113V

(2) Where are you and what connection regime are you in? If you're in the UK and conforming to the G83/2 standard for a single phase residential connection, the answer is up to 16A. (Higher output is allowed on a separate standard, G59/2 for 3 phase connections). Any excess power is simply not drawn from the panel (assuming this is a grid tie system with no local storage)

None of this applies outside the UK.

(3) Pretty much nothing visible, maybe a very slightly higher voltage and marginally cleaner waveshape (assuming your inverter is legal and conforms to the applicable standards, including harmonic distortion.

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The G83/2 specification illustrates the requirements for protection against overvoltage, out of tolerance frequency and waveform shape - and the all-important consideration of disconnecting immediately if the grid fails, to prevent electrocuting the engineers trying to repair it!

Here's an article on the rules in California to get you started on the US standards - which may well vary from State to State.

Grid regulation is a topic in itself - it isn't automatic, someone monitors it and adds or removes generation capacity to keep its parameters (voltage and frequency) within limits.

One characteristic of traditional (spinning metal) generators is that as you load them, they slow down a little, which reduces their output voltage and their contribution to the grid - transferring their load to others, which slow down in turn - arriving at a concensus on the actual mains frequency. You can watch this process in the UK in real time here. At the moment, it's reading 50.007Hz, so there's no need for additional capacity, but if it falls to 49.9Hz, phone calls will be made and some other power source will be turned on...

The importance of this for grid tie inverters is the way they can affect demand and power flows in the short term in unpredictable ways, taht the grid isn't currently designed for.

Answer 2

1) To reason about this, consider applying ohm's law to the line leading into your house. It's not possible for the voltage to be higher on the house side than the grid size by more than an amount determined by the resistance of the wire, which is going to be very small. Probably tens to hundreds of milivolts.

2) Current flow is limited by power output. I have a 3.8kW solar system in the UK which outputs up to the 16A mentioned in the other comment. It occupies my entire roof, and is just below the 4kW limit to which a different permitting and feed-in-tariff regime applies at which point it would be an "industrial" system.

100A on a US 110V system would be 11kW, quite a substantial system costing in the region of $15k.

Obviously no permitting regime is going to let you put a >100A output system on a 100A line as that would melt the line.

3) Hard to tell, although I expect it would be cleaner. You might be able to pick up a harmonic output from the inverter, showing as a slight ripple in the sine wave. It's certainly obliged to be the same frequency and RMS voltage.

Edit: I have a grid-tie system in the UK, so can't really speak about US grid or code matters.