Feeding energy to the power grid

I am doing a research about the electricity grid and a marketplace where neighbours can trade energy with each other.

I found out if your PV cell has excess energy you can with an inverter put it into the grid in 1 phase, or 3 phase if it's too much (>5kw). The question I am asked when I do presentations is: Does this work? And who does the frequency or voltage regulation?

Is it a real world problem or inverters are so good that they don't break the grid?

I also know if the grid goes out of specification the inverters are able to shut themselves down, so I got that going for me.

My professor questions if it is possible that neighbour A puts energy into the grid and then neighbour B takes it. Of course it won't be the same "energy" but the net difference on the grid will be 0 thus a "virtual" transaction. If grid operator agrees with this they might only have to pay a small transaction fee.

• Quick answer: yes it works, lots of installations are in use in the field. I've only seen single phase (230V in UK) for systems around 10kW. It is common system in Europe in general, the UK did have some pretty good Feed In Tariffs where the homeowner would sell all surplus energy to the grid. There were lots of companies that would fit the PV panels for free, let the home owner use some energy and but would sell the energy going into the grid. If I remember rightly the energy was sold to the gird at something like £0.40 per unit (as it was green energy). It works, but on a gird wide system. – Puffafish Nov 23 '17 at 8:24
• Many countries have the possibility of selling back to the grid either using meters that measure energy in both directions or simply by fitting two meters and paying for the energy produced from wind, solar pv, hydro etc etc – Solar Mike Nov 23 '17 at 8:28
• And do you have any inputs on frequency and voltage regulation that I might quote that'd convince my professor :) ? – EralpB Nov 23 '17 at 8:47
• Frequency is to match the grid. Voltage regulation is to match the grid. Depends on location. If he doesn't believe you, show him the companies that do it, if he's asking for you to do the research, then look up "feed in tariff inverters". – Puffafish Nov 23 '17 at 8:51
• You might want to point your professor in the direction of document G83.2 ofgem.gov.uk/ofgem-publications/52354/… that's the standard that inverters here in the UK have to follow. It goes into great details on how they must work. – Simon B Nov 23 '17 at 13:47

3 Answers

Grid attached solar inverters monitor the frequency of the power lines they are attached to. In fact in some jurisdictions there is a requirement that they monitor the line for about five minutes before attaching. During that time we sync a pll to the incoming frequency so that when we generate our output the inverter will be in sync with the line. We are also watching the line voltage (and of course our output current). When we attach to the line, we open our transistors and push current out onto the grid. Our local control system continues to monitor the line and adjust our output to provide a constant current to the grid regardless of where we are in the AC cycle (assuming available solar energy).

Yes, it works. The grid itself provides the frequency and voltage reference.

The inverter is programmed to feed current into its connection with the grid, in a direction that feeds power in, and a magntitude that looks like a negative resistance.

Imagine something as simple as a resistor connected to the grid, a 3kW kettle of roughly $20\Omega$ resistance for instance. When the instantaneous grid voltage is 100v, it draws 5A, when it's 300v, it draws 15A.

Now with some clever electronics, you could create a load that sensed the instantaneous voltage, and drew that amount of current. That would look like a resistor at its terminals. Those exist, and they're called power factor corrected power supplies.

You can do exactly the same thing, and sense the grid voltage, but this time feed current in rather than draw current. That's how a grid-tie inverter sends power into the grid in a well controlled fashion.

Any commercial inverter will have protection on its output so that in the event of grid disconnection, it won't generate too much output voltage.

The arrangements around money changing hands for this are mired in political issues, with incentives and tarrifs in the driving seat.

Essentially it's a complicated sort of switchmode power supply, but instead of targeting a DC voltage level it targets the AC line level. During the "off" periods of PWM, it can measure the line voltage and phase, then align its output to that.

The "does this work?" question is a bit strange. Clearly it does, there's millions of these things already installed. I have one. I don't really think about it until I recieve my regular cheque from the power company.