PV System Putting Power Back to the Grid

Though I'm only partially acquainted with these type of systems I would say your problem is not dramatic. As far as I know the power companies have to buy back your surplus electricity, the end result is your utility bill goes down.

If you feel you're putting too much power back into the grid there should be a fine tuning mechanism for your output voltage, dropped the output voltage and you should be able to balance your system a little better.

the inverter!!!

If the system is tied into the grid the power provided will be from the grid.

If you produce at the same time that you switch the hair dryer on, the power that has to be provided by the grid will be less than what you use and the remainder is taken from PV.

Think of it as a water pipe. If you switch on two faucets at the same time how does the system decide what water to run where?

Grid tied system should come with a dual way smart meter. It can measure currents in and out.

Hope that explains it a bit. The more intricate electronic business is not my field.

Whichever voltage is higher....

It doesn't determine where the power come from at all. The dryer just use the power available.

Just like every other electrical producer that's attached to the grid. They don't know who or what used power from where. Just that the meters show whats used and whats was produced and by whom.

The solar array/ inverter combination is connected to the grid via the meter.

The power from the inverter will be at the same voltage and in phase with the grid supply.

You may be grid connected either via a smart meter or an older type meter.

If the meter is an older type, then the solar will be connected at the output of the meter, and this solar power may be used to run your appliances. The power metered will be the sum of what you have used minus what you have generated from the solar. If you generate more than you use, the meter will turn backwards and so produce a credit.

If the meter is a smart meter, the solar will be separately measured and you will receive an input credit at the going rate. You will be separately billed for the total that you use.

Either way, the co-generated power will be added to your supply mains between your input terminals and the main switch, and what you use will be an aggregate of the grid and solar, if you use more than you generate, then, due to the slight voltage drop between your input terminals and the meter, the solar power will be returned to your home, if you use less than you generate, then the excess solar will be fed into the grid.

The PV panel and inverter produce an output voltage slightly higher than the Grid, so the PV power is taken first.

However PV and inverter have internal resistance. As the current drawn from the inverter increases, the inverter tries to maintain its output voltage. At a certain point it can no longer do this and the voltage from the grid is slightly higher than the inverter output, so the grid starts to deliver some power to the load. The current drawn from the PV/inverter drops slightly, the inverter o/p voltage rises, and so on. This happens at a high rate.

As the sun goes down, the inverter keeps trying to maintain the o/p voltage, while the current available from the PV at that output voltage keeps dropping. When the current that can be drawn from the PV falls low enough, ie when the current needed to operate the inverter leaves nothing to generate output, the inverter switches off.

Sorry to say that all this talk about an inverter putting real power into the grid by raising its output voltage is incorrect. This is AC, not DC, and to an AC network an inverter is just another generator, and raising the output voltage of a generator increases its reactive power (VAR) output, not its real power (kW) output.

I excerpted this from an earlier post http://cr4.globalspec.com/comment/771327 on d-q control of inverters (emphasis added):

"...All this discussion about generator voltage being "above" or "below" the utility voltage refers to the excitation level of the generator; another way to think about it is to imagine opening the connection between the generator and grid and comparing the the terminal voltage to the utility voltage- generator voltage higher = VARs out, lower generator voltage = VARs in, matched voltages = no VARs (unity power factor). By now you should be saying "...but wait, there is no excitation system inside my inverter, since there's no field, rotating mass, or iron to energize, how do I even measure or control the power angle and/or the power factor?" That's where your d-q control comes in, d stands for direct (real) power, q for quadrature (imaginary) power; d-q control affects the magnitude/timing of the firing pulses to the converter circuitry. Magnitude affects the height/width of the pulses and hence the output voltage, and timing affects where the peak/zero crossing occurs and therefore the power angle..."

Now to answer the OP's question, the relationship of the inverter's power angle relative to the electrical system's, their respective impedances, and the magnitude of their internal voltages all play a role in determining how the real energy is divided. How? Just think of two AC voltage sources, each with a series impedance, one on the left and one on the right, with a load impedance (the dryer) connected in parallel in the middle.

All things (power angle and voltage magnitude) being equal, the current contribution from each source is going to be (roughly) inversely proportional to its series impedance, so if the inverter has a much lower internal impedance then it will supply the majority of the current, up to its rating, at which point the grid makes up the difference.

Since inverters are designed to supply load, not be one, the d-q control always makes sure that it creates a sinewave voltage whose zero crossing always slightly leads the utility voltage to push Watts out, and whose voltage magnitude is always slightly greater to push VARs out, to a load, with the excess going out to the utility.

There is an intelligent type of inverter, which prioritises solar power over grid power for captive consumption, and keeps the Three Phase A C supply steady. I have seen such a system, but I do not know the electronics involved. The supplier said that we could have upto three sources of power Batteries. solar and Grid and prioritise them, for supply to the load.

As a matter of fact, without such 'Reservoirs' of power solar energy system cannot function at all due to vagaries of sunshine due to cloud, fog etc.