I'd just like to know how the darn
grid tie inverter control system works. What do they measure? voltage?
current? Every half cycle? I mean there are 3 or 4 companies that make
these things. They hopefully have smart electrical engineers designing
them. Lord knows where these guys learned how they worked. You'd think
I was asking for the formula to the Philosopher's Stone. Lighten up
folks, lets talk about voltage and current regulation in AC inverters!
Once upon a time, I took an AC power lab. There was a small (10 kw or so)
alternator which could be connected to the grid. This alternator was
mechanically driven by a DC motor whose speed was controlled by a rheostat in
series with the field. The switch which connected the alternator to the grid
had an incandescent light bulb across its contacts.
What you did was, adjust the speed of the DC motor so that the output of the
alternator was almost exactly 60 Hz; the light bulb would slowly get brighter
and dimmer as the phase of the alternator output drifted relative to the grid.
When the light bulb was at minimum brightness (not lit up, in other words), you
closed the switch so that your alternator output was connected to the grid.
There was an ammeter connected in series with the DC power source feeding the
DC motor which was driving the alternator shaft. The ammeter was a zero-center
type so you could tell the direction of the current. Now, with the alternator
connected to the grid, it was impossible to make the alternator go faster or
slower, because the grid is so much more powerful than the puny little
alternator (Grand Coulee vs. the lab alternator). If you tried to make it go
faster by adjusting the speed of the DC motor, you found that you *couldn't*
make it go faster. All that happened was that the DC motor drew more power. If
you tried to make the alternator go slower by reducing the power fed to the DC
motor, what happened was that eventually the ammeter in series with the DC motor
began reading in reverse. This meant that instead of power being supplied to
the DC motor to drive the alternator, the alternator was now driving the DC
motor (which was now a generator) and forcing power back into the DC source
(which was a big storage battery); that is, the DC motor was now charging the
battery.
But, at no time did the alternator run faster or slower. When you tried to
make it go faster, all that happened was that the output of the alternator
shifted slightly in phase--just enough to cause power to flow into the grid. If
you tried to make it go slower, the phase shifted slightly in the other
direction; enough to cause power to flow from the grid to the battery.
Much the same thing happens with a grid tie inverter. In order to supply
power to the grid, the output voltage of the inverter is made slightly higher
than the grid voltage, and in phase with the grid voltage. This causes current
flow of such a polarity as to supply power to the grid. The inverter must
measure its output current and control it to set the amount of power supplied to
the grid. The inverter also controls the phase of the current fed to the grid,
and keeps it substantially in phase with the grid voltage. It is certainly
possible to supply an out-of-phase current to the grid, and there are special
devices designed to do this for power factor correction. But, if the phase of
the current fed to the grid is more than a few degrees off, the inverter is
wasting some of its potential to sell power to the grid.
If the inverter tries to provide an output voltage lower than the grid, then
power flows back into the battery supplying the inverter.
If the grid voltage goes away and the inverter keeps inverting (while still
connected to the grid), this is called "islanding", and is dangerous to a
repairman who comes out to fix the grid problem. Underwriters Labs requires
that inverters intended for grid tie use have a circuit to detect when the grid
goes down, and turn off the inverter (anti-islanding circuit).
Also, UL requires that the current fed to the grid be a near sine wave with
less than 5% distortion, so a square wave inverter (sometimes called modified
sine wave) won't meet the UL requirements without some auxiliary means (a filter
perhaps) to get the distortion below 5%.