DIY 1KW grid tie?

=========================================================== Wow. What a bunch of dour old sourpusses. I guess you think that when I told Jasen that his admonition had been duly noted, that you didn't believe me, and you must add your learned and fatherly warning to protect the Progress Energy employees who might happen by my neighborhood. I'm curious about how these things work, so I'm asking for a sort of technical overview. You say there can't be more than .1 degrees phase difference between the local inverter and the grid voltage. This seems like a much too tight tolerance, so right now I don't believe you. Your use of the phrase 'bus-tie' doesn't seem to be used in the solar newsgroups much. Maybe it it a term used in a powerhouse where multiple generators can be switched on line? (Maybe you do have a background in power generation?) 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%.

Good post. My service is 240V and 100A, and I assume the transformer on the pole is sized for about 2 houses, so I assume the grid looks like

Perfect sync is not necessary. As I suggested, a phase error of a few degrees is perfectly tolerable. Remember, the power factor involves the cosine of the phase angle. So, noting that the cosine of 5 degrees leads to a power factor of

On 2 Jul 2006 07:35:05 -0700, in message , "BobG" scribed:

Well, you said you wanted a "DIY 1kW grid tie" to "push 10 amps back in, and who would know? Isn't it better to ask for forgiveness than ask for permission?" I took you at your word. Sorry.

yeah that sounds about right, the grid has a low impedance so the output side of the inverter should be be a controlled current that matches the the measured phase and amplitude of the grid.

Bye. Jasen