DIY 1KW grid tie?

[Alan B]

Hi Rich. I'd be happy with it running a little slower. I have this
small system I've set up in case we get another big wind and a power
outage again here in Cen Fla. My curiousity about the technical aspects
is sort of academic... seems like if I can pull 10 amps out of one leg
of the 240, I could push 10 amps back in, and who would know? Isn't it
better to ask for forgiveness than ask for permission?

You will not get much forgiveness if you seriously screw something up.
Jasen mentioned killing someone. Society sometimes forgives that after
forfeiture of some years of your life, and lots of money spent on
attorneys. Sometimes you can get off with no time served if you spend a
really awful lot of money, only to lose what little you have left when you
lose the civil court wrongful death case.

You may also see some investment in attorney fees if you try to put a
signal onto the grid that is not in complete and perfect synchronization
with the AC signal already on the grid. That's 60 Hz at zero degrees phase
shift. Not 59.99Hz, not 60.01Hz; not 0.1 degree phase shift, not -0.1
degree. 60Hz at zero degrees. Anything else, and you are inserting a
serious VAR problem.

When you call the local utility for permission to produce generation, they
will tell you the bus-tie inverters of which they approve, and will assist
you hooking it up. If you tell them you are going to build a
do-it-yourself inverter, they will be very interested, and you will get to
talk to them a lot (discussions about where you got your MSEE, what your
power system engineering background is, stuff like that), and please do not
mention that you'd like your power generation to "run a little slower."

All in all, I'd say buying an approved bus-tie inverter and following
proper channels with the utility district is by far the best use of your
time and money.

 

[BobG]

You will not get much forgiveness if you seriously screw something up.
Jasen mentioned killing someone. Society sometimes forgives that after
forfeiture of some years of your life, and lots of money spent on
attorneys. Sometimes you can get off with no time served if you spend a
really awful lot of money, only to lose what little you have left when you
lose the civil court wrongful death case.

You may also see some investment in attorney fees if you try to put a
signal onto the grid that is not in complete and perfect synchronization
with the AC signal already on the grid. That's 60 Hz at zero degrees phase
shift. Not 59.99Hz, not 60.01Hz; not 0.1 degree phase shift, not -0.1
degree. 60Hz at zero degrees. Anything else, and you are inserting a
serious VAR problem.

When you call the local utility for permission to produce generation, they
will tell you the bus-tie inverters of which they approve, and will assist
you hooking it up.

All in all, I'd say buying an approved bus-tie inverter and following
proper channels with the utility district is by far the best use of your
time and money.

===========================================================
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!

 

[The Phantom]

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%.

 

[BobG]

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
240V with a source impedance of about .12 ohms (240V/200A). So if I
wanted to feed 10 amps back into one leg of the 120, I'd have to be
able to put 10 amps thru .06 ohms, so I'd need to have 120.6V feeding
into my iso transformer to make the galvo point towrd the pole. I guess
its easy to detect when the 120 drops off the other side... the current
tries to go way up.... so seems like the secret is having a sine wave
output with fairly fine amplitude control on a per half cycle basis.
and in perfect sync. Thanks for the reply!

 

[The Phantom]

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
240V with a source impedance of about .12 ohms (240V/200A).

240v/200A gives 1.2 ohms. If the grid impedance at the service entrance was
1.2 ohms, that would mean that when you drew 200A, the voltage would drop to
zero. Since we know that with a 200A load, the voltage remains near the
nominal, then the incremental impedance must be much lower than 1.2 ohms. What
you want to do is apply a load of, say, 100A and measure the delta V. Even
though you made a little math error, it just so happens that the impedance at
the typical home service entrance is on the order of 1/10 ohms (probably a
little lower, but not much).

So if I
wanted to feed 10 amps back into one leg of the 120, I'd have to be
able to put 10 amps thru .06 ohms, so I'd need to have 120.6V feeding
into my iso transformer to make the galvo point towrd the pole. I guess
its easy to detect when the 120 drops off the other side... the current
tries to go way up.... so seems like the secret is having a sine wave
output with fairly fine amplitude control on a per half cycle basis.
and in perfect sync.

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
99.6%, and 10 degrees, 98.4%, a phase error of <= 5 degrees would be considered
excellent.

In practice, the fine amplitude control is a byproduct of making the current
the controlled variable.

 

[Alan B]

Wow. What a bunch of dour old sourpusses.

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.

 

[jasen]

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
240V with a source impedance of about .12 ohms (240V/200A). So if I
wanted to feed 10 amps back into one leg of the 120, I'd have to be
able to put 10 amps thru .06 ohms, so I'd need to have 120.6V feeding
into my iso transformer to make the galvo point towrd the pole. I guess
its easy to detect when the 120 drops off the other side... the current
tries to go way up.... so seems like the secret is having a sine wave
output with fairly fine amplitude control on a per half cycle basis.
and in perfect sync. Thanks for the reply!

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