On Apr 12, 10:27*pm, Home Guy wrote:
Bruce Richmond wrote:
The point here is that there is no such thing as the grid not being
able to accept the power you have produced. *As long as you are
connected you can always force your KW in.

I don't think the issue is whether or not you can force current into the
grid via your 120/208 VAC service connection.

Then you should look back and see that is precisely the issue being
discussed here. You wrote, "But still - you can't push more
electricity onto a network than the load is asking for (given that
your invertors are functioning correctly I guess)."

David wrote, "That second statement is correct: you can't "push"
electrons into the grid. But it doesn't matter *how* your inverters
are working; it's a basic law of physics."

I have explained in simple terms (without getting into power factors,
phase shifting, pulse width modulation, etc) the physics behind how
you can push your power onto the grid whether it is asking for it or
not.


The question is:

a) does your power source need to overcome the instantaneous line
voltage in order to achieve a flow of current (answer: *yes, and to the
extent that your power source has the capacity to do so, you raise the
output voltage as high as you can, because if you don't - then you have
excess capacity that is not going to make it out to the grid and hence
you won't gain revenue for the entire potential of your generating
system)

You do not raise it up as high as you can, you raise it just enough to
do the job. The ignition coil in your car raises the 12 volts of its
battery to many thousands of volts to force a spark across the gap of
its sparkplugs. You don't need thousands of volts to feed power into
the grid.

b) by raising the voltage on your local 120/208 grid, can your local
stepdown transformer adjust it's own operation by sensing that higher
voltage and reduce it's own output voltage in an attempt to regulate the
system back down to the desired setpoint? (answer: *I don't know -
probably not. *The neighborhood stepdown transformers probably weren't
designed to compete with sources of current being connected to their
distribution outputs).

Go back to the example I provided using two batteries. As I said,
when they are connected in parallel the 12 volt battery charges the 11
volt battery and the voltage across them will measure somewhere
between 11 and 12 volts. Connect a light to the batteries. You will
measure a slight drop in the voltage but it will still be over 11
volts. That means the 11 volt battery is still being charged and all
the power to light the light and charge the 11 volt battery is coming
from the 12 volt battery. Connect more lights (load) to the batteries
and you can drag the voltage down so that it is just over 11 volts.
So long as the voltage across the two batteries is higher than the
stand alone voltage of the 11 volt battery all the current going
through the lights will be coming from the 12 volt battery. And it
doesn't matter that the 12 volt battery has been dragged down to
within a small fraction of a volt over the 11 volt battery, the lights
see 11+ volts. Can you see now how the inverter can pump its power
into the system? By having its voltage just a bit higher than the
transformer, but well within the normal range of the line voltage, it
can take over feeding the local water heaters, cooking stoves, air
conditioners, lights, etc. No additional controlls are needed to
reduce the current coming from the transformer. The voltage
difference takes care of it.

c) So if the voltage on your local 120/208 grid is being raised slightly
because of your PV system and it's desire to push as much current back
into the grid as it can generate, then will this actually reduce the
amount of current that the regional sub-station is sending to your local
step-down transformer? *(answer: *the substation probably doesn't have a
direct line to your local stepdown transformer, and any alterations it
can make to it's output voltage is probably seen by many step-down
transformers including yours that are all wired to the same circuit. *So
in reality it's doubtful that the regional substation would even sense
that your PV system has raised the local grid voltage).

As you saw above the current flow through the transformer will be
reduced. The substation sees that as a reduced load and will behave
the same way it would any other time the load goes away. No
additional controls are needed.

d) So your PV system is raising the local grid voltage, and you're
probably pushing out 40 amps at 120 VAC or 20 amps at 240 VAC on a sunny
summer day. *So what is that extra juice doing? *Well, it's flowing
through the compressor motors of 10 to 20 of your neighbor's AC units -
whether they need it or not. *Because you've raised the local grid
voltage slightly, that translates into a few extra watts (maybe 250
watts for each house that's fed from the same stepdown transformer). *So
all the fridge compressors and AC compressor motors, lights - all linear
loads are going to blow away that extra line voltage as heat - instead
of useful work.

Nuf said?

Yes, you have said enough to make it clear how little you know.

When an electric motor is running it produces a back EMF that counters
the flow of the current.

http://en.wikipedia.org/wiki/Back_EMF

That AC compressor requires the same power it did before. Keeping it
simple that power is volts times amps equals watts. Divide the power
required by the higher volts now provided and you will find fewer amps
are required. The wasted heat is proportional to the square of the
current. So raising the voltage means there will be less waste heat
from the motor.

With resistance heaters the higher voltage flows more current, so you
get more heat, which is what you wanted anyway.