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'[PICLIST] [EE] high efficiency feed to mains'
2001\03\08@054131 by Roman Black

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face
Hi everybody, I am testing an idea to feed power
back into the mains on a small scale with hopefully
high efficiency.

The idea is to get something like a 12v 100w solar
cell, invert it directly to 240v AC and feed the
energy back into the mains. This makes that energy
available at all the house wiring.

The advantage of this would be the ability to use
a MPT (max poewer tracking) system where the current
drawn from the solar cell is constantly adjusted by
a PIC to give the max power fed into the mains.
ie, the feedback is after the inverter and this is
all done on a small scale so each solar panel has
its own MPT system. So even if solar panels are
getting different light levels efficiency is good.

Now I have repaired and built 240v ac inverters
before, but this is bit different. This needs to
feed current into the EXISTING 240vac mains waveform
with the best efficiency.

I am considering a 240v:6v 50Hz toroid transformer
with a FET h-bridge feeding in current at the low
voltage end using pwm and then monitoring the current
being fed back into the mains.
Any suggestions for best efficiency??
:o)
-Roman

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2001\03\08@062119 by Simon Nield

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roman:
>Any suggestions for best efficiency??

I don't think a standard 50Hz mains transformer will give you the best efficiency, you should be
able to use a magnetic circuit designed for the pwm frequency rather than the frequency of the
resulting waveform you are creating... EMI/RFI generation will be trickier to deal with in this
case, but that's only an issue if you are planning on doing this commercially.

I think an autotransformer type of design (i.e. not isolating) would offer further efficiency due to
the closer magnetic coupling. Not sure how practical that would be for you from a design point of
view (would there be any exposed circuitry on the panels themselves?), but I know you have the
plenty of experience with working on exposed HV circuitry :o)

sounds like a fun project.

Regards,
Simon

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2001\03\08@070611 by Jinx

face picon face
Have you looked around for some of the D-I-Y Alternative
Energy sites ? I saw a few at one time for self-made installations
for wind, solar and water generation, perhaps you'll get some
tips. The people in the ones I saw were quite keen to spread
the word and show how they did things. Might not go into nitty-
gritty details, but there are several regular publications that
could be worth looking up

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2001\03\08@075639 by Roman Black

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Simon Nield wrote:
>
> roman:
> >Any suggestions for best efficiency??
>
> I don't think a standard 50Hz mains transformer will give you the best efficiency, you should be
> able to use a magnetic circuit designed for the pwm frequency rather than the frequency of the
> resulting waveform you are creating...


Excellent point! Really excellent. I have re-thought
the design and changed to a high dc volts CURRENT
supply, that can generate up to 400v DC and inject
a measured current into the AC wave. The current can
be adjusted in real time as a 50Hz wave is very slow
by comparison. This is a really clever idea (thanks
Simon!) so I can use a tiny 40kHz pwm transformer
instead of a bulky mains transformer for the same
power output. I can also optimise the magnetics
and pwm freq for max system efficiency. And it is
cheaper!:o)

I can run two independant circuits, one for each
half of the 50Hz ac wave. Obviously this requires two
power circuits, but that is still only 2 chopper semis
which is probably better than a h-bridge for
efficiency anyway.


> EMI/RFI generation will be trickier to deal with in this
> case, but that's only an issue if you are planning on doing this commercially.
>
> I think an autotransformer type of design (i.e. not isolating) would offer further efficiency due to
> the closer magnetic coupling. Not sure how practical that would be for you from a design point of
> view (would there be any exposed circuitry on the panels themselves?), but I know you have the
> plenty of experience with working on exposed HV circuitry :o)


Thanks Simon, but unfortunately I absolutely
require full isolation. Transformers it must
be, but I do feel you have already helped a great
deal with the first suggestion. I have sat for
hours drafting special inverter circuits to
drive a reversed 50Hz mains transformer... And
I was not real happy with any of them. Now
I just need to design a high voltage adjustable
current switcher, which I can do in my sleep.
(so to speak!)
:o)
-Roman

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2001\03\08@075853 by Roman Black

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Jinx wrote:
>
> Have you looked around for some of the D-I-Y Alternative
> Energy sites ? I saw a few at one time for self-made installations
> for wind, solar and water generation, perhaps you'll get some
> tips. The people in the ones I saw were quite keen to spread
> the word and show how they did things. Might not go into nitty-
> gritty details, but there are several regular publications that
> could be worth looking up


Yeah I have had a good look, but nobody has
done anything like this. They all dump power
into DC batteries and some commercial inverters
have facility to inject power back into the
mains but no-one has really done MPT inverter
on a small scale just to put power back into
the grid with no batteries etc. Yet. :o)
-Roman

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2001\03\08@113202 by Harold M Hallikainen

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On Thu, 8 Mar 2001 23:58:51 +1100 Roman Black <spam_OUTfastvidTakeThisOuTspamEZY.NET.AU>
writes:
>
> Yeah I have had a good look, but nobody has
> done anything like this. They all dump power
> into DC batteries and some commercial inverters
> have facility to inject power back into the
> mains but no-one has really done MPT inverter
> on a small scale just to put power back into
> the grid with no batteries etc. Yet. :o)
> -Roman
>

       I recall seeing in Electronic Engineering Times a while back (year or
two) a modular PV panel with integral inverter. You could buy one, stick
it on the roof, and plug it into a convenient AC outlet. Later, you buy
another and plug it in too. Having relatively small inverters attached to
the panel eliminated any need for DC wiring, eliminated the need to buy a
big inverter, and allowed the system to grow over time. I thought it was
a pretty good idea, but haven't seen much about it recently.

Harold




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2001\03\08@145126 by Jess Hancock

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Roman, What you want to do is similar to operating two ac generators
(alternators) in parallel to supply a load.  If you want alternator #2 to
supply more power (watts), then its phase angle must lead that of alternator
#1.  If you raise the voltage of alternator #2 you will supply more reactive
power (vars, not watts).   By controlling the phase angle and voltage of
alternator #2, you can supply a controlled combination of watts and vars to
the load and adjust load sharing between the two alternators.

Doing this with a PWM system is beyond me, but I assume you will have to
advance the effective phase angle of the equivalent PWM "sinusoid" relative
to that of the mains ac voltage to obtain power (watts) transfer to the ac
system.

Jess

PS: I logged in about 1:40pm today and received about 21 PICList emails
(10:30am to 1:40pm) that did not have any text?  What about others?

{Original Message removed}

2001\03\08@161232 by Lawrence Lile

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I"ve tried this with an induction motor connected to the mains, and driven
by a gas motor (don't tell my power company).  We could rev up the motor and
watch the power meter run backwards when we were leading, it'd try to start
the gas motor when we were lagging.  Induction motors make neat connections
to the mains - they will NOT produce power on their own without some
exitation from the mains power.  We tried this as well, getting the motor up
to speed and running the power meter backwards, then suddenly disconnecting
it.  Voltage at the motor leads would quickly drop to zero even when driven
by the gas motor.  How this works I wish someone would explain.  If you want
a simple, and not inefficient, mains-synching solar system, a 12VDC motor
running an induction motor might be simple enough!  Total mains isolation,
automatic synchronization, lightning proof, and bonehead simple.  Now - how
to measure the power output of the motor, and use it to feedback to the 12V
motor speed control?  Gotta have a purpose for the PIC, y'know!

Not so interesting to us engineers, who want to make it too complicated,
with all these PWM switching circuits, but maybe a really practical method.

Lawrence Lile



{Original Message removed}

2001\03\08@175139 by Jinx

face picon face
Came across this completely by chance today while looking
for something else. It's, er, well it's different

http://www.homepower.com/rogues.htm

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2001\03\08@180157 by Harold M Hallikainen

picon face
       I wonder what the efficiency of a motor-generator system like that would
be. Back in the 1960's the marine radio on my father's boat had a
motor-generator to generate high voltage for the tubes (807s as I
recall). Every time you'd key the mic, the motor would rev up.
       Anyway, the PIC part of this would be to have it measure the power into
the motor (I'd probably measure the voltage using an on board A/D and
measure the current with one of those National current sense chips that
output PWM). Put a step up converter between the solar panel and the
motor with the PIC varying the duty cycle to the FET to give max power
into the motor.

Harold




On Thu, 8 Mar 2001 15:13:12 -0600 Lawrence Lile <.....llileKILLspamspam@spam@TOASTMASTER.COM>
writes:
{Quote hidden}

> {Original Message removed}

2001\03\09@030828 by Roman Black

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Harold M Hallikainen wrote:
>
>         I recall seeing in Electronic Engineering Times a while back (year or
> two) a modular PV panel with integral inverter. You could buy one, stick
> it on the roof, and plug it into a convenient AC outlet. Later, you buy
> another and plug it in too. Having relatively small inverters attached to
> the panel eliminated any need for DC wiring, eliminated the need to buy a
> big inverter, and allowed the system to grow over time. I thought it was
> a pretty good idea, but haven't seen much about it recently.


Thanks Harold, you are exactly right. There is a product
called Trace Microsine that does exactly that from a
100w solar panel direct to mains. I still doing research
to see if it would be a feasible product, the Trace
product is about $350 US so there is a decent profit
margin there.
-Roman

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2001\03\09@034903 by Roman Black

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Jinx wrote:
>
> Came across this completely by chance today while looking
> for something else. It's, er, well it's different
>
> http://www.homepower.com/rogues.htm


Thanks Jinx. That's definitely a "different" solar
power site. I like the guy with the gorilla mask
the best! ;o)

Interesting that in some regions the power meters
are ratcheted, and these people feeding their own
surplus back into the grid are actually paying
full price for what they are giving away!! The meter
spins the same way regardless of whether they take
or give power. Rather interesting.

I think the AC generation system is best utilised
in situations where the user will never feed back
more power than their usage. This negates any
safety or legal issues.
-Roman

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2001\03\09@035740 by Roman Black

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Jess Hancock wrote:
>
> Roman, What you want to do is similar to operating two ac generators
> (alternators) in parallel to supply a load.  If you want alternator #2 to
> supply more power (watts), then its phase angle must lead that of alternator
> #1.  If you raise the voltage of alternator #2 you will supply more reactive
> power (vars, not watts).   By controlling the phase angle and voltage of
> alternator #2, you can supply a controlled combination of watts and vars to
> the load and adjust load sharing between the two alternators.
>
> Doing this with a PWM system is beyond me, but I assume you will have to
> advance the effective phase angle of the equivalent PWM "sinusoid" relative
> to that of the mains ac voltage to obtain power (watts) transfer to the ac
> system.
>
> Jess


Hi Jess, a good point re phase angle. I was thinking
that instead of generating an AC waveform of the
right mains voltage, actually injecting a measured
current into the existing AC waveform.

This gets round all the problems of phase angle,
synchronisation and even safety issues. Obviously it
will detect zero crossing and only feed current back
at the right part of the wave and only if the wave
is there.

-Roman

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2001\03\09@073846 by Peter L. Peres

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> I have heard of inverter systems that sniff for mains power, and will
> not power a mains line that is not fed from somewhere else.  How they
> do this I don't know.

It is done using a small load and a 'T' sense device. This is mostly like
a hybrid circuit for phone lines but works at AC frequencies or even DC.

If the inverter is to feed only a house then the circuit is relatively
simple, and the 'small load' is replaced by the house consumer circuits.
If it needs to feed the grid (single endedly) then it becomes complex
because the controller needs to make sure it supplies only at most a given
proportion ( < 1.0) of the power that goes to the probe load.

The funny part is that the most desirable capability for these devices is
that to be able to travel in time, so as to accurately predict the
waveform in the future ;-).

hope this helps,

Peter

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2001\03\09@073850 by Peter L. Peres

picon face
Roman, your issue is hairy. Your converter needs to lead the mains AC in
phase by just the right amount of time to feed power into the grid. This
is not simple, it involves at least a PLL of sorts to monitor the mains
phase, and lots of protection circuitry in case the mains decides to miss
a semiperiod or jumps phase at an inconvenient moment in time.

One way to do this is to use a PWM type converter and feed current pulses
into the grid. Each current pulse is monitored (together with the voltage
at which it is injected). A smoothing device (RLC) will make a nice sine
out of this. A computer (probably not a PIC) can produce the appropriate
control signals. You will be basically building a sine wave that is
identical to the mains one but leads it in phase by just the right amount
so you inject power into the grid. This needs to be true no matter what
happens to the sine wave of the mains...

Remember that you need to match the mains in both voltage and phase in
real time. You also need to lead in phase because the time constant of
your RLC devices will delay your actual output from the true 'in phase'
time. The delay will depend on a lot of things and must be adjusted
dinamically in real time by the computer.

Every time you fail to do this you add a few burned parts to the pile and
improve your design ;-).

good luck,

Peter

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2001\03\09@073900 by Peter L. Peres

picon face
What is an 'induction motor' ?

Peter

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2001\03\09@090257 by Mark Skeels

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Roman, I don't quite follow this. Could you elaborate?

Mark


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2001\03\09@093839 by Roman Black

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Peter L. Peres wrote:
>
> What is an 'induction motor' ?
>
> Peter


Ac motor common in industry. Has 3 phase wound stator
and squirrel cage rotor, shorted alloy bars through
laminated rotor. Maybe you call them something different?
They are very common in many sizes from football to car
size!

Lawrence's post was the first time I have ever heard of
one being used as an AC generator though, and funny enough
about the only time it would work as a generator is when
feeding current back into an existing ac mains (which
is what he did!)
-Roman

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2001\03\09@095047 by Roman Black

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Mark Skeels wrote:
>
> Roman, I don't quite follow this. Could you elaborate?
>
> Mark
>
> > Hi Jess, a good point re phase angle. I was thinking
> > that instead of generating an AC waveform of the
> > right mains voltage, actually injecting a measured
> > current into the existing AC waveform.
> >
> > This gets round all the problems of phase angle,
> > synchronisation and even safety issues. Obviously it
> > will detect zero crossing and only feed current back
> > at the right part of the wave and only if the wave
> > is there.


Well, you are right that to feed power into an existing
AC wave you need an out of phase wave and the phase
angle determines current drawn or sourced. Basic AC
theory.

But, assuming the AC wave already exists, you could do
it the other way around. Feed in a measured amount of
+ current on the + half of the wave, and a measured
amount of - current on the - half. Same clean result,
different method. :o)

Obviously the controller needs to be able to sense the
zero-cross point of the existing AC wave, and also
sense whether the wave is +/- at the moment! But this is
trivial stuff. Then basically the PIC controls the PWM
drivers to feed in the correct amount of current at
that point in the wave. 50Hz remains relatively fixed
freq, if reset at the zero cross point each cycle.

So you don't have to worry about sync, this is done by
zero cross detection. You don't have to worry about
phase angles as you are controlling current directly.
And the same simple circuit that detects zero cross
and +/- would obviously notice the AC mains waveform
going off line and shut down the pwm. Remember this
is for a small app feeding only 100w or so into the
AC mains, so it's affect on the mains is minimal,
so we can still measure the mains as our control
reference.
-Roman

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2001\03\09@123448 by Lawrence Lile

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I was just working on a problem with a triac control that brought to mind a
possible bugaboo.  The triac was simply designed as a solid state relay to
turn on resistive loads.  A pulse just after zero crossing kept it on till
the next zero crossing.  Simple enough, untill........

But using this on inductive loads caused loads of headaches - missed zero
crossings, excessive power dissipation in the Triac, and reduced chopped
voltage to the load.  Turns out that "zero" isn't always "zero", depending
on whether you are talking about the voltage wave or the current wave!  In
this case, the voltage zero crossing was not the correct place to give the
triac it's little pulse.

This is relelevant because, say your power system is running a big inductive
load.  Exactly where does your inverter need to inject it's power?  In sync
with the voltage wave or in sync with the current wave?



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2001\03\09@133605 by D. Schouten

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+AD4- I have heard of inverter systems that sniff for mains power, and
will not
+AD4- power a mains line that is not fed from somewhere else.  How they do
this I
+AD4- don't know.

This is called 'island operation'. Here in Europe every solar power
on-grid inverter
must be equipped with a protection circuit to avoid power delivery
into a 'dead' mains.
This circuit must shut down the inverter within a certain time period
after mains failure.
There are two ways I know of. The first one, mainly used in Germany
and not allowed in a lot of other countries, measures the line
impedance in some way. The second one is more widely used, and works
on the theory that if the main sync source is down the inverter will
sync itself. This will result in frequency runaway, and can be
detected in order to shut the inverter down when the frequency or time
base exceeds a predefined window (e.g. +AD4- +-/-2Hz).

About the theory of always leading the mains phase in order to deliver
power into the mains, I have read in some university paper once that
you don't have to worry about reactive power in systems like these. As
long as your on-grid inverter peak power is not above a certain
percentage of the single phase mains capacity, mains will take care of
reactive power. This means that your inverter can delivery mainly pure
Watts. Getting in-phase with the mains voltage and controlling the
inverter's output amplitude is all you have to do to backfeed power
into the grid.

It's been quite a while ago since I was involved in grid inverter
technology, so please excuse me if some of the above info is not
right. :)

Daniel...

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2001\03\09@162459 by Harold M Hallikainen

picon face
       For info on "Net energy metering" (where you can have your meter go
backwards) in California, see Public Utilities Code section 2827 at
leginfo.public.ca.gov/cgi-bin/waisgate?WAISdocID=70313677+0+0+0&WA
ISaction=retrieve .
       While this authorizes backwards running meters for small renewable
resource generators who are net consumers, I think it should also be
authorized for small cogeneration operations. We could heat our building
with the "waste heat" from a small natural gas powered electric generator
that feeds power back into the grid. This'd be much nicer than using the
waste heat to heat the Pacific Ocean, as most of our power plants do.

Harold


On Fri, 9 Mar 2001 19:47:47 +1100 Roman Black <fastvidspamKILLspamEZY.NET.AU>
writes:
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2001\03\10@005203 by Roman Black

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D. Schouten wrote:

> About the theory of always leading the mains phase in order to deliver
> power into the mains, I have read in some university paper once that
> you don't have to worry about reactive power in systems like these. As
> long as your on-grid inverter peak power is not above a certain
> percentage of the single phase mains capacity, mains will take care of
> reactive power. This means that your inverter can delivery mainly pure
> Watts. Getting in-phase with the mains voltage and controlling the
> inverter's output amplitude is all you have to do to backfeed power
> into the grid.


Thanks Daniel! Excellent help and just what I wanted
to hear. :o)
-Roman

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2001\03\10@005203 by Roman Black

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D. Schouten wrote:

> About the theory of always leading the mains phase in order to deliver
> power into the mains, I have read in some university paper once that
> you don't have to worry about reactive power in systems like these. As
> long as your on-grid inverter peak power is not above a certain
> percentage of the single phase mains capacity, mains will take care of
> reactive power. This means that your inverter can delivery mainly pure
> Watts. Getting in-phase with the mains voltage and controlling the
> inverter's output amplitude is all you have to do to backfeed power
> into the grid.


Thanks Daniel! Excellent help and just what I wanted
to hear. :o)
-Roman

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2001\03\10@005203 by Roman Black

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D. Schouten wrote:

> About the theory of always leading the mains phase in order to deliver
> power into the mains, I have read in some university paper once that
> you don't have to worry about reactive power in systems like these. As
> long as your on-grid inverter peak power is not above a certain
> percentage of the single phase mains capacity, mains will take care of
> reactive power. This means that your inverter can delivery mainly pure
> Watts. Getting in-phase with the mains voltage and controlling the
> inverter's output amplitude is all you have to do to backfeed power
> into the grid.


Thanks Daniel! Excellent help and just what I wanted
to hear. :o)
-Roman

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2001\03\10@050118 by Peter L. Peres

picon face
Roman, assuming you feed only limited current into the line, if the
voltage is there: At a certain point in time, the voltage is 310V (peak).
The source impedance is ~10 ohms. There are no loads in the circuit except
a 5W radio clock which is negligible. You feed 100W of power (current
limited -> P = R*I^2/2 whence I=sqrt(2*P/R) at this exact time) into the
grid. Where does that power go. What is the voltage rise it causes. What
happens to the radio clock when it sees the rise.

(I know that the calculation does not apply for this case. I was just
simplifying).

Peter

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2001\03\10@051526 by Peter L. Peres

picon face
>Peter L. Peres wrote:
>>
>> What is an 'induction motor' ?
>>
>> Peter
>
>
>Ac motor common in industry. Has 3 phase wound stator
>and squirrel cage rotor, shorted alloy bars through
>laminated rotor. Maybe you call them something different?
>They are very common in many sizes from football to car
>size!

We call them squirrell cage motors or shorted rotor motors. ;-)

And that explains Lawrence's experience. A squirrell cage motor builds a
rotary field in the rotor area using incoming AC. If the rotor is turned
by an external motor then the rotor will act as a vector multiplier that
adds energy to the incoming ac. It does this by leading the rotor phase
wrt. the rotary field (the phase lags when working as a motor). This
elegantly solves the problem of the trigonometric calculation required to
find when and how much current to put back into the mains... but the power
co. will likely do untold things to you if they catch you do this. Also,
if the network is not loaded it will try to feed back power into the
generators eventually. If it is large enough and tries hard enough it may
cause a major power outage in your area.

Larger squirrell cage motors have a starting device that actually allows
the short in the rotor to be removed. This could be used to take such a
contraption online and offline when used as a generator.

Peter

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2001\03\10@080903 by Roman Black

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face
Peter L. Peres wrote:
>
> Roman, assuming you feed only limited current into the line, if the
> voltage is there: At a certain point in time, the voltage is 310V (peak).
> The source impedance is ~10 ohms. There are no loads in the circuit except
> a 5W radio clock which is negligible. You feed 100W of power (current
> limited -> P = R*I^2/2 whence I=sqrt(2*P/R) at this exact time) into the
> grid. Where does that power go. What is the voltage rise it causes. What
> happens to the radio clock when it sees the rise.
>
> (I know that the calculation does not apply for this case. I was just
> simplifying).


In this case the power is fed back into the AC mains
supply, possibly making the meter run backwards.
Since the AC mains has a very low impedance the voltage
won't rise significantly, instead current is fed into
the +310v line. :o)

Think of the AC mains more like a very big battery,
you can draw current out or feed it back in.
-Roman

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2001\03\10@132008 by Sean H. Breheny

face picon face
I haven't monitored this whole thread, but this puzzles me: why would you
want to feed power into the mains without the power company's permission?!
Isn't the whole idea so that you can be paid for it?

Sean

At 12:08 PM 3/10/01 +0200, you wrote:
>find when and how much current to put back into the mains... but the power
>co. will likely do untold things to you if they catch you do this. Also,
>if the network is not loaded it will try to feed back power into the
>generators eventually. If it is large enough and tries hard enough it may
>cause a major power outage in your area.

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2001\03\10@133247 by Dale Botkin

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Yes.  In most cases it depends on hwo you look at it...  if I generate 1KW
and I'm using 2KW, that's 1KW I don't have to pay for.  If I generate 3KW
I can make the meter run backward and sell the excess to the utility --
assuming the regulations where I live permit that (mine don't).  In my
case, I'd just be interested in slowing down the meter, not reversing it.
Generating enough solar or wind power here to completely meet my power
needs would require far more money than I could afford.  I do hope to
start some sort of solar or wind collection soon, though, even if only for
the geek appeal.

Dale

On Sat, 10 Mar 2001, Sean H. Breheny wrote:

{Quote hidden}

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2001\03\11@010345 by Roman Black

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Sean H. Breheny wrote:
>
> I haven't monitored this whole thread, but this puzzles me: why would you
> want to feed power into the mains without the power company's permission?!
> Isn't the whole idea so that you can be paid for it?
>


Hi Sean, it is more for the cases where you generate a
small amount of power, less than you use. In that situation
there are two options;

1. store it into batteries, then convert it to AC for
use later on, this requires expensive batteries and
has more losses as it goes through many more processes.

2. turn it straight into AC, and feed it back into your mains,
because it is less than your total usage it comes straight
off your bill, much less equipment needed and in effect
you are getting paid top dollar consumer price for your
own power.

I never suggested going against the power company's rules,
but in a LOT of areas buy-back and net-metering systems
are being allowed and this is becoming more common, ie,
this is a real growth industry which needs new
manufacturers.:o)
-Roman

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2001\03\11@022450 by Jess Hancock

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Roman, did you see my earlier post?  I was having email problems.

Hi guys - I'm shooting in the dark here.  All my PICList emails have been
blank (no text) since 10:30am yesterday.  Have had to go to ARCHIVES to read
them.

I'll try to comment on several responses:

Peter Peres:  induction motors are the vast majority of household single
phase ac (non-brush type) motors.  Also very popular in industry where
3phase is used.  Briefly, induction motors (1 phase) have two windings,
having different resistances or may use a capacitor in series with one
winding to provide current phase shift between the two windings to create a
rotating magnetic field which causes motor to start/run .

Lawrence Lile: induction motors as generators require excitation current
which is a lagging current and therefore requires a source of vars.  Placing
a capacitor across the motor leads will provide those vars but the voltage
may "run away", become excessive, and damage insulation if too much
capacitance is used and the motor is operated off mains.  Check into power
factor correction of induction motors and cautions for additional info.

Roman (your response to Mark Skeels):  You didn't specify shape of injected
pos/neg current to be injected during pos/neg half cycles.  I will assume
rectangular shape for sake of discussion, but general concepts should be
similar regardless of exact shape.

I hope the following makes a little sense at least:  At any instant during a
cycle, to inject a current INTO the ac mains, the voltage magnitude of the
current source must exceed the mains voltage.  Going back to my two
generator post earlier, this is what the phase advance of one of the
generators accomplishes.

The two (pos/neg) rectangular  (or other) pulse
waveform can be analyzed by Fourier Series and can be shown to have
fundamental (50 or 60Hz  voltage and current) components plus harmonic
components.  It is only the fundamental components which will provide power
(watts) to the ac mains system.  It is the phase angle of the fundamental
voltage component relative to the phase angle of the mains ac voltage
component which determines if power is being transferred to the ac mains or
received from it.  Without benefit of a Fourier series analysis, it would
seem that the "equivalent phase angle" of the  rectangular injected current
would need to lead the mains ac voltage in order to inject power (watts) to
the ac mains system.

Does this imply that the center of the rectangular
current pulse, timewise,  should be to the right of the 90/270 degree peaks
of the mains voltage waveform, that is, the ( emphasis - not shouting -
CENTER OF THE RECTANGULAR CURRENT PULSE SHOULD BE AT AN ANGLE GREATER THAN
90/270 degrees?  I think maybe so.

I'm not sure how zero crossings enter into this - obviously they are a
timing reference.  If harmonic currents/voltages are present and of great
magnitudes, the zero crossing timing may be affected.

Jess

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2001\03\11@143417 by hard Prosser

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Roman,
I'm quite keen on this idea myself. Hard to justify economically
@$US350/100W & $NZ0.12/unit but even so....

I've had a quick look at possible methods of doing this & the hardest part
seems to be actually feeding the power back into the mains.
Given that large mains-frequency transformers are undesirable I can't think
of any configuration that will let you feed power back into the mains over
both half-cycles except for an H-bridge or at least a 1/2 bridge. Thes will
require high voltage devices and a set of high side drivers. On the plus
side they may not need to operate too fast - even 50Hz operation may be OK
if you drive from a pulsed source.
Possibly SCRs may be the cheapest option - I might look into this idea more
closely!

I think the comments re. phase advance etc. really only apply if you are
backfeeding at 50Hz. If you are pulseing at 30kHz or so it only seems
necessary to provide a peak voltage in excess of the mains voltage to
source current. The use of a constant current or constant energy (e.g.
flyback) source would seem to be a good way of limiting output current
requirements - given that the mains will look pretty close to a  perfect
constant voltage supply. Some sort  of output filtering may be required
though.

Since the output polarity will have to reverse every 20mS or so it should
be easy to provide detection for this and provide a safety cut-out in the
event of mains failure.

I'd be only too happy to discuss this one further - on or off list.

Richard P






Jess Hancock wrote:
>
> Roman, What you want to do is similar to operating two ac generators
> (alternators) in parallel to supply a load.  If you want alternator #2 to
> supply more power (watts), then its phase angle must lead that of
alternator
> #1.  If you raise the voltage of alternator #2 you will supply more
reactive
> power (vars, not watts).   By controlling the phase angle and voltage of
> alternator #2, you can supply a controlled combination of watts and vars
to
> the load and adjust load sharing between the two alternators.
>
> Doing this with a PWM system is beyond me, but I assume you will have to
> advance the effective phase angle of the equivalent PWM "sinusoid"
relative
> to that of the mains ac voltage to obtain power (watts) transfer to the
ac
> system.
>
> Jess


Hi Jess, a good point re phase angle. I was thinking
that instead of generating an AC waveform of the
right mains voltage, actually injecting a measured
current into the existing AC waveform.

This gets round all the problems of phase angle,
synchronisation and even safety issues. Obviously it
will detect zero crossing and only feed current back
at the right part of the wave and only if the wave
is there.

-Roman

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2001\03\11@151835 by D. Schouten

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face
> I've had a quick look at possible methods of doing this & the
hardest part
> seems to be actually feeding the power back into the mains.
> Given that large mains-frequency transformers are undesirable I
can't think
> of any configuration that will let you feed power back into the
mains over
> both half-cycles except for an H-bridge or at least a 1/2 bridge.

I can't tell for the US, but in most European countries on-grid solar
inverters must contain an isolation border between the PV array and
the actual grid. However some standards do allow non-isolated
inverters, but with the restriction that no unauthorised persons can
come near the PV array.

Besides this there is an other issue which needs to be investigated.
IRC most 100W PV panels (e.g. Shell or BP solar panels) have a noload
output voltage of approx. 45VDC. The maximum power point voltage (=
the voltage where the PV panel mostly operates with, due to PWM
controlled MPPT) is about 31VDC. So when your PV array contains for
example only three panels, the nominal operating DC input voltage is
93VDC. This voltage level is of course too low for transferring energy
into a 120VAC grid, let alone a 230VAC grid like mine.

So my point is that you do need some kind of boost circuit to enlarge
the DC PV voltage in order to give you some control range in your HV
grid connected H-bridge. You could do this with some kind of high
frequency push-pull smps. This will also solve the isolation problem
as stated above. An other option is to use a mains frequency
transformer which will do the stepup as well as the isolation border.
Most people under estimate line frequency transformers by judging
these on the higher weight and 'low-tech' appearance. However, the
only reason why these type of transformers are slowely dissapearing is
due to higher costs and weight/volume. But in fact, still the most
efficient PV inverters are build using a line frequency transformer.
The higher efficiencies are accomplished by using only a single power
stage and by overdimensioning the LF transformer (must be toroidal!).
The full HF solutions are usually two or three step converters wasting
too much valuable power, even using the most up to date technologies.

Daniel...

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2001\03\11@153528 by hard Prosser

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face
Yes - I was thinking that the isolation would occur at the flyback
transformer (e.g a similar size to a 200W computer power supply
transformer). Pulse steering would be accomplished by using optos or pulse
transformers to switch the FETs/Bipolars/SCRs in order to retain isolation.

As far as overall efficiency is concened, we manufacture 2 stage recifiers
(230/110V to 470VDC  to "48V" DC) with peak efficiency of around 93% at >
2kW power levels. The output switching frequency is in the 400kHz region
This is the sort of efficiency figure I was contemplating. By using highish
frequencies and cunning circuits so called "soft switching" can be utilised
- at the expense of dearer switches.

Richard P




> I've had a quick look at possible methods of doing this & the
hardest part
> seems to be actually feeding the power back into the mains.
> Given that large mains-frequency transformers are undesirable I
can't think
> of any configuration that will let you feed power back into the
mains over
> both half-cycles except for an H-bridge or at least a 1/2 bridge.

I can't tell for the US, but in most European countries on-grid solar
inverters must contain an isolation border between the PV array and
the actual grid. However some standards do allow non-isolated
inverters, but with the restriction that no unauthorised persons can
come near the PV array.

Besides this there is an other issue which needs to be investigated.
IRC most 100W PV panels (e.g. Shell or BP solar panels) have a noload
output voltage of approx. 45VDC. The maximum power point voltage (=
the voltage where the PV panel mostly operates with, due to PWM
controlled MPPT) is about 31VDC. So when your PV array contains for
example only three panels, the nominal operating DC input voltage is
93VDC. This voltage level is of course too low for transferring energy
into a 120VAC grid, let alone a 230VAC grid like mine.

So my point is that you do need some kind of boost circuit to enlarge
the DC PV voltage in order to give you some control range in your HV
grid connected H-bridge. You could do this with some kind of high
frequency push-pull smps. This will also solve the isolation problem
as stated above. An other option is to use a mains frequency
transformer which will do the stepup as well as the isolation border.
Most people under estimate line frequency transformers by judging
these on the higher weight and 'low-tech' appearance. However, the
only reason why these type of transformers are slowely dissapearing is
due to higher costs and weight/volume. But in fact, still the most
efficient PV inverters are build using a line frequency transformer.
The higher efficiencies are accomplished by using only a single power
stage and by overdimensioning the LF transformer (must be toroidal!).
The full HF solutions are usually two or three step converters wasting
too much valuable power, even using the most up to date technologies.

Daniel...

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2001\03\11@202848 by John La Rooy

flavicon
face
"D. Schouten" wrote:

>
> This is called 'island operation'. Here in Europe every solar power
> on-grid inverter
> must be equipped with a protection circuit to avoid power delivery
> into a 'dead' mains.

Won't all these systems of deadness detectino become less reliable as
the percentage of domestically generated power increases. What if there
is a fault and the circuit delivers power anyway. Surely line workers
aren't going to bet their lives on those circuits.

John

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2001\03\11@224427 by Robert Rolf

picon face
Which is why shorting bars are used in large installations where
a line may become energized from more than one source.
My high school physics teacher had a great story about an exploding
substation transformer caused because someone forgot to remove the
short.

John La Rooy wrote:
{Quote hidden}

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2001\03\12@031659 by spam

flavicon
face
Hi -

The technology of small-scale putting power back on the net is well
known. In Denmark, 10% of the energy is produced by windmills.
They are simply phased to the power. Due to the magneto-electic
coupling between the windmill generator and the coal/gas fueled
ones the net result is that the windmill helps drive the generators,
saving fuel in that end (in 3'rd grade terminology).

We have about reached the limit of what we can do with this type
of windmills now, since they will actually be able to raise the net
frequency and at the same time lose their regulation. (They run at a
fixed speed due to the net frequency).


The usual trick is to first regulate the effective voltage up to match
the net voltage (this stage can be rather coarse). Then you
brake/regulate the generator until there is no significant voltage
between the net and your generator (you are in phase).


In the old days, this was done with a three bulbs (3-phase)
connected between the phases of the generator and the phases of
the net. You would regulate/brake until the lights were all out (they
would blink at the difference frequency, it is very easy to phase).

Then then you flip the switch. The net will then keep you in sync
with the big guys.


We have an old 10MW plant from 1912, that is still working

http://www.elmus.dk

it is running off 8 meters water level.


They have occationally just flipped the switch out of phase and it
goes BLOP! out in the turbine inlets. That is all that happens. The
turbines of course goes fast forwards or backwards to match the

phase.


Three issues:

1) Would your electricity meter actually care, whether you put
power backwards through it ? I think it ignores the sign, and you
need a reversible one.

2) The power plant has certain standards for noise etc. that have to
be met. You have to produce a lot of power to pay for the licence,
most likely.

3) It might give the electricians working on the net a neato scare
when they find that whatever is connected to your little gingerbread
house is live when they disconnected it.


Kent


<color><param>7F00,0000,0000</param>> Yes.  In most cases it depends on hwo you look at it...  if I generate 1KW

{Quote hidden}

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2001\03\12@092454 by Roman Black

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face
Richard Prosser wrote:
>
> Yes - I was thinking that the isolation would occur at the flyback
> transformer (e.g a similar size to a 200W computer power supply
> transformer). Pulse steering would be accomplished by using optos or pulse
> transformers to switch the FETs/Bipolars/SCRs in order to retain isolation.
>
> As far as overall efficiency is concened, we manufacture 2 stage recifiers
> (230/110V to 470VDC  to "48V" DC) with peak efficiency of around 93% at >
> 2kW power levels. The output switching frequency is in the 400kHz region
> This is the sort of efficiency figure I was contemplating. By using highish
> frequencies and cunning circuits so called "soft switching" can be utilised
> - at the expense of dearer switches.
>
> Richard P


Hi Richard, I too think this is an intriguing new
market. There is no way I would produce a device like
this without total mains isolation, but that's not a
huge hurdle. :o)

There seems to be two basic ways of going about it:

1. 50Hz toroidal mains transformer. Then just use
PWM to feed current into the secondary which must
be fed back into the mains. Efficiencies >95% for the
transformer, total isolation and very easy to detect
mains phase and absence. Bad point is weight and cost.
But many good points.

2. Hi freq dc step-up variable supplies, generate a high
DC voltage that can be fed into the mains direct as
DC current every +/- phase. Can be much smaller and
lighter (cheaper??) but will require a high speed high
volts switching diode for each +/- step-up circuit,
and also a high volts transistor or fet to isolate
each DC supply when the other phase polarity is active.
It also requires some communicaton from mains back to
low volts circuits, for mains detection and for switching
the isolator semis. This is a lot more high volts semis
to go bad, but would be cheaper in larger quantities
like "real" commercial production.

For smaller production and testing I am leaning back
towards the toroid transformer option 1. Safety is great,
even in the event that one main power semi might short,
there will be no catastrophic load cuurents and it is easy
to fuse. It might take some fine tuning to determine
best phase angles to feel power in as the toroid will
have different effects under different loads I imagine.
-Roman

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2001\03\12@121231 by Harold M Hallikainen

picon face
On Mon, 12 Mar 2001 09:44:23 +0100 Kent Johansen <EraseMEspamspamANAKRON.DK>
writes:

Hi -

The technology of small-scale putting power back on the net is well
known. In Denmark, 10% of the energy is produced by windmills.
They are simply phased to the power. Due to the magneto-electic
coupling between the windmill generator and the coal/gas fueled
ones the net result is that the windmill helps drive the generators,
saving fuel in that end (in 3'rd grade terminology).

We have about reached the limit of what we can do with this type
of windmills now, since they will actually be able to raise the net
frequency and at the same time lose their regulation. (They run at a
fixed speed due to the net frequency).


-----------------------HH
Response------------------------------------------
       Seems to me that the network frequency can always be brought down by
backing down of the "big" plants (hydro, gas fired, etc.). Is this not
the case?

-------------------------------------------------------------------------
------------


The usual trick is to first regulate the effective voltage up to match
the net voltage (this stage can be rather coarse). Then you
brake/regulate the generator until there is no significant voltage
between the net and your generator (you are in phase).

-------------------------------HH
Response------------------------------------
       If one were to use an induction motor as a generator (as has been
discussed here), it seems that we only need to watch the speed of the
windmill. If it's below sync frequency, we take it off line (as it is now
a big fan). If it's equal to or greater than sync frequency, we put it on
line. If it's right at sync frequency, it will ideally draw zero current.
As it tries to go above sync frequency, it will pull the line along with
it, putting power into the line.
       It seems like the neat thing about the induction generator is that it
automatically syncs itself to the incoming AC and shuts itself down
should the incoming AC disappear.
       I wonder if some mechanical maximum power tracking system could be put
on the blades of the windmill. Maybe have springs that pull the trailing
edge of the blade to perpindicular to the wind. As the wind increases,
this edge of the blade gets pushed back, changing the pitch of the blade.

-------------------------------------------------------------------------
-------------------


In the old days, this was done with a three bulbs (3-phase)
connected between the phases of the generator and the phases of
the net. You would regulate/brake until the lights were all out (they
would blink at the difference frequency, it is very easy to phase).

Then then you flip the switch. The net will then keep you in sync
with the big guys.

--------------------------------------------HH
Response------------------------------

       I remember seeing this in an old movie... Pretty neat!

-------------------------------------------------------------------------
-----------------------

We have an old 10MW plant from 1912, that is still working

http://www.elmus.dk

it is running off 8 meters water level.


They have occationally just flipped the switch out of phase and it
goes BLOP! out in the turbine inlets. That is all that happens. The
turbines of course goes fast forwards or backwards to match the

phase.


Three issues:

1) Would your electricity meter actually care, whether you put
power backwards through it ? I think it ignores the sign, and you
need a reversible one.


------------------------------------HH
Response-----------------------------------------------
       Since these are watt-hour meters and not volt-amp-hour meters, I believe
they do an instantaneous multiply of the voltage and current to get the
power in watts. If the current is flowing "the other way", the meter goes
the other way.
       This small scale cogeneration is regulated differently in different
areas. Here in California, it is called "net energy billing" and is
encouraged by the Public Utilities Code.

Harold

-------------------------------------------------------------------------
---------------------------------




2) The power plant has certain standards for noise etc. that have to
be met. You have to produce a lot of power to pay for the licence,
most likely.

3) It might give the electricians working on the net a neato scare
when they find that whatever is connected to your little gingerbread
house is live when they disconnected it.


Kent




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2001\03\12@175232 by Chris Carr

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>
> We have an old 10MW plant from 1912, that is still working
>
> http://www.elmus.dk
>
> it is running off 8 meters water level.
>
I didn't know you had a hill that high in Denmark   :-)

I am going to have serious words with my friends in Denmark, they have
not told me about this museum and I must have driven past it more than a few
times.

B&O produced Electric Shavers, that I have got to see. Is the design as good
and reliable as their Audio and Video Equipment ?

Regards
Chris

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