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[aprssig] backup pwr systems

Alex Carver agcme2002 at yahoo.com
Tue Dec 12 21:04:16 UTC 2006


> Message: 6
> Date: Mon, 11 Dec 2006 23:20:11 -0500 (EST)
> From: "Bob Bruninga " <bruninga at usna.edu>
> Subject: RE: [aprssig] backup pwr systems
> To: TAPR APRS Mailing List <aprssig at lists.tapr.org>
> Message-ID:
> <20061211232011.AFI84791 at mp2.nettest.usna.edu>
> Content-Type: text/plain; charset=us-ascii
> 
> > Best to just stick with a standard UPS for each
> item
> > (or group of items) you want to keep powered or go
> > with a grid-tie inverter system.  Having something
> that
> > plugs into the wall as you describe is a sure-fire
> way to
> > electrocute a line worker when the power goes out
> > (because you don't have an anti-islanding relay
> > installed as would normally come with a grid-tie
> inverter).
> 
> You didnt read my original post.  I proposed a
> synchronous inverter that can only generate power
> when there is already 60 Hz power there.  None when
> there is not.  I agree, safety is paramount.  Bob
> 
> 
> 
> ------------------------------
> 
> Message: 7
> Date: Mon, 11 Dec 2006 23:33:21 -0500 (EST)
> From: "Bob Bruninga " <bruninga at usna.edu>
> Subject: RE: [aprssig] backup pwr systems
> To: TAPR APRS Mailing List <aprssig at lists.tapr.org>
> Message-ID:
> <20061211233321.AFI85040 at mp2.nettest.usna.edu>
> Content-Type: text/plain; charset=us-ascii
> 
> >> My thought is just a sensing circuit that only
> fires 
> >> the inverter MOSFET's on the rising side of the 
> >> 60 cycle waveform.  And then fires them for 50ms 
> >> or so on each cycle. If there is no waveform,
> then 
> >> the inverter transistors don't fire.  Should 
> >> be very simple. 
> >
> > You would have the power company beating down your
> > door that way.  Switching the MOSFETs on only a 
> > half cycle would introduce even more harmonics 
> > into the output power and get fed back into the
> grid.  
> 
> No, that's what every light dimmer does,  And if it
> is running a 400 watt lamp load like a dining room
> light then it is the same harmonics.  A simple RFI
> filter can smooth it up so even a HAM can run on the
> same power.
> 
> THink about it.  The load of the "grid" is something
> like maybe 0.01 ohm or much less.  There is little
> that a single mosfet (or a single light dimmer) can
> do that would cause any discernable gliches on the
> waveform.  All the mosfet would do would be to
> contribute a flea power of current, and it would
> never leave my house, since with only 400W going in,
> and my houose drawing 1 kW, then nothing leaves my
> system.  I just reduce my current is all.
> 
> > Best to just stick with a standard UPS for each 
> > item (or group of items) ... or go with a 
> > grid-tie inverter system.
> 
> Yes, that was the orginal thread.  I'm looking for a
> synchronous inverter (call it a grid tie system) in
> the 400 watt class that should be able to be built
> for under $100 in electronics.  The cheapest
> grid-tie systems are in the thousands of dollars. 
> Hence by post.
> 
> >  Having something that plugs into the wall as you 
> > describe is a sure-fire way to electrocute a line 
> > worker when the power goes out (because you don't 
> > have an anti-islanding relay installed as would 
> > normally come with a grid-tie inverter). 
> 
> No, you didnt read my post correctly.  I said a
> synchronous inverter which means it cannot generate
> power on its own with out the presence of the 60 Hz
> grid in the first place.
> It is inherently safe by design.
> 
> Bob

I did read the original post.  A lamp dimmer does not
inject power back into the grid.  It controls power to
the load.  The harmonics from a lamp dimmer are
horrendous but they show up at the lamp, not the
meter.  It doesn't matter which way power is flowing. 
Harmonics will flow as well.  Your incoming power is
free of harmonics but your inverter (if poorly
designed) would be generating harmonics which then DO
flow out of the house (verified this with an actual
power engineer, this is a constant problem with
industrial loads).  The harmonics are also attenuated
because the dimmer does not generate any energy
through an outside source, it just takes energy and
modifies it.  The power grid is a low frequency RF
transmission line system as much as it is a power
system.  The DC or AC resistive load of the system has
nothing to do with the RF harmonics generated, just
the RF impedance of the system.  Think about it that
way because the power engineers do (they are
constantly worried about the impedance of the grid,
not the DC or AC resistive load).  Harmonics generated
by an amplifier in one of our radios will show up
everywhere down stream.  Same thing happens on the
grid if a poor quality inverter is tied directly to
it.  An inverter generates energy from the battery to
the output terminals.  At 400 watts, your harmonics
are not exactly low energy any longer.

Grid-tie inverters are expensive because they use
inverter designs that produce very minimal harmonics
that would get injected into the grid.  The K-Mart
blue light special inverter that most of us can afford
easily for our computers is called a Modified Sinewave
inverter.  It's made of a stack of two or three square
waves and has tons of harmonics.  But, the harmonics
are on the OUTPUT of the inverter which is only
plugged into the load.  But, for your design, the grid
is the load therefore the harmonics wind up in the
grid.

Your "synchronous inverter" is exactly a grid-tie
inverter.  We're not calling it a grid-tie just for
fun, that's actually the name for it.  It synchronizes
itself constantly with the grid to ensure that there
are no phase imbalances.  If you have a phase
imbalance, you run the risk of floating the neutral or
ground wires which would also be a major safety
concern.  Turning this into a plug-in device instead
of having a qualified person install it will do more
harm than good.  That's why they're designed to be
installed instead of plugged in.

Your $100 inverter solution really boils down to a
standard UPS with all the devices wired into it with a
handful of extension cords.  You won't get what you
want (a plug-in grid-tie) for cheap and not cause
problems somewhere.  You can't have this one both
ways.  It's either cheap and you'll have to rewire
stuff, or it's expensive but done properly such that
you don't disturb the grid.

...Or you just drop off the grid completely.  A bit
extreme but that's your third option.

KF4LVZ


 
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