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LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408
Posted: 03:00am 31 Jan 2019
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3 stages to grid power independence: Warpspeed version
Warpspeed has shared his 3 stages to grid power independence here: https://www.thebackshed.com/forum/forum_posts.asp?TID=9409&PN=1&TPN=11
It took me a while to understand and I took my time while running a few thought experiments.
It makes absolute sense for any person planning grid power independence to trace through those exact stages.
To paraphrase Warpspeed:
Stage One is to use solar to supplement rectified dc grid power to drive an inverter (without a battery). Unlike a conventional grid tie system, it feeds nothing back.
Stage two uses a high voltage battery (100v nominal) to further reduce grid dependence, if not achieve grid independence. At this stage, two different systems are available, with the new system active and the old system as reserve, so that when something blows up or requires maintenance we can do it without going back to 100% grid power.
Stage three is to optimize above system 2, get very high system efficiency: first use a higher voltage battery reducing conduction losses, and build an inverter that does not require the use of a large toroidal output transformer to minimize standby consumption.
This thread is only focused on this foreword.
We discuss Stage One here: https://www.thebackshed.com/forum/forum_posts.asp?TID=11086&PN=1&TPN=1
LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408
Posted: 03:04am 31 Jan 2019
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Placeholder now that I have edit privileges to make into wiki
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 04:33am 31 Jan 2019
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The power distribution system in the US is fairly unique, in that it uses two phases of 110 volts with reference to a grounded neutral.
What follows is how I might attempt to do something like this myself if I was located in America. The mains power distribution system in Australia is totally different so it would have no application here, or in Europe, or most other countries.
First step would be to try and work out what might be some suitable dc working voltages. If we run a diode direct from each incoming 110v supply, we can full wave rectify that and get a pretty strong ground referenced dc source of grid power for use at night, or really dismal days.
Under very light loading the dc grid supplied voltage will reach a x1.414 peak of around +155v or perhaps slightly more sometimes. If we add a series dc choke, the dc voltage might fall to the x0.637 average value or around +70v as an absolute minimum.
These voltages are all rather high, but it avoids having to use a lossy transformer for our grid source of power, and having no grid transformer would significantly cut down the cost of parts.
We would need enough solar panels connected in series so that during the day the solar panel voltage would never fall below something slightly above +155v so that the system will not revert back to grid power during the day unless the inverter load greatly exceeds available solar panel supply. That can happen on a truly awful day, but not very often.
A typical 24v panel might have a max power rated output at about +30v and a full sun no load voltage of perhaps +38v
If we connected six of these 24v panels in series, we would end up with max power voltage of +180v and full sun open circuit voltage of possibly +228v So the available dc supply input voltage to the inverter may look something like this:
+228v absolute maximum, bright sunny day zero inverter load. +180v max power point for solar panel output. +155v system begins to revert to to grid power, but with very light loading on the grid. +100v (estimated) very high inverter load, drawn totally from from the grid.
We would need to design an inverter that would operate from less than +100v to something safely above +228v, an unusually wide input voltage range ! And as the inverter load changes, the dc input voltage will be jumping around all over the place within the above range.
A very large energy storage electrolytic will be needed across the dc input to the inverter, and as energy stored will be proportional to voltage squared, a high dc voltage is a definite advantage.
This high voltage operation has many other efficiency advantages, conduction losses in series diodes and IGBTs would be absolutely minimal. But the voltages are pretty dangerous, and I feel uneasy about recommending something like this on an open Forum.
An alternative would be to use a transformer in the grid rectifier to reduce the dc voltage there, and use fewer series connected solar panels to achieve a similar effect, but all at much reduced voltage.Cheers, Tony.
yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166
Posted: 07:46am 31 Jan 2019
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I dont know where you draw the line Tony.
A speed controller for a brushed motor on an EV is pretty much identical in voltages, components and configuration. The narrow gap between the bus bars attached to the large capacitors is sobering.
Within 20 meters off where I am sitting right now is a microwave, old radios with live chassis's and vacuum tubes, a 480 volt split phase system, grid solar @ 600 volt potential. I have worked on all of them but technically I'm only qualified for high volt DC.
Maybe we could use the quote box to point out specific risks and methods to eliminate them and safe practices.
I'm confused, no wait... maybe I'm not...
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 06:22pm 31 Jan 2019
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I would seriously consider doing it this way myself if I was in the US without any hesitation. But I would also be pretty unhappy if someone fried themselves trying out these ideas.
Anyhow, some further thoughts.
A two pole ac circuit breaker would be fine, even though its carrying half wave rectified dc in each supply leg. It cannot continuously arc as it might with pure dc, because each alternate half cycle the rectifier diode shuts off the current in that leg.
There would need to be a soft start with a push button and maybe a 30 ohm high wattage resistor across one side of the circuit breaker to initially charge up the electrolytic filter capacitance. A series iron cord choke of a few mH would probably be advisable to limit the extreme current peaks into the filter capacitor. That would also provide some additional ripple filtering under heavy load.
I would test that with some resistive loading at the maximum expected inverter load. It may require an electrolytic of about 5uF per Kw, or perhaps a bit more. See how that goes. Its likely that the dc output voltage may not drop too much below the expected +155v maximum. That would set the minimum operating voltage requirement for the inverter and would be an important first step in developing this system.Cheers, Tony.
LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408
Posted: 09:49pm 31 Jan 2019
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Can we assume we have a microinverter before this inverter?
Those microinverters are between the panels and this inverter.
Those microinverters carry out MPPT based DC boost and ensure they all output 100V DC within a tolerance (instead of varying between 155v to 228v)
Would introducing these microinverters simplify or further complicate the design of this inverter you are proposing?
This inverter would have to talk to these microinverters to figure out how much power it can source from them, and from there it can figure out how much it needs to draw from the grid to supplement the output load needs.
I don't understand. PV open circuit voltage means nothing in real life from my limited tests.
When I try and draw even a few watts from the PV panel, it drops to 80% of PV open circuit voltage.
80% of 228v is 182V.
Unless our inverter extremely lightly loaded the PV output, the inverter would never see that 228v.
What am I missing?
OK, so at this point grid power and solar sourced power are working in tandem?
solar sourced power is being consumed at a 100% and the slack being provided by grid power (with very light loading on the grid)?
Do I understand this absolutely correctly?
Then I want to alleviate the danger concerns right there.
Let us now start building a module to do what we want.
To start with, we build a version of the module that works at lower voltages, say 30VDC. Yes, the efficiency of the lower voltage module will not be optimal but the math will show us that as we scale the voltage, the conduction losses will fall and the efficiency will increase?
In that case, why don't we build a version of the module that works at lower voltages, say 30VDC at the source?
It doesn't have to be 30VDC. It can be 50VDC, 60VDC.
We work out the kinks in the system. I gain a better understanding of the system as do others and then we scale it up.
what do you all think?Edited by LadyN 2019-02-02
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 11:10pm 31 Jan 2019
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I suppose it depends on the power level you are eventually aiming for.
Low voltage high current is going to be physically enormous, very expensive, and run very inefficiently. There are very practical reasons why every grid tie system on the market use dc solar voltages of 200v to 600v
The problem is surge loads. The rating plate on my refrigerator says 127 watts running power, and 350 watts during automatic defrost. The difficulty is that when the compressor starts up the first half cycle is around 17 amps peak at 235 volts.
The poor inverter gets kicked in the guts by a 4Kw surge, assuming there are no other loads running at the same time.
Now a 30v system of reasonable efficiency is going to instantly demand 150 to 160 amps from the rectifier at night, or the fridge is not going to be able to start up.
A higher voltage system such as 150 volts only has to source maybe 25 to 30 amps, and a huge bunch of electrolytics can definitely help with that.
Energy stored in a capacitor is proportional to voltage squared. A 150 volt system has five times the voltage, and 25 times the energy storage of a 30v system. So you need x25 times the capacitance, and large electrolytics are not low cost items.
During the day, the fridge can draw peak surges from the grid to help startup, unless the grid goes down. Solar can escape the problem to a certain extent, but your grid rectifier needs to have sufficient grunt to cover surge requirements day or night.
That is why I suggest running a rectifier straight off the 110v grid supply. Its easy, low cost and has (almost) infinite short term current sourcing ability. Anything else is going to be problematic.
My own grid rectifier is around 106v dc coming from a 4.5Kva three phase transformer. Its big, and it needs to be big to start up my not so very large refrigerator at night without too much voltage sag.
Edited by Warpspeed 2019-02-02Cheers, Tony.
LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408
Posted: 11:55pm 31 Jan 2019
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No, these will be Low voltage and low current. They only exist to learn how to design these systems.
Let's call them LEARN_MODULEs.
LEARN_MODULEs help me/us learn the basics of the system so that I am ready to scale them up to higher voltage.
The LEARN_MODULEs are maybe 50W output, say 50V out, 1A, but we design it along similar lines as a 120V out, 30A final module
The difference being that the LEARN_MODULEs are inefficient and one would never use them at those power levels.
However, the 120V out, 30A final module would be pretty efficient.
Would this idea work?
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 01:37am 01 Feb 2019
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Almost anything can be made to work, but how well it works is a very different thing.
If this is just a breadboard, student learning project king of thing fine.
If you are hoping to power your home from this, it needs to be practical and affordable.Cheers, Tony.
LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408
Posted: 07:14pm 01 Feb 2019
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There are no good load sharing designs on the internet that I can see. You have done this yourself already and with your help we can have a good load sharing design.
Starting with a low power design and documenting it here will allow anyone interested but uneducated in power electronics (like myself) to come up to speed without injuring themselves.
They can then take their learning and advance to a higher power design.
I have to learn how to walk first before I can run and I/we can do that with you (and the other experts on this forum).
Created a post here: https://www.thebackshed.com/forum/forum_posts.asp?TID=11076&TPN=2
I am really excited!
BenandAmber Guru Joined: 16/02/2019 Location: United StatesPosts: 961
Posted: 06:01pm 19 Feb 2019
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It is so awesome that we have people in the world that has the know-how to come up with these groundbreaking ideas people like this lead us into the future very glad to be a part of this formbe warned i am good parrot but Dumber than a box of rocks
LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408
Posted: 07:06pm 19 Feb 2019
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Absolutely.
Warpspeed has shared his 3 stages to grid power independence here: https://www.thebackshed.com/forum/forum_posts.asp?TID=9409&PN=1&TPN=11
It took me a while to understand and I sure took my time while running a few thought experiments.
His approach makes absolute sense for any person planning grid power independence to trace through those exact stages.
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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