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Grid tie inverter

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(@sid-genetry-solar)
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Posted by: @dickson
Your 48v GS 6kw with the rev C2 control board will survive a backfeed to the grid ? Then if I use FETs rated for 600A and 2 fast blow glass fuse instead of breaker to the output of my 15kw Powerjack it will also survive a backfeed to the grid ? I have not try to backfeed to the output of my inverter but Sean in the youtube say he did it by mistake and the GS inverter survive .

No, no, no, no, don't confuse three completely separate scenarios.......

a) @nilaois wanting to "backfeed" a 12kw grid-tied solar inverter to the GS inverter...which with all of the unknowns, is likely not a good idea especially on a 12v GS.

b) Sean accidentally SHORTED the output of the 12kw GS and it self-protected.  That was not backfeeding it from grid power.  If there's another incident where he did accidentally backfeed the output without smoke, that means the inverter happened to somehow be in sync with the grid--otherwise instant destruction would result.  (If you put limitless power up against limited FETs, the FETs are going to lose.)

c) Direct connecting the grid to the output of ANY off-grid (i.e. not grid-tied) inverter is simply asking for trouble.  Try multiplying the strain of a short circuit by 15x for starters--it's pretty well guaranteed to blow something out.


   
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(@dickson)
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That was not backfeeding it from grid power.  If there's another incident where he did accidentally backfeed the output without smoke, that means the inverter happened to somehow be in sync with the grid--otherwise instant destruction would result.  (If you put limitless power up against limited FETs, the FETs are going to lose.)

Then I  better NOT  try  to backfeed the  output of my Powerjack to the grid  or  short circuit the output .   I  thought  you was able to design a control board that  will  detect  a  backfeed  and  shutdown the inverter to save the  FETs .   Designing a  control board  that  can save the  FETs  cause by user  mistake  will save a lots of money  as  if that   happen  then the whole  GS 12kw inverter  is destroy  but only  the transformer and shell is  save .      Thank you  for explaining  the difference  .      


   
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(@sid-genetry-solar)
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Posted by: @dickson
Then I better NOT try to backfeed the output of my Powerjack to the grid or short circuit the output . I thought you was able to design a control board that will detect a backfeed and shutdown the inverter to save the FETs . Designing a control board that can save the FETs cause by user mistake will save a lots of money as if that happen then the whole GS 12kw inverter is destroy but only the transformer and shell is save . Thank you for explaining the difference .

A PJ probably won't survive either case.

Yes, I have the firmware to TRY to detect a backfeed and shut down--but the FETs have to survive a full half-cycle of being blasted upside down before that will trigger.  It's nearly impossible to truly protect against a backfeed event.  The issue is that if the backfeed is 180 degrees out of phase, the power will go through the blocking diodes in half the FETs, and through the "turned on" FETs in the other half.  Instant destruction will result.


   
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 JIT
(@jit)
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On 6/1/2022 at 3:53 AM, Sid Genetry Solar said:

Worth noting that the batteries can be charged through the GS inverter when grid-ties are connected to the output.  This actually is a side effect of the hardware design of an LF inverter--and the only thing the GS inverter can do to stop it is to frequency-shift "throttle" and/or shut down.  (This is why I'm a little concerned about connecting 12kw of power to a 6kw inverter.)

Frequency modulation is not the only mechanism to control backfeed power from GTI.  All UL1741/IEEE1547 compliant GTI's (i.e. all GTI's legally connected to the grid in the US) will shutoff or curtail output power if either frequency or voltage gets too high/low.  Depending on the inverter hardware design, you can modulate voltage to control backfeed power from GTI.  Also, modern GTI's have sophisticated anti-islanding detection so they will often not stay activated if the microgrid voltage is unstable or has too much harmonic content.  So, GS inverter output voltage control loop needs to have not only fast and accurate voltage response at the fundamental frequency but also compensation at harmonic frequencies.  You may want to study how other established LF inverters that support this capability works.

 

On 6/1/2022 at 3:53 AM, Sid Genetry Solar said:

Honestly, while I've designed/written firmware for grid-tie support (GTM status = grid-tie master), nobody I personally know has a grid-tie system that we can test and tweak GS firmware on!  So if you don't mind being dev help with lots of diagnostics/testing (GS and grid-tie hardware permitting), we should be able to get this working.

Honesty is good but frankly a bit surprised to see this since the GS inverter manual on the web site makes it look like a working feature.  Perhaps you should mark it as experimental until you have successfully tested it?

 


   
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(@sid-genetry-solar)
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1 hour ago, JIT said:

All UL1741/IEEE1547 compliant GTI's (i.e. all GTI's legally connected to the grid in the US) will shutoff or curtail output power if either frequency or voltage gets too high/low.  Depending on the inverter hardware design, you can modulate voltage to control backfeed power from GTI.

And that's precisely the issue: grid-tie inverters often don't indicate whether they support frequency-shift throttling.  AFAIK the spec doesn't require it.

Yes, I'm fully aware that grid-tie inverters have to shut down if frequency (or voltage) go out of range--but an on/off throttle is considerably more common (especially on older units).  The uncertainty coupled with the oversizing of the grid-tie inverter in this case...is what really comprises the issue.

 

1 hour ago, JIT said:

Honesty is good but frankly a bit surprised to see this since the GS inverter manual on the web site makes it look like a working feature.  Perhaps you should mark it as experimental until you have successfully tested it?

So I have tested the GS inverters' response to backfed power--yes, it will throttle the frequency upwards until either it shuts down ("GTM Regulate Fail") due to exceeding 62Hz, or until the backfed power goes down/away.  And if the load increases beyond the backfed power (i.e. power is being pulled from the battery), the GS inverter will edge the output frequency down to 60Hz until either the backfed power comes back up--or until it reaches normal 60Hz operation.

On paper--and with bench experiments--the function is tested and known to work.

What hasn't yet been tested is how the GS inverter would interact with an actual grid-tie system--and this may be highly dependent on the type of grid-tie-system.


   
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(@sid-genetry-solar)
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1 hour ago, JIT said:

You may want to study how other established LF inverters that support this capability works.

Without getting into the super-high-trim big-name-brand inverters (Outback, Schneider, etc., etc.), the only LF inverter I'm aware of that even acknowledges that this sort of functionality exists are certain Sigineer inverters with a "frequency shift dry contact input" that you have to externally control.  Without an external smart controller that really knows what's going on, if you connect a grid-tie inverter to the output of one of these Sigineers and forget about it, you'll probably boil your batteries to pieces (or launch the next lithium-fueled rocket, depending on the chemistry).

https://www.sigineer.com/features/dry-contacts-providing-ac-output-frequency-switch-between-60hz-62-5hz/

GS inverters already have an automatic frequency shift shutoff.  Again, I don't know how this will play with each individual different grid-tie system.  And in the case of the OP, the unknowns and potential power scale/amperages just don't play well with the GS inverter.


   
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 JIT
(@jit)
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Posted by: @sid-genetry-solar
And that's precisely the issue: grid-tie inverters often don't indicate whether they support frequency-shift throttling. AFAIK the spec doesn't require it.

It's not hard to tell.  Perhaps you don't know the difference between UL1741 and UL1741SA (SA is short for Supplement A).  For UL1741 compliant GTI, frequency-watt (i.e. frequency shift) curtailment is not required.  For CA Rule 21/UL1741SA compliant GTI, frequency-watt curtailment is required.  Similar requirements for voltage-watt curtailment. 

 

Posted by: @sid-genetry-solar
Yes, I'm fully aware that grid-tie inverters have to shut down if frequency (or voltage) go out of range--but an on/off throttle is considerably more common (especially on older units). The uncertainty coupled with the oversizing of the grid-tie inverter in this case...is what really comprises the issue.

Inverters supporting GTI backfeed power 2x the nominal inverter output power capacity are available.  So, it's definitely doable depending on the inverter HW/SW design.  BTW, OP's GTI can only put out 10kVA max so the max GTI backfeed power is most likely well under 10kW probably closer to 1.5x the nominal inverter output power capacity.

 

Posted by: @sid-genetry-solar

Bench testing is a good start but frankly not adequate for quality power electronics engineering.  I would suggest you do at least similar level of testing with real target loads/systems as you did for the 12kW inverter (i.e. testing with a real central A/C) before release as a generally available function.  I like what you guys are doing and I am fan of agile development in general but not so much for power electronics.  Apps crashing on a phone is a bit different from power electronics failures.  Perhaps you don't have enough customers yet but sooner or later you could have a serious liability problem.

Also, real GTI behave quite differently due to their built-in anti-islanding detection mechanisms that differ across vendors and even across models.

 


   
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 JIT
(@jit)
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22 hours ago, Sid Genetry Solar said:

Without getting into the super-high-trim big-name-brand inverters (Outback, Schneider, etc., etc.), the only LF inverter I'm aware of that even acknowledges that this sort of functionality exists are certain Sigineer inverters with a "frequency shift dry contact input" that you have to externally control.  Without an external smart controller that really knows what's going on, if you connect a grid-tie inverter to the output of one of these Sigineers and forget about it, you'll probably boil your batteries to pieces (or launch the next lithium-fueled rocket, depending on the chemistry).

https://www.sigineer.com/features/dry-contacts-providing-ac-output-frequency-switch-between-60hz-62-5hz/

GS inverters already have an automatic frequency shift shutoff.  Again, I don't know how this will play with each individual different grid-tie system.  And in the case of the OP, the unknowns and potential power scale/amperages just don't play well with the GS inverter.

Sigineer has a crude hack they put in for Jack Ricard/EVTV.  It's not a good reference.  I understand Outabck and similar are high end but not sure why you would exclude them.  If anything, you should study what they do (instead of hacks like PJ) and provide similar functions with better cost/benefit tradeoffs for your customers.  I would suggest you study Victron inverters (e.g. Multiplus, Quattro).  Or even the OzInverter.


   
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(@sid-genetry-solar)
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43 minutes ago, JIT said:

Sigineer has a crude hack they put in for Jack Ricard/EVTV.  It's not a good reference.  I understand Outabck and similar are high end but not sure why you would exclude them.  If anything, you should study what they do (instead of hacks like PJ) and provide similar functions with better cost/benefit tradeoffs for your customers.  I would suggest you study Victron inverters (e.g. Multiplus, Quattro).  Or even the OzInverter.

Best I could find on the OzInverter was their usage of a voltage rise to shut off grid-ties--but this is not a very common specification for grid-tie inverters.  (Yes, they should shut down if the voltage goes out of range--but if I understood some briefs of the latest certification specs, some of the updated specifications increased the "out of voltage range timeouts" to try to not brown out as easily.)  Adding to that is that common AC line voltages can be 220v--and easily past 260vAC.  We've got several customers that have 260v line AC--either a grid-tie won't run, or if it does, how high does the input voltage have to go to cause it to shut down?

For that matter, I have never heard of grid-tie inverter throttling based on line voltage--as line voltage is not close to much of a "defined standard" as line frequency is.  Without an in-depth examination, I would presume that the OzInverter is an on/off system.  (Batteries full, raise the voltage and kick out the grid-ties.)  That's not the linear throttling I am hoping to achieve.

 

All of the others inverters are too expensive for me to buy.

Besides, there's no joy in "copying someone else"--that's an old trick that China exists on.  I would rather learn how to do it the best way and be original.


   
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(@thebutcher)
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IIRC the inverter limit in my state of QLD Australia, it's 255VAC.  The pretty loose spec considers from about 216V up to 253V to be valid.  Grid tied inverters must progressively throttle as line voltage rises up towards 253V and beyond and completely shutdown at 255V.  This allows better supply stability rather than having a bucket of inverters abruptly turn off and the voltage drop etc.


   
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(@sid-genetry-solar)
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8 hours ago, TheButcher said:

IIRC the inverter limit in my state of QLD Australia, it's 255VAC.  The pretty loose spec considers from about 216V up to 253V to be valid.  Grid tied inverters must progressively throttle as line voltage rises up towards 253V and beyond and completely shutdown at 255V.  This allows better supply stability rather than having a bucket of inverters abruptly turn off and the voltage drop etc.

Can you provide the official document that specifies/characterizes this behavior?  (Obviously, Australia certs would likely be different than U.S. certs.)  The only "official" specs I've found have had to do with frequency shift.


   
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 JIT
(@jit)
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23 hours ago, Sid Genetry Solar said:

Best I could find on the OzInverter was their usage of a voltage rise to shut off grid-ties--but this is not a very common specification for grid-tie inverters.  (Yes, they should shut down if the voltage goes out of range--but if I understood some briefs of the latest certification specs, some of the updated specifications increased the "out of voltage range timeouts" to try to not brown out as easily.)  Adding to that is that common AC line voltages can be 220v--and easily past 260vAC.  We've got several customers that have 260v line AC--either a grid-tie won't run, or if it does, how high does the input voltage have to go to cause it to shut down?

Many GTI's allow custom configs within the absolute limits set by UL1741.  Typically UL1741SA compliant GTI with default config will start curtailment at 252-254V and shutoff at 264V after ~10 cycle timeout or immediate shutoff at 288V.   UL1741 compliant GTI with default config will typically just shutoff at 264V.

 

23 hours ago, Sid Genetry Solar said:

For that matter, I have never heard of grid-tie inverter throttling based on line voltage--as line voltage is not close to much of a "defined standard" as line frequency is. 

For an island grid (i.e. disconnected from the utility grid) where this function applies, the normal voltage range should be very well defined.  It's controlled by your inverter.

 

23 hours ago, Sid Genetry Solar said:

Besides, there's no joy in "copying someone else"--that's an old trick that China exists on.  I would rather learn how to do it the best way and be original.

Not suggesting you just copy someone else.  If you did that then I doubt you could do it at a meaningfully lower cost.  There is nothing preventing you from being original by studying what others have done.  It would more likely help you design something better.  The problem with most low end inverter manufacturers in China is that they don't really understand what they are looking at.

 


   
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 JIT
(@jit)
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On 6/1/2022 at 1:39 PM, Sid Genetry Solar said:

.......and the FETs in the 6kw inverter are conservatively rated for 600A.  If he was at 24v or higher, they would probably survive.

I assume 600A is the FET's continuous current limit?  If so, the pulsed current limit should be much higher ... 


   
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(@sid-genetry-solar)
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16 minutes ago, JIT said:

I assume 600A is the FET's continuous current limit?  If so, the pulsed current limit should be much higher ... 

Conservative continuous limit, yes. 

Yes, pulsed is much higher BUT...the pulsed rating is generally specified either nonrepetitive, or a very, very small time.  Current GS inverter FETs' pulsed rating is literally 300uS @ 2% duty cycle (NCEP039N10 datasheet, page 2, footnote 3.)  Reversing the math works out to a 2% duty cycle at 66Hz--in other words, not quite exactly inverter duty.

As a result, for inverter purposes, the "pulsed" rating of a FET is borderline useless in estimating maximum power handling abilities--for practical purposes, an "A/C compressor startup surge" falls into "continuous duty" from the FETs' perspective.  Same for a grid-tie inverter that will take several seconds to be throttled down.

The issue is instantaneous heat generation vs the thermal resistance between the actual FET junction and the heatsink pad on the case (we haven't even gotten to the heatsink the FET is mounted to).  If too much heat is generated too quickly, the FET will explode as a result of this thermal resistance--even if the FET was submerged in ice water.

 

Think further down the line too--supposing the FETs could handle the blast of power, it's ALL going to be gated into your batteries.  Can they handle it for a few seconds?

 

 

33 minutes ago, JIT said:

The problem with most low end inverter manufacturers in China is that they don't really understand what they are looking at.

Ain't THAT the truth...

Seriously.  I was quite shocked to discover how little said manufacturers actually know about the product--to mention nothing of electricity itself in the first place.

 

34 minutes ago, JIT said:

Many GTI's allow custom configs within the absolute limits set by UL1741.  Typically UL1741SA compliant GTI with default config will start curtailment at 252-254V and shutoff at 264V after ~10 cycle timeout or immediate shutoff at 288V.   UL1741 compliant GTI with default config will typically just shutoff at 264V.

Do you have a link to a written spec sheet where this is covered?

BECAUSE if this is indeed a guaranteed spec, I could very easily design the inverter to work with it (within reason).  And if so, I would happily agree with the OzInverter gurus that it's better than a frequency shift.  Either option could be used--I could make it a config option on the WiFi board.

 

This does bring a ray of hope to the initial scenario here--because IF you can determine how/where your grid-tie inverter starts throttling at (especially if you have a copy of the standard it must follow), you could technically configure the output voltage of your GS inverter to a reduced max wattage throttle on the GTI.

We could go from there.  My concern is about FET failure resulting from them signing up for an ice bucket challenge, but instead getting a water tower felled on them.

 

36 minutes ago, JIT said:

For an island grid (i.e. disconnected from the utility grid) where this function applies, the normal voltage range should be very well defined.  It's controlled by your inverter.

Yeah.  Controlled by the inverter...by dumping the excess power in your batteries!


   
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(@thebutcher)
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Posted by: @sid-genetry-solar
Can you provide the official document that specifies/characterizes this behavior? (Obviously, Australia certs would likely be different than U.S. certs.) The only "official" specs I've found have had to do with frequency shift.

Unfortunately standards documents are not free here, wonder aint it, but this might give you some insight.

https://pure-electric.com.au/news/how-correct-reactive-power-settings-your-inverter-can-increase-your-solar-revenue


   
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