Connect grid tied solar inverter to DC+ Bus of VFD?

[looking]

What solutions are there to feeding a high DC voltage rated (eg 1000VDC) certified grid tied solar inverter such as a Yaskawa Solectria PVI23-480; from an alternate DC+ bus supply. For instance the 670 VDC input from a solar string; might be replaced by a a 6 pulse rectifier; (possibly from a multi kilowatt VFD fed from a 480VAC synchronous gen set) which would in effect supply the DC+ requirement of the grid tied solar inverter.
Whilst extremely straight forward... how would the lack of current control into the grid tied converter be handled. Suggestions and comments deeply appreciated.

 

[Baluncore]

Welcome to PF.

It might seem simple at first, but high DC voltages are very difficult to control. Simply switching on/off takes special techniques, as do circuit breakers. DC voltages above 600V are expensive to control. 1kVDC will present you with real challenges. You are entering a very dangerous world.

The MPP controller in the grid-tie inverter will be confused by the supply impedance of rectified AC from an alternator. The inverter is expecting a constant current input but will receive a low impedance rectified voltage with a high ripple current. The impedance inversion also precludes placing a battery bank between solar panels and a grid-tie inverter. I have considered similar modifications, but the risks significantly outweigh the benefits.

It is better to install separate independent systems.
The danger of lethal shock and of fire are just too high.

 

[looking]

Welcome to PF.

It might seem simple at first, but high DC voltages are very difficult to control. Simply switching on/off takes special techniques, as do circuit breakers. DC voltages above 600V are expensive to control. 1kVDC will present you with real challenges. You are entering a very dangerous world.

The MPP controller in the grid-tie inverter will be confused by the supply impedance of rectified AC from an alternator. The inverter is expecting a constant current input but will receive a low impedance rectified voltage with a high ripple current. The impedance inversion also precludes placing a battery bank between solar panels and a grid-tie inverter. I have considered similar modifications, but the risks significantly outweigh the benefits.

It is better to install separate independent systems.
The danger of lethal shock and of fire are just too high.

Thankyou for your wise and sober summary. Every word is duly noted

BUT grid tieing synchronously generated AC (of say 20-100 KVA) in an acceptable manner to the grid utility poses several concerns beyond the tens of thousands of dollars required. Could grid tieing be accomplished in any innovative way if cost is the least of any constraints

The fact is that there are no such installations on the province wide grid of 3542MW generating capacity. I can safely assume that applies both country or continent wide. And venting and flaring waste flare gas has to be one of the greatest wastes of resources that goes back 70 plus years and has not yet ended.

For anyone willing to pay whatever the cost of an innovative solution ... The question is: Are there any "standards approved" devices (eg UL or CSA with the trademarked C accompanying) that could safely and reliably connect a properly rated solar grid tied inverter to a "common" DC+ bus system provided by a 6 pulse rectifier built into an CSA approved and properly sized variable frequency drive.

No batteries involved please; just a standalone AC genset running like a top producing water and CO2; and grid tied through a usable and acceptable converter/"black box"/inverter system.

 

[Baluncore]

You do not need the inverter if you run an engine on waste gas with a synchronous generator connected to the grid.

 

[anorlunda]

You do not need the inverter if you run an engine on waste gas with a synchronous generator connected to the grid.

Bingo; that's exactly right. @looking, do you undersatnd @Baluncore 's point? An inverter converts DC to AC. If you have an AC generator, and a AC power grid, no inverter is needed.

But you will need auxiliary equipment to protect the generator and the grid from malfunctions, and to synchronize, and to meter, and potentially to respond to the commands of the grid operator. You would need analogous auxiliary equipment using an inverter too.

There are numerous commercial solutions meant to tie homes with rooftop solar to the grid. They integrate the inverter and auxiliary functions into the same box and call it "interface". Homeowners typically buy or rent the interface from the power company.

But I don't know of any similar commercial packages to interface AC generators to the grid. That is probably because AC generators come in all sizes from kW to GW.

Your idea of capturing waste heat to make electricity may be welcomed by the local power company. But you need to talk with their engineers about their specific requirements before you connect. If you want to spread the idea across the province and the country, you must comply with the specific requirements of each local power company. If there is an approved interface available, those utility engineers can tell yuou about it.

So, my answer is to ask your questions to the local power company, not to PF.

 

[looking]

Of course the above observation is 100% correct

But; from my first hand experience; I can say that for smaller scale synchronous generation (approx. 5KW to <100KW) there are no examples of "synchronous generators connected to the grid."

That would change if some way were found to use certified existing products to do things differently. It apparently won't happen by using conventional relay controls and off site direct control somewhat equivalent of commissioning much larger synchronous utility owned units. That is a non starter and would only be proven untrue if we could be pointed to at least a single case where anyone has actually overcome the dilema of no small scale synchronous generation connected to the grid.

Maybe there are no solutions; but also how can it hurt to identify any currently available alternative possibilities (if any); and give it a fair consideration.

 

[looking]

Our messages have crossed whilst I was preparing a reply

QUOTE There are numerous commercial solutions meant to tie homes with rooftop solar to the grid. They integrate the inverter and auxiliary functions into the same box and call it "interface". Homeowners typically buy or rent the interface from the power company.

But I don't know of any similar commercial packages to interface AC generators to the grid. That is probably because AC generators come in all sizes from kW to GW. UNQUOTE

I have carefully tried to say "grid tied solar inverter" at all times. The "Interface" is already included in the grid tied package AND other than the metering is not in any way the responsibility of a grid operators liability. We both agree that such an interface isn't probably available as a commercial package for small scale grid tied synchronous AC generation .

But the facts all point to the conclusion that both my problem (and everyone else's similar predicament) is not being advanced; by saying you don't need an inverter for AC generation. Maybe nothing is further from the truth.

 

[looking]

Last sentence should be "by saying you don't need an inverter for AC grid tied generation. Maybe nothing is further from the truth"

 

[anorlunda]

But; from my first hand experience; I can say that for smaller scale synchronous generation (approx. 5KW to <100KW) there are no examples of "synchronous generators connected to the grid."

My first hand experience is the opposite. I once lived in Potsdam NY. We had 22 hydro plants (with 3-4 generators each) on the Raquette River alone. (We used to joke that it was the damdest river in the country.) In New York state alone, there are more than 300 grid connected small hydro plants.

Diesel generators on islands. Wind farms. There are tens of thousands of small synchronous generators connected to the grid in North America. Where does your experience come from?

Edit: not wind farms.

 

[looking]

I'll conclude by responding to this quote from above

QUOTE But you will need auxiliary equipment to protect the generator and the grid from malfunctions, and to synchronize, and to meter, and potentially to respond to the commands of the grid operator. You would need analogous auxiliary equipment using an inverter too. UNQUOTE

I think that the strong points of a "solar grid tied inverter" include automatically synchronizing connection to the grid; lack of additional requirements of auxilliary equipment for grid connection; and no necessity of responding to commands of the grid operator (which is currently typical with less than 100 KW generating capacity) are all good reasons for wanting to feed an identical "solar grid tied inverter; with the energy that initially comes from synchronous generation... but through some approved black box creates a close enough DC bus output that would fool any "Solar grid tied inverter" into thinking it was connected directly to a solar panel array.

Ergo the "solar grid tied inverter" can't tell the difference and I'm sure the electrical utility would have no legitimate concern about whether the sun or flare gas was the energy source. Both sources are considered in same light for environmentally appropriate special generation programs.

Again small scale grid tied AC synchronous generation is a non starter; got to try to get to point of actually looking for alternative ways such as what apparently solar energy has accomplished in regards to grid connection

Thanks

 

[looking]

My experience comes from being told by the electrical utility some two years ago (and I have asked since if anything has changed) is that " if approved under the special programs for <100Kw flare gas generation; the application would be the first one in the utilities history"

Proof by not submitting the application (for various reasons including "carbon taxes"; contract details; "demand charge potential; upgrades as demanded etc). Apparently; other potential generators agree; as not much of a rush by potential similar other recruits (to my knowledge).Sure there are a handful of 5 MW Rankin cycle waste heat recovery generation projects nearby. Sure there are windfarms (but not many grid tied working connections make sense less; and I'm talking tens of KW generating capacity ) and even smaller ones may utilize equipment like Aurora "grid tied wind inverters" similar to their "grid tied solar inverters". And there is hydroelectric generation; but most of those are orders of magnitude away from what I'm talking about.

What is needed is something like the convenience and ease of connecting the numerous common "grid tied solar inverters" for anyone who wants to use internal combustion engines that commonly happen to have a synchronous generator attached.

The solar panel craze isn't 20 years old (more like 5 years or less here)... so maybe my inquiry is just a few years premature.

And Geez I never saw such prompt responses on any other forum. I do appreciate it all.

 

[anorlunda]

To do maintenance on the power lines leading to the neighborhood, the utility needs the ability to shut off the power. But a household grid connected power source could energize, or re-energize the local wires, causing electrocution hazard for the utility work crew. That is just one of several reasons why the utility can and does insist on those auxiliary functions.

 

[looking]

And that protection comes from the last piece of consumer supplied equipment just before the utility owned metering equipment. In a solar photovoltaic system it is called a "grid tied solar inverter" CSA approved of course.

In a smaller wind generation project that piece of equipment would commonly be a "grid tied wind inverter" CSA approved of course

And in a few tens of kilowatts of synchronous generating capacity; in theory; the power could be rectified and with the proper "interface" (black box or any name that anyone comes up with for a CSA approved electrical apparatus) could feed that same "grid tied solar inverter" mentioned above. There ain't nothing completely magical about creating a voltage and current similar enough to what a "grid tied solar inverter" expects as input from a solar array. All having to be CSA approved before applying to connect to the electrical grid.Anyone else who has any ideas about a suitable "black box" Please weigh in. Thanks.

 

[jim hardy]

A slight;y oversped induction motor becomes a generator and will return power to the mains so long as it can draw its magnetizing current from them.
Read up on "Induction Generator". It's dirt simple..
But the thought needs careful consideration - it'll hurt their power factor and might violate the business agreement with their utility.

It could interact with power factor correction equipment(capacitors) and backfeed the utility as mentioned by @anorlunda, so look before you leap.

Down in South Florida the sugar mills used to have small inhouse steam power plants to burn the cane residue and save on their electric bill. They had agreements to sell excess power to the utility.

 

[Baluncore]

Anyone else who has any ideas about a suitable "black box" Please weigh in. Thanks.

If you take an off-the-shelf grid-tie inverter and supply it with energy, then it will require an intelligent switching converter that produces an output voltage somewhere between 300 and 600 volts. That output must appear to be a quiet constant current of between zero and about 20 amps. Changes in available power must appear as a gradually changing current. Both maximum output voltage and maximum output current need to be programmed to suit the inverter. Control signals from that switching converter need to feed back to regulate the engine power, also with at least an emergency stop. The alternator on the engine will need field control to regulate voltage, while fuel input will need to be controlled by the switching converter current source.
With feedback within feedback, and many different engine and alternator possibilities the design will need to be adaptable to many possible configurations.

 

[looking]

If you take an off-the-shelf grid-tie inverter and supply it with energy, then it will require an intelligent switching converter that produces an output voltage somewhere between 300 and 600 volts. That output must appear to be a quiet constant current of between zero and about 20 amps. Changes in available power must appear as a gradually changing current. Both maximum output voltage and maximum output current need to be programmed to suit the inverter. Control signals from that switching converter need to feed back to regulate the engine power, also with at least an emergency stop. The alternator on the engine will need field control to regulate voltage, while fuel input will need to be controlled by the switching converter current source.
With feedback within feedback, and many different engine and alternator possibilities the design will need to be adaptable to many possible configurations.

Excellent and to the point comments.
Until now I was always thinking of feeding the grid tie inverter component from a "15 amp" 3 phase breaker source off the existing standalone fixed speed rocksolid 480VAC genset.

But having a different separate dedicated smaller genset to supply the necessary power to an isolated "solar grid tied inverter" will be investigated with all your comments above taken into account. Just maybe the front end (DC+ bus "output" of an Allen Bradley VFD.) could be one of the components to feed the grid tie converter.

I'll maybe begin with a 1HP VFD and experiment to prove that the DC + voltage can be suitably controlled through feed back signals that regulate field voltage on alternator end; as well as determining if current control is maintained within inverter parameters by other feedback signals. Thanks

How does that sound??

 

[looking]

A slight;y oversped induction motor becomes a generator and will return power to the mains so long as it can draw its magnetizing current from them.
Read up on "Induction Generator". It's dirt simple..
But the thought needs careful consideration - it'll hurt their power factor and might violate the business agreement with their utility.

It could interact with power factor correction equipment(capacitors) and backfeed the utility as mentioned by @anorlunda, so look before you leap.

Down in South Florida the sugar mills used to have small inhouse steam power plants to burn the cane residue and save on their electric bill. They had agreements to sell excess power to the utility.

That was what I initially thought was the first choice.So two years ago or so I built a 480V 20 HP electric motor drive coupled directly to an identical 480V 20 HP induction motor to function as the asynchronous generator when driven at slightly over its 60 HZ synchronous speed. The drive motor was switched in after a time delay for the asynchronous end to come up to "running" speed through the live utility feed. And in a short test it did work and produced some 16,000 watts of power as I recall. It does indeed solve the synchronization to "line" utility problem.

However; because of a very poor power factor which no doubt was contributed by the "asynchronous induction generator" power facor of 0.8; as ll as a large transformer needed to reduce down to a 208V AC 3ph utility service voltage...this solution was abandoned.. Another reason was that although such an asynchronous AC generator is "inherently" quite safe for utility islanding concerns; there are possible scenarios where capacitance can initiate self-generation even though the utility supply is down. Thus there were going to be requirements of various controls and power quality issues to be resolved to whatever the utility deemed satisfactory.

Maybe readers can see why I am looking for an off the shelf solution that is reported to meet all the possible concerns for grid connection...as it does appear to be the case for solar grid tie; and even smaller wind grid tie inverter apparatus.

The utility is obliged to allow tie ins. But they can and do make all the rules; including a "blank cheque"; and a contract that they get to change at any time. Nothing guarantees that you will ever have any thing but expenses at the "end of the day".

 

[jim hardy]

there are possible scenarios where capacitance can initiate self-generation even though the utility supply is down. Thus there were going to be requirements of various controls and power quality issues to be resolved to whatever the utility deemed satisfactory.

Thanks - i wasn't sure if you were aware.

And you were a few year ahead of me with the idea.

Thanks for the feedback !

good luck

old jim

 

[Baluncore]

How does that sound??

I don't think you have understood that the PV grid-tie inverter needs to be fed from a steady current source that behaves just like a PV panel.

The rectified DC bus from a VFD is simply the input reservoir capacitor voltage, which has very low impedance, but you need very high impedance.
You might be able to program a VFD to generate a constant output current, then rectify that current for input to the grid-tie inverter.

 

[jim hardy]

I don't think you have understood that the PV grid-tie inverter needs to be fed from a steady current source that behaves just like a PV panel.

Is that because of the inverter's MPPT characteristic ?

 

[Baluncore]

Is that because of the inverter's MPPT characteristic ?

Yes. The input structure of the grid-tie inverter expects a DC current from the PV string that rises and falls slowly.

 

[looking]

J

I don't think you have understood that the PV grid-tie inverter needs to be fed from a steady current source that behaves just like a PV panel.

The rectified DC bus from a VFD is simply the input reservoir capacitor voltage, which has very low impedance, but you need very high impedance.
You might be able to program a VFD to generate a constant output current, then rectify that current for input to the grid-tie inverter.

Just throwing this out for comment.

What about feeding two VFD's in series. The first one fed with a reasonable "abundance" of fused amps at rated full 480AC volts from the generator. First VFD would be programmed to control max current that the "grid tied solar inverter" can handle (which I guess is what Baluncore suggested in the last post). And then direct those first VFD output lines into the input of a second identical VFD, and then trying to do something with the DC+ voltage of the second inverter for any impedance mismatching?

The first VFD would be a good current limiter; but maybe just a good fault controller/detector for over current. Probably just a dumb idea but just throwing it out there. Maybe just another non starter idea. I'll look more into the Baluncore possibility of "programming a VFD to generate a constant output current, then rectify that for input to the "grid tie converter".
The Allen Bradley VFD's I'm looking at boast Common DC+ buses which can handle regeneration from other VFD motors etc. I'm not worried about losing 2% to 3% efficiency losses with each stage. Right now I'm wasting 100% of potential useful electricity.

.

 

[jim hardy]

Yes. The input structure of the grid-tie inverter expects a DC current from the PV string that rises and falls slowly.

Hmm that makes a lot of intuitive sense.
Unlike a plain transformer, a solar cell cannot deliver excessive current into an overload .
To use the vernacular, you cannot suck out more energy than the sun is putting in.
So a MPPT algorithm will look for voltage sag in the transformer and not find it...?

I would have had to learn that the hard way... Thanks !

old jim

 

[Baluncore]

What about feeding two VFD's in series.

Why do you need to use two VFDs when you can program one to limit motor current and voltage?

The output voltage of some VFDs may be limited by the input AC, but many VFDs have a voltage boost capability, so the maximum voltage must be programmed.

A VFD may have a PF correction rectifier in the front-end, which you should not upset. That PF correction circuit will be gentle on your generator.

Avoid contact with the VFD rectified Common DC+ bus. You simply do not need it.

A switching constant current generator is as easy to build as a switching DC voltage regulator.
You only need use a VFD if it can generate a constant DC current. Study the programmers manual.

 

[Tom.G]

As for the 'problem' of getting the power company to accept power, here in the USA there is 'Net Metering' available. This is where a residential Solar installation draws power from the grid when needed and feeds the grid when generating excess power. With that in mind, it seems any problems would be of a technical nature of impedance matching your power source to the input needs of the grid interface electronics. Even dynamic impedance matching would seem to be solvable.

(Note: I haven't actually tried it though!)

Cheers,
Tom

 

[essenmein]

What solutions are there to feeding a high DC voltage rated (eg 1000VDC) certified grid tied solar inverter such as a Yaskawa Solectria PVI23-480; from an alternate DC+ bus supply. For instance the 670 VDC input from a solar string; might be replaced by a a 6 pulse rectifier; (possibly from a multi kilowatt VFD fed from a 480VAC synchronous gen set) which would in effect supply the DC+ requirement of the grid tied solar inverter.
Whilst extremely straight forward... how would the lack of current control into the grid tied converter be handled. Suggestions and comments deeply appreciated.

As mentioned in previous replies the trick is going to be providing the inverter with a DC source that the MPPT algorithm can work with.

This can be quite interesting in a number of ways since you now have at your disposal a load (the inverter) that will try to find the optimal operating point (current vs input volts) of your energy source. So if you set your energy source up right then the MPPT inverter can find the optimal operating point of a given finite power source. For example it would just as happily load down a wind generator to its optimal point for a given wind speed as it would a solar panel.

However, your source must be inherently limited , ie its output voltage must reduce with increasing load current, that's generally how MPPT works, keep increasing current draw, measure resulting voltage drop, multiply V*I, if this number is still getting bigger increase current draw a little more, repeat until V*I starts going down, they also get tripped up by "double peak" type energy sources where there are local maxima that is not the actual maxima. So if your energy source does not significantly change its output voltage with load, and then, when its over loaded pops a breaker, then the MPPT will just go at full power the whole time, which might not be what you want.

So the question is, what is your energy source and how "manageable" is its output.

Also a little note on safety, 600V+ dc demands respect. You need a rescue hook and someone there in case you need it, if you don't know what that is I'd seriously read up on electrical safety before proceeding, if proceeding at all.

 

[looking]

As mentioned in previous replies the trick is going to be providing the inverter with a DC source that the MPPT algorithm can work with.So the question is, what is your energy source and how "manageable" is its output.

Also a little note on safety, 600V+ dc demands respect. You need a rescue hook and someone there in case you need it, if you don't know what that is I'd seriously read up on electrical safety before proceeding, if proceeding at all.

The energy source is a natural gas driven 100KW synchronous AC generator with 12 lead configurable output voltage leads to configure any desired single phase or 3 phase output voltage. The governor responds to load changes of adding 10 hp motors almost instantly and with next to "no hunting". Stability isn't a problem"n But PLEASE let's consider stopping the presses. Just maybe there is an "epiphany moment staring us all in the face.

About 2007, a company called Magnetek developed what has been sometimes described as a somewhat indestructible grid tie wind inverter. All with approved proper conformity papers that are acceptable to utility grids. The news gets better when they were taken over by Power-One (and marketed under the Aurora brand) which was later acquired by the well known and respected company of Allen Bradley (now ABB). Allen Bradley has apparently contniued to use this (or similar) technology in their grid tied wind inverter offerings.

The Aurora "grid tied wind inverter" products are perhaps best generally described in that companies various Product Manuals that are "intended to provide installers and users with all the necessary information about installation, operation, and use of Aurora Wind inverters."

Now granted the manual is a United Kingdom version and the Aurora PVI-6000-OUTD-UK-W isn't the Aurora PVI-6000-OUTD-US-W one; but I'll bet its better than nothing until I can locate the US version. Here are the quotes directed to the primary market target of Synchronous permanent magnet 3 phase synchronous generators (driven by air foil wind turbine blades); and connected to a DC rectifier (or the alternative "Wind Box interface") that ABB still recommends to feed any of the "grid tie wind inverters" that may be available "new or used"; and fitting the manufacturers description.

This looks extremely promising to me

QUOTE
User Manual Page 11 of 74 (PVI-6000-OUTD-UK-W Rev.1.0)

2 SYSTEM DESCRIPTION The AURORA inverter is capable of feeding power to the grid converting the power generated by a wind turbine. The wind turbine (using a generator) converts the mechanical energy from wind into the 3-phase AC voltage. The voltage and frequency of the AC generated by the wind turbine are variable and depend on the wind speed. To be exported to the utility grid the power need to be converted to the frequency and voltage level of the grid. When used in parallel with the grid, the alternate current generated by the inverter is directly fed into the distribution circuit, which is also connected to the public power distribution grid. The wind energy system can thus powers all the connected users, such as lighting devices, household appliances, etc. When the wind turbine is not generating enough energy, the power required by the connected users is provided by the public power grid. If the energy produced by the wind turbine exceeds the quantity required by the user’s loads, then extra power is directly fed to the grid, becoming available to other users. According to national and local standards and regulations the energy produced can be sold to the electrical utility or credited to the user against future consumption, thus providing financial savings. 2.1 Key elements of a wind energy system: “WIND TURBINE” and “GENERATOR” Two elements are necessary in order to harvest the wind energy: a wind turbine that will spin based on the wind speed and a generator. The turbine forces the generator to rotate thus producing energy that can be exported to the grid. The most common turbine design has a horizontal rotor with 2 or 3 vertical fiberglass blades forming the propeller which may have a fixed or a variable tilt. The generator is fixed on the rotor. Usually for the small wind system a synchronous permanent magnet generator is used. The current generated by this type of generators has variable voltage and frequency depending on the turbine speed. To be exported to the grid this needs to be is converted to current. The conversion is done in 2 steps: A) The 3-phase AC voltage from the generator is filtered and converted to the DC voltage. This conversion can be done inside the turbine or using an interface box between the generator and the inverter. User Manual Page 12 of 74 (PVI-6000-OUTD-UK-W Rev.1.0) B) The resulting DC output is connected to the AURORA input and converted into AC power with the appropriate voltage and frequency to be exported to the grid.

Fig. 2 - Wind Turbine – Generator WARNING: The DC voltage input to the inverter shall not exceed 600Vdc for any reason, in order to avoid damage to the equipment. NOTE: A minimum input voltage of 50Vdc is required for the AURORA inverter to start the grid connection sequence. Once connected, the AURORA inverter will transfer the maximum power available for any input DC voltage value in a 50V to 580Vdc range to the grid. The max output current from the generator or an interface device shall be always within the inverter limits. In the AURORA inverter the total max input current is 36Adc. UNQUOTE

OK so 480VAC is out of the question. I can live with that as the current genset is a fair bit larger than needed for just one "grid tie wind inverter" But note that manufacturer claims a couple hundred inverters can be fed off same synchronous 3 phase; and my fuel energy input will be max'd out with other high efficiency gas furnace demands in this kind of weather; and I don't want to be paying for utility transformer upgrades anyway etc etc.

Lots of (maybe good) news for everyone to think about. I'll note that without all your other input to this thread that this kind of information may not have been connected; dot to dot; and surfaced to the audience.. AND there is no doubt a whole lot more potential discussion remains. Thank you Thank you again

;

 

[looking]

As for the 'problem' of getting the power company to accept power, here in the USA there is 'Net Metering' available. This is where a residential Solar installation draws power from the grid when needed and feeds the grid when generating excess power. With that in mind, it seems any problems would be of a technical nature of impedance matching your power source to the input needs of the grid interface electronics. Even dynamic impedance matching would seem to be solvable.

(Note: I haven't actually tried it though!)

Cheers,
Tom

Tom: Please see the response below..The "Net metering" appears to be what all the neighbors in this neck of the woods are jumping on and subscribing to. Its mainly solar panels and "grid tie solar invertes". My opinion is that our weather is too extreme (especially winters) and 15% of less than 10Kw of solar panels generating capacity ; and 20 some year payouts is break even at best.
In regards whether or not impedance matching could be achieved; not many of us have the ability and/or desire to get those components "certified" for "conformity". Designing and getting something approved for such a use just ain't going to often happen.

 

[essenmein]

To me the issue is not about the grid tie aspect of the inverter, this is common and done all the time, the 4 quadrant load machine on our dyno regenerates back to the grid for example (basically back to back VFD).

The real issue is how MPPT solar inverters work. Basically what it comes down to is this question: How does the proposed inverter know how much power to push onto the grid?

With solar MPPT its easy, the inverter "pulls" as much as it can from the solar panel by always hunting for the peak V*I on the input as light changes (ie the MPPT algorithm).

So how would your natural gas generator set "tell" the inverter how much it can deliver to the grid?

The generator has a governor that will push more power as the load needs, if you connect this the grid, that load is for all intents infinite, ie, from the grids perspective, you can run 100% power and it will happily take it.

However I assume your goal is not to be a power generator that is simply delivering all your power to the grid.

So how will the controls work that set the output of the generator and then determine what spare energy can be sent to the grid, so you can work out the current command to give to the grid tie inverter? (since inverter output voltage is set by the grid, current command is basically a power command)

 

[looking]

It was pointed out that "grid tie solar inverters" are specifically stated by commenters (and indeed the manufacturers literature) as not being recommended for anything other than photovoltaic panel energy input..

A literature search shows that wind grid tie inverters are designed for accepting 3 phase synchronous energy input (to a "Wind Box" or rectifier) that then feeds what the grid tied wind inverter is designed to accept.

I would expect it to remain true that the current output of the synchronous generator would need current output control; but surely the other mentioned deficiencies of the former "grid tied solar inverter " would have disappeared.

 

[looking]

With a grid tie wind inverter; does a person not have to worry about damaging a relatively small inverter by being connected to a genset that at all times can exceed the Max current handling capabilities of the inverter.?

I assure you the genset is properly protected from current overload and its voltage could be adjusted under feedback control (I guess)...and the grid (though not the secondary transformers) can handle anything the genset might be able to output.
I humbly ask for the methods to control current input to the "rectifier/Wind Box" and the grid tied wind inverter itself.

 

[Windadct]

I have tried to read though all of this - but I am missing why this is "stuck" a solar inverter.
For small scale wind there are some "off the shelf" VFD based grid tie inverters... It should be ONE step Gen -> VFD -> Grid...

The inverter - is the load, how would the inverter see more current than it can take?

The key and necessary feature is Anti-Islanding - where the inverter can sense that the grid is connected and if the Grid V is lost it shuts down. ( It is this feature that makes most solar installs NOT a back-up source since it will not operate unless there is Grid Voltage)

Gen -> VFD -> Rectifier-> Solar inverter -> Grid ?

 

[Baluncore]

Gen -> VFD -> Rectifier-> Solar inverter -> Grid ?

Solar PV grid-tie inverters are common, cheap and often have unused PV string inputs. If you use a solar grid-tie inverter then you need to make your energy source look like a constant current PV string.

If you do not use a solar PV grid-tie inverter the source and control requirements will be different.

 

[looking]

Please reread post #27.

I tried to switch from a "grid tied solar inverter solution" to a "grid tied wind inverter solution" because of the opinion of people who had understandable concerns about my lack of electrical impedence matching, DC+ bus safety concerns and other matters.

I don't currently understand "current control" and required feedback to a synchronous driven genset sufficiently (if any); but before actually hooking up an experimental circuit; I will do further study and I will try to be careful.

But if you reread post #27; you may conclude that I am onto something that just hasn't apparently been thought out previously within the internet I am familiar with.

If a "rectifier/grid tied wind inverter" can be hooked up to the same characteristics of power that a wind generator might produce in a continuous steady breeze; and
"The inverter - is the load, how would the inverter see more current than it can take? "
then are guess my worry concerns are unfounded.

I am trying to understand. I do have expertise in several other areas and truly wish being an electrical engineer was one of them.

In a nutshell; I consider a permanent magnet AC 3 phase synchronous generator hooked to wind airfoil blades to be nothing more than a widely variable speed version of a governor controlled AC 3 phase similar synchronous fixed speed governor controlled natural gas driven genset..

 

[Baluncore]

What solutions are there to feeding a high DC voltage rated (eg 1000VDC) certified grid tied solar inverter such as a Yaskawa Solectria PVI23-480; from an alternate DC+ bus supply.

This thread began by discussing the application of a solar grid-tie inverter as the grid interface.

I tried to switch from a "grid tied solar inverter solution" to a "grid tied wind inverter solution" because of the opinion of people who had understandable concerns about my lack of electrical impedence matching, DC+ bus safety concerns and other matters.

You have now switched to a wind inverter solution. I have no experience with wind solutions so I cannot help you with that. When you departed from solar solutions I backed off, there are others better placed to advise you. Our experience makes us more careful, your inexperience allows you to believe that anything is possible.

Your admitted lack of expertise in the electrical engineering field makes it unlikely that you will be able to safely implement either system in the short term. You are certainly not the first to think of the idea. If it was as simple as plugging blocks together, then it should have appeared on the market long ago.