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

[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.

 

[Tom.G]

@looking
I agree that the wind inverter approach is definitely worth looking into. Perhaps talking to the manufacturer about supplying it with your generator is a good next step. On the surface I don't see any problem with that approach, that's why I suggest contacting the manufacturer.

As for those that had a concern about supplying such an inverter with 'too much current', you can be pretty sure that the built-in overload protection won't allow that to happen!

Many of us on this site are rightly very conservative in our recommendations, if we haven't done/studied something in minute detail we cautiously say 'No'. Often that is the best course of action, but not necessarily always so. For instance, if you were a high-school student I would not be making this post.

See what the manufacturer says and please let us know. We like to learn too!

Cheers,
Tom

 

[essenmein]

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.

Could you clarify what hasn't been previously thought out in post 27?

I'm still not sure what overall you want to achieve, for example do you want 100% of your generator to go to the grid? Do you want excess power to go to the grid?

 

[looking]

@looking
I agree that the wind inverter approach is definitely worth looking into. Perhaps talking to the manufacturer about supplying it with your generator is a good next step. On the surface I don't see any problem with that approach, that's why I suggest contacting the manufacturer.

As for those that had a concern about supplying such an inverter with 'too much current', you can be pretty sure that the built-in overload protection won't allow that to happen!

Many of us on this site are rightly very conservative in our recommendations, if we haven't done/studied something in minute detail we cautiously say 'No'. Often that is the best course of action, but not necessarily always so. For instance, if you were a high-school student I would not be making this post.

See what the manufacturer says and please let us know. We like to learn too!

Cheers,
Tom

Thank you Tom. Very very well summarized. I did talk to the ABB help line (just before joining this forum) although all staff were in a meeting; I was promised that someone would phone back. No response yet, but I do have patience and also their phone number. And I have learned; and am still learning from this thread.
Whilst not "cock sure" about knowing everything about anything, rest assured that I could have become an "expert" in most fields I may have chosen; but the limits of time and interest dictate not pursuing all interesting topics at one time.

I'm pretty sure I will eventually get a standards approved certified solution and it may well be "simply" using a 6, 12 or 18 pulse rectifier (with accompanying DC smoothing capacitors and circuitry);connected to a "grid tied wind inverter"

I will certainly post any response from respected manufacturers supplying such electrical equipment; because now it appears others believe that this is not a unique and first time time quest for a solution. Heck it may even have fairly wide application.

 

[looking]

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

This is not intended to be a smart response. But I do think that within post #27 above; you will find a direct quote of the Systems Description of the Aurora "Grid tie wind inverter system". Now an electrical engineer would have drafted that manufacturers literature (now ABB which does know it's business) that I quoted. As also quoted that User and installation Manual says it is "intended to provide installers and users with all the necessary information about installation, operation, and use of Aurora Wind inverters."

Now I can comprehend every last word in what repeated from that quoted section.

I would think its a good short summary for anyone who has no experience with wind solutions. It should only take the time to read maybe 300 words and I highly recommend this short read to anyone.

 

[looking]

Could you clarify what hasn't been previously thought out in post 27?

I'm still not sure what overall you want to achieve, for example do you want 100% of your generator to go to the grid? Do you want excess power to go to the grid?

Thank you. Very reasonable request.
I want something for synchronously generated AC 3ph power that is as straight forward and uncomplicated to connect to a utility (from an electrical inspectors point of view) as the "grid tie wind inverter" that the electrical inspector would approve for a similar sized wind generation system.

I note the difference between my objective and the wind generator system is that the wind generator speed (thus voltage, frequency and power output) is much more variable. What other basic difference does anyone note when you refer to the manufacturers User and Installation manuals for the Aurora PVI-6000-OUTD-xxW) manual quoted above in my post#27).
Otherwise its a 3 phase AC power generation that needs to be rectified and fed to a grid tie inverter (in both my case and every other wind generation system using same basic theory of operation).

And for simplicity sake; I want 100% of what the "dedicated" generator output going to the grid. The constraints of available fuel supply management, any required utility transformer capacity upgrades etc. are problems I would welcome. Probably I could already help others out on those issues.

 

[essenmein]

Thank you. Very reasonable request.
I want something for synchronously generated AC 3ph power that is as straight forward and uncomplicated to connect to a utility (from an electrical inspectors point of view) as the "grid tie wind inverter" that the electrical inspector would approve for a similar sized wind generation system.

I note the difference between my objective and the wind generator system is that the wind generator speed (thus voltage, frequency and power output) is much more variable. What other basic difference does anyone note when you refer to the manufacturers User and Installation manuals for the Aurora PVI-6000-OUTD-xxW) manual quoted above in my post#27).
Otherwise its a 3 phase AC power generation that needs to be rectified and fed to a grid tie inverter (in both my case and every other wind generation system using same basic theory of operation).

And for simplicity sake; I want 100% of what the "dedicated" generator output going to the grid. The constraints of available fuel supply management, any required utility transformer capacity upgrades etc. are problems I would welcome. Probably I could already help others out on those issues.

Looking at the front page for these inverters on the ABB site:
"
The PVI-6000-TL-W is ABB's most used small wind turbine inverter designed with proven high performance technology. This dual stage transformerless wind inverters offers a unique combination of high efficiency, installer-friendly design and very wide input voltage range ensuring high energy harvesting.

The high speed and precise power curve tracking algorithm allows to best match the power curve of each turbine."

These paragraphs tell you two things, 1) its two inverters, one to control the generator and provide DC link voltage, and another to push that power to the grid (dual stage transformerless). 2) Much like a solar inverter, it provides a MPPT type algorithm to maximize energy harvested from a wind turbine (High speed power curve tracking).

For a constant speed ICE powered generator you do not have a "power curve" that can be tracked. (unless part of your plan is to vary the speed of the ICE? don't know).

If you simply rectifiy the output of your generator to make a DC bus voltage you will have poor power factor and lose efficiency, maybe this is an acceptable amount though.

Personally I think the type of system you are looking at is just a regular industrial motor drive that can provide 4 quadrant control (ie motor and generate in both rotating directions, and can regenerate back to the grid, this is an absolutely normal thing.

Once you have this, you simply set a torque command opposite to the direction of rotation to load your generator.

 

[looking]

Could it be that the MPPT power curve could be a inputted as a straight line. If you know for sure that you have a very stable voltage output and current is always going to be adequate; then why not program the inverter to expect a consistent "power curve"

As an aside I do note the grid tie wind inverter is for PM synchronous generators (permanent magnet AC generation 3 phase). Of course synchronous AC 3 phase generators don't have magnets for rotor excitation like the wind generators would have. I'd think though its the AC output waveform of the generator that would be paramount. Would the reactive power component in the genset be important compared to the wind power PM generator power output?

I don't worry about poor efficiencies at this stage; but power factor is important (though the grid tie inverter should approach the 1.0 that no one will complain about)...Thanks

 

[looking]

Looking at the front page for these inverters on the ABB site:
"
The PVI-6000-TL-W is ABB's most used small wind turbine inverter designed with proven high performance technology. This dual stage transformerless wind inverters offers a unique combination of high efficiency, installer-friendly design and very wide input voltage range ensuring high energy harvesting.

The high speed and precise power curve tracking algorithm allows to best match the power curve of each turbine."

These paragraphs tell you two things, 1) its two inverters, one to control the generator and provide DC link voltage, and another to push that power to the grid (dual stage transformerless).
.

Just a note to say the complete system needs also either a "rectifier" or one of ABB's "Wind Box" interfaces ( that performs the AC rectification to a DC+ Bus output; required between the wind generator and the "grid tie wind inverter"

 

[essenmein]

Just a note to say the complete system needs also either a "rectifier" or one of ABB's "Wind Box" interfaces ( that performs the AC rectification to a DC+ Bus output; required between the wind generator and the "grid tie wind inverter"

Interesting, I miss read that. Seems then that basically these are not that different from a solar inverter. The "two stage" must mean a DC-DC boost type converter and inverter.

Regarding the MPPT curve, it uses how the voltage changes with current to work, if the voltage is not changing with current most likely it will just run flat out the whole time.

 

[looking]

Now's the time to

Interesting, I miss read that. Seems then that basically these are not that different from a solar inverter. The "two stage" must mean a DC-DC boost type converter and inverter.

Regarding the MPPT curve, it uses how the voltage changes with current to work, if the voltage is not changing with current most likely it will just run flat out the whole time.

Not saying I'd do this; but "most likely it will just run flat out the whole time" would be the ideal result for me if that is within operating parameters. I'll certainly ask the manufacturer's "spokesperson" when I retry that contact .

What I do know for sure is that a "grid tie solar inverter" is specifically recommended by at least one manufacturer (ABB/Power-one) as suitable only for hooking to solar photovoltaic panels (which they then immediately interestingly call FV ). Others more commonly always say PV.

But...The point I'd emphasize is that the Aurora ABB "grid tie wind inverter" is certainly meant to connect to synchronous 3 phase AC wind generators; albiet generators which contain Permanent magnets to as ABB claims; eliminate excitation power required by other synchronous generation designs (and thus partly improve efficiency up to "30%)

 

[essenmein]

Not saying I'd do this; but "most likely it will just run flat out the whole time" would be the ideal result for me if that is within operating parameters. I'll certainly ask the manufacturer's "spokesperson" when I retry that contact .

Actually thinking about it, a solar inverter would do the same thing (run flat out), looking at how MPPT algos work, at time = 0 it looks at the open circuit voltage of the cell or string of cells, then it starts to pull current, and watches this voltage, at some point it reaches a knee point where increasing the current further and the voltage collapses, this is the constant current region of the cell, maximum power occurs at that knee, so an MPPT algo sits there changing the load current slightly while watching the cell voltage to ensure it is still on that peak power operating point.

pictures:
http://www.aurorasolarenergy.com/iv-curve-of-a-solar-panel/

Now you can see if your voltage does not change, it will keep increasing the current drawn from your source, but since it won't find a maximum since the voltage doesn't collapse, it will run up to its current limit and stay there.

Might be perfect for you, each time you switch on one of these inverters on you add its power rating to your output. Ie if they are 5kW inverters you switch it on and it puts the full 5kw on the grid (assuming your source can supply).

 

[essenmein]

(which they then immediately interestingly call FV ). Others more commonly always say PV.

Photo in english, Foto in german etc maybe?

 

[looking]

Actually thinking about it, a solar inverter would do the same thing (run flat out), looking at how MPPT algos work, at time = 0 it looks at the open circuit voltage of the cell or string of cells, then it starts to pull current, and watches this voltage, at some point it reaches a knee point where increasing the current further and the voltage collapses, this is the constant current region of the cell, maximum power occurs at that knee, so an MPPT algo sits there changing the load current slightly while watching the cell voltage to ensure it is still on that peak power operating point.

pictures:
http://www.aurorasolarenergy.com/iv-curve-of-a-solar-panel/

Now you can see if your voltage does not change, it will keep increasing the current drawn from your source, but since it won't find a maximum since the voltage doesn't collapse, it will run up to its current limit and stay there.
Might be perfect for you, each time you switch on one of these inverters on you add its power rating to your output. Ie if they are 5kW inverters you switch it on and it puts the full 5kw on the grid (assuming your source can supply).

Might be perfect for you, each time you switch on one of these inverters on you add its power rating to your output. Ie if they are 5kW inverters you switch it on and it puts the full 5kw on the grid (assuming your source can supply.
https://www.solacity.com/power-one-abb-aurora-solar-wind-inverters/ seems to agree 100% for the Aurora "grid tie wind inverters" . It does take an isolation transformer for all except the first "inverter". Also because the transformer triggers a false "ground fault" one has to turn off the inverters monitoring for that function. They call it "stacking".

Now; back to the "rectifier"/"wind box interface" to create the DC+ Bus supply for the inverter. It is pointed out that proper fusing of the DC converter is essential (in case the diode bridge fails and AC gets directed to the inverter and let's out "magic smoke" and other serious instantaneous consequences.
I'd sure like some additional pointers to information that confirms one should not under any circumstance entertain using a VFD and its outwardly looking well filtered DC+ power supply. There's credible info (even from a manufacturer) to indicate that a diode bridge will work for the "grid tie wind inverter"

There are lots more used solar inverter than wind ones; but I'll keep the "solar inverter running flat out" in mind ( for the future) if it gets past the experimental stages.

 

[Baluncore]

This is not intended to be a smart response. But I do think that within post #27 above; you will find a direct quote of the Systems Description of the Aurora "Grid tie wind inverter system".

Sorry, but PF is not my first priority.
Being “spoon fed” with selected quotes from a document reduces me to what you think is important. The problem is that if you do not understand the problem, you may have unwittingly selected an inappropriate quote, which does not advance the discussion. If your quotes do not solve your problem, then why should I also waste time reading them.

If you provided an obvious link to a particular relevant document, then I could examine that document when I have time.

 

[essenmein]

I'd sure like some additional pointers to information that confirms one should not under any circumstance entertain using a VFD and its outwardly looking well filtered DC+ power supply. There's credible info (even from a manufacturer) to indicate that a diode bridge will work for the "grid tie wind inverter"

Do you mean connect the phase connection of the VFD to your generator and then the DC connection to the grid tie inverter?

It would result in much less current harmonics (which reduce effy), it would only be a bit more tricky to set the control up but other wise this works fine. I feel like a broken record here but this is a common configuration, we have two Emerson drives doing just that.

Edit: Added link.
https://www.machinedesign.com/motorsdrives/regeneration-adjustable-frequency-drives

 

[looking]

Already done sir. Nineteen lines into Post # 27 that you quoted from above
User Manual Page 11 of 74 (PVI-6000-OUTD-UK-W Rev.1.0)

I trust that a Google search will yield exactly what I quoted; and not a word is out of place.

Sorry, but PF is not my first priority.
Being “spoon fed” with selected quotes from a document reduces me to what you think is important. The problem is that if you do not understand the problem, you may have unwittingly selected an inappropriate quote, which does not advance the discussion. If your quotes do not solve your problem, then why should I also waste time reading them.

If you provided an obvious link to a particular relevant document, then I could examine that document when I have time.

 

[Baluncore]

User Manual Page 11 of 74 (PVI-6000-OUTD-UK-W Rev.1.0)
I trust that a Google search will yield exactly what I quoted; and not a word is out of place.

I don't question the veracity of your quote, just it's selective relevance to a solution.
Why must everyone repeat your search? I have more important things to do, I only needed your link.

 

[looking]

Do you mean connect the phase connection of the VFD to your generator and then the DC connection to the grid tie inverter?

It would result in much less current harmonics (which reduce effy), it would only be a bit more tricky to set the control up but other wise this works fine. I feel like a broken record here but this is a common configuration, we have two Emerson drives doing just that.

Edit: Added link.
https://www.machinedesign.com/motorsdrives/regeneration-adjustable-frequency-drives

Yes to your above question in your post 6132340. Kohler or Onan synchronous genset 208 VAC 3 phase output; wired to a disconnect switch and fused supply feeding a VFD sufficiently sized to supply DC Bus voltage (required under full load by inverter). This DC output wired through a fused connection (in case of rectifier failure) and fed to the DC input of the "grid tie wind inverter."



Got to keep the Max DC voltage level below 600VDC to protect the "grid tied wind inverter" so am considering the above lower voltage; which would also not initially necessitate the upgrading of the utility transformer..

A sincere thankyou for your comments. I did try the manufacturers hotline today; but could only leave a message. It looks like a leave a message and telephone tag scenario. Lots of time. .

 

[looking]

I don't question the veracity of your quote, just it's selective relevance to a solution.
Why must everyone repeat your search? I have more important things to do, I only needed your link.

Sir: You did already have the link. It seems to be becoming quite revelant; and I didn't edit or make anyone repeat the search. Like the rest of us we all can miss something the first time around.

No ill will on this end.

 

[essenmein]

Yes to your above question in your post 6132340. Kohler or Onan synchronous genset 208 VAC 3 phase output; wired to a disconnect switch and fused supply feeding a VFD sufficiently sized to supply DC Bus voltage (required under full load by inverter). This DC output wired through a fused connection (in case of rectifier failure) and fed to the DC input of the "grid tie wind inverter."



Got to keep the Max DC voltage level below 600VDC to protect the "grid tied wind inverter" so am considering the above lower voltage; which would also not initially necessitate the upgrading of the utility transformer..

A sincere thankyou for your comments. I did try the manufacturers hotline today; but could only leave a message. It looks like a leave a message and telephone tag scenario. Lots of time. .

Cool, you will need some way of managing the bus voltage, invariably during load changes, the DC link voltage between the two inverters will fluctuate a lot if there is a brief power imbalance. At least for us, where the "generator" is the load machine on a dyno, so this needs to do +/-~50kW for both rotating directions (4 quadrant), and it needs to respond quickly to speed and torque changes. So we have a good sized DC link capacitance, and then there is a brake resistor bank (50kW) that switches in if the link voltage exceeds some amount (I think around 800V), ie this only happens on fast changes say when we take the test system from full power generate to full power motor and the VDF pushing the power back to the mains doesn't respond fast enough. A battery string on the DC link would be equally effective as they kind of act like a big voltage source/sink.

 

[looking]

Hitachi has a very good 4 page application note about "Powering Inverters from a DC Power Supply.
https://www.ametherm.com/inrush-current/how-to-stop-inrush-current

Perhaps two appropriately rated NTC Thermistors in series with the DC Bus would limit inrush currents

 

[jim hardy]

Hitachi has a very good 4 page application note about "Powering Inverters from a DC Power Supply.

I had some experience along those lines back in the early 1970's.

Thermistors do a fine job against inrush current, I've used them myself.

I don't know what type of inverter you contemplate
so i'll just toss this out as a possible opportunity for that proverbial " ounce of prevention "...

In my day of forced commutation line frequency Jones choppers
http://www.completepowerelectronics.com/how-jones-chopper-works/

The running current drawn from the DC bus was a long way from steady continuous DC.
They took a huge short gulp of current at each SCR firing , something like twice rated (nameplate was about 65 amps each for our set of three),
and that was enough to modulate our 130 volt station battery bus with 'spikes' somewhat over 100 volts peak-to-peak at 120 hz.
Of course that wreaked havoc with other electronics trying to run from the same battery bus.

We added input filter capacitors to the tune of one microfarad per milliamp of rated input current .
With that sort of input capacitance on a substantial inverter you have to address inrush,
ours had a 'precharge' feature to charge the input filter slowly before energizing the main electronics...

Probably that's not an issue anymore, i don't know much about today's PWM MOSFET machines so can't say for sure
but my message is: " there's more to powering inverters than inrush ---"
so don't leap into this without awareness of the running current demand that your inverter is going to lay on your source...

old jim

 

[looking]

I had some experience along those lines back in the early 1970's.

Thermistors do a fine job against inrush current, I've used them myself.

I don't know what type of inverter you contemplate
so i'll just toss this out as a possible opportunity for that proverbial " ounce of prevention "...

In my day of forced commutation line frequency Jones choppers
http://www.completepowerelectronics.com/how-jones-chopper-works/

The running current drawn from the DC bus was a long way from steady continuous DC.
They took a huge short gulp of current at each SCR firing , something like twice rated (nameplate was about 65 amps each for our set of three),
and that was enough to modulate our 130 volt station battery bus with 'spikes' somewhat over 100 volts peak-to-peak at 120 hz.
Of course that wreaked havoc with other electronics trying to run from the same battery bus.

We added input filter capacitors to the tune of one microfarad per milliamp of rated input current .
With that sort of input capacitance on a substantial inverter you have to address inrush,
ours had a 'precharge' feature to charge the input filter slowly before energizing the main electronics...

Probably that's not an issue anymore, i don't know much about today's PWM MOSFET machines so can't say for sure
but my message is: " there's more to powering inverters than inrush ---"
so don't leap into this without awareness of the running current demand that your inverter is going to lay on your source...

old jim

Good ideas...and cautions too

I checked the DC Bus output voltage of the VFD and found exactly the 1.41 times the 3 phase ACinput voltage expected as a DC Bus voltage. Before doing anything with tying a "grid tie inverter. I will do some more checks of the DC Bus voltage of the VFD under similar output currents (using a 3 phase motor of about same current draw ) as what a 6000W "grid tie wind inverter might take.

I'm expecting that the capacitors in the "DC converter section" of the VFD may well perform satisfactorily by themselves.

Time will tell. Thanks

 

[essenmein]

Just be aware the 1.41 thing is for rectification only, if the VFD or inverter is actually running and commanding negative torque for positive rotation (ie generating), then the DC bus (output) voltage cannot be determined from the AC line voltage of the connected machine.

 

[looking]

Got a request into Ametherm regarding their new lines of MS35 Inrush current Limiters.(NTC Thermistors).They are up to 50 amp current ratings and cold resistance values of 0.5 to 20 ohms resistance. Maybe the MS35 3R030 part number will fit the bill.

I explained in detail; the "latest" proposal of what I was contemplating and we'll see if their customer support, free samples and problem solving are all top notch. Their opinion will be very interesting (when received)