Adding a wind turbine to existing on-grid PV system

[marekm]

I'm considering how to add a 3-phase 2kW 350V wind turbine to existing on-grid PV.
Some say it will not work properly and I must have a dedicated (expensive) wind turbine inverter, I want to know why and try to find some cheaper way around it.
The existing PV system consists of 2 strings, each 8 panels, each 450W (Voc=50V Vmp=41V Imp=11A), connected to a Chinese 8kW on-grid inverter with 250-850V MPPT input range (typical string Vmpp is about 300-330V).
The wind turbine 3-phase output will be rectified by 6-diode 3-phase bridge, up to about 500V 4A DC at nominal 15m/s wind.
Safety first, I plan to make an independent emergency brake circuit (basically fire an SCR to put a high load until it almost stops if too high RPM is detected, simple LM2917 circuit should do the trick, pure analog solution with no buggy firmware to crash).
But now here is my crazy idea - use the PV string as dump load (normal non-emergency for moderate wind speeds, basically a high-power shunt voltage regulator well cooled by wind), by connecting the 3-phase bridge DC output (with some series inductance to reduce ripple current) in parallel to it.  The inverter should see this like sun was shining when it's not, and try to minimize power loss in PV panels by drawing just enough current to extract maximum power.
Now, this is the part people say will not work, MPPT algorithm will not handle the wind turbine properly - but why?
It should not stop it completely, as minimum MPPT voltage of the inverter is 250V.   So at lower wind speeds it will just spin producing very little power until it exceeds 250V.
Power is proportional to cube of wind speed, so 2kW at 15m/s and 500V DC is only 250W at 7.5m/s and 250V DC (just an approximation, but should be reasonably close).
Can the ripple current from 3-phase rectifier cause the MPPT to misbehave (detecting it as fast power changes and trying to compensate)?
Better solution could probably be to add a step-up DC/DC converter that draws power as cube of voltage and produces constant current for PV inverter, but this complicates things and brings EMI issues etc.  So it may be acceptable to lose some efficiency with a simpler circuit.  What do you think?

[trobbins]

Suggest you look up anti-islanding compliance requirement of any power generator connected to grid.  In many countries that requires a compliant protection contactor (ie. $ along with approval permit and cost).  Even then, you need to sync the wind turbine to the grid - is that what you have?

MPPT looks for a valid I-V curve from the PV string.  Do you know what I-V curve your wind gen provides after rectification?  PV inverter likely looks for isolated PV string and does an IR test - is your wind-gen and rectifier suitably floating to at least 1kVdc with negligible leakage?

[marekm]

Anti-islanding is already handled by the existing PV inverter, it's on the NC-RfG certified list, no changes here.  I'm only connecting to the DC input of the inverter.
The wind turbine is variable speed PMSG 3-phase, I haven't bought it yet, the currently offered ones (priced about 1k EUR) are specified as "2kW 350V" and branded "Ista-Breeze".
https://allegro.pl/oferta/elektrownia-wiatrowa-ista-breeze-2kw-350v-on-grid-13375059470
The seller can't really say much more about them, they just sell it and have no idea how to count the number of pole pairs etc.
Anyway, first I'm recording wind speed statistics at that location for another few months before making final decisions to buy it or not.
Not connecting it directly to the grid, of course.  Its windings should be isolated (just like PV panels), as the inverter is transformerless.
The inverter would see a parallel connection of the PV string and rectifier output, so my hope is it wouldn't be too different from PV string alone - the rectifier adding some extra current from the wind, and wind turbine speed limited by load increasing with voltage.
Or is the rotational inertia a problem, like when the inverter makes a step voltage change to which PV responds much faster but wind turbine needs some time to spin up/down?  But as far as I can see how the MPPT works, it seems to make small adjustments, not large steps.

[Someone]

Or is the rotational inertia a problem, like when the inverter makes a step voltage change to which PV responds much faster but wind turbine needs some time to spin up/down?  But as far as I can see how the MPPT works, it seems to make small adjustments, not large steps.

You would need to be sure of the specific MPPT implementation, some periodically do open circuit voltage. But even without that most MPPT algorithms will expect a very rapid I/V relationship which the turbine will not produce, other complications such a the ripple of a generator may throw out a simple MPPT algorithm.

[uer166]

my hope is it wouldn't be too different from PV string alone

That's not really what PV MPPTs expect, a normal algo would almost surely fail here. A panel is a nice I/V curve with near instantaneous response, which the generator would not be. At the very best you might make an either/or system at suboptimal MP (either the MPPT tracks solar, or the turbine). I suggest getting a proper turbine MPPT grid-tie inverter and paralelling it at the grid side, which would keep system optimal.

[trobbins]

I guess you're planning on using a steering diode to avoid reverse dc current to the rectified wind, as your aim was not to use a steering diode for the PV as you were hoping to dump current into the PV strings. 

At low PV, if wind increased then your rectified wind voltage could exceed the PV MPPV and hence wind current would start to flow.  One scenario could be that the MPPT disables power flow (eg. fault or error or ...), and so PV voltage could easily be pushed above PV OCV (as you inferred), and unless otherwise constrained/feathered could approach PV inverter max PV rating, and/or PV module bypass diode PIV.  I'd anticipate the current flowing through the PV string in the fourth quadrant would still be dependent on the insolation level - so the effective loading presented by the PV string could be lower than you may want to hope for, especially at night.

[marekm]

The 3-phase rectifier itself blocks reverse current, its diodes are reverse-biased if there is too little wind.  The PV string should limit the voltage to 450-500V, well below inverter ratings (MPP 850V, OCV 1000V).  Most of the wind here is when there is little sun, and there isn't really that much wind to justify added cost of separate wind inverter.  The wind generator + rectifier + inductor is intended to appear to the PV system as a parallel current source (adding current as if there was more sunlight).  My hope is to achieve this with practical inductor size/cost, like a 400W sodium lamp ballast (rated 230V/50Hz/4.45A which I estimate to be about 150mH) - or could that still be too small?

BTW, I've just found a probably little known but interesting 3-phase voltage doubler circuit with 12 diodes and 3 capacitors - US5444357A patent (expired in 2013).  It could double the rectified wind generator voltage at low wind speeds when there is also little load current, and at high currents (when the capacitors are too small) it becomes close to the ordinary 6-diode rectifier, so should appear a bit more like a current source.  I'll try to simulate it.

[uer166]

I've designed MPPT converters for solar, and if someone connected a large value inductor with a generator source to one of mine I'd be utterly terrified.

Remember panels are NOT current sources with unlimited voltage headroom. By placing an inductor like that, you risk blowing up the PV MPPT when it goes from full load to no load.

I'm also fairly certain this would defeat good MPPT algorithms such as a full panel scan for searching max global power point, since the generator response looks nothing like a PV string. There is a desire to make that search as fast as possible, so at least in mine there would be no allowance for settling time other than minimum requirements.

By all means try it and report back, but be prepared to not only buy an expensive turbine MPPT inverter, but also replace your existing damaged PV MPPT inverter.

[uer166]

Another thing is by rectifying the generator with dumb diodes into a cap, you slash your generation by a decent amount due to crappy power factor. I would expect to have some kind of PFC-esque  bridge front-end on a real generator interface to utilize the power available properly.

[marekm]

If the PV MPPT goes from full load to no load, the voltage spike from the inductor would be clamped by the PV string.  There is a possible failure mode if the PV goes open circuit (like damaged by bad weather), so it would probably be wise to add some clamp to the inductor (normally there is only DC + some ripple across it).

How fast is really the fast panel scan?  Fast enough that current in the inductor will not change too much during that time?  Of course we don't know what the specific inverter does (cheap no-name China Export, similar ones are also sold with ThinkPower logo) but what is it usually - on the order of a mains cycle, a few cycles?

As for the power factor, the 3-phase 6-pulse rectifier is not that bad actually, generator windings leakage inductance should do a little passive PFC, no need for a cap at the output (the doubler would have small capacitors as it's only for low power levels when doubling is needed).  Yes if we were talking about megawatts that would matter a lot, but it's just 2kW peak (at 15m/s which is rare) and at half that wind speed it's just about 250W (1/8 power = 1/2 voltage * 1/4 current) and 1/16 loss in the windings resistance.

Plan B if I can't make PV+wind work properly is to heat a hot water storage tank with the wind power, resistive heater with a series capacitor seems well matched (assuming both voltage and frequency proportional to wind speed, power would be close to cube of wind speed).

[trobbins]

Perhaps do a little proof of concept by applying DCV across one of your PV panels at night and see what loading it provides as DCV increases - that may make you rethink your proposal.

[uer166]

Why do you think PV string will clamp anything? I mean eventually it'll go into breakdown, but that is much, much higher voltage than a reasonable MPPT will be able to accept.

[Dacian]

Bad idea to install a wind turbine.
First the spec mention's a 2.2m diameter turbine so that will be about a 1.2kW wind turbine best case in nominal 12m/s.
I made a search and it seems their nominal power spec is for 15m/s witch is just not industry standard and was done to look much better than it is.
Even 12m/s is 43km/h and you will not see that sort of wind speed in most place where people live.
I had a smaller 300W version so about a quarter of this and while is fairly windy here in middle of Canada on a flat land with no building or anything to interfere around. I just disconnected after fist year of use and used the cables to install more solar panels.
When it was cloudy for a full week there was also no wind. If there was wind the clouds went away and solar will produce way more.
In high winds it can get damaged even with electromagnetic brake enabled. And you also need a very strong wind turbine tower as forces involved are crazy.
Noise and vibration may also be an issue for you and maybe your neighbors. It was no issue for me as turbine was small and detached from house that is super insulated but I cold still hear it especially in stronger winds and I have no neighbors in visual distance.   

[marekm]

PV cells are basically large photodiodes, or diodes forward-biased by light-dependent current source.  PV panels have many such cells connected in series-parallel, with bypass diodes added (but normally reverse-biased, except if part of the string is shaded).  So it's not a breakdown, the cells are still forward-biased, at -Isc in darkness (4th quadrant) the voltage should be close to normal Voc.  I have actually measured it on a small "12V 30W" (Voc=21V Isc=1.9A) panel, it starts to draw a few mA at about 15V in darkness, as I would expect from 30 forward-biased Si diodes in series.

As for good or bad idea, for now I have installed a wind sensor https://wiki.dfrobot.com/RS485_Wind_Speed_Transmitter_SKU_SEN0483 and logging wind speed every few seconds - in a few months should have more data to decide.  PV works very well here in spring and summer, not so much in autumn and winter but at that time there is often much more wind (though we too had "Dunkelflaute" - little sun, little wind, lots of coal burning).  Valid point about vibration and noise, but does the turbine really make much more noise than the wind itself?

[uer166]

Huh I learned something new today. Always thought that a PV diode cell cathode is the positive side of the panel, which appears to not be the case. (i.e. you seem to be correct, and it should clamp).