Electronics > Power/Renewable Energy/EV's

Allowable input to solar converter

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markfrencken:
Hi, I'm new to this forum so I'll introduce myself and the issue I'm struggling with...

In the Netherlands, electricity is going to hit 1 Eu/kWh likely in the next couple of months for most people when renewing their contract.
I'm a chemical technology engineer with quite some interests and experience in mechatronics and would like to investigate the use of a diesel generator to shave off the peak loads (and use the waste energy to heat my house) to enable using a 'day ahead'electricity contract. Day ahead means you know the hour by hour prices of electricity and when 0.4 l/kWh is cheaper compared to the net, I want to cap my net energy usage by feeding in with the generator. I do have solar panels, but these typically do not produce during peak price hours and batteries are not yet economically feasible, hence the approach.

Now the question: the diesel generator can deliver 1 phase 230V @ 50 Hz, with a capacity of 6 kW.
I do not want to run it at max load, so I would like to connect 2 2kW SMA inverters (with galvanic isolation) to the rectified output of the generator. I'll take the conversion losses etc at face value for the enormeous simplicity of this approach at the net side (if the net drops, I will  not have power I understand but I also do not need complicated and convoluted switchover solutions etc. I might be looking into that direction when I invest in a battery solution).

Rectifying the gen output is one thing, and due to the galvanic isolated SMA's I can choose to feed in on different phases of the 3 pahse system without worrying about earthed neutrals etc.

The SMA converters I want to use also give me the option to select either MPPT tracking or constant voltage for input. This of course also helps a lot.

What I'm not sure about and cannot find in specifications is the sensitivity of the (or any) inverter against ripple voltage. My proposed 'solution' will introduce quite some ripple @ 100Hz that I'm unsure about whether or not a converter can cope with.
I can filter the rectified output with for example a PI filter, but that could cause spikes during switching of the converter and cause all kind of issues as well.

How sensitive are (in general) these solar converters for ripple voltage, ifI were to use a MPPT tracker setting I can imagine it would / could severely affect performance of the control loop, using constant voltage I have no idea but expect it to be better?

What kiind of filter would be the best choice for this application? A PI filter, or something active even? How about voltage spikes when teh converter is switching off ?

Circlotron:

--- Quote from: markfrencken on September 29, 2022, 10:31:04 am ---and when 0.4 l/kWh is cheaper compared to the net, I want to cap my net energy usage by feeding in with the generator.

--- End quote ---
An alternative and simpler method that doesn't need inverters to couple to the AC mains is to use a normal induction motor as a generator. As a motor they resist being slowed below synchronous speed, and as a generator they resist being rotated above synchronous speed. Have your diesel engine coupled to this motor, bring it up to 1500 rpm (4 pole 50Hz) or 3000 rpm (2 pole 50Hz), close the big switch and dial up the power on the diesel while watching the AC amps doesn't go above rated motor amps. No synchronising necessary.

https://en.wikipedia.org/wiki/Induction_generator

Faringdon:
I think you cant just rectify the output of the generator and then feed that to the inverters. Well, you can , but its unwise.
A better way, would be to connect a 4Kw offline SMPS to the gen output…and then connect the two inverters to the dc output of that.
You can either buy a 4kw smps….or buy several parallelable lower power smps’s and do like that, to get the 4kW.

Or you could just use a Boost PFC stage and run the two SMA's from the output of that.....which woudl be say 390Vdc or so...Is the gernerator output pure sine or quasi-sine?

Just rectifiying the gen output will  mean a very low power factor, (between gen and rectifier) and you wont be able to get much power out of it.

when you say you want to rectifiy the gen output.....I understand you mean, rectify and smooth...ie with capacitors following it......so just imagine, 1...the poor power factor...and 2....the hideously high peak currents going on in that rectifier.

By the way, if you can get a gov't grant or something, then ill come over and we can build a 4kW SMPS up for you......i have a quick, cheap way of building high power smps's........a quick way to get the wanted power at wanted voltage........not beautifully packaged etc, but do the job. You can maybe get a gov grant for this....specially now....no gas and all that.

markfrencken:
Hi, thanks for the input.
Unfortunately, any government grants in the Netherlands are only given out to ready made product installers, not DIY, so the grant is currently already absorbed in the prices ;)
Having said that, your remarks have set me to the path of using PFC circuitry of existing large power supplies like for example HP/delta ever psu's. On the other hand, passive massive solutions also have a positive ring to them for the robustness. and, moreover, using a massive inductor before the rectifier would (at least to some amount) mimic the impedance of actual solar panels whilst at the same time largely correct the peak currents of the rectifier/capacitor bank. Now on to looking for a 10 Amp 50 mH inductor, which might be already present in the genset since it was oriinally a floodlight system with metal halide lamps.

Faringdon:
...You could just build yourself a 4kW Boost PFC and  supply the SMA's from that.
As you can see from attached LTspice, PFC circuits are just boost converter ccts with some control...not difficult....you can parallel them with eg ucc28070A......or do a load of cheap easy BCM PFCs in parallel with a good controller....BCM PFCs are very very easy.....You can buy controllers which can paralell them.

You can also get a very good power factor by having eg a constant off time controller with  eg L6564 in COT mode....very easy way to give you a 4kW 390V Bus from which to run your SMA's.....COT just follows the mains sine wave....so you dont even have the current error amplifier to worry about....its almost impossible to get that kind  of PFC wrong......PFC is not perfect like that, but is very very good...0.95 or so.

These Cosel PFCs can be paralleled, but you coudl build  PFC yourself as discussed.
https://en.cosel.co.jp/tool/tag/pdf/SFE_DPF.pdf

A passive PFC circuit for 4kW is going to be absolutely enormous......so much easier to hack a high fequency Boost  PFC up and go like that....just do a simple L6564 COT mode one first to get you started...then develop a CCM Boost PFC one...or several in pllel.

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