question

marekp avatar image
marekp asked

Can I connect DC generator to MPPT 250/70 instead PV panels

I have EasySolar-II GX 48/3000/35-32. and 48V/200Ah LFP battery.

The generator is a 115V/400Hz model with the rectifier bridge and capacitor at the output.

This generator can provide 115V AC at 65A, so after the bridge voltage will be between 115 and 200V DC depending on the load.

I would like to use it as a DC source for the MPPT 250/70 included in my EasySolar-II to charge my 48V LFP battery.

Can it be done safely?

MPPT Controllersgennerator
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3 Answers
wkirby avatar image
wkirby answered ·

Presenting an electromagnetic "machine" to the input of an MPPT has been done before. People have used hydro electric sources in front of an MPPT controller. See here:
https://www.victronenergy.com/blog/2019/01/03/diy-ingenuity-hydro-power-mkii-part-i/
https://www.victronenergy.com/blog/2019/01/23/diy-ingenuity-hydro-power-mkii-part-ii/

You just need to be aware that the MPPT's are designed for solar panels, so they will do some things that you might not expect. They perform a scan periodically to tailor their power point to the changing sky conditions. When connected to a machine, the RPM range will suddenly change without warning. A generator, advantageously, has a governor to limit the RPM when the MPPT unloads to perform a scan.
A solar MPPT can also impose quite a load on the generator as it scans for the maximum power point. This can be limited by reducing the maximum charge current of the MPPT in the settings.

Number one DO NOT allow the generator output Voltage to exceed the MPPT maximum input Voltage. Even a short Voltage spike will destroy the MPPT front end. This is very unlikely to happen in a well designed solar setup, but magnetic sources can spike especially if they are suddenly unloaded. You already have the 250V MPPT and your DC input Voltage is nominally lower, it may be worth considering over Voltage limiting quite a bit below the 250V to prevent anomalous spikes.

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marekp avatar image marekp commented ·

Thank you for your input and all the technical insight into workings of MPPT charger.

Generator, I have, is a solid military machine that have no problem holding RPMs at full 7.5kW and no load thanks to AVR unit.

What would you suggest I use to minimize inductive spikes?

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wkirby avatar image wkirby ♦♦ marekp commented ·

An engine driven generator with controlled RPM is a huge advantage.
You could employ good size capacitor bank which makes up part of the rectifier to absorb any transients. 400Hz rectified will produce a smoother output per capacitor bank size than lower frequencies do.
You could add TVS diode protection just before the input to the MPPT. A fuse will be needed between the capacitor bank and TVS diode.
Selecting a TVS diode is a balance between keeping the reverse standoff Voltage high and the reverse breakdown Voltage a decent amount (maybe 10%) below the MPPT maximum input Voltage. The TVS diode should offer protection within a few picoseconds, but this is not a guarantee that the MOSFET front end of the MPPT won't respond (catastrophically) during this time frame. So, the lower the breakdown Voltage the better. Low breakdown Voltage usually means low standoff Voltage. You don't want the standoff Voltage to be within the normal operating Voltage as this is the point where the TVS starts to conduct which will waste energy or burn the diode out.
TVS diodes are self resetting, but the fuse is not, so nuisance tripping is to be avoided.

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marekp avatar image marekp wkirby ♦♦ commented ·

Thank you again.

Tried to find this TVS diode for my voltage range but it looks like it is impossible.

If the "Reverse voltage stand-off" is above 200V, the "clamping voltage" is 324V and that is to high for 250V MPPT,

For "clamping voltage" below 250V (243V) the "Reverse voltage stand-off" is 150V, to low for possible working 200V.

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wkirby avatar image wkirby ♦♦ marekp commented ·
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marekp avatar image marekp wkirby ♦♦ commented ·

My search of the Farnell site did not produce this one.

Found it after entering its model number.

The price is around 30 Euros.

This 190V will not be a problem?

It is 10V below the max working voltage.

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wkirby avatar image wkirby ♦♦ marekp commented ·

This is the problem with electronic components. They are based on physics. So yes, 190V is where it starts to conduct, not ideal, but by TVS standards it is really good for your application, maybe it saves your expensive MPPT.

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marekp avatar image marekp wkirby ♦♦ commented ·

Thank you for sharing your knowledge.

Before I buy this TVS, I will measure exact voltages produced by my generator.

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dazey77 avatar image
dazey77 answered ·

Can't you just tap into the AC side and use an off the shelf charger that handles 110-240v?

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marekp avatar image marekp commented ·

Do you know of one that takes 115V AC at 400Hz and charges 48V LFP battery?

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Trina avatar image Trina marekp commented ·

I have a friend who had to replace his generator after he got his 48v lithium batteries replacing his flooded ones on his solar setup.

So there's the sticky part -the lipos are picky and very expensive and the generator wasn't designed with that in mind at all.

But besides Google I don't know enough except I hope a victron tech will pipe in as I'll be in a similar setup sometime and an all-in-one easysolar unit I can plug my Honda EU into would be really nice vs yet another thing to add in cost and complication.

I'll be watching this thread!

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heidi avatar image heidi marekp commented ·

Most of the charger (SMPS) won't care about the 400Hz, cause they utilize a full bridge rectifier direktliy behind the input filter. Because of that topology they will also run on DC grid. As long as it is a universal charger (90-240V) the control loop will handle it.

Yes this will end up in a one-sided load for the rectifier, but I'm sure they are not that tightly calculated as other parts in the system.


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marekp avatar image marekp heidi commented ·

How the input filter, designed for 50-60 Hz, would react to 400 Hz input?

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heidi avatar image heidi marekp commented ·

The input filter isn't a filter for 50Hz, the filter (for example a pi filter plus common mode choke) is designed to reduce EMI emitted by the switcher (50-300kHz) inside the SMPS, it is more or less a low pass filter.

Additionally there is at least a passive PFC, sometimes an active PFC, that is tracking voltage/phase, but 400Hz isn't HF, I would not worry about it.

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marekp avatar image
marekp answered ·

Is i possible for someone from Victron to "chip in" and advise about possibility of connecting other than PV panels as a source of DC into MPPT charger?

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ejrossouw avatar image ejrossouw commented ·

Just some thoughts. By using the device for purposes other than designed for and not in line with the manufacturer's instructions, you will not get support and also immediately invalidate a warranty. I also imagine, setting a precedent would also not be in their best interest and neither is dishing out detailed design information. I am totally in favour of experimenting, but as long as you accept it will be at you own peril / expense.

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marekp avatar image marekp ejrossouw commented ·

I understand what you are saying but, from the technical point of view there should be no difference for MPPT where the current is coming from. MPPT would simply adjust the current, to get max power from the source. The characteristic of the generator is similar to that of the PV panel. The more current the less voltage would provide.

It would be helpful if someone from Victron took time and comment this idea.

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