Diesel generator into MPPT on PV inverter - current limiter

[flyingdingy]

As our electricity prices are going nuts (over $1 pr. kWh, and heavily increasing) it is now alot cheaper to run a diesel generator.
As I already have a 20.6kWp PV system on my roof, I would love to utilize my PV inverter to combine the two power inputs, giving full flexibility of utilizing all the solar energy, diesel generator energy, and the possibility of using grid energy when welding and such.
My diesel generator is single phase 230v AC 16amp / 3600W.

What I want to do is to connect my generator via a rectifier (convert to DC) to a free MPP tracker/string on my PV inverter (Growatt MID 20KTL3-XL)
The inverters voltage range is 200-850V DC and maximum 26A pr. MPP.

But, the generator has max efficiency at 70% load. So I need a method to limit the output current of the generator / input current to the inverter to aprox 12 amp. This should take care of almost all my energy needs, and the surplus energy to be sold back to the grid.

Is this possible? .... how?

 

[KlausST]

Hi,

For me this is the wrong way to go.
* you are mistreating the MPPT (meant for solar)
* and "limting the output current" basically is done by dropping the voltage. But dropping the voltage means waste of energy.

I´d use a battery charger for: AC_in --> battery_out
Choose one specified for your expected power/ current

Klaus

 

[flyingdingy]

Thank you - yes, a battery charger of above 200V might be the way to go!
Thinking about it now it is rather obvious....

I know, it is not the way a MPPT is supposed to be used - but the inverter is grid tied, so any surplus energy is sold to the grid. To use it the "proper" way would require a hybrid-inverter with a powerbank and dedicated circuits...
Mistreating it like this allows me to utilize all functions, and sell to the grid the surplus energy.

 

[KlausST]

Hi,

Mistreating it like this allows me to utilize all functions, and sell to the grid the surplus energy.

I don´t think so.
MPPT is meant to work with photovoltaic cells. ... which can be seen as "high impedance current sources".
But an AC generator is no "high impedance current source" it rather is a "low impedance voltage source".
So in best way it won´t work. In worst case something will burn.

From an efficiency view I doubt converting 230V AC down to 12V DC (or whatever the battery voltage is) and then back to 230V AC.... is quite not the best way to do.

As far as I know - in my country - it´s illegal (without special permit) to use a diesel engine to push energy to the grid.

Klaus

 

[flyingdingy]

What I was thinking now is a DC power supply of 250v DC with input connected to the generator (230v AC) and output to the MPPT (250v DC) at 12a / 3000W

220V DC power supply, 3000W • DWE

The MW3001-PS250 is a power supply with an input voltage of 230V AC and an adjustable stabilized output voltage of 220V DC.

www.dwe-oss.eu

I must confirm if this is legal or not - it might not be considering the "green aspect"

Or just do the right thing and get a MultiPlus-II from the Victron ESS system...

1. ESS Introduction & features

www.victronenergy.com

 

[KlausST]

Hi,

What I was thinking now is a DC power supply of 250v DC with input connected to the generator (230v AC) and output to the MPPT (250v DC) at 12a / 3000W

I guess you don´t understand how MPPT works.
--> the MPPT will not work with a constant voltage (low impedance)

Klaus

 

[crutschow]

How would you synchronize the Diesel AC voltage to the grid, even if you could?

 

[betwixt]

I have ~2KW of PV and a 6KW Diesel generator so I face a similar problem. Thankfully I'm signed up on a 2 year fixed price contract for electricity at about $0.5 per KWH with 18 months to run.

There is absolutely no way a grid tied inverter will track the generator output frequency and as already pointed out, driving DC to an MPPT system defeats having it there at all. The only way I can find to solve the problem is to use the Diesel output to drive a pure sine inverter and use that instead of grid power. A pure sine inverter will be crystal controlled so its output frequency should be accurate and stable, allowing the grid-tied inverter to lock on to it. It isn't a cheap solution and I don't know of any commercially available pure sine inverters that run off 230V AC. Its a pity because ones with 12V/24V inputs step up to higher voltage internally before the output stage and all that circuitry would be redundant if the higher voltage could be used directly from an external source.

Brian.

 

[Easy peasy]

The grid tied inverter should meet local standards requirements for disconnection when the real mains fails - this is the main safety issue.

As to powering it off a stiff voltage source such as a charger - at 250VDC in this case - it will work ( the MPPT ) but will sit at full power, until the Vout from the charger sags a bit.

If you can obtain a charger with a controllable Imax out ( i.e. a current limit, ideally a slightly soft current limit ) then the MPPT in the GT inverter may, stress may, settle at the Imax point - so you can control the power to the grid by adjusting the Imax out of the charger.

Best of luck.

 

[flyingdingy]

Hi,

I guess you don´t understand how MPPT works.
--> the MPPT will not work with a constant voltage (low impedance)

Klaus

Well - that is the main reason for posting in a forum with knowledgable people...
But I think I got a fair grasp on how MPPT works (checking for the optimal voltage for every given situation of the PV with varying temperature, coverage and so forth...) - kinda irrelevant for this given situation as I will be feeding a constant voltage and current.
What I do not know is how the MPPT/inverter would react to this static voltage situation.
What I am hoping will happen is that it finds the "Maximum power point" at the setting set at the DC power supply and are happy with that.

How would you synchronize the Diesel AC voltage to the grid, even if you could?

That is one of the issues I want to avoid by going through a DC power supply in to the PV inverter.
The PV inverter would act as the synchronization unit.
Diesel AC->DC powersupply -> PV Inverter/MPPT -> Grid

The grid tied inverter should meet local standards requirements for disconnection when the real mains fails - this is the main safety issue.

As to powering it off a stiff voltage source such as a charger - at 250VDC in this case - it will work ( the MPPT ) but will sit at full power, until the Vout from the charger sags a bit.

If you can obtain a charger with a controllable Imax out ( i.e. a current limit, ideally a slightly soft current limit ) then the MPPT in the GT inverter may, stress may, settle at the Imax point - so you can control the power to the grid by adjusting the Imax out of the charger.

Best of luck.

Correct, that is my main concern now - what happens to the surplus power from the DC powersupply in case of a grid fallout / islanding situation.
The islanding function of the inverter should not be impacted, as the supply will be in to the MPPT and therefore to the inverter act like a PV module.
The DC powersupply I refered to ( https://www.dwe-oss.eu/product/220v-dc-power-supply-3000w/ ) can set either constant voltage or constant current, but also Overload Protection Type "continuous constant current limiting".
But ideally would be a powersupply / charger with both constant voltage and constant current

 

[KlausST]

Hi,

given situation as I will be feeding a constant voltage and current.

No. The current is not constant. Just the voltage could be seen as almost constant.
This is completely differnt to a photovoltaic cell.

PV: increase the current ... and the voltage will drop. An there is a point where an percentual increase in current lead to a higher percentual drop in voltage causing less power. The MPPT point is found. The same is true in the opposite direction.
The total power that can be delivered by the PV cells is physically limited.

***
Voltage from a generator.
The voltage is considered to be stable. The more current the higher the power. No limit.
O.K. there is a limit. And the MPPT of a generator is when the generator impedance and the load impedance are equal.

What I am hoping will happen is that it finds the "Maximum power point"

So let´s say 230V and the generator´s internal impedance is 1Ohm. Then the MPPT point is when load also is 1 Ohms.
= 2 Ohms total loop impedance. (ignoring phase shift).
So the MPPT is at 230V and 115A = 26.5kW. Where 13.25kW (50%) really can be "used" and the other 50% are dissipated as heat = loss.
While the 1 Ohm is realistic for your generator ... it is not realistic to draw 26kW from your generator ... and one does not want to dissipate 50% of power.
Since the generator does not "limit" the delivered power (like a PV does) .... the MPPT will make the current to rise until the fuse drops. Resulting in no usable power.

There may be MPPT controllers with internal current limit. But be careful. If it just states "max 1kW" .. does it mean you must not connect more than 1kWp solar panels .. or there is an internal limit? Careful manual reading is essential.

You are free to desgin by "hoping" .. it´s just not my way.

Klaus

 

[flyingdingy]

Thank you for a good informative reply!
Learn something new every day.
I do mostly understand what you are describing - but is the DC power supply taken into account?

To my (limited) electronics understanding, I was under the impression that the DC powersupply would act as the "consumer" as far as the generator is concerned, and with volt and current limiting in the DC powersupply / charger, limited to i.ex. 3kW, be within the specs of the generator.
This is under the presumption that it is actually possible to limit both volt and current through the power supply / charger...

 

[KlausST]

Hi,

but is the DC power supply taken into account?

Indeed I more talked about about the rectified AC of the generator.
A common DC power supply gives even more stiff (low impedance) output .. making things worse.

It also adds it´s own power loss.

If you want a power supply like a benchtop with adjustable output voltage and current ... then one needs to read it´s datasheet carefully about power loss (in both modes: voltage and current limiting), timing ... and so on.

Klaus