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Can I connect a hydro-electric generator to a Axpert inverter?


Cassie

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My son has a strong stream of water coming down the mountains (head; over 150m) 24/7.    To start as an experiment he is considering installing a small hydroelectric 1000w 220v generator and store energy in 4x 105ah gel batteries.

  1. Can he connect this generator to a simple 5kva Axpert and use it's MPPT to charge 4 batteries?  (he has a surplus Axpert)
  2. In view of the fact that this generator will run 24/7,  will it take the batteries to SOC of 100% via the Axpert (assuming that due to the amount of water and the head a max output of 800 - 1000w will be possible)?
  3. As far as I can remember the Axpert doesn't have a generator input, can he just use the AC input?
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Other people will chip in, but here is my take on things.  The hydro generator will have the same problem a petrol generator has.  An unstable 50Hz frequency and fluctuating voltage.  It is ironic that most loads would tolerate that (within reason) but an Axpert and many other type of inverters will not.  Sad but true.  

My believe is that you could run the 230V through a full-wave bridge rectifier followed by a electrolytic smoothing capacitor. This would give you about 320V DC.  This voltage should sit bang in the centre of a high voltage MPPT.

Your 1000 odd watts should be well within the power/input current limit of an Axpert with a high voltage MPPT.

@Coulomb will be very quick to warn us that you cannot do so for high power generators (say 5KVA) as they would be able to source too much current for the MPPT to handle.  I am sure he is right, but I am still scratching my head as to why a very intelligent DSP that is in charge of the MPPT, is not clever enough to self-limit the incoming power.

If the above is not possible, the only other solution would be to bypass the Axpert and charge the battery with a dedicated battery charger.  Getting a 1KW battery charger would be hugely expensive.

PS. A 50A bridge might cost you R 150.  And a high voltage capacitor a bit more.  So this could be implemented at a very reasonable cost.  You would need to mount a heatsink onto the bridge rectifier.

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I'm not an expert in this, but here is my 2¢ worth.

An MPPT is designed with PV panels in mind, obviously. One of the characteristics of PV panels is that as you load them more, their voltage drops and you get a smooth increase in power. Load them less, and the voltage springs back. But the critical thing is that you gradually get more power as you put more load on. If you like, the panels have a relatively high internal resistance. You can short circuit them, and the current will be quite limited; Isc for any panel is only a little larger then Imp.

I don't know much about hydrogenerators, but it seems to me that they would be more like an electric motor/generator. An electric motor has a quite low internal resistance, and short circuiting the output of a motor will generally result in damaging current levels. Though maybe if the mechanical input is weak (I imagine that peleton wheels are pretty weak for example), maybe you can get it to look more like a PV panel than a motor.

My concern is that the solar MPPT will adjust its PWM ratio, effectively adjusting the "back EMF" of the PV input by a small amount, but this small change in back-emf will generate a large change in current, because you have two voltage sources connected by a small internal resistance.

Motor controllers deal with this issue, so it can be handled, but my concern is that you may need a rather different design than a standard solar MPPT. Motor controllers operate over small time scales using the inductance of the motor windings to limit the current. Once you convert the generator output to DC, you can't use that technique. I think what you want is effectively a motor controller, and to draw power from the generator smoothly, you'd to the same thing that your electric car does when it regenerates and pushes power into the battery. In this application, you don't have use for what is normally forward power flow: battery to motor, as that would use energy to make the stream flow faster.

But a motor controller is a very different beast to a solar MPPT. For one thing, the motor controller is three phase.

A wind generator's charge controller would possibly be the closest off-the-shelf thing that might work in this application. You could presumably connect such a thing directly to the inverter's battery. The speed might be all wrong; I imagine that wind turbines spin at much slower speeds than a generator. You might be able to overcome that with mechanical gearing.

I hope that this makes some sense.

You really need to find someone with expertise in this area. I'm not that person.

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48 minutes ago, Coulomb said:

I don't know much about hydrogenerators, but it seems to me that they would be more like an electric motor/generator. An electric motor has a quite low internal resistance, and short circuiting the output of a motor will generally result in damaging current levels. Though maybe if the mechanical input is weak (I imagine that peleton wheels are pretty weak for example), maybe you can get it to look more like a PV panel than a motor.

@Coulomb Yes I agree.  In theory one would be able to add a series resistor after the bridge rectify but this might have to be a very high wattage resistor mounted on a heatsink.  Not ideal for efficient power transfer.  I need to see if I can calculate the internal resistance of a typical panel to see where one would end up.  Obviously whatever the resistance, one would need to multiply this by a number of "panels" that would typically reach the 320VDC level.  Adding a series resistor would be alright for a 100 or 200W system.  If one talks about 1KW+ it gets difficult.  For me, any power manipulation in the KW range is "difficult".  The principals are the same.  The circuits are the same.  It's just that the holes in the semiconductors, the holes in the pocket and the brown marks in the undies are potentially much bigger.

I would have expected such hydro-systems to use a 3-phase generator.  I believe that most wind generators use 3-phase.  Converting that to DC would obviously reduce the ripple voltage. 

I do not follow your logic about needing a circuit that is similar to a motor controller   A MPPT is basically a SMPS that has an integrated (firmware controlled) battery charger.  So I don't see what the problem would be to feed the MPPT hardware with a constant voltage and manipulate the battery charge current not only for battery safe operating area but ALSO for limited input power (so as to not overload the generator).  I am under the impression that one should be able to re-configured it for such constant DC voltage with firmware only, and retain the hardware architecture.  In other words, use the MPPT hardware to implement a "normal" battery charger.   This would only be possible for someone that has the source code - i.e. the manufacturer.  Perhaps this would be redundant on inverters with only one MPPT, but that sort configuration choice should at least be made available to inverters that have more than 1 MPPT.

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29 minutes ago, Modina said:

@Coulomb Yes I agree.  In theory one would be able to add a series resistor after the bridge rectify but this might have to be a very high wattage resistor mounted on a heatsink.  Not ideal for efficient power transfer.  I need to see if I can calculate the internal resistance of a typical panel to see where one would end up.  Obviously whatever the resistance, one would need to multiply this by a number of "panels" that would typically reach the 320VDC level.  Adding a series resistor would be alright for a 100 or 200W system.  If one talks about 1KW+ it gets difficult.  For me, any power manipulation in the KW range is "difficult".  The principals are the same.  The circuits are the same.  It's just that the holes in the semiconductors, the holes in the pocket and the brown marks in the undies are potentially much bigger.

I would have expected such hydro-systems to use a 3-phase generator.  I believe that most wind generators use 3-phase.  Converting that to DC would obviously reduce the ripple voltage. 

I do not follow your logic about needing a circuit that is similar to a motor controller   A MPPT is basically a SMPS that has an integrated (firmware controlled) battery charger.  So I don't see what the problem would be to feed the MPPT hardware with a constant voltage and manipulate the battery charge current not only for battery safe operating area but ALSO for limited input power (so as to not overload the generator).  I am under the impression that one should be able to re-configured it for such constant DC voltage with firmware only, and retain the hardware architecture.  In other words, use the MPPT hardware to implement a "normal" battery charger.   This would only be possible for someone that has the source code - i.e. the manufacturer.  Perhaps this would be redundant on inverters with only one MPPT, but that sort configuration choice should at least be made available to inverters that have more than 1 MPPT.

Your way of thinking is spot on as per information from Sunsynk to use a 3ph rectified input from a wind generator directly to a MPPT. They advise a 1kW 1 ohm resistor. I see no problem why a 1ph genny cannot be restified and used in the same way. What Sunsynk don't show is using enough caps to smooth the ripple. Perhaps they feel it is not needed. 

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A three phase generator will have very little (rectified) ripple.  The phases are overlapping and never reach the zero volt point in the way a single phase does.  A SMPS should ideally run far over 20KHz and should be able to handle large input ripple, but it is always advisable to keep the ripple reasonably low.  In a single phase application one would definitely need to place a smoothing cap after the rectifier.  If there is the  requirement to increase the source impedance, that would have to be done after the capacitor. 

A 1KW 1ohm resistor???  That can't be right.  If we assume an input current of 40A (at a high voltage that would be a massive input power) a one ohm resistor would **only** dissipate 40W.

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1 hour ago, Modina said:



A 1KW 1ohm resistor???  That can't be right.  If we assume an input current of 40A (at a high voltage that would be a massive input power) a one ohm resistor would **only** dissipate 40W.

I am not going to debate it with you. Rather take it up with Sunsynk and tell them their manual is asking for the wrong size of resistor. This picture is from their manual. 

My notes have it P=40^2*1=1600W not 40W.

IMG_20230405_212054.jpg.c9fdbefd870214861535f7302e11f9b1.jpg

Edited by Scorp007
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Sorry @Scorp007.  I lost my mind.  You are right.  But what a waste of power.  Something like that is a heater and shouldn't be called a resistor.  SunSynk must be thinking of seriously large wind turbines.

I wonder if the current through the field windings of a generator could be manipulated to vary the "apparent" output impedance.  In this way no power would need to be burned.  But I am no power or motor specialist.  I rather shut up before making another dumbass of myself. LOL.

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17 hours ago, Modina said:

Sorry @Scorp007.  I lost my mind.  You are right.  But what a waste of power.  Something like that is a heater and shouldn't be called a resistor.  SunSynk must be thinking of seriously large wind turbines.

I wonder if the current through the field windings of a generator could be manipulated to vary the "apparent" output impedance.  In this way no power would need to be burned.  But I am no power or motor specialist.  I rather shut up before making another dumbass of myself. LOL.

It act partially as a dump resistor. When you talk of up to 5kW of power being sent to a MPPT on a 8kW inverter then 1kW of anything is small. 

Also refer to what level is called high voltage😅

IMG_20230406_153853.thumb.jpg.7eaa999315215743ae73a4ec40dc6bd7.jpg

 

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I'm an electronic engineer.  12V is high voltage for me. 😆
Once upon a time, I designed some EEG amplifiers used for neurofeedback....  Brain waves are measured in a few uV.
My bible is The Body Electric by Robert Becker.  Bone has piezo-electric and semiconducting characteristics.  That book is the reason why I dislike strong H & E-fields, RF radiation and that sort of thing.

Maybe you can use that heater that you call a resistor for both wind gen functions.  To increase the source impedance and as a real dump when the wind gets too strong or batteries are full.  Might have to place a tap somewhere in the middle for MPPT usage.  And the full length of the wound resistor as the dump.

Edited by Modina
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  • 6 months later...
On 2023/04/03 at 5:20 PM, Cassie said:

 

My son has a strong stream of water coming down the mountains (head; over 150m) 24/7.    To start as an experiment he is considering installing a small hydroelectric 1000w 220v generator and store energy in 4x 105ah gel batteries.

  1. Can he connect this generator to a simple 5kva Axpert and use it's MPPT to charge 4 batteries?  (he has a surplus Axpert)
  2. In view of the fact that this generator will run 24/7,  will it take the batteries to SOC of 100% via the Axpert (assuming that due to the amount of water and the head a max output of 800 - 1000w will be possible)?
  3. As far as I can remember the Axpert doesn't have a generator input, can he just use the AC input?

Hi Cassie, this can be done. but you need a special PID controller for the setup. The best way is to tie into the mppt dirctly after rectification. the volatges do go high and i think that the axpert have a low voltage mppt. it is tricky but can be done.see my above post of our micro hydro's we have done this with before. awesome small machines. 

Im not sure of the flow he has, but at 150m it will be a pelton turbine he will need to install.

 

Edited by Brian_SA
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44 minutes ago, Brian_SA said:

Hi Cassie, this can be done. but you need a special PID controller for the setup. The best way is to tie into the mppt dirctly after rectification. the volatges do go high and i think that the axpert have a low voltage mppt. it is tricky but can be done.see my above post of our micro hydro's we have done this with before. awesome small machines. 

Im not sure of the flow he has, but at 150m it will be a pelton turbine he will need to install.

 

Question? where will you use the PID controller to control what part of the of the PID loop, also i would think you would need a set parameter for your PID to control. Will be the speed of the turbine or the max voltage of the MPPT.

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6 minutes ago, Antonio de Sa said:

Question? where will you use the PID controller to control what part of the of the PID loop, also i would think you would need a set parameter for your PID to control. Will be the speed of the turbine or the max voltage of the MPPT.

We use the PID control to feed the power through to the inverter once we have the voltage stable on the ballast load. We control on voltage and ensure once the voltage is rectified we don't overshoot the max MPPT rating.

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7 minutes ago, Brian_SA said:

We use the PID control to feed the power through to the inverter once we have the voltage stable on the ballast load. We control on voltage and ensure once the voltage is rectified we don't overshoot the max MPPT rating.

From your explanation the control variable is the voltage, what do you use as the derivative parameter. Do you use a PLC or a dedicated controller?

Edited by Antonio de Sa
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  • 6 months later...
On 2023/04/05 at 8:28 PM, Modina said:

A three phase generator will have very little (rectified) ripple.  The phases are overlapping and never reach the zero volt point in the way a single phase does.  A SMPS should ideally run far over 20KHz and should be able to handle large input ripple, but it is always advisable to keep the ripple reasonably low.  In a single phase application one would definitely need to place a smoothing cap after the rectifier.  If there is the  requirement to increase the source impedance, that would have to be done after the capacitor. 

A 1KW 1ohm resistor???  That can't be right.  If we assume an input current of 40A (at a high voltage that would be a massive input power) a one ohm resistor would **only** dissipate 40W.

Good day @Modina

 

I have a client that I have connected a 1.5kw 3phase river pump, going through a 3 phase rectifier, I am getting a fluctuating voltage between 310V and 330V. I have connected the pump to a Growatt axpert 5kw inverter with a high voltage mppt. The inverter sees the power coming in, but only takes about a 100w from the motor. Is there anything I might have missed?

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