help with step down transformer for wild ac

Thread Starter

liefryan

Joined May 4, 2021
8
I have a hydro turbine that generates 100-600v (depending on load) 3phase ac with a long cable run, i need to make a step down transformer to a max of 150v, any help would be much appreciated.
 

Papabravo

Joined Feb 24, 2006
16,777
I have a hydro turbine that generates 100-600v (depending on load) 3phase ac with a long cable run, i need to make a step down transformer to a max of 150v, any help would be much appreciated.
Are we converting 3-phase to single phase in the process? If not I don't see how you can expect 150VAC output from 100 VAC input. AFAIK transformers operate on fixed conversion ratios which would make them inappropriate for your needs. More likely would be an offline AC to DC converter, followed by a true sine wave inverter to produce 150VAC output. That would be a fairly ambitious design and construction project, especially if you have never done any work with a SMPS.
 

Thread Starter

liefryan

Joined May 4, 2021
8
Are we converting 3-phase to single phase in the process? If not I don't see how you can expect 150VAC output from 100 VAC input. AFAIK transformers operate on fixed conversion ratios which would make them inappropriate for your needs. More likely would be an offline AC to DC converter, followed by a true sine wave inverter to produce 150VAC output. That would be a fairly ambitious design and construction project, especially if you have never done any work with a SMPS.
Hi, thanks for the reply.
I'm aiming for a maximum of 150v, and i could go from 3 phase to single phase or use a 3phase bridge rectifier either one, i'm looking to use a charge controller (max input 150v DC) to charge my 48v battery bank
 

Papabravo

Joined Feb 24, 2006
16,777
Hi, thanks for the reply.
I'm aiming for a maximum of 150v, and i could go from 3 phase to single phase or use a 3phase bridge rectifier either one, i'm looking to use a charge controller (max input 150v DC) to charge my 48v battery bank
If we have a transformer with a 150VAC output and a 600 VAC input that implies a turns ration of approximately 4:1. When the input drops to 100 VAC you will have an output of 25 VAC, which rectified gives you 35.4 VDC which won't do much for recharging your 48 VDC battery bank. How did you plan to work around this detail? I'm not really clear on what you're trying to do, but I don't think you have the correct approach. That is why I suggested an alternative that has some potential for meeting what I think are your requirements.
 

Ian0

Joined Aug 7, 2020
3,221
My suggestion would be to rectify the three-phase, then put it through a suitable MPPT, to charge the batteries. Then use a sinewave inverter to produce the AC.
 

Thread Starter

liefryan

Joined May 4, 2021
8
i see what you're saying, at the moment i have the wild ac rectified and feeding a grid tied inverter with very wide input voltage range then ac coupled to my main off grid inverter charger, this setup works well but i am trying to work out a way of using the same hydro turbine on a different setup with an off grid inverter that doesn't support ac coupling, i hope i have explained it well enough?. maybe a voltage limiting circuit could be built?
Thanks again.
 

Thread Starter

liefryan

Joined May 4, 2021
8
My suggestion would be to rectify the three-phase, then put it through a suitable MPPT, to charge the batteries. Then use a sinewave inverter to produce the AC.
My suggestion would be to rectify the three-phase, then put it through a suitable MPPT, to charge the batteries. Then use a sinewave inverter to produce the AC.
That is basically what i want to do, the problem is that max input voltage on the mppt is 150v and turbine voltage can exceed 500v
 

Ian0

Joined Aug 7, 2020
3,221
Victron will do Solar MPPTs up to 250V, I imagine that they would work with a water turbine, because the power is limited in a similar way i.e. by the water flow, instead of by the amount of sunshine.
I presume that the frequency of your turbine varies with the voltage, and that would be important in finding a suitable transformer.
That means that the transformer won't saturate as the volt-time constant stays about the same as the voltage and frequency vary; but as the frequency goes up core losses will start to increase.
 

Thread Starter

liefryan

Joined May 4, 2021
8
Victron will do Solar MPPTs up to 250V, I imagine that they would work with a water turbine, because the power is limited in a similar way i.e. by the water flow, instead of by the amount of sunshine.
I presume that the frequency of your turbine varies with the voltage, and that would be important in finding a suitable transformer.
That means that the transformer won't saturate as the volt-time constant stays about the same as the voltage and frequency vary; but as the frequency goes up core losses will start to increase.
you can get 250v mppt's but unfortunately the price is prohibitive for me :( and i already have a 150v mppt
 

Ian0

Joined Aug 7, 2020
3,221
That would restrict the maximum input voltage, as the minimum inout voltage would be the battery voltage.
Again, whether you could use an off-that-shelf transformer to condition the input voltage would depend on the frequency, because as custom non-50/60Hz transformer would make a 250V MPPT look cheap.
You also haven’t said how much power is involved.
 

Thread Starter

liefryan

Joined May 4, 2021
8
If we have a transformer with a 150VAC output and a 600 VAC input that implies a turns ration of approximately 4:1. When the input drops to 100 VAC you will have an output of 25 VAC, which rectified gives you 35.4 VDC which won't do much for recharging your 48 VDC battery bank. How did you plan to work around this detail? I'm not really clear on what you're trying to do, but I don't think you have the correct approach. That is why I suggested an alternative that has some potential for meeting what I think are your requirements.
Ok i think the step down transformer idea is not the best, what do you think about using a voltage sensing relay to turn on a resistive load if the ac voltage rises above a a set voltage (say 140v to protect my mppt charge controller}? this would be cheaper and easier, can you see any floors with that? would ac frequency be an issue?
 

Ian0

Joined Aug 7, 2020
3,221
That would probably lose most of your power. You would be better off without the MPPT.
If you could go straight into the battery from the rectifier, the battery would limit the voltage and the frequency, but limiting it at 48V isn’t ideal either. I can’t make any suggestions without knowing the power and frequency.
 

Thread Starter

liefryan

Joined May 4, 2021
8
That would probably lose most of your power. You would be better off without the MPPT.
If you could go straight into the battery from the rectifier, the battery would limit the voltage and the frequency, but limiting it at 48V isn’t ideal either. I can’t make any suggestions without knowing the power and frequency.
The maximum power is 300w, after rectifying the ac the mppt in the grid tied inverter keeps and voltage somewhere between 100-200v, i haven't actually measured the frequency as it goes up and down with the voltage, i would think that the Victron MPPT would keep the rectified dc at roughly 100v for a 48v battery bank and the voltage sensing relay (with resistive load) would only be a safety precaution? it is only with no load that the turbine can spin up to really high voltages (600+v)
 

Thread Starter

liefryan

Joined May 4, 2021
8
That would probably lose most of your power. You would be better off without the MPPT.
If you could go straight into the battery from the rectifier, the battery would limit the voltage and the frequency, but limiting it at 48V isn’t ideal either. I can’t make any suggestions without knowing the power and frequency.
Direct battery connection would mean dropping the voltage on the long cable run to 48v and cause inefficiency in the cable, hence the high voltage turbine (washing machine :} although still only 6A so it could still work
 
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