LVD low voltage disconnect on 24v wind turbine

DrnalineDrnaline Posts: 10Registered Users
I searched the forum, didn't find any thing relevant. I added a second turbine to my system and naively put an under sized diode. Which burnt and feed back, killing by battery bank. Was able to save two of the battery's, the other two were junk. Batteries are 8D commercial 12V.
    Replaced the two bad ones. And was scared to restart the system without some kind of protection. First thing to come to mind was low voltage disconnects which disconnect anywhere from 10.8 to 11.2. But most if not all LVDs I see have over voltage protection. They will cut off at a certain voltage like 32-35v. I have seen my original turbine put out at much as 37-38 volts on a digital readout.
     Is it dangerous to have this type of cut off at that high of voltage?



all questions, comments and suggestions are welcome. 

Comments

  • BB.BB. Posts: 27,354Super Moderators admin
    Your turbine, at high wind speeds can output as high as 100+ volts. It is the battery that "sets" the output voltage of the turbine (i.e., the battery just draws so much current charging, the voltage cannot go much over 32 volts or so--Unless you are overcharging or simply driving too much current into the battery bank).

    The cutoff voltage should be around 22 to 23 volts (typical) for a 24 volt battery bank. That is already a well discharged battery. 10.5 volts cutoff (for a 12 volt battery bank) is just too low (unless you have very high/short surge current--such as starting a well pump).

    On the other side--High voltage cutoff. Most horizontal axis wind turbines, if they are disconnected from the battery bank/loads (no current flow) can over speed and self destruct during medium to high wind speeds. So placing a "simple" controller that turns off if the battery over charges can cause problems too (you do not want blades flying through the air or the nacelle falling from the tower).

    If there is somebody at the property most of the time--A large volt meter and/or high/low voltage alarm that you can monitor is probably your best bet. If you see a battery bank voltage that does not look right--Then you can investigate the problem and fix it before the battery bank (or something else) is damaged.

    You have seen the problem with diodes--They are just as likely to fail shorted as open. Industry standard (for simple on circuit board backup battery charging circuits) is to put two diodes in series so that if one fails, the second will still protect the battery from over charging.

    You are seeing the problems that design engineers run into. You start throwing hardware at a problem and find out that adding more hardware can have other consequences/safety/reliability issues (more hardware, more things to fail, more ways to fail badly).

    My suggest is to get a good quality rectifier assembly (with good heat sinking) and a voltage monitor/alarm. And not to go with a high/low voltage cutoff.

    Also--You have fuses/circuit breakers on all the + wiring that leaves your battery bank (to prevent short circuits from causing wiring fires)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • plasmahunt3rplasmahunt3r Posts: 18Registered Users
    I am not sure I agree with your assessment of why the batteries burnt up.  When a diode fails, it usually just stops rectifying AC into DC and then allows AC to pass.  So you will get both negative AND positive to your battery and that will zap your batteries.  So I only agree that your Diode failure zapped your batteries.

    Low voltage wouldn't be any problem.  Your batteries just won't charge. 

    High voltage will destroy your batteries just as easily as AC to your batteries would destroy your batteries. 

    I use a shunt regulator (circuit found in this forum) which sets limits on charging voltage.  Or you can use a commercial charge controller.
  • BB.BB. Posts: 27,354Super Moderators admin
    I believe the diode(s) were failing shorted--And then the battery bank would feed backwards through the windings of the alternator/wind turbine--Discharging the batteries.

    It is also possible the diode(s) failed open, then the alternator would simply not charge the battery bank. And you get dead batteries.

    Either is a bad thing to have happen to a battery bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • plasmahunt3rplasmahunt3r Posts: 18Registered Users
    Ah I see.  Multiple diodes would have to fail to feed backwards through the windings.  I was looking at a single diode failure.  But if his Bridge Rectifier was too small, then its posible that all diodes failed.

    Shows the importance of over rating the Bridge Rectifier (both in voltage and amps) and sitting it on a heatsink. 
  • plasmahunt3rplasmahunt3r Posts: 18Registered Users
    If you still need a low voltage disconnect, there are circuits you can build.  Just search for "Low Voltage Disconnect Circuit".  Also there are modules you can buy: 
    http://www.newmarpower.com/Low_Voltage_Disconnects/Low_Voltage_Disconnects.html
  • BB.BB. Posts: 27,354Super Moderators admin
    Yes--As you say, a properly rated diode array and good heat sinking are very important. Electronics do not heat or high surge currents.

    And wind turbines are subjected to highly variable wind conditions. All it takes is a good wind storm to ruin your day.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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