10 Replies Latest reply on Jan 19, 2016 8:25 AM by michaelkellett

    Overvoltage protection

    dunfarming

      I have a problem with a variable ac wind generator

      (15kW) which keeps blowing rectifiers. It normally generates at 300-350 v but can on a gust momentarily go over 400 which is the limit on the rectifiers which feed the inverters for onward network transmission.  I have rigged up, using a crouzet voltage control relay linked to a contactor, a bypass to protect inverters but it occurs to me that a capacitor bank or super cap would be able to 'absorb' the gust energy so;

      am i correct in this assumption? if no, any suggestions for alternatives.  If yes, how do we size them and are we talking series or parallel? What would be the likely pitfalls/adverse consequences?!

        • Re: Overvoltage protection
          mcb1

          It normally generates at 300-350 v but can on a gust momentarily go over 400

          AC or DC?

           

          The better method would be to detect the voltage and dump a large load onto it.

          I presume sustained winds cause the blades to feather or rotate to keep it within the design speed.

           

          Mark

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          • Re: Overvoltage protection
            jw0752

            Hi Ken,

            I know very little about wind mills but it seems to me that adding mass to the rotor would slow the reaction to wind gusts and even store some of the energy from the gust and spread it out over time. Also do you have a way to adjust the field of the alternator so that it can regulate the output voltage? If you are running with voltage close enough to the tolerance of your output circuitry that gusts of wind cause failure there may be something wrong with the design or the adjustment. As I mentioned I am not familiar with the specifics of windmills but just looking at your question from a peripheral understanding.

            John

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              • Re: Overvoltage protection
                dunfarming

                Hi John, blades are 6 m long and 20m up in the air and all 'balanced' to spin at 90rpm so wouldnt want to mess with them.  It's a permanent magnet alternator, so no field coils to adjust. Newer models have wider voltage tolerance but it would cost thousands to change all the rectifiers/inverters so just looking for a more cost effective solution.

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                  • Re: Overvoltage protection
                    jw0752

                    Hi Ken,

                    I do not see the capacitors as a solution to your problem. They would have to be on the rectified side of the diodes so they would actually be an increase in load at the time of stress. One solution that I can see is to put an extra set of diodes in series or in parallel with your existing diodes. If you feel that it is the voltage that is killing the diodes put an extra set in series with the existing set. This will double the voltage specification. If you feel that it is the current that is killing them put the extra set in parallel with the existing set. This will double the current capacity of the system. Without pictures or more information I am not sure how this would work mechanically but from the point of view of the electronics it should work.

                    John

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                • Re: Overvoltage protection
                  dunfarming

                  Hi Mark, AC variable depending on rpm, hence the rectification. Difficulty with dumping a large load on it is that current flow in windings is already near max on the gusts, it would also be wasteful of the energy. My thinking is capacitor would soak up the gust and then release it back as volts drop.