Overvoltage protection

Question

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?!

jw0752 · Answer

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

jw0752 · Answer

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

mcb1 · Answer

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

Former Member · Answer

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.

Former Member · Answer

Thanks Mark, I have been looking at these - they're ideal (apart from the price) and form my plan B! they have a max of 600v instead of 400v and some nice built in protection so would certainly do the job

Former Member · Answer

Has anybody any experience of these: CA-603-R Delta Industrial Surge Capacitor 600Vac, 3 phase, 4-wire - it looks as if they might do the job

michaelkellett · Answer

You'll be wasting your money with one of these (capacitor things) to try and deal with wind driven events for the reason John suggests. If I wanted capacitors I would look for a supplier who offered proper technical data. It seems to me that your problem is that the rectifier is just not suitable for the generator - the over voltage trip seems a reasonable work around. MK

jw0752 · Answer

Hi Ken, If you are interested in trying the capacitors it would not hurt but typically capacitors are designed to help with spikes and surges with a time factor of milliseconds or less. You mentioned power surges caused by gusts of wind in your question and I would expect these to have a time factor of seconds in duration. For example a surge that lasts 10 ms and tries to go to 500 volts maybe absorbed and ameliorated by a capacitor while one that rises with the increase in power from a wind gust will fill the capacitor and still raise the voltage to 500 over several seconds. If you try the capacitors and they help it will be an indication that the wind gusts were not the real cause of the problem. John

Overvoltage protection

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