Help needed: Derating PFC Capacitors

1. Jun 26, 2012

Hektor

I'm converting a first generation, fixed speed wind turbine from 60Hz operation to 50Hz operation. There are passive PFC capacitor banks in the downtower box on the turbine. How are the capacitor banks derated for 50 Hz operation? Is it as simple as, for example, downsizing from 15 to 12.5 kVAR (17%)? Thanks for any help!

2. Jun 26, 2012

psparky

I think you are actually upsizing....due to the impedance of 1/JωC
So more like 17.5 KVAR. They should be able to handle more inductive reactive power....although now it may be too much in a capacitive way.

But I'm pretty sure that is wrong. Just some random thoughts.

Dice it.

3. Jun 26, 2012

Hektor

I was thinking that because xL = jwL, inductive reactance would go down as frequency decreases. And as current will be staying the same, there will be less inductive reactive power (VAR). Then, I figured I would need less capacitive reactive power to compensate.

4. Jun 26, 2012

psparky

Perhaps both are correct. They are both a logical conclusion.

Or not....someone else will speak up soon...

5. Jun 26, 2012

dzimmerm

Think of it this way. What is the easiest life for a capacitor? It would be having only pure DC at its inputs. 60 changes a second is more work than 50 changes a second. If you look at how much energy the device is exposed to , that would also be less. That would imply for the same level of filtering at 60 Hz you would need a bit more capacitance than at 50 Hz as the number of positive pulses per unit of time, assuming a full wave rectified output fed to the capacitor, would be less.

In a nutshell, you will not be hurting the capacitors by feeding them 50 Hz but your downstream DC will be slightly less filtered.