Back-up energy storage with in-rush current limiting for wind turbine pitch drive

1. A wind turbine pitch drive system comprising:

an electric grid configured to supply electrical power;

a motor configured to drive a pitch actuator;

an electronic converter configured to control the motor, the electronic converter including a DC-link capacitor bank;

a back-up energy storage unit configured to supply electrical power;

a switching device configured to selectively connect the DC-link capacitor bank to the back-up energy storage unit; and

means for protecting the electronic converter from damaging in-rushing current by controlling the in-rushing current from the back-up energy storage unit to the DC-link, wherein the in-rushing current is the current that occurs when the back-up energy storage unit is substantially charged and the DC-link to which the back-up energy storage unit is connected is substantially discharged, and energy supply is switched to the back-up energy storage unit; and

wherein the means for protecting comprises a frequency generator configured to generate a high frequency pulse based on a detected difference in voltage level between the back-up energy storage unit and the D-C link.

2. The system according to claim 1 , wherein the back-up energy storage unit comprises at least one ultra-capacitor.

3. The system according to claim 1 , wherein the back-up energy storage unit comprises at least one battery.

4. The system according to claim 1 , wherein the switching device is an insulated gate bipolar transistor.

5. The system according to claim 1 , wherein the switching device uses the high frequency pulse generated by the frequency generator to apply pulse width modulation to control the in-rushing current.

6. The system according to claim 1 , wherein the switching device uses the high frequency pulse generated by the frequency generator to apply pulse frequency modulation to control the in-rushing current.

7. A method for protecting a component of an electronic converter in a wind turbine pitch drive system that includes: an electric grid configured to supply electrical power; a motor configured to drive a pitch actuator; an electronic converter configured to control the motor, the electronic converter including a DC-link capacitor bank; a back-up energy storage unit configured to supply electrical power; a switching device configured to selectively connect the DC-link capacitor bank to the back-up energy storage unit; and a frequency generator configured to control the switching device,

the method comprising:

detecting a difference between a voltage level of the DC-link capacitor bank of the electronic converter and a voltage level of the back-up energy storage unit such that the voltage level of the back-up energy storage unit is higher than the voltage level of the DC-link capacitor bank;

generating a high frequency pulse, from the frequency generator, based on the detected difference in voltage level; and

using the high frequency pulse to intermittently switch the switching device on and off to control an in-rushing current from the back-up energy storage unit.

8. The method according to claim 7 , wherein the using of the high frequency pulse to intermittently switch the switching device comprises applying pulse modulation.

9. The method according to claim 8 , wherein the pulse modulation applied is pulse width modulation.

10. The method according to claim 8 , wherein the pulse modulation applied is pulse frequency modulation.

11. The method according to claim 7 , wherein the detecting of a difference between the voltage level of the DC-link capacitor bank of the electronic converter and the voltage level of the back-up energy storage unit comprises measuring the voltage level of the back-up energy storage unit.

12. The method according to claim 7 , wherein the detecting of a difference between the voltage level of the DC-link capacitor bank of the electronic converter and the voltage level of the back-up energy storage unit comprises measuring a voltage level at the DC-link capacitor bank.

13. The method according to claim 7 , wherein the detecting of a difference between the voltage level of the DC-link capacitor bank of the electronic converter and the voltage level of the back-up energy storage unit comprises measuring a voltage level at the electric grid.