| Abstract |
Mechanical brake is typically used in wind turbine, which typically suffers from frequent wear-out, more trip opportunities and high maintenance cost. To addressing this issue, a redundant electrical braking system is proposed in this paper. It comprises a conventional dynamic brake connected to the converter DC link and a fully controllable resistive load connected to the generator terminals. The resistive load can be engaged by an active power semiconductor and a NC(Normally Closed) contactor. The key advantages with the proposed braking mechanism are: (1) Relief mechanical braker's loading significantly since electrical braking is the primary braking method and mechanical braking only functions as back-up; (2) The total braking torque is controllable, which allows some pre-determined braking torque along with decreasing turbine speed could be followed during braking process. The specific designed braking torque curve could minimize the drive train fatigue; (3) High reliability could be obtained based on the proposed control logics. Even under the worst case, where all electrical components fail, the resistive load on generator terminals can still be engaged through a Normal-Closed mechanical contactor. The engagement of this resistive load could prevent the turbine from free-wheeling and stop the turbine working with mechanical braker. The optimum load sharing between dynamic brake and resistive ac load is discussed. A state machine control system is presented. |
