Analysis of instability of direct powered DC-compound machines in pitch systems of large wind turbines

DC machines are commonly used as pitch drives enabling the possibility to drive into the feathering position also if power electronics fails. The pitch system of a wind turbine has to fulfill two functions: Limiting the turbine power during strong wind conditions but also braking the turbine via pitching into the feathering position. The second task is safety critical and therefore the pitch system must be equipped with a backup storage in order to react autonomously under grid failure conditions. State of art for electrical pitch systems are lead fleece accumulators due to big experience and cost attractiveness of this technology. Compared with standard DC machines, compound types combine the advantages of a shunt and series machines for direct battery connection: They are naturally able to resist low and regenerative torque conditions like shunt machines by also able to provide a torque even if the battery voltage is low e.g. in case of defect battery cells. But compound machines can show an unstable behavior if the machines changes from motor to regenerative operation during direct powered emergency drive. If this effect appears, the machine generates a dynamic regenerative current peak that causes electromechanical transients stressing the whole pitch drive train. In order to analyze and understand this phenomena a detailed dynamical model of the DC compound machine was developed.