This paper presents how to operate a double fed induction generator (DFIG) with a rated power of approximately 30kW with unmodified low cost standard converters. The stator of the machine is directly connected to the grid and the rotor is fed with variable voltage and frequency from two standard low cost voltage source converters. Actually such machines are used in wind generators with a rated power of more than 500 kW with special build converters. In the presented solution the rotor of the DFIG is fed with two DC-Link connected standard voltage source converters. The original software is designed to calculate speed and phase of the rotor by receiving a speed signal from the rotor shaft. Therefore it is necessary to add two small application modules to modify the firmware externally to realize the calculation of slip frequency and phase in real time. The equivalent network parameters of the DFIG are determined by the converter itself by using an autotune function. With this parameters it is possible to develop an analytical model of the system to set up a closed loop active and reactive power control. The test system provides the ability to control both power types decoupled and with an approximately linear characteristic.

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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.