Wind energy conversion system based on dual stator winding induction generator to supply autonomous AC load

Due to the wind speed fluctuations in wind energy conversion systems (WECSs) and because of problems of the fixed speed wind turbine-generators, the variable speed wind turbines (VSWTs) are preferred to them. Most of the common types of VSWTs are WTs based on the doubly-fed induction generators (DFIG) and permanent magnet synchronous generator (PMSG). In DFIG wind turbines, the partial rated converters are used with lower cost, whereas, due to existence of brushes and slip rings they have less reliability and have higher maintenance cost. WTs based on PMSG have advantages as high efficiency and power density as well as brushless structure of rotor; however, they suffer from the high price of rare permanent magnet materials and the property reduction of these materials over time. Furthermore, the high cost of full power electronic converters and the infeasibility control of the excitation is the other disadvantages. To take benefits of both DFIG and PMSG, the dual stator winding induction generator (DSWIG) for VSWT applications has been recently proposed. This type of generator in comparison to DFIG due to the elimination of the brushes and slip rings has a robust structure and less maintenance cost. The rotor type in DSWIG is squirrel cage, therefore, in comparison to the nested-loop rotor type in the brushless doubly fed induction generator (BDFIG), the DSWIG has a simpler structure. On the other hand, in comparison to the PMSG, in this generator, the rare permanent magnet material does not exist and the excitation is controllable. The stator of DSWIG has two distributed three phase windings that share common pole pairs in air-gap. In this paper, DSWIG based WT equipped with related converters and battery is used to supply an independent AC load. In the proposed power topology, each of the DSWIG stator winding is connected to a voltage source converter that is called static excitation converter (SEC). The output active powers of both stator windings are transmitted to the load through load side inverter. 

In the proposed topology, the DC link voltage of three voltage source converters is common and in order to provide a balance between the generated active power by DSWIG and consumed active power by the AC load, the battery energy storage with bidirectional DC/DC converter is used. In this paper, for each converter, an appropriate control strategy comprising inner and outer control loops are used. In the proposed control strategy, the WT output power is equally shared between the two stator windings. The outer control loop of SECs is generator speed control and by implementing this control loop, the WT operates in maximum power point tracking (MPPT) mode. The inner control loops of SECs are extracted by using field oriented control (FOC) with indirect rotor flux orientation, in which, the amplitude and angle of rotor flux are estimated.

For examining the performance of the system under study and verifying the theoretical analyses, time domain simulations in the Matlab-Simulink environment are presented.

By proposed power topology and designed control system, with regulation the DC link voltage, once the WT power is more than the load power, the additional power is stored in the battery, and once the load demand is more than the WT power, the battery provides the power shortage, and thus, proper operation of voltage source converters is provided. Desired amplitude and frequency for the AC load is provided by the load side inverter in spite of load and wind speed variations. In a future study about the proposed power topology in this paper, the state of charge (SOC) of the battery as a control variable will be examined and the control loops with considering it will be designed.