New State Observers and Sensorless Control of Wound Rotor Induction Generator (WRIG) at Power Grid with Experimental Characterization Ioan Serban G.D. Andreescu, L. Tutelea, C. Lascu, I. Boldea Frede Blaabjerg Robert Bosch GmbH University Politehnica of Timisoara Aalborg University Corporate Research and Advance Engineering CR/ARE 3 Electric Drives Faculty of Automation and Computers/Electrical Engineering Institute of Energy Technology 70049 Stuttgart 300220 Timisoara 9220 Aalborg East GERMANY ROMANIA DENMARK serban@ieee.org dandre@aut.utt.ro ; boldea@lselinux.utt.ro fbl@iet.aau.dk Abstract – The paper deals with a wound rotor induction generator (WRIG), also known as the doubly-fed induction generator. A complete experimental set-up is presented and analyzed. It is composed of: WRIG, two power electronics converters connected in the rotor side of WRIG, a line filter, and the data acquisition and control system (dSpace DS 1103). Both converters are commercial units and are vector controlled using appropriate interfaces. They are back-to-back connected, sharing the same DC bus, one supplied through a line filter from the power grid, and the other one with the output on the rotor of the generator. The stator of the generator is directly connected to the power grid. Two stator flux observer topologies were investigated and compared, one with the voltage model in parallel with the current model and the other one with both models connected in series. A speed estimation strategy, which works also during the synchronization procedure, was implemented and tested. It is based on model reference adaptive system (MRAS) principles. All control strategies, the flux observers and the MRAS, were developed in Matlab-Simulink ® and implemented using a dSpace ® DS1103 single-board control and acquisition interface. Different tests were performed, and sample results are presented and discussed in the paper. The schemes used are illustrated in the paper, and the experimental results are shown and analyzed. I. INTRODUCTION Nowadays, as the penetration of wind power is higher and higher, the wind turbines have to be more controllable [1], and a way to do this is by variable speed generators. By pitching the rotor blades, a variable speed turbine can control the power output at theoretically any wind speed. To transfer the wind energy from a variable speed turbine to the constant frequency electrical grid, an electrical generator and a power converter are needed [2]. The doubly fed induction generator with a bi- directional power flow converter in the rotor has, doubtlessly, advantages with respect to other solutions. II. THE WRIG SYSTEM AND ITS CONTROL The main components models in the WRIG system are described in what follows. Electrical block-diagram of the whole system is shown in Fig. 1. Fig. 1. The WRIG system. For the DC link capacitors pre-charging, a resistor was used (not illustrated here) between the grid and the line filter; the resistors were short-circuited after the voltage in the DC link reached approx. 500 V. The WRIG-to-grid synchronization procedure is described in Section IV. For driving the WRIG is used a 3 kW/ 1475 rpm induction motor supplied with a 4.3 kVA Danfoss VLT5004 commercial voltage source inverter with the original interface for constant speed control mode. In this way, the variable wind speed could be easily simulated. As the generator nameplate data and parameters are given in the Appendix, in what follows the grid-side converter (GSC) and the machine-side converter (MSC) are presented. A. The Grid-Side Converter (GSC) The grid-side converter is used to control the DC link voltage and the input power factor regardless of the level and the direction of the rotor power (Fig. 2). A vector control strategy in stator voltage reference [5] frame is employed for this purpose. The converter is current regulated with the direct axis current used to control the DC-link voltage, meanwhile the transverse axis current is used to regulate the displacement between the voltage and the current, and thus the input power factor [6], [7]. The angle of the grid voltage is: ( ) α β θ s s e u u / tan -1 = . Between the grid converter and the grid itself, a line filter was introduced to reduce the higher harmonic content in the line current, produced by the 4260 1-4244-0136-4/06/$20.00 '2006 IEEE