Research On The Induction Generator System Based On The Power Electronics Converter

With the development of the renewable energy and the standalone power system, the induction generator system based on the power electronics converter has attracted more and more attention.Two types of induction generator have been studied in this thesis. One is the standard squirrel cage induction generator (SCIG). And the other is the new style dual stator winding induction generator (DWIG). The instantaneous torque control (ITC) technology is introduced in the SCIG system to generate DC voltage via the power electronics converter. The ITC controlled SCIG not only has the satisfying performance, but also possesses the dual function of starter and generator. The DWIG generating system only possesses the generating function with the following attributes: (1) the power-winding feeds the load via the diode bridge while the control-winding only supplies the reactive power, thus the capacity of the power electronics converter on the control-winding side, which is termed as the static excitation controller (SEC), can be greatly reduced; (2) The dc voltage can be obtained during the wide speed operation; (3) Different voltage levels can be obtained by tuning the turn ratio of the two-sets of windings.The thesis focuses on the DWIG generating system which operates over a wide speed range. As the foundation, the self-inductance, mutual inductance between the windings and the mutual leakage inductance of the DWIG are deeply analyzed. The mathematic model for the DWIG is then set up.The fixed excitation capacitor bank C is connected across the power-winding to reduce the capacity of the SEC. Investigations into the reactive power released by the C represent that the minimal capacity of SEC can be achieved by appropriate selection of C , when the DWIG operates over a large speed range. The comprehensive design using the powerful computation of computer is proposed to search the appropriate C when the power-winding feeds the load through the diode bridge.According to the practical working-conditions of the generator, it is deserved to explore the characteristic of DWIG for wide-speed-range operation. However, the published literature mainly lies on the working conditions with a constant rotor speed or a narrow speed variation. With the help of instantaneous power theory, aspects related to the DWIG control mechanism for a wide-speed-range operation are investigated in this thesis, consequently the hysteresis current control oriented by the stator-winding flux vector is consequently proposed. The parameters of the circuits and the PI regulators are well designed. Simulation and experimental results for a 270V/18kW DWIG system with a speed-range of 4000rpm to 8000rpm are presented for the verification. To reduce filter inducdance between the SEC and DWIG, the space vector modulation(SVM) technology is introduced to substidute the digital hysteresis current controller. In this case the fixed switching frequency, high rate of the dc-bus voltage usage and lower harmonics in current are achieved.The voltage control mechanism of the DWIG is analyzed, based on which the influence of the control winding instantaneous power on the output dc voltage is disclosed. The strategy of the direct power control (DPC) for the DWIG is proposed. No inner current loops and no PWM modulator block are required. Fast control on the control winding instantaneous power is achieved, because the required voltage vectors, which restricts the errors of the active or reactive power into the certain band, is directly selected from an optimal switching table. Experimental results on the DWIG prototype system are provided to demonstrate the effectiveness and robustness of the proposed control strategy during the variation of rotor speed and output power.The following of the thesis will converge towards the applications of DWIG system. The 28V/18kW medium power generator is widely used in the standalone power system. The particular structure of the DWIG can be employed to control the great output current. The voltage level of the control-winding is enhanced higher than that of the power winding, thus the control-winding current which the SEC regulates will reduced. In this way, the regulation of output voltage on the power-winding side can be achieved fairly easily. The current hysteresis control strategy is employed to control the low voltage large current DWIG. The simulation results have verified the validity and effectiveness.As for the wind-power generation, the simulation platform for the DWIG based wind power generation system is established. The control principle for the system at different wind speeds is discussed and maximum wind power extraction control scheme is analyzed and verified by the simulation. A 20kW DWIG prototype direct at the wind power generation has been built up, taken the rotor speed and output voltage in accord with the wind power generation. The current hysteresis control technique is assumed to test the basic attributes including the variable speed operation, load step change, heating and efficiency.