FTEEE-1699 Sliding Mode Control of PMSG Wind Turbine Based on Enhanced Exponential Reaching Law – IEEE EEE Project 2016-2017
Sliding Mode Control (SMC) based scheme for a variable speed, direct-driven Wind Energy Conversion Systems (WECS) equipped with Permanent Magnet Synchronous Generator (PMSG) connected to the grid. In this work, diode rectifier, boost converter, Neutral Point Clamped (NPC) inverter and L filter are used as the interface between the wind turbine and grid. This topology has abundant features such as simplicity for low and medium power wind turbine applications. It is also less costly than back-to-back two-level converters in medium power applications. SMC approach demonstrates great performance in complicated nonlinear systems control such as WECS. The proposed control strategy modifies Reaching Law (RL) of sliding mode technique to reduce chattering issue and to improve THD property compared to conventional reaching law SMC. The effectiveness of the proposed control strategy is explored by simulation study on a 4 kW wind turbine, and then verified by experimental tests for a 2 kW set-up.
RENEWABLE energy has been considered as an alternative energy source because fossil fuels are limited and make pollution problems. Renewable energy integration in power system raises a lot of challenges in research and practice. One of the most favourable sources of renewable energy technology is Wind Energy Conversion System (WECS) technology. This technology has been improved from the capacity of few tens of kilo Watts to several mega Watts over the past few decades. Worldwide investment in WECS area is going to be expanded in the future. Today, WECSs equipped with Permanent Magnet Synchronous Generator (PMSG) are becoming more popular in wind energy community, especially in offshore applications due to the elimination of gear box and excitation system.
Proposed ARL sliding mode technique in this research performs the controller gain correction based upon the discrepancy between the actual and desired system states (error signal). When the error value is high, the gain is increased such that to force the system state to move toward the desired state as fast as possible. When the error signal becomes small, the applied gain is going to decrease in a way that once the gain tends to zero, the error inclines to zero. It is worth noting that the ARL approaches suggested in the literature do not amend the gain in a wise manner in the wide range of error values from zero to high error quantities. Therefore, the proposed approach in this paper is more pragmatic than the methods proposed.
Propose an SMC based controller, called Enhanced Exponential Reaching Law (EERL) sliding mode approach which can meet the requirements in power applications. The contributions of the paper are three folds: First, EERL reduces the reaching time of system trajectory to the equilibrium point even the initial condition of the system parameters are far from the sliding surface. Second, EERL has the capability to further mitigate chattering issue of sliding mode approach with respect to the conventional SMC. Then, EERL improves current THD of the grid-connected inverter which is vital for Distributed Generation (DG) integration into the power grid.
- Dynamic model of the PMSG based wind turbine
- Controller design for PMSG wind turbine
- Simulation Results of PMSG Wind Turbine
- Report Generation
Dynamic model of the pmsg based wind turbine:
In the literature, models with different level of complexity are available for the drive train subsystem. The available drive train models of the WECS are six-mass model, three-mass model, two-mass model and one-mass (lumped) mode. One-mass model is chosen in this paper to simplify state space model of the system. The generation capacity of a WECS is usually specified by a curve based on the generator output power versus actual wind speed which is called WECS power curve. The WECS operation can be described in four different regions based on the wind speed. At wind speeds between v! min and nominal speed (region II), the available wind power would be lower than generator rated power.
Most studies in the field of WECS are dedicated to the development of appropriate control techniques to increase its ability to supply and regulate active and reactive power in both grid connected and islanding modes. Several control approaches have been applied for PMSG based WECS control to reduce generator currents, to extract maximum power from wind turbine, to keep DC link voltage constant, to control the reactive power injected to the grid, and to decrease current harmonics at PCC. These control strategies of PMSG wind turbines can be generally arranged into two categories.
Controller design for PMSG wind turbine:
In order to control PMSG based WECS, it is required to generate appropriate switching signals for boost converter and NPC inverter based on the PMSG parameters and connection point requirements. This expression can be applied to every system with governing equation similar to, considering the availability of inverse of g function. Noticeably, there exists inverse functions of the above-mentioned equations of PMSG based WECS components. It is employed to generate duty cycles for boost converter and NPC inverter switches.
Simulation Results of PMSG Wind Turbine:
One major impediment of diode rectifier in PMSG wind turbine is the high harmonic contents in the generator stator current which leads to the generator torque and DC link voltage ripples. Ripple in DC link voltage is smoothed by incorporating capacitor C0 between diode rectifier and boost converter. Several studies have been directed to minimize the PMSG torque ripple connected to diode rectifier. Nonetheless, it must be devised to design the PMSG and related drive-train in such a way that they could tolerate generator torque ripple. Simulation results show that the maximum electromagnetic torque ripple of PMSG is approximately 0.17 per unit.
A sliding mode control based on enhanced exponential reaching law was proposed and investigated on the grid-connected PMSG wind turbine system. The examined topology consists of: diode rectifier, boost converter, NPC inverter and L filter. This topology can be used in low power and medium power wind turbine systems. Boost converter is controlled by maximum power point tracking idea of PMSG, while NPC inverter control is developed using the unity power factor concept. The key features of the proposed controller are: chattering minimization respect to conventional and exponential reaching law sliding mode controllers, proper voltage balancing performance of DC link capacitors, improving transient traits of WECS, and appropriate dynamic error tracking. The verification of the proposed reaching law for SMC approach was validated by both simulation and experiment.
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