Research On Control Strategies For Grid-Connected Converter System Of Direct-Driven Wind Turbine

In the background of the increasingly serious global energy crisis and environmental issues, wind power generation technology has attracted great attentions of many countries in the world and has developed rapidly in recent years. In the two mainstream wind turbine generators at present, the direct-driven permanent magnet synchronous generator (PMSG) based wind turbine, due to its advantages of low mechanical loss, high operational efficiency, gearless, low maintenance cost and high reliability, has become one of the major development direction of modern variable-speed constant-frequency wind turbines. With the growing capacity of grid-connected wind turbines, the interaction between power grid and wind power turbines becomes more and more evident, which indicates that the in-depth study research on the control strategies of grid-connected converter system of PMSG-based wind turbine is of great realistic significance and value.In the control issues of the grid-connected converter system of PMSG-based wind turbine, this paper mainly focuses on the nonlinear control schemes of PWM converters in PMSG-based turbine and the control strategies of grid-connected converter system of PMSG-based wind turbine under abnormal operational conditions in grid. Because of multi-input multi-output nonlinearity of PWM voltage source converter, conventional double-loop PI vector control scheme cannot satisfy the high performance requirement due to its drawbacks of insufficient power decoupling, weak dynamic response characteristic and highly dependence on model. From the viewpoint of power grid safety operation during abnormal operating conditions such as short circuit faults, the low-voltage fault ride-through and anti-disturbance capability of wind turbines is required. As the conventional control scheme of wind turbines is usually designed based on grid normal operating condition, research on the optimal control strategies which make wind turbines more adaptive to complicated grid conditions is very necessary. In summary, with the research object of PMSG-based wind turbine integration converter system, this paper studies the nonlinear control schemes of converters in the grid-connected system and the control strategies under grid voltage dip and unbalanced fault conditions.As the complete mathematical model of direct-driven wind turbine constitutes the foundation of research on the control strategies of grid-connected converter system, this paper establishes the detailed model of wind turbine aerodynamics, mechanical drive train, blade pitch angle control, PMSG and two-terminal back-to-back converter system. The vector control and feedforward compensation control method are adopted to discuss the conventional control strategies of generator-side and grid-side converters. Simulation results validate that the basic control targets such as maximum power point tracking (MPPT) and constant DC-link voltage can be realized by the conventional control system discussed in this paper.For the nonlinear control issues of converter system in PMSG-based wind turbine, this paper firstly adopts the differential flatness-based (DFB) theory to carry out research. According to the essential concept of DFB theory and the mathematical model of two-terminal converter system of PMSG-based wind turbine, the flat property of converter system is verified and, based on which, the DFB control strategies of generator-side and grid-side converters are designed. On the basis of designing the normal DFB controller, the control structure of the error feedback compensation loop in DFB controller of grid-side converter is optimized by improving the conventional PI error compensation control method. Furthermore, a compound nonlinear control strategy of grid-side converter which combines the active disturbance rejection control (ADRC) technology in outer-loop and the DFB theory in inner-loop is proposed to enhance the anti-disturbance capability of direct-driven wind turbine grid-connected system.In the aspect of nonlinear control for the grid-side converter of PMSG-based wind turbine, the back-stepping design approach is also applied as basis in the discussion of this paper. On one hand, this paper adopts the extended state observer (ESO) to estimate the total disturbance of direct-driven wind turbine grid-side system. By introducing the estimated disturbance into the derivation process for the back-stepping control law of grid-side converter, an ESO-based back-stepping control strategy which compensates the system total disturbance is presented to improve the grid-integration dynamic characteristics of direct-driven wind turbine under outer disturbance conditions. On the other hand, through combining the sliding mode control with back-stepping method and considering the high frequency chattering problem existed in conventional sliding mode control, this paper presents a grid-side hybrid control scheme which synthesizes back-stepping control and non-singular terminal sliding mode control. In addition, the asymptotic stability of direct-driven wind power grid-side system under the proposed hybrid control is validated based on Lyapunov stability theory.In order to study the low voltage ride-through (LVRT) control strategy of direct-driven wind turbine during voltage dip cause by grid faults, this paper illustrates the basic concept of LVRT and the fundamental principle for direct-driven wind turbine to achieve LVRT. A combined control strategy for the back-to-back converter used in direct-driven wind turbine is proposed, which synthesizes generator-side segmental speed control and grid-side multi-mode power control. The active power transmitted from the generator is restrained by adjusting the rotor speed, which enables the generator-side active power trace the changing trend of grid-side active power. During the low voltage period, the grid-side converter is used to provide reactive power support for the grid by redistributing the active and reactive power. The simulation results demonstrate that using the proposed control strategy can effectively improve the LVRT capability of direct-driven wind turbine and its reactive power support to power grid.As to the problems of grid-side active power and reactive power double-frequency oscillation and DC-link overvoltage in direct-driven wind power system under asymmetrical grid fault conditions, conventional fault ride-through (FRT) control strategy cannot achieve the multi control targets at the same time, due to the respective calculation of control references for different control objectives. This paper presents two integrated control schemes with the combination of main controller and additional control loop. In the main controller, an optimal DC voltage feedback compensation control loop and a feedforward control component which reflects the active imbalance between the two terminals of DC-link are designed by Lyapunov asymptotic stability theory and feedforward current control method respectively. The two kinds of DC voltage control are combined with DFB control to construct two kinds of main controllers for grid-side converter. For the additional control loop, the grid-side power characteristic under unbalanced grid fault conditions is analyzed based on the grid-side mathematical model of direct-driven wind turbine in synchronous rotational frame and the transformation method between the positive components in the positive rotational frame and the negative components in the negative rotational frame is derived. Based on this coordinate transformation method and the grid-side double-frequency voltage equation of direct-driven wind turbine represented by negative second harmonic components, the control equations consists of the second harmonic components in active and reactive power are derived. According to the control equations, two kinds of additional harmonic compensation control loops are designed by the adoption of Quasi-PR controller in this paper, which can restrain the double-frequency pulsations of active power and reactive power in a same time. Simulation results indicate that the proposed asymmetrical FRT control strategy can simultaneously restrain the double-frequency pulsations in grid-side active power, reactive power and DC voltage of direct-drive wind turbine, reducing the DC-link overvoltage and improve the asymmetrical FRT capability of direct-driven wind turbine.