Abstract
In order to keep constant DC-link voltage of a flywheel energy storage system (FESS) discharge in a wide rotational speed range, the control structure of the FESS is comprised of an inner current loop and an outer DC-link voltage loop. Since the dynamic equation of the DC-link voltage in the FESS discharge is nonlinear, it is difficult for some controllers to make the DC-link voltage in discharge be constant as the rotational speed is varying in a large range. Considering the nonlinearity of the DC-link voltage in discharge and the fast discharge requirements of the FESS, an immersion and invariance manifold (I&IM) adaptive nonlinear controller for a constant DC-link voltage is proposed via methodology of immersed in the invariant manifold. The stability of the control algorithm and the influence of the parameter error on the stability are verified by the Lyapunov stability theory, and the influence of the parameters error on the steady state and transient characteristics of the closed-loop system is analyzed numerically. It is proved that the closed-loop system satisfies the global uniform asymptotic stability conditions at the equilibrium point, and the error of the model parameters does not affect the equilibrium point of the system. Finally, the effectiveness of the I&IM adaptive nonlinear controller were studied by simulation and experiment. The results show that the DC-link voltage in discharge remains stable when switching the system load in cases of different rotational speeds and loads.
Highlights
Flywheel energy storage system (FESS) is an energy storage system where mechanical energy is stored in a rotating flywheel that is integrated with a motor/generator and driven by a bidirectional power converter
Several types of motor/generator are available for the FESS, among which the permanent magnet synchronous motor/generator (PMSM/G) is often chosen for its high efficiency, high power factor, high power density and good dynamic performance [4]
The motor used in the FESS should operate in wide rotational speed range due to its discharge mechanism, and the rotational speed sudden change in a short discharge period is a challenge for the controller design of the FESS
Summary
Flywheel energy storage system (FESS) is an energy storage system where mechanical energy is stored in a rotating flywheel that is integrated with a motor/generator and driven by a bidirectional power converter. Considering the nonlinear property of the DC-link voltage in discharge and the uncertain parameters in the PMSM/G based FESS, an affine nonlinear model is built involving the dynamic model of the inner current control loop in the system. The subsystem with the state variables (φ, z, x1) is globally asymptotically stable at the equilibrium (0, 0, 0), while x2 will converge to the corresponding value and vary according to the instantaneous load power Pload It is known from the above discussion that the stability of the I&IM adaptive controller is verified via the Lyapunov stability theory, but we hardly obtain the convergence characteristics of the stator current from theory. The I&IM adaptive controller can make the state variable x1 converge to the origin asymptotically, i.e., keep the DC-link voltage of FESS discharge constant at the desired level when load power varies. The estimated parameter θtracks the real value θ well, and its transient performance has the characteristics of a first-order system, as assigned in (31)
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