Sliding mode control of grid-connected power converters for microgrid applications

SMC is known for the low sensitivity to disturbances and parameter variations, making it an effective method to deal with the nonlinear behavior of complex systems. It has been demonstrated to be a highly promising solution for power electronic systems, such as switching DC/DC power converters, grid-connected power converters, and motor drives. The sliding mode approach is expected to become increasingly popular in the field of power converter control. This chapter will be organized as follows. Section 1 begins with an introduction to the fundamental theory and methodology of SMC to familiarize readers with the main principle and background of SMC. Section 2 will discuss the mathematical models of three-phase grid-connected AC/DC power converters and DC/DC buck converters. Next, in Section 3, the design of the SMC and output feedback control for aforementioned power converters will be presented, and the simulation results comparing the performance of the proposed control with the conventional proportional-integral controller will be discussed. Finally, some conclusions are drawn in Section 4.

In this chapter, we focus on output feedback control and observer design for two types of commonly used microgrid power converters, that is, three-phase two-level AC/DC power converters and DC/DC converters. As is well known, the sliding mode control (SMC) is naturally well suited for the control of variable structure systems. Since power converters inherently include switching devices, making them belong to variable structure systems; therefore, it is straightforward to apply SMC that yields a discontinuous control law. Moreover, given that power converters are usually modeled using the state space averaging method, SMC forms an efficient analysis and design tool for the control of switched mode power converters, because it offers excellent large-signal handling capability.

Conventional linear control is small signal based; it only allows one to optimally operate the converters for a specific range of operating conditions and often fails to achieve satisfactory performance under large parameter/load variations, that is, large signal operating condition. SMC as a kind of nonlinear control method is suitable for controlling the power converters, which is able to achieve better regulation and dynamical performance for a wider range of operating conditions. The main reason is that there is no need to have a linear model of the power converter for nonlinear controller design. However, the main obstacle associated with the application of SMC is its variable frequency nature, which makes the design of output filter difficult. Nonetheless, if this problem is properly handled, SMC is a powerful control design method for power converters and has huge potential in industrial applications.

The three-phase two-level ACDC power converter acts as a rectifier or voltage source inverter (VSI). When acting as a rectifier, it is the primary interface for a wind turbine (WT) generator, converting the WT source AC voltage to a varying output DC voltage following the maximum power point tracking (MPPT) control. As a microgrid is normally an AC microgrid, the VSI is the most important module of the converters, which works as a DC/AC converter. VSI interfaces among the renewable energy source (RES) DC-link output, energy storage system (ESS) DC-link output, and the microgrid. It converts DC voltage to AC voltage with the microgrid voltage magnitude and frequency, in order to inject the active and reactive power to the microgrid. The control objectives of AC/DC converter are: (1) DC-link voltage regulation; and (2) AC current tracking with the possible lowest harmonic distortion.

The DC/DC converter converts a source of direct current from one voltage level to the fixed or adjustable one. In a microgrid, a DC/DC converter is used in the following scenarios: (1) working as a primary interface for photovoltaic panels, which converts the PV source DC voltage to an adjustable output DC voltage based on the MPPT control; (2) for a DC microgrid, the DC/DC converter acts as an interface among the microgrid, DC loads, RES DC-link output, and the ESS, such as batteries and flywheels. In this scenario, the DC/DC output voltage is a fixed one. To control the DC/DC converter, great control effort should be made to regulate the output voltage to the desired value, due to input voltage disturbances and abrupt load variation. From a control point of view, the control design for boost converters is more difficult than the buck type, because its standard model is a nonminimum phase system. Traditionally, the control problems of the DC/DC converters are solved by using pulse width modulation (PWM) techniques, in which an external high-frequency signal is used to modulate a low-frequency desired function to be tracked. In view of practical implementation, SMC is much easier than a PWM control, since the maximum frequency of commercially available switching elements increases higher and higher.

In this chapter, a novel observer-based control is proposed for both three-phase two-level grid-connected power converters and DC/DC buck converters. The proposed control technique forces the input currents to track the desired values, which can indirectly regulate the output voltage while achieving a user-defined power factor. The presented approach has two control loops. A current control loop based on the sliding mode technique and a DC-link voltage regulation loop which consists of an extended state observer (ESO) plus SMC are adopted. The load connected to the DC-link capacitors is considered as an external disturbance. An ESO is used to reject asymptotically this external disturbance. Therefore, its design is considered in the control law derivation to achieve a high performance.