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Design of Robust Higher Order Sliding Mode Control for Microgrids

This paper deals with the design of advanced control strategies of sliding mode type for microgrids. Each distributed generation unit (DGu), constituting the considered microgrid, can work in both grid-connected operation mode (GCOM) and islanded operation mode (IOM). The DGu is affected by load variations, nonlinearities and unavoidable modelling uncertainties. This makes sliding mode control particularly suitable as a solution methodology for the considered problem. In particular, a second order sliding mode (SOSM) control algorithm, belonging to the class of Suboptimal SOSM control, is proposed for both GCOM and IOM, while a third-order sliding mode (3-SM) algorithm is designed only for IOM, in order to achieve, also in this case, satisfactory chattering alleviation. The microgrid system controlled via the proposed sliding mode control laws exhibits appreciable stability properties, which are formally analyzed in the paper. Simulation results also confirm that the obtained closed-loop performances comply with the IEEE recommendations for power systems.

Adaptive suboptimal second-order sliding mode control for microgrids

ABSTRACT This paper deals with the design of adaptive suboptimal second-order sliding mode (ASSOSM) control laws for grid-connected microgrids. Due to the presence of the inverter, of unpredicted load changes, of switching among different renewable energy sources, and of electrical parameters variations, the microgrid model is usually affected by uncertain terms which are bounded, but with unknown upper bounds. To theoretically frame the control problem, the class of second-order systems in Brunovsky canonical form, characterised by the presence of matched uncertain terms with unknown bounds, is first considered. Four adaptive strategies are designed, analysed and compared to select the most effective ones to be applied to the microgrid case study. In the first two strategies, the control amplitude is continuously adjusted, so as to arrive at dominating the effect of the uncertainty on the controlled system. When a suitable control amplitude is attained, the origin of the state space of the auxiliary system becomes attractive. In the other two strategies, a suitable blend between two components, one mainly working during the reaching phase, the other being the predominant one in a vicinity of the sliding manifold, is generated, so as to reduce the control amplitude in steady state. The microgrid system in a grid-connected operation mode, controlled via the selected ASSOSM control strategies, exhibits appreciable stability properties, as proved theoretically and shown in simulation.

Master-slave second order sliding mode control for microgrids

This paper deals with the design of advanced control strategies of sliding mode type for microgrids. Each distributed generation unit (DGu), constituting the considered microgrid, can work in both grid-connected and islanded operation mode. The DGu is affected by load variations, nonlinearities and unavoidable modelling uncertainties, because of the presence of a voltage-sourced-converter (VSC) as interface with the main grid. This kind of uncertainty terms makes the sliding mode controller perfectly fitting the control problem to solve. In particular, a second order sliding mode (SOSM) control scheme, belonging to the class of Suboptimal SOSM control, is proposed. Moreover, in order to face some undesired overshoot on the currents fed into the load, due to the reconnection to the main grid, as well as to step variations of current references, a constrained SOSM control is designed. Simulation results confirm that the proposed robust controllers provide closed-loop performance complying with the IEEE recommendations for power systems.

Third order sliding mode voltage control in microgrids

In this paper, we propose a robust voltage control scheme for microgrids based on a suitable designed third-order sliding mode (3-SM) controller. The use of 3-SM allows to reject matched disturbances and unmodeled dynamics, due to the presence of a voltage-sourced-converter (VSC) as interface with the main grid. The motivation for using a 3-SM control approach, apart from its property of providing robustness to the scheme in front of a significant class of uncertainties, is also given by its capability of enforcing sliding modes of the controlled system with chattering alleviation. The microgrid system controlled via the proposed 3-SM approach proves to exhibit appreciable stability properties. Specifically, the voltage error with respect to the required reference is steered to zero in a finite time. The comparison with respect to second order sliding mode (SOSM) and PI controllers shows the beneficial effects of the proposed strategy, and simulation results confirm that our control law provides closed-loop performance complying with the IEEE recommendations for power systems.

Distributed Second Order Sliding Modes for Optimal Load Frequency Control

This paper proposes a Distributed Second Order Sliding Mode (D-SOSM) control strategy for Optimal Load Frequency Control (OLFC) in power networks, where besides frequency regulation also minimization of generation costs is achieved. Because of unknown load dynamics and possible network parameters uncertainties, the sliding mode control methodology is particularly appropriate for the considered control problem. This paper considers a power network partitioned into control areas, where each area is modelled by an equivalent generator including second-order turbine-governor dynamics. On a suitable designed sliding manifold, the controlled system exhibits an incremental passivity property that allows us to infer convergence to a zero steady state frequency deviation minimizing the generation costs.

Decentralized Sliding Mode Control of Islanded AC Microgrids With Arbitrary Topology

This paper deals with modeling of complex microgrids and the design of advanced control strategies of sliding mode type to control them in a decentralized way. More specifically, the model of a microgrid including several distributed generation units (DGUs), connected according to an arbitrary complex and meshed topology, and working in islanded operation mode, is proposed. Moreover, it takes into account all the connection line parameters and it is affected by unknown load dynamics, nonlinearities and unavoidable modeling uncertainties, which make sliding mode control algorithms suitable to solve the considered control problem. Then, a decentralized second-order sliding mode control scheme, based on the suboptimal algorithm is designed for each DGU. The overall control scheme is theoretically analyzed, proving the asymptotic stability of the whole microgrid system. Simulation results confirm the effectiveness of the proposed control approach.

Event-triggered second order sliding mode control of nonlinear uncertain systems

This paper presents a novel Second Order Sliding Mode (SOSM) control algorithm for a class of nonlinear systems subject to matched uncertainties. By virtue of its Event-Triggered nature, it can be used as a basis to construct robust networked control schemes. The algorithm objective is indeed to reduce the number of state transmissions over the network, in order to alleviate the network congestion and reduce possible packet loss, jitter and delays, while guaranteeing satisfactory performance in terms of stability and robustness. The proposed Event-Triggered Second Order Sliding Mode control strategy is theoretically analyzed in the paper, showing its capability of enforcing the robust ultimately boundedness of the sliding variable and its first time derivative, and consequently the practical stability of the uncertain nonlinear system, in spite of the significant reduction of the number of state transmissions with respect to a conventional SOSM control approach. The satisfactory performance of the proposed scheme are also assessed in simulation.

Event-Triggered Sliding Mode Control algorithms for a class of uncertain nonlinear systems: experimental assessment

An experimental assessment of the recently introduced event-triggered sliding mode control approach is presented in this paper. The major design requirement, in this approach, is to reduce the number of transmissions over the network, while guaranteeing that the sliding mode control is stabilizing with appropriate robustness in front of matched uncertainties. In the present paper a novel Event-Triggered Sliding Mode Control algorithm is first introduced and discussed and then it is compared with two different Model-Based Event-Triggered Sliding Mode Control algorithms. Finally, their experimental assessment is reported, obtaining satisfactory performance consistent with the theoretical treatment and fulfilling all the design requirements.

Decentralized Sliding Mode voltage control in DC microgrids

The present paper deals with the design of a decentralized control scheme that relies on advanced control strategies of Sliding Mode (SM) type to regulate the voltage in islanded Direct Current (DC) microgrids. More specifically, the model of an islanded DC microgrid composed of several Distributed Generation units (DGus) interconnected according to an arbitrary topology including loops, is presented. The model takes into account the power lines dynamics and is affected by unknown load demand and unavoidable modelling uncertainties. First, a Second Order Sliding Mode (SOSM) control algorithm, belonging to the class of Suboptimal SOSM control, is proposed to solve the voltage control problem. Then, in order to obtain a continuous control signal that can be used as duty cycle of the power converter, a third order Sliding Mode (3-SM) control strategy is presented.

Sliding mode control for Maximum Power Point Tracking of photovoltaic inverters in microgrids

In this paper the design of sliding mode controllers for Maximum Power Point Tracking (MPPT) of a photovoltaic inverter in microgrids is presented. A master-slave configuration of the microgrid is considered in islanded operation mode where the photovoltaic Distributed Generation unit (DGu) serves as a slave. The DGu is also affected by nonlinearities, parameters and modelling uncertainties, which make the use of the sliding mode control methodology particularly appropriate. Specifically, a sliding mode controller, relying on the so-called unit vector approach, is first proposed to control the photovoltaic inverter. Then, a Second Order Sliding Mode (SOSM) controller, adopting a Suboptimal SOSM algorithm, is proposed to alleviate the chattering phenomenon and feed a continuous modulating signal into the photovoltaic inverter. Simulation tests, carried out on a realistic scenario, confirm satisfactory closed-loop performance of the proposed control scheme.