Networked control of AC microgrid

Time-delay is unavoidable, encountered in many practical systems such as networked control systems, communication networks, manufacturing, and biology. On the one hand, it often represents a source of instability and oscillation. On the other hand, the appearance of delay can have a stabilizing effect, and delayed feedback control can be used to stabilize some unstable systems. Considering its theoretical and practical importance, increasing attention has been focused on the analysis and synthesis of time-delay systems, and various new approaches have been proposed to reduce their conservatism. The aim of reducing the conservatism of delay-dependent stability criteria is to establish new stability criteria to provide a maximal allowable delay as large as possible. Among different techniques, the reduced conservatism is mainly obtained by constructing an improved Lyapunov functional and employing a tighter bound on some weighted cross products. A great number of effective methods have been provided such as delay-partitioning method, convex combination method, Wirtinger-based inequality, and Bessel–Legendre inequality.

This paper is concerned with stability criteria of networked control systems (NCSs) with both the time-varying network-induced delay and data packet dropout taken into account. A continuous time model of NCSs with time-varying and uncertainties is proposed. Based on the dividing of the time-varying delay, a new Lyapunov functional is constructed, and a delay-dependent stability criterion is derived to guarantee the globally asymptotic stability of the networked control systems (NCSs). Expressed in terms of linear matrix inequalities (LMIs), the stability condition can be checked using the numerically efficient Matlab LMI Control Toolbox. Compared with some report methods, no slack matrix variable is introduced and the proposed methods gave a much less conservative maximum allowable delay bound (MADB). A numerical example is provided to demonstrate the effectiveness of the proposed method.

This paper deals with time delays which exist in networked control systems due to presence of communication network. Time delay impact on networked control system stability is analyzed, and some relations for maximum allowable delay bound estimation are given as well. It is shown, on example of DC motor control, that the system behaves in desired manner and remains stable when appropriate values of regulator parameters are chosen to match estimated maximum time delay.

Delay-dependent stability criteria for a class of networked control systems with multi-input and multi-output☆

In this article, we characterize the joint effects of plant uncertainty, transmission delay, and fading channel on the stabilization problem of networked control systems (NCSs). We assume that the controller remotely controls the plant and the control packet is transmitted over a delayed and fading channel. We formulate the integrated system as an uncertain discrete-time stochastic system with both input delay and multiplicative noise. The innovative contributions of this article are described as follows. We propose a set of necessary and sufficient conditions for stabilizing an uncertain-free system. Moreover, for the general uncertain model, we derive the necessary and sufficient stabilization condition in terms of symmetric matrix homogeneous polynomials, which can be verified by linear matrix inequality (LMI) feasibility test. As an application, under the parallel transmission strategy, we derive an explicit formula for computing the maximum allowable delay bound, which is uniquely determined by system parameters and signal-to-noise ratio.

This article reviews two techniques that use delay for control: time-delay approaches to control problems (which initially may be free of delays) and the intentional insertion of delays into the feedback. We begin with a now widely used time-delay approach to sampled-data control. In networked control systems with communication constraints, this is the only method that accommodates transmission delays larger than the sampling intervals. We present a predictor-based design that enlarges the maximum allowable delay, which is important for practical implementations. We then discuss methods that use artificial delays via simple Lyapunov functionals that lead to feasible linear matrix inequalities for small delays and simple sampled-data implementations. Finally, we briefly present a new time-delay approach—this time to averaging. Unlike previous results, this approach provides the first quantitative bounds on the small parameter, making averaging-based control (including vibrational and extremum-seeking control) reliable.

In this paper, an H∞ direct output feedback control algorithm is employed to design the control system in reducing structural seismic responses. Control forces are obtained from the multiplication of direct output measurements by a pre-calculated time-invariant feedback gain matrix. To achieve optimal control performance, the strategy to select both control parameters γ and α is developed. Analytical results show that the decrease of γ or the increase of α makes better control performance, but requires larger control forces. It is also found that lower and upper bounds of γ and α exist. The selection beyond these values will cause control system instability. In this paper, analytical expressions of the upper and lower bounds of γ and α are derived. Structural seismic responses can thus be reduced effectively with appropriate selection of γ and α. Moreover, control force execution time delay cannot be avoided. Relatively small delay time not only can render the control ineffective, but also may cause system instability. In this study, explicit formulas to calculate maximum allowable delay time and critical control parameters are derived for the design of a stable control system. Some solutions are also proposed to increase the maximum allowable delay time. The desired control performance can be guaranteed even with time delay.

In this paper, an H∞ direct output feedback control algorithm through minimizing the entropy, a performance index measuring the tradeoff between H∞ optimality and H2 optimality, is employed to design the control system in reducing structural responses due to dynamic loads such as earthquakes. The control forces are obtained from the multiplication of direct output measurements by a pre-calculated time-invariant feedback gain matrix. To achieve optimal control performance, the strategy to select both control parameters γ and α is extensively investigated. The decrease of γ or increase of α results in better control effectiveness, but larger control force requirement. For a single degree-of-freedom (SDOF) damped structure, exact solutions of output feedback gains and control parameters are derived. It can be proved analytically that the LQR control is a special case of the proposed H∞ control. Direct velocity feedback control is effective in reducing structural responses with very small number of sensors and controllers compared with the DOFs of the structure. In active control of a real structure, control force execution time delay cannot be avoided. Relatively small delay time not only can render the control ineffective, but also may cause system instability. In this study, explicit formulas to calculate maximum allowable delay time and critical control parameters are derived for the design of a stable control system. Some solutions are also proposed to increase the maximum allowable delay time.

This article addresses the problem of controller design for networked control systems over digital communication. The systems under consideration are stabilised via state feedback, where the effects of sampled signal, state quantisation, network-induced delay and packet dropout are considered. The proposed delay-dependent stability criteria are formulated in the form of a linear matrix inequality, which ensure asymptotic stability and a prescribed H ∞ performance level for networked control systems with admissible uncertainties. Maximum allowable delay bound of networked control systems is obtained by solving a convex optimisation problem. Furthermore, a numerical example is given to illustrate the effectiveness of the main result.

This paper presents a distributed control scheme for achieving secondary frequency control in AC microgrids. The control scheme is designed for microgrid electrical networks comprised of distribution lines with equal/unequal resistance to reactance ratio; in other words, it works well even when ohmic losses are taken into account in the electrical network. The control problem is formulated and an overview of the proposed distributed secondary frequency control scheme is provided. A criterion for choosing controller gains that stabilize the system frequency is also provided. Finally, results on the testing of the control scheme on a controller hardware-in-the loop (C-HIL) testbed are provided.

In this paper, we present an improved delay-dependent absolute stability criterion for Lur’e systems with time delays. The guarantee of absolute stability is provided by Lyapunov-Krasovskii theorem with the Lyapunov functional containing the integral of sector-bounded nonlinearities. The Lyapunov functional terms involving delay are partitioned to be associated with each equidistant fragment on the length of time delay. Employing the Jensen inequality and S-procedure, the sufficient condition is derived from time derivative of the Lyapunov functional. Then, the absolute stability criterion expressed in terms of linear matrix inequalities (LMIs) can be efficiently solved using available LMI solvers. The bisection method is used to determine the maximum allowable time delays to ensure the stability of Lur’e systems in the presence of uncertain time-invariant delays. In addition, the stability criterion is extended to Lur’e systems subject to norm-bounded uncertainties by using the matrix eliminating lemma. Numerical results from two benchmark problems show that the proposed criteria give significant improvement on the maximum allowable time delays.

Ensuring the cybersecurity of secondary control in AC microgrids is crucial for maintaining system stability and operational resilience. Cyberattacks targeting the communication links of secondary controllers can lead to severe disruptions in frequency regulation, voltage control and power sharing mechanisms. This paper proposes a cybersecurity‐resilient control strategy based on the short‐time correlation transform (STCT) to enhance the robustness of distributed secondary control against cyber threats. The proposed strategy leverages the magnification properties of STCT to accurately identify compromised communication links, isolate them from the control network and reinforce the integrity of unaffected links for reliable data transmission. The STCT‐based strategy is independent of attack type and location, making it highly adaptable to dynamic microgrid environments. The effectiveness of the proposed strategy is validated through extensive simulations on multiple microgrid topologies under various attack scenarios. Results demonstrate that the approach successfully mitigates cyberattacks, preserves system stability and restores microgrid frequency to its reference value with minimal delay. Furthermore, due to its lightweight computational requirements and compatibility with practical control implementations, the proposed strategy is well‐suited for real‐time applications. This study provides a significant advancement in microgrid cybersecurity by introducing a highly efficient, scalable and attack‐agnostic protection mechanism for distributed secondary control.

The power gating technique is useful in reducing standby leakage current, but it increases the gate delay. For a functional unit, its maximum allowable delay (for a target clock period) limits the smallest standby leakage current its power gating can achieve. In this paper, we point out: in the high-level synthesis of a non-zero clock skew circuit, the resource binding (including functional units and registers) has a large impact on the maximum allowable delays of functional units; as a result, different resource binding solutions have different standby leakage currents. Based on that observation, we present the first work to draw up the timing driven power gating in high-level synthesis. Given a target clock period and design constraints, our goal is to derive the minimum-standby-leakage-current resource binding solution. Benchmark data show: compared with the existing design flow, our approach can greatly reduce the standby leakage current without any overhead.

Connecting a complex control system with various sensors, actuators, and controllers as a networked control system by a shared data network can effectively reduce complicated wiring connections. This system is also easy to install and maintain. The trend is to use networked control systems for time-sensitive applications, such as remote DC motor actuation control. The performance of a networked control system can be improved if the network can guarantee quality-of-service (QoS). Due to time-varying network traffic demands and disturbances, QoS requirements provided by a network may change. In this case, a network has to reallocate its resources and may not be able to provide QoS requirements to a networked control application as needed. Therefore, the application may have to gracefully degrade its performance and perform the task as best as possible with the provided network QoS. This paper proposes a novel approach for networked DC motor control systems using controller gain adaptation to compensate for the changes in QoS requirements. Numerical and experimental simulations, and prototyping, are presented to demonstrate the feasibility of the proposed adaptation scheme to handle network QoS variation in a control loop. The effective results show the promising future of the use of gain adaptation in networked control applications.

Delay-range-dependent robust stabilization for uncertain T–S fuzzy control systems with interval time-varying delays