In this brief, a novel current-limiting droop controller for paralleled dc-dc boost converters loaded by constant impedance Z, constant current I, or constant power P loads in a dc microgrid is proposed to guarantee closed-loop stability and power sharing. Using an improved version of the recently proposed nonlinear current-limiting controller, an inherent current-limiting property is guaranteed for each converter independently of the load type or magnitude variations. Then, sufficient conditions to ensure closed-loop stability for the entire dc microgrid system with a constant Z, I, or P load are analytically obtained. Hence, compared with existing droop control methodologies, the proposed controller ensures accurate power sharing, tight voltage regulation, and closed-loop stability with a current limitation when connected to Z, I, or P load, for multiple paralleled boost converters, which introduce nonlinear dynamics. To verify the effectiveness of the proposed controller and the stability analysis, simulation results for the three parallel operated dc-dc boost converters with Z, I, and P loads and experimental results for two parallel operated dc-dc boost converters with a P load are performed under several changes of the load power demand.

A nonlinear controller for single-phase grid-tied inverters, that can operate under both a normal and a faulty grid with guaranteed closed-loop stability, is proposed. The proposed controller acts independently from the system parameters, does not require a phase-locked loop, and can achieve the desired real power regulation and unity power factor operation. Based on nonlinear input-to-state stability theory, it is analytically proven that the inverter current always remains below a given value, even during transients, independently from grid variations or faults (short circuit or voltage sag). The desired performance and stability of the closed-loop system are rigorously proven since the controller has a structure that does not require any switches, additional limiters, or monitoring devices for its implementation. Therefore, nonlinear stability of a grid-tied inverter with a given current-limiting property is proven for both normal and faulty grid conditions. The effectiveness of the proposed approach is experimentally verified under different operating conditions of the grid.

Summary: Proportional Integral Derivative (PID) controllers still represent the core control method for achieving output regulation of either linear or nonlinear systems in the majority of industrial applications. However, conventional PID control cannot guarantee specific state constraint requirements for the plant, when the system introduces uncertainties. In this paper, a novel nonlinear PID control that achieves output regulation and guarantees a desired state limitation below a given value for a wide class of nonlinear systems with constant uncertainties is proposed. Using nonlinear ultimate boundedness theory, it is shown that the proposed state‐limiting PID (sl‐PID) control maintains a given bound for the desired system states at all times, ie, even during transients, whereas an analytic method for selecting the controller gains is also presented to ensure closed‐loop system stability and convergence at the desired equilibrium. Two nonlinear engineering examples that include an electric motor and a dc/dc converter are investigated using the conventional PID and the proposed sl‐PID to validate the superiority of the proposed controller in achieving the desired output regulation with a given bounded state requirement.

Droop-controlled inverters are widely used to integrate distributed energy resources (DERs) to the smart grid and provide ancillary services (frequency and voltage support). However, during grid variations or faults, the droop control scheme should inherit a current-limiting property to protect both the inverter and the DER unit. In this brief, a novel structure of the recently developed current-limiting droop (CLD) controller is proposed to accomplish two main tasks: 1) guarantee current limitation with maximum power injection during grid faults and 2) rigorously guarantee asymptotic stability of any equilibrium point in a given bounded operating range of the closed-loop system for a grid-connected inverter. Since the maximum power of the DER unit can be utilized under grid faults with the proposed enhanced CLD and then inspired by the latest fault-ride-through requirements, it is further extended to provide voltage support to a faulty grid via the maximum injection of reactive power. This is achieved by simply adjusting the reactive power reference opposed to the existing control schemes that require adjustment of both the real and the reactive powers. Hence, a unified current-limiting control scheme for grid-connected inverters under both normal and faulty grids with a simplified voltage support mechanism is developed and experimentally verified in this brief.

The cyber–physical system (CPS) architecture provides a novel framework for analyzing and expanding research and innovation results that are essential in managing, controlling and operating complex, large scale, industrial systems under a holistic insight. Power systems constitute such characteristically large industrial structures. The main challenge in deploying a power system as a CPS lies on how to combine and incorporate multi-disciplinary, core, and advanced technologies into the specific for this case, social, environmental, economic and engineering aspects. In order to substantially contribute towards this target, in this paper, a specific CPS scheme that clearly describes how a dedicated cyber layer is deployed to manage and interact with comprehensive multiple physical layers, like those found in a large-scale modern power system architecture, is proposed. In particular, the measurement, communication, computation, control mechanisms, and tools installed at different hierarchical frames that are required to consider and modulate the social/environmental necessities, as well as the electricity market management, the regulation of the electric grid, and the power injection/absorption of the controlled main devices and distributed energy resources, are all incorporated in a common CPS framework. Furthermore, a methodology for investigating and analyzing the dynamics of different levels of the CPS architecture (including physical devices, electricity and communication networks to market, and environmental and social mechanisms) is provided together with the necessary modelling tools and assumptions made in order to close the loop between the physical and the cyber layers. An example of a real-world industrial micro-grid that describes the main aspects of the proposed CPS-based design for modern electricity grids is also presented at the end of the paper to further explain and visualize the proposed framework.

A new nonlinear control framework that guarantees the desired regulation (voltage, current, or power) with an inherent current-limiting capability for different types of dc/dc power converters is presented in this brief. This framework is based on the idea of applying a virtual resistance in series with the inductor of the converter, which changes according to nonlinear dynamics that depend on the control task. Without requiring any knowledge of the converter inductance, capacitance, or the load, the controller structure is appropriately formulated for each power electronic system based on the nonlinear model of the converter. Using input-to-state stability theory, it is proven that the inductor current remains below a maximum value at all times, even during transients, independently of load and input voltage variations. This offers an inherent current-limiting property of the converter under faults, input voltage sags, and unrealistic power demands without the need of external protection mechanisms, saturation units, or current limiters. Extensive simulation and experimental results validate the effectiveness of the proposed control scheme and its current-limiting property, with comparison to traditional control strategies.