Fulwani, Deepak M

For the implementation of distributed cooperative control of the dc microgrid (DCMG), dependency on the communication layer is inevitable. Consequently, the DCMG, a typical cyber-physical system, is susceptible to cyber-attacks, which can hinder the achievement of overall control objectives, and the system may even get destabilized. To detect and mitigate cyber-attacks on actuators as well as sensors, a resilient mechanism is proposed in this article. First, the detection mechanism is proposed using a finite-state machine model-based technique that exploits a dynamic function. This function keeps investigating the actual and estimated values of actuator and sensor signals to authenticate the attack detection. Second, mitigation of attacks is carried out using a robust sliding-mode functional observer (SMFO). The signal of the compromised node is replaced by the SMFO estimated value in the event of an attack detection. The SMFO-based proposed technique is insensitive to measurement noise and parametric variations. The proposed approach is validated by simulation as well as experimental studies on a prototype of a 4-node DCMG.

The study proposes a method to design a non-linear sliding surface to achieve better transient response for a class of single-input and single-output (SISO) non-linear uncertain system represented in a Brunowsky canonical form. The proposed surface can also be used for linear uncertain systems with matched perturbations. The proposed surface increases the damping ratio of the closed-loop system from its initial low value; as the output approaches the setpoint from its initial value. Initially, the system is lightly damped resulting in a quick response and as the output approaches the setpoint, the system is overdamped to avoid overshoot. The existence of sliding mode is proved and a new control law is proposed to enforce sliding motion. The scheme is able to achieve low overshoot and short settling time simultaneously which is not possible with a linear sliding surface. To ease the synthesis of the non-linear surface, linear matrix inequalities-based algorithm is proposed. Effectiveness of the proposed scheme is illustrated by the simulation results. © 2011 The Institution of Engineering and Technology.

A class of nonlinear uncertain systems with saturated actuator is considered in this paper. Sliding surface matrix is obtained using parameterized Riccatti equation. The proposed surface ensures that control limits are respected in a region of state space. This region can be made sufficiently large by choosing appropriate value of design parameters. © 2012 IEEE.

Direct current microgrids (DCMGs) are swiftly moving toward the realm of communication-dependent distributed cooperative control strategies. The incorporation of cyber layer for robustness, scalability, and reliability makes the system vulnerable toward cyber-attacks. The extent of damage caused by these attacks on DCMG is substantial, to the point where ceasing the operation may become necessary. This article proposes a resilient strategy for the detection and mitigation of the most prominent false data injection attacks (FDIAs) on actuators of nodes of DCMG. An accurate error-free detection is guaranteed using a state machine-based model that makes use of a dynamic signature function that monitors the actuator signal and its estimated value. Linear functional observer (LFO) based mitigation scheme is proposed, in which the affected node is switched to LFO upon a true attack detection. The proposed technique is consistent during transients. Experimentation and simulation studies are carried out for various practical situations for a four-node DCMG to validate the proposed theory.

This paper proposes an input-based event-triggered mechanism for a network-controlled discrete-time singularly perturbed system, where the latest measurements are transmitted when a predefined condition is satisfied on the input. The significant contribution of the proposed work is the separation of control updates for slow and fast dynamics such that the proposed event-triggered mechanism updates and broadcasts the current measurements of slow and fast dynamics in their respective time scales and reduces the usage of network resources. Further, the proposed input-based event-triggered mechanism avoids redundant updates of the control input and reduces actuation and resource utilization in some instances. Furthermore, the ultimate boundedness of the system states is established. The simulation uses a quarter-car suspension system to demonstrate the proposed theory.

The switched boost inverters are single stage topologies that boost dc voltage and convert it to required ac voltage. It requires lesser number of components and is more efficient compared to conventional two stage dc-dc-ac methods. The dc-ac conversion results in second order harmonic currents (SHCs) to be reflected at the source end. It causes various problems such as heating or failure of sources, oscillations in maximum power point tracking. The active or passive filters for SHCs may increase component count as well as increase overall cost of the system. In this paper, a sliding mode control based voltage and current control method method is proposed so as to reduce the SHCs in Switched Boost Inverter (SBI). Also, the transients during dc or ac load variations are kept within allowed range. This results in excellent voltage regulation during load changes. The proposed controller is validated through simulation for different types of load and upto 50 % load variations.

Distributed control of converters is important to achieve plug and play functionality in a dc microgrid. It must ensure proportional load sharing among converters and voltage regulation of the dc bus. Droop control is a conventional distributed control method to achieve load sharing among converters in a dc microgrid. It is realized by reducing the voltage reference, linearly or dynamically, as the load increases. This degrades the voltage regulation as the overall bus voltage decreases as the load increases. In this paper, a sliding mode control based adaptive voltage tuning method is proposed such that the voltage reference is adjusted dynamically above and below the desired dc bus voltage to achieve load sharing. This results in excellent voltage regulation, also the proposed control is realized locally hence it facilitates plug and play functionality. The per unit input current value is exchanged among the neighboring converters. The proposed controller is validated through simulation.

There have been several advanced topologies proposed by the community for micro-inverter applications. However, many such applications suffer from unwanted second-order harmonic current ripple at dc input. Moreover, in the absence of suitable passive filter or ripple compensator, the second-order harmonics ripple may propagate into the dc source. This results in several problems to the system, related to system efficiency, life, cost, size, reliability, and stability. This paper proposes an adaptive sliding-mode controller to shape the output impedance of the boost-circuit of quasi-switched boost inverter such that the propagation of the ripple from dc link to the dc-input source is resisted. The quasi-switched boost inverter is one of the advanced and suitable topologies for the micro-inverter applications. The adaptive nature of the proposed controller improves the transient performance of the system at the line-load transients unlike some existing solutions, which affects dynamics adversely to achieve ripple mitigation objective. The controller ensures voltage regulation within {5%} at dc link. The proposed control technique is verified using a lab-prototype of 500 W quasi-switched boost inverter.

The emulation of virtual resistance finds various applications in the power processing industry. This paper proposes a novel notion of adaptive sliding mode based loss free resistor (ASLFR). This is achieved by allowing the input power of the power-out power-in (POPI) system to vary, in order to accommodate the load demands. In this paper, the concept is illustrated for power-factor correction (PFC) applications. The ASLFR is used to achieve the dual purpose of harmonics-free rectification along with excellent system response under load and line transients. The scheme serves itself as an efficient single-stage PFC solution. A generic mathematical formulation of the scheme is presented, which can be used for different converters. Then on, a boost topology, operating in continuous conduction mode, is chosen to demonstrate theoretical developments and to showcase the effectiveness of the scheme. The robustness of the proposed controller to any line or load variation is established. A fast voltage recovery with almost no undershoot/overshoot is achieved at transients by using the proposed controller. Additionally, a qualitative analysis is provided to demonstrate the expediency of the proposed ASLFR. The theoretical claims are well supported by simulation as well as experimental results.

The distributed control of dc microgrid requires communication of the voltage or per-unit current data between the neighboring nodes. This data is processed by the secondary controller to generate a reference for the primary controller. Any uncertainty in the communicated data leads to the oscillations in the dc bus voltage, disproportionate load sharing, or instability due to erroneous references generated. In this work, an ISMC based secondary controller is proposed, which tunes the voltage reference within the regulation range and also mitigates the bounded uncertainties in the communicated microgrid load data. The ISMC has the advantage of elimination of the reaching phase, as the desired node trajectories start from sliding manifold. This makes the secondary control robust throughout the operating range and facilitates proportional load sharing in uncertain operating conditions. The proposed secondary control compares the actual node parameters with the desired global reference values and generates the control signal, which is added with the primary controller's control signals. The proposed control methodology requires only local parameters for formulation. This eases the control design process. Further, a sliding mode control based primary control (SMPC) is proposed to regulate the voltage of a node and facilitate plug and play among the microgrid interfacing converters. The proposed robust SMPC has excellent control during load transients and plug-in and out of nodes from the dc bus. The proposed control is verified using simulations and experiments on a three-node dc microgrid.