In recent years, the application of renewable energy resources (RES) with DC output has increased, and RES integration as DC islanded microgrids (DC ImGs) has attracted the attention of many researchers. However, DC ImGs face many challenges, and voltage stability is extremely critical for efficient power distribution. This challenge becomes more prominent when exogenous disturbances, as well as time-delay, exist in the system mainly because of the communication network. In this study, we develop a mathematical model of the time-delay DC ImG. To compensate for the effect of the time-delay, three control strategies are introduced—stabilizing, robust, and robust-predictor. The controller’s stability is guaranteed based on the Lyapunov-Krasovskii theorem, whereas for the exogenous disturbance rejection, the L2-norm of the system is reduced. Furthermore, to obtain the proposed controllers’ gains, linear-matrix-inequality constraints are formulated. The performances of the controllers are investigated through numerous simulations, and a detailed analysis is presented.

The integration of renewable energy resources to DC microgrid has captured the attention of the researchers in recent years. One of the active field of application of DC distribution is the islanded DC microgrid (DC ImG). The DC ImG present numerous challenges to researchers. Among many challenges, the regulation of voltage and stability of the system is indispensable to efficient operation. The voltage stability problem becomes more prominent when the system is exposed to disturbances and possess uncertainties in parameters. However, challenges can be overcome by skilful design of a robust controller for the system. Therefore, in this paper, an output-feedback based centralized robust control scheme is proposed. The proposed controller is designed to maintain good control performance in the presence of parametric uncertainties and exogenous disturbances. The uncertainties of the DC microgrid is modelled as a linear time-varying state-space system. The upper and the lower bounds of the time-varying parameters are determined by a Lebesque-measurable matrix. To attenuate the exogenous disturbances of the system $H_{\infty }$ based output-feedback controller is designed. The system stability is assured by the Lyapunov function candidate. The output-feedback controller needs only the voltage measurement; therefore, it requires less communication bandwidth as compared to the state-feedback. To obtain the controller parameters linear matrix inequality constraints are formulated and solved. The performance of the proposed controller is verified via simulations and compared with the existing schemes.

A low voltage direct current (LVDC) distribution network is a promising technology to meet the standards of future energy demands for smart loads. An LVDC distribution network can not only supply efficient, smooth and clean energy, but also makes the integration of renewable energy resources in the distribution system easy. A major obstacle in the implementation of the LVDC distribution network is the protection of the network during abnormal grid conditions, such as transients and faults. This paper analyzes DC fault characteristics considering an LVDC distribution network, highlights the worst case scenario during a fault and protection related issues and proposes the protection schemes for the LVDC network. In the proposed protection scheme, a fault is detected and located through superimposed components. To minimize the effect of the DC fault on the distribution network, distributed fault current limiters are introduced and the final decision to disconnect or reconnect the affected line is made on the basis of the type of fault. In addition, a reclosing scheme for a temporary fault is proposed to avoid high inrush currents and false tripping, which eventually increases the reliability. A fast communication-based backup protection is also suggested, and to reduce dependency, a secondary backup is used in the case of communication delay or failure. The proposed scheme is verified using the modified IEEE 13 node test system, which is implemented in ATPDraw. The results show that the proposed scheme can successfully detect, locate and limit a DC fault in an LVDC distribution network with different fault resistances or locations. Moreover, the network is restored successfully in the case of temporary faults.

Power generating units in the form of distributed generation (DG) are integrating with microgrid (MG) systems to supply reliable power to users. The trend of an integrated power supply is increasing continually to meet growing energy demands. Besides certain advantages of the MG system, some interconnection and system protection issues pose a challenge in terms of implementation. These problems need to be addressed in order to facilitate smooth operation of power systems. Superconducting fault current limiters (SFCLs) provide the best performance and unique protection features to electrical power systems during various fault conditions. This paper describes modeling, characterization, and implementation of SFCL in an MG system to enhance protection coordination. The proposed SFCL is modeled in MATLAB/ Simulink software and the performance of the system is validated through simulation results. It is also shown that the proposed SFCL provides a means of better relay coordination during various fault conditions. This is achieved by incorporating well-calculated design parameters of SFCL and its placement at optimal locations.

The day-by-day increase in digital loads draws attention towards the need for an efficient and compatible distribution network. An LVDC distribution network has the capability to fulfill such digital load demands. However, the major challenge of an LVDC distribution network is its vulnerability during a fault. The need for a high-speed fault detection method is inevitable before it can be widely adopted. This paper proposes a new fault detection method which extracts the features of the current during a fault. The proposed fault detection method uses the merits of overcurrent, the first and second derivative of current, and signal processing techniques. Three different features are extracted from a time domain current signal through a sliding window. The extracted features are based upon the root squared zero, second, and fourth order moments. The features are then set with individual thresholds to discriminate low-, high-, and very high-resistance faults. Furthermore, a fault is located through the superimposed power flow. Moreover, this study proposes a new method based on the vector sum of positive and negative pole currents to identify the faulty pole. The proposed scheme is verified by using a modified IEEE 13 node distribution network, which is implemented in Matlab/Simulink. The simulation results confirm the effectiveness of the proposed fault detection and identification method. The simulation results also confirm that a fault having a resistance of 1 m $\mathsf{\Omega }$ is detected and interrupted within 250 $\mathsf{\mu }$ s for the test system used in this study.