Active Power Distribution Scheme in a Hybrid AC/DC Microgrid Integrated with Composite Energy Storage Devices

Abstract

The growing power requirement and the limited availability of fossil fuels make it necessary to use renewable energy resources (RERs) as a substitute. The penetration of renewable sources in the existing distribution system becomes a big challenge for the safe and predictable operation of the microgrid. In this work, a solar photovoltaic (PV) system is integrated because of its low cost and high efficiency. However, the intermittent characteristics of RESs along with the arbitrary load deviations greatly affect the quality of power supplied to the consumers and the steadiness of the system operation. Hence, it necessitates the inclusion of energy storage systems with both high energy and high power handling capability to coexist in microgrids. Here, by considering the complementary characteristics of supercapacitors with a high power density (500 − 5000 𝑊/𝐿) and batteries with high energy density (50 − 80 𝑊ℎ/𝐿), a combination of these two storage devices is used with renewable sources as a composite energy storage devices (CESDs). The purpose of CESD is to lessen the power disparity produced by the intermittency of renewable sources and varying load demands. Optimal utilization of storage devices is very important in microgrid applications as a backup power source to enhance the resiliency and reliability of the system for critical loads. Hence, the power distribution in the composite energy storage system is a major concern and also it remains a challenging issue in conventional techniques. The problem is resolved by implementing an improved mixed droop technique (IMDT) with optimized steady-state along with momentary performances of CESDs in this research. With the proposed technique, the supercapacitor compensates all the fast varying power oscillations appeared in the system, whereas the battery supplies only the average power at steady-state. This control strategy advances the consistency of the system, dynamic restoration of the DC link voltage, and reduces stress of current from the battery units. The microgrid performance is dependent on the parameters of the IMDT, hence proper design guidelines are also defined in this work to get better performance. Also, a robust sliding mode nonlinear controller (SMC) is implemented instead of the conventional PI controller for the switching regulation of the DC/DC bidirectional converters connected across the CESDs. Sliding mode contro is mainly intended to maintain the required performance of the microgrid despite any changes in system parameters or model inaccuracies and any external disturbances. SMC is effective in compensating any disturbances, ensuring that the system remains stable and follows the desired trajectory or maintains its position. Hence, a hybrid control algorithm is developed in this work by combining both IMDT and SMC for the distribution of active power between the battery and supercapacitor units. The inclusion of RESs and energy storage devices in the conventional power grid necessitates rigorous study of the power equilibrium and microgrid stability. Hence, this research work introduces an active power distribution scheme (APDS) for both isolated DC microgrid systems and grid interactive hybrid AC/DC microgrid (GIHM) systems. The power-sharing technique is designed by checking the power disparity between the PV generation and load requirements and the available state of charge (𝑆𝑂𝐶) of energy storage units. The APDS helps to utilize the PV and energy storage devices optimally by reducing the utility grid dependency and also maintains the 𝑆𝑂𝐶𝑠 of the storage devices within the safe limit to enhance their lifetime. This power-sharing scheme provides improved performance by maintaining the unity power factor grid operation, faster restoration of DC link voltage, and reduced harmonic. Also, the small-signal modeling of the system using a double-loop control scheme is presented to analyse the stability of the proposed system and to obtain the accurate estimation of the PI parameters by calculating the proper bandwidth and phase margin of both current and voltage loops in the system. The obtained open-loop transfer functions of each loop are provided with their individual Bode response. Also, a PV-integrated electric vehicle charging microgrid (EVCM) along with combined storage systems and a conventional single-phase utility grid is proposed in this research to facilitate both grid-to vehicle (G2V) and vehicle-to-grid (V2G) functions. The bidirectional power flow of the EV supports the microgrid in peak load hours and during low PV generation periods. For charging of EV battery (EVB), an innovative dynamic charging current-constant voltage (DCC-CV) method is planned to minimize the transient in the system and to avoid the overcharging of EVB. For the discharging of EVB, a fixed power supply concept is employed. To estimate the performance and feasibility of the designed APDS in isolated DC microgrid, hybrid AC/DC microgrid, and EVCM, the respective system models are tested in MATLAB/Simulink, developed prototype in the laboratory using DS1103, and OPAL- RT simulator with an extensive analysis of obtained results.