# Modeling and Simulation of Autonomous DC Microgrid with Variable Droop Controller
## 1 Introduction  
The integration of renewable energy sources with advanced distributed storage technologies has enabled microgrids to deliver clean, reliable, and cost-effective energy to society [1, 2]. Microgrids offer additional benefits, including voltage support, loss minimization, and enhanced power quality. Initially, AC microgrids gained more popularity than DC microgrids, primarily because of the transformers, which are the the core component of AC systems, naturally allowed adaptation to varying voltage and power levels as required. AC microgrids continue to dominate the modern power industry, as mostexisting power grids are AC-based, and the widespread adoption of purely DC microgrids has been limited. However, the advent of highly efficient, cost-effective power electronic converters (PECs) and the growing variety of DC loads has established a significant role for DC microgrids in the industry. DC microgrids offer distinct advantages, including being free of synchronization issues, reactive power challenges, and harmonic distortions, which makes them increasingly reliable in modern power systems. The benefits and challenges of DC microgrids, particularly in terms of efficiency, cost reduction, and integration with renewable energy, are discussed in [3, 4]. In [5], modeling of a DC microgrid with various types of DGs and storage is performed and the scope of the system for sustainable power is investigated. Figure 1 shows a general architecture of the DC microgrid.  
Various types of DGs are integrated with the DC bus using PECs. DC loads are connected directly, while AC loads are interfaced via AC-DC converters. The bus voltage and power levels vary depending on the specific application. DC microgrid control focuses on maintaining bus voltage stability and ensuring proportional power sharing between the sources. Maintaining stability, especially in autonomous mode, presents a significant challenge in microgrids. To address this, various control strategies have been developed. Voltage control of the DC bus primarily involves regulating the interfacing PECs. In DC microgrid control, decentralized droop control is a well-established technique for islanded operation. The basic principle behind droop control is to enhance the low output resistance of the PEC, which is often the primary cause of inadequate load sharing in a microgrid. This introduces a resistive droop characteristic into the system. However, the literature reveals that fixed droop control cannot fully address the inherent dependency of bus voltage deviation on load current. A decentralized variable droop controller for an autonomous DC microgrid is proposed to improve bus voltage regulation under varying load dynamics. The effectiveness of current sharing with the proposed control technique is also investigated.  
This paper is organized as follows: Section 2 offers an in-depth review of the operation and control of DC microgrids. The proposed system is introduced in Section 3. The simulation results and a detailed analysis of the results are given in Section 4. The paper is finally concluded in Section 5, which also provides a summary of the main contributions and possible future research areas.  
Figure 1: DC microgrid architecture.