# Power Management Strategy for an Autonomous DC Microgrid
## 1 Introduction
Unlike in the above research work, the DG operates constantly in Reference [19] to contribute to the power balance of an isolated DC microgrid, which includes renewable sources and electrochemical storage. To compensate power fluctuations and decrease DG fuel consumption, a discrete Fourier transform method for the coordinated power dispatch of the electrochemical storage and DG is proposed. Although the continuous operation of the DG avoids its start-up time, this operation mode results in inefficient loading and increased greenhouse gas emissions. In References [20; 21], electrochemical storage and a supercapacitor (SC) are used to improve the performance of wind-DG generation by absorbing disturbance due to wind speed variations. However, the proposed power management strategy is based on a DG operated continually to regulate bus voltage and without considerations of fuel consumption. Moreover, a controlled DC source is used for DG behavior emulation. In Reference [22], the DG's frequent switching control and the non-linearity of the DG fuel consumption curve are considered, showing that significant fuel savings can be achieved compared to where the DG is used alone to supply the same load requirements. However, the sluggish dynamic response of the DG is not considered, and consequently, the given simulation results are based on an ideal power source. Furthermore, Reference [23] considers that since the DG is equipped with an electronic speed governor and automatic voltage regulator, its dynamic behavior is not a concern and DG modelling is therefore limited to operating cost and greenhouse gas emissions. Nonetheless, DGs cannot output instantaneous power and their cold start-up takes several seconds or even minutes [24] before they output stable power, depending on the machine's capacity (i.e., about 10 s for DGs smaller than 1000 kVA [25]). Therefore, fuel-based generators need a certain warm-up period to reach a safe operating temperature and stabilize output voltage before being connected to the grid. However, power interruptions during DG start-up can cause interruption of the entire microgrid power system. Power interruptions are very costly and unacceptable to critical power consumers, such as hospitals, data centers, and security systems. Through to its rapid power response and high-power density, the SC is a solution that may solve the discontinuity problem of power supply during DG start-up. Several research works propose SC to improve dynamic characteristics and to compensate for the sluggish dynamic behavior of a variable-speed commercial DG [11] or fixed-speed commercial DG [26], both used as single isolated power systems. However, many SC are equipped with a passive balancing system for protective purposes, but this causes self-discharge that must be considered in long-term operation. In References [27; 28], a power management strategy demonstrates by experiment that an SC can correctly balance DC bus power during DG start-up, although there is no consideration of DG fuel consumption and the total energy cost. The aforementioned research work includes an as-yet not comprehensive study taking into account the various issues caused by DG and SC characteristics, in the context of an autonomous DC microgrid. To overcome all these issues, a specific power management strategy for an autonomous DC microgrid involving a DG is necessary. To advance knowledge, this article furthers research based on References [26; 27; 28] and proposes a comprehensive power management strategy for an autonomous DC microgrid based on a PV source, electrochemical storage, a supercapacitor, and a DG. The main contribution of this paper centers on a power management strategy for an autonomous DC microgrid, and economic analysis for the DG and microgrid. This control aims to achieve power balance while accounting for the slow start-up characteristic of a DG, the self-discharge of a SC, dynamic load management, and the economic operating mode of the DG.