# Power Management Strategy for an Autonomous DC Microgrid
## 1 Introduction  
The smart grid is a next generation power system [1; 2], in which the microgrid gathers the loads, storage, and distributed energy resources (DERs) operating as a single controllable electrical power system [3]. The microgrids' power generation is mainly for self-consumption to decrease influence on the utility grid. With the wide adoption of renewable DC power sources, the rapid progress of power electronics technology, and the gradual increase of DC loads in commercial, industrial, and residential applications, the DC microgrid largely attracts public attention due to its merits over the AC microgrid [4; 5]. The DC microgrid can operate in grid-connected mode and in off-grid or islanded mode if a blackout occurs in the utility grid. It can also operate as an isolated system in a remote isolated area. When operating in autonomous mode, i.e., islanded or isolated operating mode, the DC microgrid requires backup power sources to handle the intermittency of renewable energy sources. Electrochemical storage, which is commonly used to provide backup capacity and store electrical energy generated from photovoltaic (PV) panels, is provided by lead-acid battery banks because of their comparatively low cost and good performance [6] and second life lithium-ion battery banks from electric vehicles [7]. However, due to size and economic constraints, both the power andenergy capacity of electrochemical storage are usually limited [8]. The diesel generator (DG), based on fossil fuel or biofuel, controllable and independent of weather conditions [9], is introduced as another backup source due to its multiple advantages [10]. However, the slow start-up of the DG and its frequent switching on/off, which is unavoidable, require a well-conceived control method. At the same time, the DG may operate in under- or over-loading conditions. Fuel efficiency related to output power is therefore an inevitable constraint, since most DGs use fixed speed synchronous generators [11]. In this context, several power management strategies and control methods for PV-DG-storage-based DC microgrids have been proposed. To supply the necessary power in the case of insufficient PV power and electrochemical storage power, many studies propose the DG working in either continuous operation or frequent switching. In Reference [12], a nonlinear adaptive back-stepping controller is proposed to balance the power and stabilize the voltage of the DC bus. In Reference [13], PV power and DG power are controlled by a linear quadratic controller. In Reference [14], both a centralized and a decentralized demand-response multi-agent control and management system are conceived to minimize operating costs and consumers' inconvenience concerning the electricity shortage. In Reference [15], the pumped-storage hydroelectricity is used to store or release energy when PV and wind turbine generating power is sufficient or short; wherein if the total renewable power cannot respond to the load power demand, the DG would serve as backup power. However, DG start-up time and DG loading conditions are not considered in these articles. Otherwise, in Reference [16], both the load-following control strategy and cycle-charging dispatch control strategy are considered and compared. Two tests using real weather data and load power demand have been carried out to validate the proposed strategies. However, the DG is emulated by a source that provides instantaneous power. Regarding the evolution of specific fuel consumption versus the specific DG generating power, Reference [17] demonstrates that adequate power management can produce fuel cost savings compared to using only a DG, but a DGs start-up time is still not considered. In Reference [18], optimal power management aiming to minimize both fuel costs and electrochemical storage wear costs is presented. In this strategy, the DG runs in load-following mode without demand response considerations.