# Improved Control of Forest Microgrids with Hybrid Complementary Energy Storage
## 1 Introduction  
With the increasing efforts of forestry encouragement and protection policy, the forest area and corresponding stock volume in China have achieved double growth. The carbon sink capacity of the forest has been greatly enhanced, and its ecological barrier function is stable, with good momentum for growth [1]. However, the gradual expansion of forest area has presented a major challenge to the power supply and distribution systems in forest areas. Capacity expansion of forest power networks always covers large areas with long transmission lines, which leads to huge construction investment and high loss in the power line. In addition, the maintenance area of the forestry power grid is expanded, and the corresponding human, material, and financial resources invested in operation and maintenance are increased. Therefore, the management pressure of power companies has risen remarkably [2; 3; 4]. As a low-cost utilization method of renewable energy, the microgrid, which is based on renewable energy, has the advantages of small initial investment, reliable power supply, small transmission loss, and flexible operation. It is a useful supplement to the utility grid, and helps to promote sustainable development of energy [5]. The microgrid can not only operate in grid-connected mode, but also inislanded mode, which makes it suitable for niche applications such as power supplies in isolated forest locations [6]. By effectively utilizing the abundant wind energy, biomass energy, and other natural energy locally, clean and efficient power can be provided for the loads in forests, such as monitoring and alarm systems, wood processing machinery, or electrical equipment on watchtowers. Therefore, the problems of power construction, operation, and management in remote forest areas can be solved. Being different from islanded microgrids constructed in environments such as islands, microgrids in forests have their own ecological particularity. A large amount of residue from forest clearing, logging, and wood processing cannot usually be cleaned up and transported in time, which is a hidden hazard that may trigger forest fires [7]. Scientists in the United Kingdom and Sweden have proposed using the surplus produced by cleaning and afforesting to generate economic benefits, so as to promote the restoration of the forest ecosystem. American scientists have also paid great attention to wood power generation, and invested plenty of money on related research [8; 9]. Using forestry residues for power generation can not only dispose of forest waste nearby, saving transportation costs and avoiding fires, but can also provide natural and environmentally friendly energy support for power loads in remote forest areas. The forest microgrid in this paper, which is composed of a biomass power generation unit, wind power unit, and energy storage units, adopts a DC bus to improve stability and reliability during operation. In addition, in a DC microgrid there are no such problems as frequency deviation, reactive current circulation, and power angle stability, which are normally encountered in AC systems. The distributed generators and energy storage units in islanded DC microgrids are connected to the DC bus through power electronic converters, with asynchronous motors providing rotational kinetic energy as a natural inertial support. Therefore, the forest microgrid is an intrinsic small inertial system. In the case of large power disturbance, a sudden change of DC voltage will pose a threat to the voltage-sensitive load, leading to load shedding. In addition, fast variation of DC voltage caused by random fluctuations of distributed energy will also affect the quality of the output power. Therefore, it is of practical significance to investigate advanced control schemes of DC microgrids, in order to improve the transient response and ensure its safe and stable operation. Scholars have proposed various control schemes of energy storage devices to suppress the power fluctuations of microgrids [10; 11; 12; 13; 14; 15; 16; 17; 18; 19].