**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 [2–4].

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 in

islanded 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 [10–19]. In Reference [10], a synergistic operation between converters of battery energy storage and a photovoltaic generator to assist management of microgrids is presented and in Reference [11], current-controlled bidirectional DC/DC converters were applied to connect each lithium ion battery bank as well. Although the control methods of converters in the above literature can ensure the basic stability of corresponding systems, time delay is inevitable, in which traditional proportional integral adjustment or corresponding improved forms based on deviation of the controlled variables are utilized. An experimental investigation of an energy storage unit which incorporates electric energy storage in the form of hybrid capacitors and hydraulic energy storage in the form of pressure vessels in a photovoltaic powered seawater reverse osmosis desalination system was proposed by Karavas, whereas there was only innovation in the energy storage form [12]. In Reference [13], a wireless droop control method for distributed energy storage units in AC microgrids is presented, which employs the SoC-based droop control method locally to prolong the service life of the energy storage, and in Reference [14], the energy storage was scheduled to work in a grid supportive manner, with a grid adaptive power management strategy being formulated to generate current references for energy storage systems and microgrid-connected converters. Both methods emphasize different priorities on various control targets, with indifference to improving control speed to enhance transient stability, which is critical in islanded microgrids. In References [15–17], optimal controls for microgrids with hybrid energy storage system were carried out using model predictive control (MPC), which allowed maximization of the economic benefits of the microgrids, minimizing the degradation causes of storage systems, or fulfilling other, different system constraints. The focuses of these papers are economical schedules in the upper control level, having nothing to do with improvement of the response speed or the transient characteristics of microgrids. A hierarchical control of a hybrid energy storage system, composed of both centralized and distributed control, was proposed in [18], and a method consisting of a virtual resistance droop controller and a virtual capacitance droop controller for

energy storages with complementary characteristics was proposed in [19]. Although more accurate current references for DC converters were generated, the control speed of converters did not improve, which weakened the effectiveness of the control strategy. In References [20,21], the authors used a two-level control scheme to control the charge/discharge power of the storage unit, in which the reference in the first level was obtained through a robust optimal power management system. Moreover, the research on inertial control of the energy storage system was limited to adding a supercapacitor into the microgrid to improve the equivalent inertia, whereas the effect of disturbance suppression was not obvious [22–24]. characteristics was proposed in [19]. Although more accurate current references for DC converters were generated, the control speed of converters did not improve, which weakened the effectiveness of the control strategy. In References [20,21], the authors used a two-level control scheme to control the charge/discharge power of the storage unit, in which the reference in the first level was obtained through a robust optimal power management system. Moreover, the research on inertial control of the energy storage system was limited to adding a supercapacitor into the microgrid to improve the equivalent inertia, whereas the effect of disturbance suppression was not obvious [22–24].

*Appl. Sci.* **2019**, *9*, 2523 3 of 19

microgrids. A hierarchical control of a hybrid energy storage system, composed of both centralized

controller and a virtual capacitance droop controller for energy storages with complementary

In this paper, a coordination control strategy of hybrid complementary energy storage in a forest microgrid is proposed, to improve its transient operation stability and the fault ride-through capability. The battery, the supercapacitor, and the wind turbine energy storage unit constitute the hybrid energy storage system, all of which undertake different tasks in maintaining power balance under various operation modes. To better ensure the control speed and efficiency of the battery and the supercapacitor under power fluctuation conditions, the adaptive droop coefficients and predictive converter control method are firstly proposed. In the following, an inertia enhancement control strategy of the wind turbine, utilizing its rotating kinetic energy, was investigated to improve its fault ride-through capacity in urgent conditions, such as load shedding. In addition, overall control of the hybrid complementary energy storage system was studied based on the fundamental DC voltage sectional control to coordinate various energy storage units. In this paper, a coordination control strategy of hybrid complementary energy storage in a forest microgrid is proposed, to improve its transient operation stability and the fault ride-through capability. The battery, the supercapacitor, and the wind turbine energy storage unit constitute the hybrid energy storage system, all of which undertake different tasks in maintaining power balance under various operation modes. To better ensure the control speed and efficiency of the battery and the supercapacitor under power fluctuation conditions, the adaptive droop coefficients and predictive converter control method are firstly proposed. In the following, an inertia enhancement control strategy of the wind turbine, utilizing its rotating kinetic energy, was investigated to improve its fault ride-through capacity in urgent conditions, such as load shedding. In addition, overall control of the hybrid complementary energy storage system was studied based on the fundamental DC voltage sectional control to coordinate various energy storage units.

This paper is organized as follows. Section 2 presents the basic operation mode and coordination control of DC microgrid. Section 3 describes the hybrid complementary energy storage system, while the overall control of the forest microgrid is discussed in Section 4. Simulations are presented in Section 5. This paper is organized as follows. Section 2 presents the basic operation mode and coordination control of DC microgrid. Section 3 describes the hybrid complementary energy storage system, while the overall control of the forest microgrid is discussed in Section 4. Simulations are presented in Section 5.

## **2. Operation Mode and Coordination Control of Microgrid in Forest Area 2. Operation Mode and Coordination Control of Microgrid in Forest Area**

simple structure and low construction cost, as is shown in Figure 1.
