# Improved Control of Forest Microgrids with Hybrid Complementary Energy Storage
## 1 Introduction
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 [PERSON], 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; 16; 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; 23; 24]. 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.