*3.1. The Necessity of the Hybrid Complementary Energy Storage*

The DC microgrid based on power electronic converters is a system with small inertia, which presents large fluctuation when being subjected to power disturbance. Although microgrids are normally equipped with an energy storage device to ensure stability of the system, when confronted with temporary high power fluctuations, the control system may fail to respond in time due to control delay. In this condition, load shedding or system protection will be caused. By coordinating the three kinds of energy storage units proposed in the DC forest area microgrid, adopting the appropriate energy storage unit to suppress system disturbances under different operation modes or disturbance conditions, the response characteristics of the system can be improved. Accordingly, as the transient stability of the system is enhanced, large fluctuations of DC voltage can be restrained.

As batteries have the advantage of high energy density, they are suitable for long term energy storage to compensate for power gaps when system input is limited in emergency mode. However, their charge–discharge efficiency is low, which is not recommended for frequent action to suppress power fluctuations. Contrarily, supercapacitors have the merits of high power density and long cycle life, as well as high charge and discharge efficiency [26]. Therefore, they are appropriate for suppressing the power fluctuations with high frequency and small amplitude that are typical in free mode and emergency mode, to assist the biomass unit in power regulation. In addition, by exploiting the large rotating kinetic energy of wind turbine, large disturbances of the system can be suppressed, which can effectively improve the fault ride-through capability of the microgrid and avoid load shedding.
