## 3 Structures of Microgrids
### Energy Storage System  
Energy storage system (ESS) application increases power availability, resilience, and stability for load demand. In the event that the primary power supply source fails or is insufficient, ESS functions as a backup power system. Load shedding, load leveling via energy storage system charging or discharging, power fluctuation smoothing, voltage and frequency fluctuation issues, and power quality improvement are additional areas where ESS can be useful, as shown in Figure 16. A classification of energy storage systems is presented in Table 1.  
Figure 16: Discharge time and application of energy storage systems [48].  
Figure 15: Islanded and grid-connected modes under unbalanced mitigation [46].  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Technology** & **Type** & **Invention Date [49]** & **Response Time [80]** & **Advantages [51]** & **Drawbacks [51]** \\ \hline Compressed Air Energy Storage & Mechanical & In 1978, Germany developed the world’s first compressed air energy storage plant. & 1-15 min & High capacity & Need for fuel and underground cavities. \\ \hline Pumped Hydro Storage & Mechanical & 1907 was the first use of pumped storage & s-min & High capacity & Geographical restrictions \\ \hline Thermal Energy Storage: Hot-water storage: Phase material storage & Thermal & & - & High energy density & Lower efficiency \\ (PCM); and Thermochemical Energy Storage (TCS). & Thermal & & & & \\ \hline Hydrogen storage and synthetic natural gas (SNG) & Chemical & The history of hydrogen started in 1766. & - & Minor environmental issues. & High investment costs. \\ \hline Hydrogen Fuel cell & Chemical & 1839, by Sir [PERSON] & \textless{1 s} & Long term storage & expensive catalyst \\ \hline \hline \multirow{3}{*}{flyweet} & mechanically store energy as kinetic energy & \multirow{3}{*}{1883, by [PERSON]} & \textless{1 ms-s} & High power and efficiency & Lower energy density \\ \cline{3-5}  & & & & & \\ \hline \hline \multirow{3}{*}{\(\bullet\) Second-life batteries } & \multirow{3}{*}{\(\bullet\) Lead acid battery: 1899 by [PERSON]} & \multirow{3}{*}{\(\bullet\) Neckel-calmin batteries} & \multirow{3}{*}{\(\bullet\) Neckel-calmin batteries} & \multirow{3}{*}{\(\bullet\) Neckel-calmin batteries} & \multirow{3}{*}{\(\bullet\) Neckel-calmin batteries} & \multirow{3}{*}{\(\bullet\) Neckel-calmin batteries} & \multirow{3}{*}{\(\bullet\) Neckel-calmin batteries} \\  & & & & & & \\ \cline{3-5}  & Sodium-sulfur batteries & & & & & \\ \cline{3-5}  & Lithium-on batteries & & & & & \\ \cline{3-5}  & \multirow{3}{*}{\(\bullet\) Vandium redox-flow batteries} & \multirow{3}{*}{Electrochemical} & \multirow{3}{*}{\(\bullet\) [PERSON] Whitening in 1976. The} & \multirow{3}{*}{\(\bullet\) 20 ms-s} & Higher power and energy density & Cycle life. Temperature Dependent \\ \cline{3-5}  & & graphite anode (negative pole) for lithium-ion & & & & \\ \cline{3-5}  & Flow batteries & & batteries was developed by [PERSON]. & - & & \\ \cline{3-5}  & & & The first commercial lithium-ion battery was released by Sony in 1991. & - & & \\ \hline Capacity & Electrical & & & & & \\ \hline Supercapacitor & Electrical & & & & & \\ \hline Supercapacitor & Electrical & & & & & \\ \hline Supercapacitor & Electrical & & & & & \\ \hline Superconducting magnetic energy storage & Electrical & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Energy storage systems.