# Utilizing Energy Storage for Reliability Solutions in Active Distribution Systems
## 4 Case Studies and Discussion
### End-User Reliability Profile with ESS  
This section explores the role of ESS toward mitigating different reliability events for a customer sensitive to short-duration reliability events. In addition, the reliability impacts of other resources are also discussed. The critical loads are fed with utility supply during normal condition. ESS takes up the load in isolated microgrid mode in case of utility supply disturbances within a fraction of cycle [8]. The supply is transferred back to utility after utility supply becomes normal. Based on this assumption, the reliability benefit from ESS is assessed executing the algorithm developed in Section 2 and 3. The component availability model of ESS is developed as described in the authors' previous work [23] is used. This study considers Sodium Sulfur based \"PQ-50\" module [23] batteries rated at 1 MW and 7 MWhr capacity. The efficiency and allowed minimum SOC are taken as 90% and 10%, respectively. The hardware unavailability for ESS is assumed 2%. ESS-rated capacity is matched to support the entire load, hence the variation in load is neglected in this study. It should be noted that reliability performance of load point is poor, as it is located far from the substation in a radial distribution feeder. Figure 5 shows the reliability indices of IL9 with and without ESS without considering the operation of DERs/microgrids. The \(y\)-axis of Figure 5 shows the magnitude of annual outage time, damage cost, and frequency of disruptive sags, momentary, and sustained interruptions. Results indicate that ESS is effective in improving reliability performance in both short- and long-duration reliability events. It should be noted that the frequency of short-duration reliability events is reduced significantly compared to the sustained interruption-based indices. It is because the discharge duration and SOC of ESS can significantly limit the mitigation of sustained interruption but have little impact on short-duration reliability events. The mitigation of sustained interruptions greatly depends on the initial SOC at the time of contingency, efficiency, and discharge duration of BESS. In addition, reliability improvement depends on hardware availability as well. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sector Type** & \multicolumn{4}{c}{**Interruption Duration (minutes)**} \\ \hline  & 1 & 20 & 60 & 240 & 480 \\ \hline Industrial & 1.625 & 3.868 & 9.085 & 25.16 & 55.81 \\ Commercial & 0.381 & 2.969 & 8.552 & 31.32 & 83.01 \\ Residential & 0.001 & 0.093 & 0.482 & 4.914 & 15.69 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sector customer damage function [16]. Another study is conducted to analyze the impact of initial SOC at the time of occurrence of contingency and the rated discharge duration of ESS to mitigate long-duration outages. Figure 6a,b, respectively, shows that load point unavailability increases with a decrease in SOC and rated discharge duration in the case of sustained forced outages. The sustained interruption frequency follows a similar trend. This means if ESS is utilized for peak shaving or other application instead of standby mode, ability of ESS mitigating sustained outages reduce. However, it does not significantly affect the mitigation for short-duration reliability events. Given that batteries suffer from degradation and need a periodic replacement due to the limited life cycle, other ESS technologies, such as Flywheel, Magnetic energy storage systems (SMES), Supercapacitor, etc., are also used to protect critical loads. These technologies, however, cannot retain energy for a long duration and, therefore, need to be used with other resources (battery, backup generator, etc.) to reduce the financial impact due to long-duration reliability events. The hardware failure of ESS also plays a vital role in the relative reliability improvement. Both short- and long-duration reliability event-based indices are affected by component failures.