# Utilizing Energy Storage for Reliability Solutions in Active Distribution Systems
## 4 Case Studies and Discussion
### DERs/Microgrids and Reliability Improvement  
In this section, role of DERs/microgrid in mitigating reliability events is discussed. Microgrids (MG1 and MG2) and DERs are integrated (in Seg 3 and Seg 4) into the distribution network as shown in Figure 3. For the purpose of illustration, microgrids and DERs are assumed to consist of PV arrays, wind generators, and an ESS. Historical time series data for wind speed and PV irradiance are taken from Reference [24] for Los Alamitos, California, and the USA for the Year 2003. Corresponding renewable power outputs are obtained using an analytical set of equations [25]. It is acknowledged that the use of more elaborate models considering physical and environmental parameters can also be used to obtain renewable DG output. The rated capacity of wind generator, PV arrays, and ESS for each segment and microgrid are shown in Table 2. SOC and hardware unavailability of ESS used in Section 4.1 are utilized. Table 3 shows the reliability improvement for customers benefited from DER/microgrid operations. The reliability profile of customers within MG1 indicates that the implementation of microgrid helps reduce the impact of voltage sag, momentary interruption, and sustained interruptions. The customers within MG2 experience a similar improvement in their reliability profile. The islanding operation of DERs in Segment 3 reduces the associated annual outage time (Unavailability, \(U_{SI}\)). However, frequency of momentary interruptions (\(F_{MI}\)) for these customers appears to be increased. It is due to the restoration of supply with DERs through automatic switching, as mentioned in Section 2. There is no change in the frequency of disruptive sag event (\(F_{DSE}\)) since voltage sag mitigation with such DERs is not considered. Customers within Segment 4 of the test network experience similar reliability improvement as that of Segment 3. The total customer damage cost (\(D_{tot}\)) reported in Table 3 shows that the integration of DERs/microgrid reduces financial losses incurred to customers. It should be noted that the reliability profile of customers located at different parts of the network is dependent on various factors in addition to times to failure and repair of distribution network elements, such as network topology, protection settings, operation strategy of the network (radial/meshed, provision of back-feed supply), voltage sensitivity of customer equipment and processes, and location, capacity, and mix of resources of DERs/microgrids, etc. The methodology developed in this paper take such factors into consideration to quantify the impact of different reliability events to distribution system customers. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Segments** & **Wind Generator** & **PV Arrays** & **ESS** \\ \hline MG1, MG2 & 0.6 MW & 0.6 MW & 1.4 MW/9.8 MWhr \\ Seg3, Seg4 & 0.3 MW & 0.3 MW & 0.7 MW/4.9 MWhr \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Resources of DERs/microgrids within segments/microgrids (MG). Figure 7: Impact of ESS hardware unavailability on the financial loss of IL9. In the deregulated environment, utilities are rewarded/penalized based on their system reliability performance as part of the performance-based rate-making. The integration of DERs/microgrids improves the reliability of the worse performing load points as well as the overall network as reported in Figure 8. Results show decrement in system indices of annual outage time, damage cost, and the frequency of disruptive sag event, momentary and sustained interruptions with DERs/microgrids operation. It should be noted that the reliability profile of load points within segment 1 and 2 remain unchanged since there is no DERs/microgrids integrated. The system reliability profile takes into account all load points and number of customers associated with them. Thus, the contribution of DERs/microgrids in enhancing system reliability profile is lower than that observed in Seg 3, Seg 4, MG 1, and MG 2.