# Utilizing Energy Storage for Reliability Solutions in Active Distribution Systems
## 1 Introduction  
The quality of supply is getting increased attention in modern society with the widespread use of sensitive equipment and industrial processes. These devices/processes are subject to misoperation even with short-duration reliability events: voltage sags and momentary interruptions. Financial losses due to short-duration reliability events are significant for an industrial/commercial customer [1]. The sustained interruption-based reliability indices are generally utilized in system planning and regulatory compliance structures [2]. Such practice, however, fails to provide incentives for utilities to invest in system upgrades to improve the reliability performance considering short-duration reliability events. However, regulators and electric utilities are taking into account disruptive voltage sag and momentary interruption in addition to sustained interruptions, together termed as Reliability Events (RE) [1], in system planning and regulatory policies with ongoing changes in the modern distribution system [2]. Commercial and industrial customers generally install a standby supply system in the form of the uninterruptible power supply (UPS) or emergency generators to deal with short- and long-duration reliability events [3]. Sensitive customer loads can be connected to multiple supply points, such that in case of a disturbance in the primary supply point, the secondary supply will take up the load with fast switching action [4]. Voltage sag mitigation with power electronic-based devices, e.g., dynamic voltage regulator (DVR), dynamic sag corrector (DSC), Thyristor voltage regulator (TVR), flexible ac transmission system (FACTS) devices, etc., are also common in modern distribution system [5; 6]. With the advent of smart control, monitoring, and communication systems, distributed energy resources (DERs), which include energy storage system (ESS) and distributed generation (DG), arealso being utilized to mitigate different reliability events. DERs can help reduce the impact of sag event by injecting an additional reactive power in case of voltage dips during system contingencies [7]. The energy storage installed at customer premises can override short- and long-duration reliability events [8; 9]. Microgrid, which consists of controllable loads and DERs integrated at the point of common coupling (PCC) of distribution network, operates in isolated mode during utility supply disturbances (sags and interruptions), thereby protecting critical load against reliability events [7; 10]. DERs that are not part of such microgrids and integrated along a distribution feeder can also operate in an islanded mode to reduce long-duration outages [11]. Various aspects of utilization of ESS and other distributed energy resources to mitigate different reliability events have been explored in the existing literature. Reference [3] provides an analytic method to assess the contribution of energy storage and a backup generator to reduce the impact of sustained outages. A new method of protecting sensitive load against momentary interruptions using inverter-coupled ESS is developed in Reference [9]. The potential of sodium-sulfur batteries to protect customer devices/processes are investigated in Reference [8]. Authors in Reference [7] have discussed the role of DG in the mitigation of reliability events with the microgrid-like operation and an additional current injection method. References [5; 6] have presented methodologies to model the contribution of power electronic-based devices to lower the impact of voltage sag on an individual load point and overall network. A Monte Carlo simulation-based approach is developed to quantify the role of ESS in reducing sustained outages experienced by customers in Reference [11]. The impact of intermittent renewable energy resources on the distribution network and the use of energy storage to mitigate this issue have been explored in Reference [12].