## 4 Case Study  
### System Definition  
The Cyber-Physical System (CPS) in this case study is a cloud-based monitoring and control system for large residential lithium-ion (Li-on) energy storage systems. An overview of the said CPS is shown in Figure 5 to illustrate the interactions among the system hardware components; note that it is not intended to be exhaustive but showing some necessary signals (including commands, measurements, actions, etc.) transmitted between components. Please also note that technology-specific details are abstracted owing to commercial sensitivity; though not explicitly shown in Figure 5, various telecommunication means (e.g., cellular networks, computer networks, and the Internet) are in fact involved to enable seamless signal exchange among a plethora of the system components and sub-components.  
In the following, the term _system_ is used to denote the CPS of this case study. In the _system_, Lithium-ion (Li-ion) batteries are used to store the energy. Such large-scale energy storage, as compared to small and medium-size applications like consumer electronics, is more likely to be exposed to extreme conditions. Li-on batteries can fail owing to cell's chemical imbalance, internal short-circuit, or external causes such as exposure to high heat, mechanical damages, and electrical abuse conditions. Failure of Li-on batteries is probably among the most undesired risks, as it can lead to fire or explosion, and hence significant losses. A good discussion on potential hazards related to Li-on battery systems as well as their mitigation may be found in [44]. Please note that the analysis is carried out on a commercial system and not a research grade equipment; therefore, the analysis is practically validated on a system that is commercially viable.  
As shown in Figure 5, there are four major components in the _system_, namely EMU (Energy Monitoring Unit), PCS (Power Conversion System), BMS (Battery Management System), and EMS (Energy Management System). The EMU communicates with PCS/BMS  
Figure 5: A simplified architecture of the cloud-based monitoring and control system for residential Li-on battery storage systems.  
via Modbus RTU using serial RS-485 cables. The EMU is the master while PCS and BMS are the slaves. Periodically, EMU uploads to EMS the data it requests from both PCS and BMS. On certain occasions, EMU may receive operational commands (e.g., shutdown or reboot EMU) from EMS. The PCS serves as an interface between the battery and the grid/load, and determines the rate of charging/discharging. It converts AC power into DC power (or vice versa) in the event of battery charging (or discharging).Furthermore, it monitors local measurements from current and voltage transformers, and contains some features to protect the system from over-current, phase imbalance, etc. Periodically, PCS reports its status and applies the required configuration, as requested by EMU. The BMS measures the battery's conditions (e.g., cell voltage, cell temperature, and current). Based on the measured values, it estimates the battery's SOC (State of Charge) and SOH (State of Health) with a pre-defined process model of the battery behaviour. It regulates/controls charging and discharging of the battery, thereby ensuring the battery system is operating safely. Appropriate charging and discharging of battery is essential for maximizing the lifespan and performance of the _system_. Overcharging of battery could result in physical damage (including explosion) while over-discharging would increase the batteries' aging rate. Periodically BMS responds to EMU's request for PCS status data. The EMS is basically a website hosted on a AWS (Amazon Web Services) cloud server, which allows the authorized personnel (using valid login accounts) to perform the following activities: (1) see both historical and real-time data regarding PCS and BMS; (2) shutdown/restart EMU; (3) carry out energy management functions such as configuring the charging/discharging limits; and (4) override the set-points on PCS and BMS via EMU.