Fast and Reliable Load-Shedding Scheme for Wastewater Treatment Plant - A Case Study

Innovations in the fields of automation and networking have helped traditional power system substations evolve. Intelligent electronic devices (IEDs) accompanied by optimized and smartly engineered communications networks have provided engineers with opportunities to better design and implement various algorithms. Therefore, in the event of a disturbance or fault, the power system stability and process survivability are maintained. Power systems are proven to have more stable operation while connected to a utility; however, the challenge arises when the power system is islanded and suffers from a loss or an excess of generation. In an islanded configuration, fast and selective shedding of loads and/or generators based on system topology is critical in responding to system disturbances to avoid blackouts and ensure minimum process downtime. This paper presents a real-world implemented load-shedding scheme (LSS) for a North American wastewater treatment plant. The LSS was deployed in two tiers of primary and secondary controls via redundant substation-hardened controllers. The primary shedding system is based on calculation of a predictive power deficit or surplus for various predetermined contingency events. The primary system issues shedding decisions upon contingency detection, whereas the secondary shedding system is based on triggers asserted by underfrequency and/or overfrequency protective relays. The paper also provides an overview of the implemented network scheme; however, a detailed discussion regarding engineering and performance will be included in the authors' future work.