A Wide Area Load Shedding Scheme to Prevent Voltage Instability
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- Institutt for elkraftteknikk 
These days, power systems are operated close to their stability limit, and disturbances such as transmission line contingencies can cause the system to lose stability. If the system loses stability, countermeasures has to be taken. Otherwise, disruption in parts of, or in the whole system occurs. In this thesis an Emergency Load Shedding scheme, (ELS) using voltage stability indicators is proposed. Due to the scarcity of available mitigation actions in the considered area in Northern Norway, load shedding is used as the only corrective action. As there are large social and economic costs associated with load shedding the goal of the ELS is to keep the amount of load shed as low as possible. First, four methods for estimating the Thévenin impedance (system impedance) of a power system is studied and tested. Two are based on local measurement at a single substation, while the other two uses information about system topology as well. Using the stability criterion imposed by the theorem of maximum power transfer a rule for stability can be found. The load impedance divided by system impedance needs to be greater than one (Impedance Stability Index, ISI > 1). The performance of the indicators is tested by a gradual load increase of a load, and evaluating if the Impedance Stability Index correctly crosses its stability limit when the system becomes unstable. From the results the topology based Duong-Uhlen and the measurement based Corsi-Taranto method showed the best performance. Using the two best performing indicators an Emergency Load shedding Scheme is proposed. Indicators are implemented at critical nodes found through simulation. Two options for load shedding is proposed when the system becomes unstable to prevent system instability.