The transient and sub-transient impedances of the synchronous generator are considered so that more accurate transient stability analysis can be obtained. The steam turbine couples with the rotor of the synchronous generator to generate electric power, and they are operated according to the heat demand. The commercial cogeneration systems use the natural gas, coal or oil to drive the steam turbine. Table 3 Characteristics of the IEEE 9-bus test system The loads, totaling 6.0 MW, are distributed along the bus 5,6,7,8 and are modeled as constant impedance static and ZIP dynamic loads. Three DG units with nominal power of 4.85 MVA are connected to bus 1, 3 and bus 4, respectively, and simulated with 1.5 MW power output before islanding operation. The system consists of three DG units, eight transmission lines, one transformer and four loads. The modified IEEE 9- bus system comprises a 60kV, 50 Hz grid which feeds an 11 kV network through a 60/11kV transformer. The singleline diagram of the modified IEEE 9-bus test system in DIgSILENT PowerFactory and RSCAD are shown in Fig. The modified IEEE 9-bus test system is used to illustrate the islanding operation of an active distribution networks, and to validate the proposed multi-agent based controller for the islanding operation of active distribution systems.
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