By Whitaker, Jerry C

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Extra info for AC power systems

Example text

36 shows the blade arrangement of an impulse-type turbine. Steam enters through nozzles and flows through the first set of moving rotor blades. The following stationary blades change the direction of the flow and direct the steam into the next set of moving blades. The nozzles increase the steam speed and reduce pressure, as shown in the figure. The impact of the high-speed steam, generated by the change of direction and speed in the moving blades, drives the turbine. In a fossil fuel plant, the generator converts mechanical energy from the turbines into electrical energy.

An excitation control system is a feedback loop designed to maintain the voltage at the machine terminals at a set level. 30 illustrates the mechanisms at work. Assume that a disturbance occurs in the system, and as a result, the machine terminal voltage Vt drops. The excitation system boosts the internal voltage EF. This action can increase the voltage Vt and also tends to increase the reactive power output. From a system viewpoint, the two controlling mechanisms of excitation and the governor rely on local information (the machine terminal voltage and rotor speed).

The turbine converts the heat energy of the steam into mechanical energy. Modern power plants typically use one high-pressure and one or two lower-pressure turbines. High-pressure steam enters the high-pressure turbine to flow through and drive the turbine. The exhaust is reheated in the boiler and returned to the lower-pressure units. Both the rotor and the stationary part of the turbine have blades. The length of the blades increases from the steam entrance to the exhaust. 36 shows the blade arrangement of an impulse-type turbine.