9 Generators, Motors and Switch Gears

The electrical motor is the most common type of power that is used to start the gas turbines in a combined cycle power plant. There are some applications where the start-up power is provided by steam turbines and diesel engines. Diesel engines are used in plants where black starts are needed. A black start is needed where there is no electrical power available for start-up power. The new large gas turbines use the generator as a motor for start-up requirements. The generator acts as a motor to bring the turbine to a point where the turbine is fired. Once the turbine is fired and reaches a given rpm of about 40% to 60% of design speed, the motor is declutched and then converted to a generator.

The generator is used to convert the rotating mechanical energy of a gas turbine or steam turbine into electrical energy. In the gas turbine, the hot air is expanded through the turbine impinging on the turbine blades causing them to rotate converting the hot gases compressed in the compressor and heated in the combustor into mechanical energy. The high-pressure steam created from the hot exhaust gases from the heat recovery steam generator is channeled to the steam turbine and impinges on the turbine blades causing them to rotate, thus converting the energy in the steam into mechanical energy. The turbine rotors are connected to the generator rotor through a coupling and as the turbine blades rotate, they cause the generator rotor shaft to rotate also. The generator converts the rotating mechanical energy transmitted to it into electrical energy. In majority of the combined cycle power plants, the gas turbine and the steam turbine both have separate generators. There are some turbines where the gas turbine, steam turbine are on the same shaft and thus, there is only one generator, all three components on a single shaft.

Electric motors and generators perform very similarly. Synchronous motors convert electrical power to mechanical power and synchronous generators convert mechanical power to electrical power. This chapter deals with some of the basic characteristics of the electrical motors and generators.