Before a synchronous generator can be connected to the mains it must be synchronized with the mains. Up to now it has been assumed that the stator frequency is equal to the mains frequency. During the start-up of a synchronous generator, the rotor speed and therefore the stator frequency is different from the mains frequency. If a synchronous generator were to be connected immediately to the mains, strong compensation reactions would appear such as very high compensation currents. This would cause

Load Angle
Figure 5.26 Curve of the Torque of a Synchronous Machine with Cylindrical Rotor as a Function of the Load Angle & and the Internal Voltage Vp

unacceptable strain on the mains and could even damage the generator. Therefore, the following synchronization conditions must be fulfilled before connecting a synchronous generator to the mains:

• same phase sequence of mains and stator (same sense of rotation)

• equality of the frequencies of stator and mains

• equality of the voltage amplitude of stator and mains

• equality of phase characteristic of the stator and mains voltage.

The synchronization of a modern synchronous generator usually happens fully automatically. The following synchronization conditions must be fulfilled as a minimum (VDEW, 1994):

voltage difference AV < ±10% VN

frequency difference Af < ±0.5 Hz phase angle difference A^ < ± 10°.

Asynchronous machines, which are described in the next section, do not need such a synchronization process.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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