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
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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