Parallel Operation of synchronous generator
Parallel Operation of synchronous generator:
Fig: Parallel operation of synchonous generator
- It is rare to find a 3-phase alternator supplying its own load independently except under test conditions. In practice, a very large number of 3-phase alternators operate in parallel because the various power stations are interconnected through the national grid.
- Therefore, the output of any single alternator is small compared with the total interconnected capacity. For example, the total capacity of the interconnected system may be over 40,000 MW while the capacity of the biggest single alternator may be 500 MW. For this reason, the performance of a single alternator is unlikely to affect appreciably the voltage and frequency of the whole system.
- An alternator connected to such a system is said to be connected to infinite busbars. The outstanding electrical characteristics of such busbars are that they are constant-voltage, constantfrequency busbars.
- Fig. shows a typical infinite bus system. Loads are tapped from the infinite bus at various load centres. The alternators may be connected to or disconnected from the infinite bus, depending on the power demand on the system.
- If an alternator is connected to infinite busbars, no matter what power is delivered by the incoming alternator, the voltage and frequency of the system remain the same.
- The operation of connecting an alternator to the infinite busbars is known as paralleling with the infinite busbars. It may be noted that before an alternator is connected to an infinite busbars, certain conditions must be satisfied.
- For proper synchronization of alternators, the following three conditions must be satisfied :
1. The terminal voltage (effective) of the incoming alternator must be the same as bus-bar voltage.
2. The speed of the incoming machine must be such that its frequency (= PN/120) equals bus-bar frequency.
3. The phase of the alternator voltage must be identical with the phase of the bus-bar voltage. It means that the switch must be closed at (or very near) the instant the two voltages have correct phase relationship.
- Condition (1) is indicated by a voltmeter, conditions (2) and (3) are indicated by synchronizing lamps or a synchronoscope.