The physical method
Having discussed the principles of synchronizing generator to the grid, we will look briefly on the physical method of accomplishing it.
In theory, there are at least two simple measurements or indications, which can be used for synchronizing generator to a grid.
When two voltages satisfy the conditions of being equal in magnitude, have the same frequency and an angle of zero between them, then around the voltage loop they add to be zero at each and every instant of time on the sine waves.
Consequently voltmeters connected across each of the synchronizing breaker contacts will both read zero. Lights placed in the same position will also be totally out when all the synchronizing conditions are satisfied.
The instrument, which will provide this information, is the synchroscope. Figure 1 shows the connection of a synchroscope between the generator being synchronized and the grid.
We will still need to use two voltmeters to check that the generator and the grid voltage are the same (noting that these two voltmeters are not the ones referred to above, since those ones were placed across the synchronizing breaker contacts).
The position of the synchroscope pointer indicates the difference in angle between the generator voltage and the grid voltage. When there is a zero angle between the two voltages, the synchroscope pointer is in the vertical or 12 o’clock, position.
The speed of rotation of the pointer indicates the difference in frequency of the two voltages. The pointer will rotate in the Slow or counter clockwise, direction when the generator frequency is below the grid frequency. The pointer will rotate in the Fast or clockwise, direction when the generator frequency is greater than the grid frequency.
The last two paragraphsindicate that when:
- The pointer is vertical or at 12 o’clock,
- The pointer is steady, not rotating; then the two voltages are in phase and the frequencies of the generator and grid are equal.
In practice the synchronizing breakers are closed when the generator is just slightly fast and at about the 5 minutes to 12 position moving toward 12 o’clock. This allows a little bit of time for closing the synchronizing breakers and it assures that the generator will not act as a motor once the synchronizing breaker is closed.
It is important to check the correct operation of the synchroscope before each synchronizing is attempted.
To do this, the generator is operated at less than synchronous speed and the synchroscope must rotate in the slow direction. Similarly when the generator is operated at a speed greater than synchronous, the synchroscope must rotate in the Fast direction.
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Reference // Science and Reactor Fundamentals – Electrical CNSC Technical Training Group