In order to synchronize a generator to the grid, four conditions must be met.
The phase sequence (or phase rotation) of the three phases of the generator must be the same as the phase sequence of the three phases of the electrical system (Grid). The only time that the phase sequence could be wrong is at initial installation or after maintenance.
There are two possible problem sources. The generator or transformer power leads could actually be interchanged during maintenance or the potential transformer leads could be interchanged during maintenance.
The magnitude of the sinusoidal voltage produced by the generator must be equal to the magnitude of the sinusoidal voltage of the grid. If all other conditions are met but the two voltages are not the same, that is there is a voltage differential, closing of the ac generator output breaker will cause a potentially large MVAR flow.
Recall that before a generator is synchronized to the grid, there is no current flow, no armature reaction and therefore the internal voltage of the generator is the same as the terminal voltage of the generator.
If the generator voltage is higher than the grid voltage, this means that the internal voltage of the generator is higher than the grid voltage. When it is connected to the grid the generator will be overexcited and it will put out MVAR.
If the generator voltage is less than the grid voltage, this means that the internal voltage of the generator is lower than the grid voltage. When it is connected to the grid the generator will be underexcited and it will absorb MVAR.
The frequency of the sinusoidal voltage produced by the generator must be equal to the frequency of the sinusoidal voltage produced by the grid.The synchroscope would be rotating rapidly counter clockwise.
If the generator breaker were to be accidentally closed, the generator would be out of step with the external electrical system. It would behave like motor and the grid would try to bring it up to speed. In doing so, the rotor and stator would be slipping poles and damage (possibly destroy) the generator as described previously.
The same problem would occur if the generator were faster than the grid. The grid would try to slow it down, again resulting in slipping of poles.
The high points and zero crossings of the sinusoidal voltages occur at the same rate of speed. However, if you notice in 2 NotesN otes: with the grid and a phase angle exists between them. This would appear as a non-rotating synchroscope (both generator and grid at same frequency), where the pointer would appear stuck at about 9:00 o’clock (generator lagging grid).
If the generator breaker were to be closed at this time, the grid would pull the generator into step. However, this again would cause a large current in-rush to the generator and high stresses on the rotor/stator with subsequent damage to the generator.
If the generator were leading the grid, it would try to immediately push power into the grid with the same destructive forces as mentioned. Hence the generator must be brought to a point where the grid voltage waveform exactly matches what it is producing.
As previously mentioned, the phase angle between the voltage produced by the generator and the voltage produced by the grid must be zero. The phase angle (0 to 3600) can be readily observed by comparing the simultaneous occurrence of the peaks or zero crossings of the sinusoidal waveforms.