Preparing to synchronize a generator to the grid (on photo: General Electric steam turbine
generator)
Figure 1 - Synchronizing a Generator to the Grid
http://electrical- engineering- portal.com/preparing- to- synchroniz e- a- generator- to- the- grid April 7, 2013
Preparing to synchronize a generator to the grid
Edvard
Conditions
In order to synchronize a generator to the grid, four conditions must be met:
1. Phase Sequence
2. Voltage Magnitude
3. Frequency
4. Phase Angle
1. Phase Sequence
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
Figure 2 - Generator Slower than Grid
possible problem sources.
The generator or transformer power leads could actually be interchanged during maintenance
orthe potential transformer leads could be interchanged during maintenance.
2. Voltage Magnitude
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 under-excited and it will absorb MVAR.
3. Frequency
The frequency of the sinusoidal voltage produced by the generator must be equal to the
frequency of the sinusoidal voltage produced by the grid.
In Figure 2 above the generator is slower than 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.
Figure 3 - Generator at Same Speed
asGrid but not in Phase
Figure 4 - Generator in Phase with Grid
Figure 3 shows the condition where the generator
and grid have matching speed. The high points and
zero crossings of the sinusoidal voltages occur at the
same rate of speed.
However, if you notice in 2 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.
4. Phase Angle
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 360°) can be readily observed by comparing the simultaneous
occurrence of the peaks or zero crossings of the sinusoidal waveforms.
If the generator breaker is closed when they match exactly, the connection will appear
smooth and seamless.
At that instance (Figure 4 below), the pointer on the synchroscope would indicate 12:00
oíclock.
The worst case occurs if the generator is exactly out-of phase, with a phase angle of
180° and the synchroscope pointing at 6:00 o’clock.
Synchronisation of Generators to a Busbar
(VIDEO)
Cant see this video? Click here to watch it on Youtube.
Resource: Science and Reactor Fundamentals –
Electrical CNSC Technical Training Group