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Preparing to synchronize a generator to the grid

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Preparing to synchronize a generator to the grid
Preparing to synchronize a generator to the grid (on photo: General Electric steam turbine generator)

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
Synchronizing a Generator to the Grid
Figure 1 – Synchronizing a Generator to the Grid

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 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.

Generator Slower than Grid
Figure 2 – Generator Slower than 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.

Generator at Same Speed asGrid but not in Phase
Figure 3 – Generator at Same Speed asGrid but not in Phase

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.

Generator in Phase with Grid
Figure 4 – Generator in Phase with Grid

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

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About Author

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on