Synchronising a turbine to the grid: the conditions to meet

Coupling a turbo-alternator to the grid is not a switch you flip. Four conditions must be met at the same instant, or the machine takes a violent jolt.

Why you don't close the breaker at just any moment

Connecting the alternator to the grid means joining two voltage sources. If they are not matched at the moment of coupling, the difference shows up as a brutal current inrush and a mechanical restoring torque on the shaft. At best, you trip. At worst, you damage the coupling, the windings or the turbine.

Hence the principle: you bring the machine as close as possible to the grid conditions, then close the breaker at the right instant, when the gap is near zero.

The four synchronisation conditions

• Frequency. The turbine must spin at the speed that gives grid frequency. For a two-pole alternator on a 50 Hz grid, that is 3,000 rpm. You aim for a frequency very slightly above the grid so the machine picks up load right after coupling.
• Voltage. The alternator terminal voltage must equal the grid voltage, adjusted through excitation.
• Phase concordance. At the moment of coupling, the voltages must be in phase (angular difference close to zero).
• Phase sequence. The rotation order of the alternator phases must match the grid. It is checked at wiring, but it remains a condition.

A synchroscope (or an automatic synchroniser) shows the phase and frequency gap and indicates when to close.

What the interlocks check

On a real plant, the operator does not decide alone. Before allowing coupling, the protection logic checks a set of conditions: speed within range, steam pressure and temperature above their minima, frequency/phase match. As long as one condition is missing, the synchronisation command is refused. It is a safety, not a constraint.

What happens before synchronisation

Synchronisation is the end of a sequence. Before it: slow rotation (barring) to avoid shaft distortion, gradual heating, then a speed ramp up to nominal. In parallel, steam must be available at the right parameters (minimum pressure and temperature) and bearing lubrication assured. Skipping a step risks a trip at the first demand.

And after coupling

Once the machine is on the grid, you take load progressively. The turbine can then feed its extractions (steam drawn off for the process) and, depending on the plant, switch to islanding if the external grid disappears. Operation does not end at synchronisation: it begins.

Why it is practised before doing it for real

Synchronisation strings together conditions invisible to the eye and precise timing. Understanding it on paper is not enough: you have to have brought a machine to the coupling point, seen a condition fail, and understood why the system refuses to close the breaker.

A simulator lets you repeat this sequence as many times as needed: speed ramp, meeting the conditions, coupling, taking load, without ever putting a real turbine at stake. And when coupling is refused, you immediately see which condition was missing.