We have seen how the system behaves when subject to large disturbances in the previous example. In the example, we had assumed that the machine was already connected and was initially in equilibrium (i.e., was in synchronism with the infinite bus).

It is natural of you to ask : How was the machine "synchronised" when it was first connected to the grid?

When a synchronous machine is connected to a grid, it is important to follow a particular procedure so that the transients which occur are minimised and the incoming generator is "pulled into synchronism".

This includes ensuring that the generator voltages and the system voltages are in the same phase sequence and the voltage magnitude of the generator and the grid at the point of interconnection are nearly equal.

Additionally:

a) The electrical speed of the incoming generator should be almost equal to the grid frequency.

b) The phase angular difference between the generator voltage and the grid voltage at the point of interconnection at the instant of interconnection should be small.

The situation is analogous to connecting two moving masses by a spring. To avoid excessive stretch in the spring when the masses are inter-connected (which may cause the spring to break), one would need to have the two masses to be close to each other and to be moving at nearly the same speed at the time of interconnection. If the interconnection is done smoothly, without breakage of the spring, then the masses move together at the same speed after transients die out. Thereafter, the masses are held together by the spring unless a large disturbance causes the spring to break.

In practice, a plant operator starts rotating the generator by introducing a prime mover torque. He ramps up the speed to a value which is very close to the grid frequency by adjusting the prime mover torque. He then switches on the field excitation and brings voltage close to the grid voltage at the point of interconnection by adjusting the field voltage. Using an instrument called a synchroscope, he monitors the phase angular difference and switches on the interconnecting circuit breaker at the instant when the phase angular difference is very small.

You can simulate the process of synchronisaton by downloading all of the following MATLAB/SIMULINK simulation files: 1 , 2, 3, 4,5 ,6. Run only the sync.mdl (1) file for simulation purpose.