Synchronising Circuit Breaker Closing Window

It is essential that the phase sequence of the incoming generator matches that of the Point of Common Coupling (PCC) busbar and also that all of the following conditions are met before the circuit breaker of the incoming generator is closed on to that busbar.

1. Frequency must match within ........0.1 Hz.

2. Rate of change of frequency.......... 0.1 Hz. / second.

This 'rate of change of frequency' is very significant, as an unstable and erratic engine speed - example; a cold gas engine - produces a circumferential rotational speed which accelerates & decelerates in a fast oscillatory manner. Such an unstable running condition may satisfy a 'Rate of change of frequency' monitor considering an average condition, and so may allow synchronisation to take place under a momentary situation where in fact the generator's rotor is grossly miss-positioned. This will result in a compensating very high 'alignment' torque, which can damage mechanical components within the generator or the engine to generator drive/coupling system.

3. Voltages must match within
3.1 For Generator to Mains : ..........+/- 3%
3.2 For Generator to Generator : ......... +/- 0.5%

4. Circuit Breaker closing angle ..............see below.

The electro-mechanical stress levels generated under an unnecessarily wide closing angle can at best cause rotating diode surge suppressors to be damaged, or at worst cause stators to rotate or shafts to be twisted, making the actual recommendation for the value of closing angle subjective, and ultimately a matter for careful consideration by the commissioning engineer.

If the generator is to be synchronised regularly eg: daily; then setting to close limits will result in lowest possible stress and resulting generator longevity.... +/- 5deg.

If the generator is infrequently synchronised, eg: put in synchronism and remains operational for many weeks before being again synchronised ..... +/- 10deg

OR, has a gas engine and so is relatively unstable during the synchronising procedure, but operates continuously for long periods ........ +/- 15deg


Exceeding +/- 10deg should only be implemented after VERY careful consideration, and all attempts to set a closer angle result in failure.
A variety of techniques can be used to ensure these conditions are met. Cummins Generator Technologies recommends the use of a check synchroniser as the minimum requirement.

A fully automatic synchroniser, used in conjunction with a check synchroniser is preferable.

Failure to meet the above conditions 1, 2, 3, and 4 when closing the circuit breaker, will generate excessive mechanical and electrical stresses, resulting in equipment damage.