1. Generator Synchronization Definition and Redefinition

Fundamentals

The definition of the synchronization process in terms of matching two sources requires that there are two power sources: one that is running (usually the grid) and the other which is incoming (usually the generator). After the matching is achieved, a connecting device (circuit breaker) is closed to join these two power sources in coordinated operation. The matching is applied to:

- equal phase sequence,

- equal voltages,

- equal frequencies,

- equal phases.

How this process is conducted in power converters? The converter control board is sensing the grid voltage and adjusts the converters quantities to be equal - it adjusts the converter voltage, varies the converter frequency using PLL (phase-locked-loop) to track the phase of the grid voltage and as a result the voltage, frequency and phase of the converter equals the grid. Once the "locked" condition is achieved (the converter operates at fixed voltage and frequency synchronously to the grid), some delay is introduced to make sure it is stable and then the closing command is issued. The breaker is switched on and the control changes it's operating mode to grid-connection.

In generator synchronization, a (usually) dedicated digital electronic device is introduced as a separate controller which performs the same task. Seems like you just extract the synchronization code and put it in a separate hardware. But, once you do it, you'll get in trouble. The synchronizing controller is just a device inserted in a very complicated control environment (hierarchically structured), having no direct knowledge of what the voltage controller is actually doing, nor what the frequency (speed) controller is actually doing. What's more, unlike the power converter, the frequency (speed) of the generator is never stable unless connected to the grid. Due to magnetic coupling with fluctuating rotational field, the generator voltage is also slightly varying despite the voltage regulator actions. If you have no stable frequency and no stable voltage, how can you ever achieve locked synchronization? The answer is - you can't. You can never achieve perfectly synchronized generator despite your hardest efforts. It is simply out of your control because the equipment your synchronizer is trying to control works in a way you don't know or you don't understand. However, you can loose the definition a little and save the day. You can say:

- I insist on equal phase sequence during synchronization, it is mandatory,

- I tolerate voltages to differ by a small amount at the moment of circuit-breaker closing; the voltage regulator will do its best to keep the voltage constant but it's not that easy,

- I tolerate frequencies to differ by a small amount at the moment of closing; the speed controller will do its best to keep the frequency constant but it depends on so many parameters,

- I tolerate phase difference to differ from zero by a small amount at the moment of closing; if it varies, it must be variable in a way that zero-difference instant may be predicted.

Once you introduce these rules, the generator synchronization becomes achievable. But, how small are those tolerances? Also, is there any other equipment involved, requirements to be met and parameters to be defined? It turns out there is a lot more than you can see from the definition. And it will be described in following articles.