I'm a operator of gas turbine with capacity 145 MW. when the gas turbine starts and the generator will be synchronized to system by synchrotact 4, it alway use automatic synchronization.
what Im asking here is what emergency condition where the automatic synchronizer can't take a place so that require a manual synchronization,.,.,.?
infact that the synchronize device that we use can operate in auto operating mode or manual operating mode.
Good question. Increasingly, in many parts of the world, grid regulators are requiring that generators and their prime movers (in your case, the gas turbine) be automatically synchronized to the grid these days. The reasoning is that it's done (more or less) at a controlled rate, that the amount of power which will be produced at the instant of synchronization can be programmed and therefore anticipated, and the possibility for closing out of phase is significantly reduced.
Having said that, it's still important for operators to know how to synchronize manually, which is becoming a lost art. Automatic synchronizers are "machines", or instruments if you prefer, either electronic or electro-mechanical or some combination of the two, and that means that they may not work some day, that an internal component may fail, and render the device useless. Which would be the primary reason for manually synchronizing the turbine, that the auto synchronizer had failed.
What are the failure modes? One or both of the potential transformer input circuits (there are usually two of them) could fail. One of the outputs to either the turbine control or the exciter regulator (also called the "AVR") could fail. The self-diagnostics of a digital electronic auto synchronizer could detect an internal problem that would prevent operation. The power supply to the auto synchronizer could fail. The output to close the generator breaker could fail.
Now, you bring up an interesting point that your auto synchronizer performs both manual and automatic synchronization. That's interesting, because one wonders if the auto synchronizer portion fails if the manual synchronizer portion would still work. In other words, are they capable of independent operation in the event of a failure of one or the other.
Most sites and the turbine control systems I'm familiar with have two independent synchronizers working together during automatic synchronization, for redundancy.
Both synchronizers will use the same inputs, but the automatic synchronizer usually (not always) has two outputs to adjust generator speed (frequency) and also generator terminal voltage when synchronizing. (That's the automatic part of auto synchronization.)
When an operator is manually synchronizing the unit, the operator is responsible for adjusting the generator speed (frequency) and for adjusting the generator terminal voltage prior to actually initiating the generator breaker closure at the appropriate time.
When a manual synchronization is being performed, one of those synchronizers is usually still necessary as a "back-up" to the operator, and is usually required by most grid operators. In other words, there should be an independent means to ensure that the operator does not and cannot close the generator breaker out of phase with the grid. And, a synchronizer of some type is necessary to do that; sometimes it's just called a synch-check relay but there is usually a requirement for some kind of back-up function to prevent closure at an inappropriate time.
So, it would seem you should be asking your plant supervision and/or the instrument technicians or plant engineer if there is some "back-up" synchronizer that will be available if the automatic synchronizer isn't working.
Hope this helps!