Why we take respectively 2 phase of both PT in synchronization with synchroscope for matching voltage and frequency? why we don't take three phase? and also why we synchronized incoming machine with running machine in clockwise mode of synchroscope?

 

Devesh,

The answer to the first question is pretty simple. It's adequate and it's less expensive than having the equipment (synchroscope and metering) to compare all three phases. The machines aren't going to change phases--and once the high voltage phasing and the PT measurement/feedback to the synchronizing circuit is verified against any other machine or a grid nothing should change. <b>AND</b> if anything is done to possibly affect phasing--like disassembling the high voltage connections <b>OR</b> disturbing the PT connections (input or output) after verification of phasing then it's incumbent on the people working on the phases/measurement equipment to re-verify phasing and/or PT measurement/feedback before re-synchronizing.

So, because of verification which is done prior to initial synchronization and also after any work is done which might affect the phasing or the PT measurement or feedback/input to the synchronization circuit the presumption is that using a single phase (or the output from an open delta PT arrangement which is still only two phases but more certain) is adequate protection. Of course this relies on people understanding and insisting on verification when changes or work is done on the phases or the measurement/feedback circuit. (The polarity of the PT feedback is VERY important and is often overlooked when control systems are upgraded.)

When the synchroscope is spinning in the clockwise direction that means the energy input to the generator from the prime mover is more than that required to maintain synchronous speed. When the breaker closes this means the power flow out of the generator will be positive not negative--and that's important for the protection of the prime mover. The generator doesn't care if power is flowing out (positive) or in (negative), but when it's negative then the generator is acting like a motor and will increase the speed of the prime mover--which is BAD for some prime movers.

You could theoretically close the generator breaker when the synchroscope is stopped at the 12 o'clock positio, but many electromechanical and some digital synchronizing relays require some slip (movement) as verification that there is actually PT voltage/frequency feedback from both sets of PTs. This would result in zero power flow out of the generator, and while that's okay positive power flow is the best. (At least it has been for more than a century in most parts of the world.)

Hope this helps!

 

OUTGOING GEN. PT SEQUENCE IS DIFFERENT (RY) FROM THE INCOMING GEN PT SEQUENCE (RB) AND THE SNCHROSCOPE AND SYN CHECK RELAY GAVE SYN SIGNAL AND GEN SYNCHRONISATION HAPPEN AND TRIPPED INSTANTLY WITH HEAVY CRUURNT DRAWN. HOW THIS IS EVEN POSSIBLE

 

What you mean by PT SEQUENCE? is that related to rotophase (phase direction )

Did you check rotophase of the 3 phases (bus line and generator line)

 

I am talking about the PT input of the Generator given to the synchroscope being RB and bus PT input given to the synchroscope being RY. due to the different phases to the synchroscope, synchronization should not happen, but the synchronization command given to the Generator and breaker was closed, immediately generator was tripped in Overload and synchronization failed PMG MCB tripped ......
my question is that how this synchronization was possible , why syc command given to GT

 

Ok
Can you tell us more about unit/plant configuration

Also what are controls/protection systems installed ?

 

Also i suggest to open a separate thread for this issue ........

We can then see more clearly what is happening to the sync command

Also can you tell us why sequence /phases are different from Generator to Line bus??