I am not sure how a Synchronism check relay works? Any good sites with explanations would be of great help….. As the name and description given in manuals, it states that this relay checks the voltage, magnitude, phase angle and frequency between the lines connected to the in and out of a breaker that is ready for a close operation or in other words it checks the voltages between the incoming line (line going to the breaker) and the bus( the part that is going out of the breaker and connected to the bus). But in the actual case –only the incoming line to the breaker will be hot and the outgoing line from the breaker to bus will not be hot and will become hot only if we close the breaker. So in that case – how will it actually compare the voltage, magnitude, phase angle and frequency as we have a live line and a dead bus condition all the time and there by closing the breaker? When and Where does the comparison take place?

 

If the line connecting your breaker to the bus is dead, no sync is possible or necessary.
Only when connecting two energised elements is it possible to perform a sync check.
In this instance, provided that the relay has been configured for it, a close will be permitted under what is called "live bus-dead line" conditions. The reverse of this would of course be dead bus-live line.
Closing of breakers under different conditions are considerations best studied by an experienced engineer as the consequences of improper operation can be catastrophic to say the least.

 

As i understand synchronization takeplace between two live systems and if is dead then only it is energization of dead system from the live one and not synchronization of the two. Further for synch process it is compulsory that generator is started and it attains its volts,frequency equal (as set in the relay ) to live bus and the phase difference between the two is zero (ideal cond) as nearly as set in the relay. So actually both systems are hot (running) and through PT,s their state are compared in the relay. Comparizon at low level is safe and practicable as compred to dangerousg high volt