Synchcheck/synchroscopes and phase rotation

[Rodmcm]

I was amazed to learn that your average synchcheck relay will not detect the wrong phase rotation on one side (say a generator). Even the phase to phase ones! Also as I understand it the SEL300G does not detect this. How can this be! Surely, at commissioning the wrong rotation is a very real possibility. Can anyone explain why there is this the blind spot in design. I would have theought that it is so easy to check on phase to phase units.

 

[davidbeach]

Nope, can't be done if you're only looking at a single voltage on one side. The best bet, during commissioning, is to energize a set of 3 phase VTs from one side and determine rotation. Then open a breaker and close to the other side and energize the same set of 3 phase VTs and determine rotation with the same instrument using the same connections. A hard wired 3 phase voltage input to a relay is best, but a rotation check meter can be used if it is not disconnected between energizations.

 

[Rodmcm]

Yes,Back livening is a common technigue, but with two phase measurement then surely checking that (Red/Red and Yellow/Yellow) are both at near zero volts also checks for incorrect phase rotation

 

[rbulsara]

Single phase sync check relay is a common practice (to save cost too) but with a caveat. Those systems are supposed to be correctly commissioned for phase rotation. Once correctly commissioned, most generators are not expected to reverse their direction unless you change the prime mover or the alternator, and therefore deemed sufficient in most cases.

Single phase sync check relay is not intended to be a phase rotation check device or even a phase loss device. As with all designs, their validity depends on intended application and skills of the operators.

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

Or the bus connections of course...

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

then surely checking that (Red/Red and Yellow/Yellow) are both at near zero volts also checks for incorrect phase rotation

Nope.
Do a little research on synchronizing lamps and you will see this is not so.
Or if you prefer vectors, draw two delta or star diagrams to represent the two three phase systems. One diagram may then be rotated to simulate a small frequency difference as is normal during synchronization. You may rotate one diagram so that any phase of one system is in sync with the corresponding phase of the other system, at which time the other two phases will be badly out of sync.

One advantage of using one phase to sync is simplicity of switching. We had 5 generators and one syncroscope. A single pole- five position switch was all that was required to switch the scope' input between the five generators when syncing to the bus.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

A synch check relay is not looking at voltage between phases across the open. What a synch check relay is looking at is the magnitude and phase angle of two independent voltages, one on each side of the open, and comparing magnitudes, phase angle, and slip rate.

 

[waross]

On the small stuff, some sync check relays don't even check the voltage. There just needs to be enough voltage to check the phase angle. These are commonly used to permit very fast open transfers.


Bill
--------------------
"Why not the best?"
Jimmy Carter