Assuming that you are referring to pole slipping protection for synchronous generators, then the answer is that pole slipping (out of step) protection is usually provided by an impedance relay scheme that is set up generally as follows -
- A mho circle element
- Two impedance "blinders"
- Timer logic
The scheme is set so that the impedance locus will pass through the two parallel blinder characteristics for an unstable swing, with the mho circle set to act as a fault detector. During an unstable swing, the impedance locus will take a predetermined time to pass between the blinder charcteristics - this is processed by the timer logic which issues a trip command.
During an external fault, it is possible that the impedance locus will also fall within the operating zone of the scheme, but it will remain stationary and will not traverse bothe blinder characteristics. The logic will recognize this and block tripping. For a stable swing condition, the locus will not traverse the blinder zone fully and tripping will also be blocked.
I know that the above may be somewhat obsciure, but the topic is not an easy one - I have tried to give you a very general overview, which is difficult without diagrams.
This protection will be needed where there is a possibilty of generators falling out of step. The points made above by jbartos may be helpful in avoiding the situation; I would add the following -
- Improve fault clearing times to avoid machine instability
- Operate the machines with an adequate stability margin - avoid high or leading power factor operation
- Perform a comprehensive study of machine AVR and excitation systems to evaluate appropriate gain settings and to analyse whether power system stabilizers or other means of system stabilization are needed.
It's definitely standard protection for large generators. The consequence of a pole slip can be devastating for the generator (require a rewind). On the other hand, it seems like a fairly rare event.
At our two power plants we have the distance relay with blinders as described by peterb. I think it's an sk-something.
I believe the pole slip is more likely where the generator is "weakly" connected and power lines are loaded near their stability limit... ie lots of kw flowing through long transmission lines with few parallel paths. Also clearing time of faults as peterb mentioned... all of the generators will accelerate as voltage is depressed and generator electric power output reduced during a fault. When the generator angles swing too far from each other the danger of pole slip occurs. Bottom line... (as per peterb) a simulation of the power system is required to determine vulnerability.
Thanks for the kind words, electricpete. You are also right on with the GE reference - additionally, check out GE publication GER3179, Out of Step Protection for Generators, (1997/07) at:
To the previous post. Yes, it is true. The asynchronous generators have all poles slipping and it does not pose any problem as long as it is in a reasonable tolerance.
It was just to show a generic option or an avoidance of the synchronous generator pole slip.
There is also a paragraph "Resynchronizing" on page 434 in Reference:
Say M. G., " Alternating Current Machines," John Wiley & Sons, New York, 1978
That addresses the automatic resynchronization.
