how we can differentiate faults and disturbances in a power system??

disturbances will be cleared and stability returns with tripping and reclosing (auto/manual), while faults persist and continues and leading to sectionalizing for load-supply continuities. and more

thank you...what's about swell and sag ??fault or disturbances.

How about really high resolution metering and recording? Swells and sags per IEEE 1159 definitions (highly recommend obtaining and reading the entire document given your questions) are disturbances of relatively short length. They normally don't cause any protection devices at transmission and distribution any "worry" -- though the customer load centers can certainly have trouble due to facility protection design (inherently more conservative).

Persistent monitoring can answer every question you could conceivably have about a power system however.

I haven't read IEEE 1159. But should one really classify swell/ sag as fault or disturbance? It is actually an undesirable condition, bordering a steady state!

Disturbance, since sag for example really simply reflects a momentary reactive power flow caused by load (usually high starting currents for very large induction machines). Since a motor start isn't a "fault," in my opinion it would be unhelpful to call that a fault. So it would have to be a disturbance.

Thanks..Paul Haddick ,S. D. Varwandkar

Fault, its mean short circuit phenomena and Disturbance related of power quality (sag, swell, harmonics etc.) in your system. You can get more information the differentiation if you have Digital Fault Recorder.

If duration of dip/sag/fault is more,contactors will drop & many motors will come to a standstill till restat command is given.

Faults are actually short circuits that trip the protective device (circuit breaker) or blows the fuse. Faults maybe: Line to Line, Line to Ground, 2 Lines to ground, or 3 phase faults (all 3 lines faulted). The protective device will trip due to the fault. Arcing faults may not immediately trip the protective device. It may take some time for the insulation to fail due to arcing and then finally trip the breaker or blow the fuse out.

Disturbances are momentary or short time, or prolonged, or maybe reccurring anomallies on the system: such as surges, voltage dips, voltage spikes, voltage swells, voltage sags, harmonics, stray current, under frequency, under voltage, etc.They are disturbing the system power quality and performance but not necessarily tripping the circuit breaker or fuses, but in some cases of extreme magnitude, it may cause the system automatic controls and protective devices to cut off the power supply in order to protect the equipment it serves.