Underfrequency, mostly for Load-Shedding applications, has been considered an electrical system protection parameter. About 20-25 years ago, especially with the proliferation of larger ST and GT prime-movers, emphasisis on underfrequency applications shifted to machinery protection.
A major area of concern has been the fact that underfrequency operation, even for small speed dips, increases the risk of blade failue due to resonance. And, risk for damage is not ony a function of speed deviation, but also duration.
Nimish, if you are condidering the application of underfrequency tripping I recommend using redundancy to reduce the probability of unwarranted (false) trips, and integration to cover the duration aspect.
most generators are set-up to trip on underfrequency for generator protection. Since underfrequency/underspeed conditions cause higher generator stator and rotor temperatures it is necessary to trip the generation source if system frequency degrades below a certain setpoint to protect generator components. The difficulty is making sure that system relay settings are appropriate to trip system load (load shedding) before tripping generation, since typically low frequency is due to a heavily loaded system where every bit of generation needs to be kept online!!
Underfrequency i.e. underspeed protection, is a part of protection set-up for the turbines. As in one of the earlier replies to the query, it has been mentioned that this is critical from the load-sheding point of view, which is necessary to cut the consumer side load (starting from non-critical loads)to restore the system frequency lest the entire system gives way for a major outage.
Other that this, considering the fact that at lower frequency/speeds (when mostly the system voltage do not show a major deviation) the excitation of the machine would be unchanged and hence the field current would definetly be on the higher side than what would be ideally required in cases of reduced frequency. A higher excitation to reduced frequency means a disturbed V/F (since for an ac device the flux is proportional to the applied voltage and inversely proportional to the frequency) which could result in over-fluxing. The phenomenon would result in saturation of magnetic circuit and result in heating of stator and rotor iron and high current in rotor ultimately resulting in I2R (I square R) losses. If not controlled, this could be detrimental to the running of the generator.
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