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# Machine Riddle No.29- Generator transient and sub transient behavior

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• #598
Ahmmed

What are the transient and sub transient concepts? Which phenomena cause transient and sub transient behavior of a generator in short circuit conditions?

What are the transient and sub transient concepts? Which phenomena cause transient and sub transient behavior of a generator in short circuit conditions?

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• #2223
Hamid

When a generator is operating at no load, that is, with the generator breaker open, the only flux produced in the machine is a result of the DC current in the field windings on the rotor. This rotor flux bridges the air gap between the rotor and the stator and travels around the perimeter of the stator at rotor speed. As it sweeps across the stator, it passes through the A, B, and C phase stator windings, inducing a voltage in each stator coil. The instantaneous flux in the

#3058
Ragamalas

The answer given by Mr Hamid is quite absorbing, but it does not give the full idea why the reactance of the machine varies from sub transient to transient to steady state. In my opinion, a fault can be defined as a sudden change of impedance of a circuit. Hence, sub transient or transient must occur during load change also. The answer to why they occur lies in the fact that flux takes a finite time to respond to a change of current depending on the magnetic behavior of the core material. Hence, the ratio of change of flux to change of current (i.e. inductance) in the circuit varies during the first few cycles leading to sub transient reactance. To counter balance the stator flux so set up during the sub transient phase, the field circuit again needs finite time leading to transient behavior. After transient phase, the steady state is achieved. If there is any error in my understanding, I request you to please correct me.

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