Because magnetic circuit
and magnetic resistance for direct axis of synchronous generator is different
from that of variable axis, the stator and rotor are in relative motion, so the
mutual inductance coefficient and self-inductance coefficient of stator and
rotor are not constant. It is difficult to analyze and solve by using
differential equations, based on the principle of flux conservation in
superconducting closed loop, explain from the physical concept, and derives the
short circuit current in each winding when sudden short circuit occurs.
Characteristics
of sudden short circuit for diesel generators
1.In the case of
steady-state short-circuit, the steady-state short-circuit current is not large
because of the large synchronous reactance, while in the case of sudden
short-circuit, the sudden short-circuit current is large because the
super-transient reactance limiting its current is very small and contains DC
component. The peak value of the sudden short-circuit current can reach more
than ten times of the rated current.
2.With the emergence of
this impulse current, the winding of the electric motor will be affected by a
great impulse electromagnetic force, which may cause the winding to be deformed
or even the insulation of the winding to be damaged.
3.In the process of
sudden short-circuit, the electric motor is affected by strong short-circuit
torque, which can cause vibration.
4. Overvoltage occurs in
stator and rotor windings of motors.
Characteristics
of physical phenomena in generators during sudden short circuit
1.When in steady-state
short circuit, the armature current is constant, and the corresponding armature
magnetic potential is a constant amplitude rotating magnetic field rotating at
synchronous speed, therefore, it will not induce electromotive force and
generate current in the rotor winding. From the current relationship, it is
equivalent to the open circuit state of transformer.
2.When short circuit
occurs suddenly, the magnitude of armature current varies, and the
corresponding magnitude of armature magnetic field varies. Therefore, the
transformer between stator and rotor can induce potential and current in the
rotor winding, and then affect the change of current in the stator winding.
From the point of view of electromagnetic relationship, it is equivalent to the
sudden short-circuit state of transformer.
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