Civil Engineering Reference
In-Depth Information
FIGURE 6-11
Induction machine starting and accelerating characteristic.
A severe transient current can flow in the system
if the induction generator operating in a steady state suddenly gets discon-
nected due to a system fault or any other reason, and then reconnected by
an automatic reswitching. The magnitude of the current depends on the
instant of the voltage wave when the generator gets reconnected to the grid.
The physical appreciation of this transient comes from the constant flux
linkage theorem. A coil having no resistance keeps its flux linkage constant.
Since the winding resistance is small compared to the inductance in most
electrical machines, the theorem of the constant flux linkage holds, at least
in the beginning of the fault. If the reswitching was done when the stator
and rotor voltages were in phase opposition, large transient currents are
established to maintain the flux linkage, which then decays slowly to small
values after tens of milliseconds. Meanwhile, the transient electromechanical
torque may be large enough to give the machine and the tower a severe jolt.
The actual amplitude and sign of the first peak of the transient torque are
closely dependent on the rotor speed and duration of the interruption. In
the worst case, the first peak may reach 15 times the rated full-load torque.
Frequent faults of this nature can cause shaft breakage due to fatigue stresses,
particularity at the coupling with the wind turbine.
Reswitching Transient:
When a short circuit fault occurs at or near the generator
terminals, the machine significantly contributes to the system fault current,
particularly if it is running on light load. The short circuit current is always
Short Circuit:
