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SUB TRANSIENT STATE ARMATURE REACTION

Again assume that the generator is loaded and operating in a steady state. In this situation the magnitude of the stator current is allowed to change rapidly, as in the case of a short circuit in the stator circuit. The additional flux produced by the stator winding will try to penetrate the surface of the rotor poles. Most oil industry generators are provided with damper bars to reduce the excursions in rotor speed during major disturbances. The bars are made of copper or copper alloy and placed longitudinally in the face of the rotor poles. They function in a manner similar to a squirrel cage induction motor when there is a transient change in rotor speed relative to the synchronous speed. As soon as the additional flux passes through the pole faces it will induce currents in the damper bars and the solid pole tips, by the process of transformer induction. These induced currents will set up flux in opposition in order to maintain constant flux linkages with the stator.

During this transient condition, or more appropriately called a sub-transient condition, the additional flux is forced to occupy a region consisting of air and the surface of the rotor poles. This is a high reluctance condition which gives rise to reactances of low values.

Some generators have the damper bars connected to a ring at either end of the pole structure, which provides some damping action from the quadrature axis. This provides a set of short-circuited coils in the quadrature axis, which are air cored and able to repel the flux that is attempting to enter their region.

By the same reasoning as for the ‘transient’ reactances so the sub-transient reactances are derived, and are called the direct axis sub-transient reactance X’’d and the quadrature axis subtransient reactance X’’q.

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