Question

What is the transient stability of the electric drive? full details

Answer 1

Concept of Transient Stability::

While analysing the steady state stability of an electric drive, only its initial and final conditions are examined on the basis of the speed torque characteristics of the motor and load, without paying any attention to the inertia torques and to the time taken to change over from the initial condition to the final one.

The nature of the motion of the
drive during this period is also not
considered. The essence of the
study of transient stability of an
electric drive or, in general, any
moving system is to take into
account the influence of the above
mentioned factors. Such a study
enables us to estimate more accurately the performance of the drive with respect to its stability of motion. With slow arid steady changes in load, the driving motor may be loaded upto its maximum capacity of torque or power. For example, an induction motor can be loaded upto its pull out torque, determined after taking into account any fall in voltage of the supply lines.

In case of transient processes, which takes place quite fast, the pull out torque of the motor no

longer will be the permissible limit
of load torque, since the inertia
torque due to the kinetic energy of
the rotating masses also would
come into play.The inertia torque aids the motort torque while the speed decreases tho motor torque

and opposes the motor torque,
while the speed increases. Thus,
the equality between load torque
and the motor torque, which forms
the basis for the determination of
steady state stability limit, no
longer holds good during transient
processes.
A study of the transient stability of
an electric drive, both during its
design and operation, enables us
to use the equipment more
rationally. For example, in case of
drives having flywheels, the size of
the latter can be decreased, if the
design, were to be done on the
basis of transient stability limit.
However, in practice, this is done
only partially, allowing the transient
stability limit to be used as a
reserve capacity, when sudden changes in load occur.

More Information:

Transient stability in Electrical Drives:

Transient stability in Electrical Drives – From the preceding discussion it is clear that a drive is in its state of equi­librium at constant speed if the developed motor torque is equal to the sum of load torque and friction. The basis of the investigation of the dynamics of a drive from one steady-state equilibrium condition to another has already been discussed in detail. Electrical drives have closed loop control systems. Investigations on the stability of drives must include all the controls available along with the motor and mechanical system. A system is in an equilibrium condition, if there is no disturbance. In the presence of a disturbance the equilibrium conditions are disturbed and the drive tries to take a new equilib­rium position under the new input conditions. The system is said to be stable if sometime after the appearance of a disturbance it attains a new equilibrium condition. The system is said to be unstable if it comes to rest or has a continuous increase in speed following the disturbance. i.e. the system is unable to take up a new equilibrium position. The disturbances may be external or internal to the drive.

The stability of a drive may be affected very much by the parameters of controllers, motors, etc. A system which is stable for one set of parameters may become unstable for another set. The stability investigations of a drive system having closed loop controls and represented by means of a block diagram will be discussed in detail in a later chapter dealing with the control techniques of drives.

The stability of a drive is influenced by the inertia of the rotating masses and inductances of the motor, in the same manner as the transient behaviour. Therefore in cases of sudden changes of parameter causing the drive to change its equilibrium state, the effects of these components cannot be ignored. The stability behaviour of the drive taking into account the effects of these param­eters is called Transient stability in Electrical Drives. On the other hand, if the changes from one state of equilibrium to another take place too slowly to have the effects of the above parameters, the stability conditions refer to steady-state stability.

Before discussing the stability characteristics of a drive in a closed loop or open loop operation it is first necessary to investigate the stability of the motor driving the load. The motors have both steady-state and Transient stability in Electrical Drives conditions existing in them.