this ppt about Discrete control of excitation system

1. DISCRETE CONTROL OF
EXCITATION SYSTEMS
Presentation By :- Karansinh Parmar

2. FLOW OF PRESENTATION
Introduction
Detailed information
Block diagram
Operation
Advantages
Conclusion
Reference

3. INTRODUCTION
 Fast acting static exciters with high ceiling voltages
coupled with high gain AVRs can improve transient
stability.
 However they can also result in oscillatory instability by
negative damping of low frequency rotor oscillations.

4. CONTINUE…
 Power System Stabilizers (PSS) with control inputs derived from rotor
speed, power or terminal frequency are used to modulate the generator
voltage and damp oscillations.
 The application of PSS does not result in improvement of transient
stability.
 The improvement in transient stability can be achieved by the
provision of discrete signals to control excitation during a
transient disturbance.

5. DETAILED INFORMATION
 Ontario Hydro has developed a scheme where a signal
proportional to the rotor angle is used (during the transient
period) in addition to the terminal voltage and rotor speed
signals.
 The rotor angle signal cannot be used continuously as
it results in dynamic instability.

6. BLOCK DIAGRAM
Fig.: Discontinuous Excitation Control

7. OPERATION
 The angle signal is derived by integrating the speed signal used as i/p signal
to PSS as shown in figure.
 The angle signal is generated during a transient disturbance by closing the
switch S when there is a sudden drop in terminal voltage followed by the
rise in rotor speed above a preset value.
 The relay contact S is opened when the speed drops below the preset level
or the exciter comes out of saturation.

8. CONTINUE…
 The output of the integrator will then decay exponentially with time
constant T.
 The value of this constant is chosen such that the output is
proportional to the rotor angle for the relevant oscillation
frequencies.
 The effect of the discrete control is to maintain the field voltage and
consequently the terminal voltage at a high level during the positive
swing in the rotor angle.

9. CONTINUE…
 However, the terminal voltage is limited to 1.15 pu.
 From studies carried out, the discrete control of excitation
was found to be as effective as fast valving scheme in a
particular application where the interarea mode dominates
over local modes.
 This rise in terminal voltage of 1.15 p.u. lasts for 1 sec.
affects step-up transformers & UATs.

10. CONTINUE…
 Differential protection for transformer windings should be
improved due to higher magnetizing current.

11. ADVANTAGES
 The problem associated with PSS of reduction in excitation
because of negative rotor speed deviation is overcome by
this method.
 Alternative schemes give priority to improve the first swing
transient stability just after a large disturbance occurred.
 It helps in damping post fault trajectories.
 Provides max. field voltage when desired.

12. CONCLUSION
 Discrete control of excitation system during transient
disturbance improves the transient stability of the power
system.
 It maintain field voltage & as a result terminal voltage at
high values during +ve swing in rotor angle.
 Protections in various parts of the system have to be
modified.

13. REFERENCE
“Power System Dynamics,
Stability & Control”
-by K.R.Padiyar
Published by:- B.S.Publications

14. THANK YOU