This document provides an introduction and classification of power system stability, including rotor angle stability, voltage stability, and frequency stability. It defines each type of stability and describes some of the basic phenomena associated with each. Rotor angle stability deals with the ability of synchronous machines to remain in synchronism after a disturbance and includes small-disturbance and transient stability. Voltage stability is defined as the ability to maintain steady state voltages and is affected by the balance between load demand and supply. Frequency stability concerns the ability to maintain steady state frequency following a severe upset.

1. Introduction to Power System Stability and
Control
By
Mangesh S Kulkarni
Sharad Institute of Technology College of Engineering, Yadrav-Ichalkaranji
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2. Classification
Power System Stability
Rotor Angle
Stability
Small-
Disturbance
Angle
Stability
Short Term
Transient
Stability
Frequency
Stability
Short Term
Long Term
Voltage Stability
Large-
Disturbance
Voltage Stability
Short Term
Small-
Disturbance
Voltage Stability
Long Term
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3. Rotor Angle Stability
• “The ability of synchronous machines to remain in synchronism
after being subjected to disturbance.”
• Basic Phenomena associated with angle stability:
– Imbalance Between Accelerating and Decelerating torque.
– Temporary (negative) surplus energy is stored in rotating
masses.
– Capture range of synchronizing torque is limited by pull out
torque.
– Stability may be lost.
• Small Disturbance (or Small-Signal) rotor angle stability
• Large Disturbance rotor angle stability (Transient Stability)
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4. Voltage Stability
• The ability of PS to maintain steady state voltages at all buses after
being subjected to disturbance.
• Depends on the equilibrium between load demand and load supply.
• Basic Phenomena associated with voltage stability:
– High (reactive) loading reduces the voltage in an area.
– Temporary load reduction.
– Transfer capacity to the area is reduced.
– Load demand recovers.
– Voltage reduction and collapse
• Large- disturbance voltage stability
• Small- disturbance voltage stability
• Short term voltage stability
• Long term voltage stability
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5. Frequency Stability
• The ability of PS to maintain steady state frequency following
a severe system upset.
• Reasons of occurrence of frequency instability:
– Inadequacies in equipment responses.
– Poor coordination of control and protection equipment.
– Insufficient generation reserve.
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6. Swing Equation
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During any disturbance, the rotor decelerates or
accelerates with respect to the synchronously rotating air gap
mmf, creating relative motion. The equation describing the
relative motion is known as the swing equation, which is a
non-linear second order differential equation that describes
the swing of the rotor of synchronous machine. The power
exchange between the mechanical rotor and the electrical grid
due to the rotor swing (acceleration and deceleration) is
called Inertial response.

7. Equal Area Criterion
• The Equal area criterion is a “graphical technique used to
examine the transient stability of the machine systems (one or
more than one) with an infinite bus”. Here, the stability
conditions are stated by equating the two area segments which
is present in the power angle diagram.
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Where,
Pa=accelerating power

8. Factors Affecting Transient Stability
• Transient stability is greatly affected by,
1. Type and location of Fault
2. Inertia constant M
3. Clearing angle δ
• Methods of improving Transient Stability limit,
1. Increase of system voltages, use of AVR.
2. Use of high speed excitation systems.
3. Reduction in system transfer reactance.
4. Use of high speed reclosing breakers.
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9. Recent Trends
• Some of the recent methods of maintaining stability are,
I. HVDC links
II. Breaking Resistors
III. Short Circuit Current Limiters
IV. Turbine Fast Valving or Bypass Valving
V. Full Load Rejection Technique
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