The document discusses voltage stability in power systems, focusing on the ability to maintain steady-state voltages during disturbances. It covers various aspects such as reactive power flow, mathematical formulation of stability problems, and methods to prevent voltage collapse. Key strategies include voltage analysis, reactive power compensation, and control mechanisms to avoid voltage instability.

1. Voltage Stability

2. Contents
• Comparison of angle and voltage stability.
• Reactive power flow and voltage collapse.
• Mathematical formulation of voltage stability
problem.
• Voltage stability analysis.
• Prevention of voltage collapse.

3. Introduction
“Power system stability is the ability of an electric power
system, for a given initial operating condition, to regain a state
of operating equilibrium after being subjected to a physical
disturbance, with most of the system variables bounded so that
practically the entire system remains intact”

4. Introduction

5. Voltage Stability
“It is the ability of the system to maintain steady state voltages
at all the system buses when subjected to a disturbance. If the
disturbance is large then it is called as large-disturbance
voltage stability and if the disturbance is small it is called as
small-disturbance voltage stability”

6. Comparison of angle and voltage stability
Static Voltage Analysis:
• Load flow analysis reveals as to how system equilibrium
values (such as voltage and power flow) vary as various
system parameters and controls are changed.
• Power flow is a static analysis tool wherein dynamics is not
explicitly considered. Many of the indices used to assess
voltage stability are related to NR load flow study

7. Comparison of angle and voltage stability
counter measures to avoid voltage instablilty are:
1. Generator terminal voltage increase (only limited control
possible)
2. Increase of generator transformer tap
3. Reactive power injection at appropriate locations
4. Load-end OLTC blocking
5. Strategic load shedding (on occurrence of undervoltage)

8. Reactive power flow and voltage collapse
• Long Transmission Lines
• Radial Transmission Lines
• Shortage of Local Reactive Power

9. Mathematical formulation of voltage stability
problem

10. Mathematical formulation of voltage stability
problem

11. Mathematical formulation of voltage stability
problem

12. Mathematical formulation of voltage stability
problem

13. Mathematical formulation of voltage stability
problem

14. Other Criteria of voltage stability

15. Other Criteria of voltage stability

16. Voltage stability analysis
The voltage stability analysis for a given system state involves
examining following two aspects.
1. Proximity to voltage instability: Distance to instability
may be measured in terms of physical quantities, such as
load level, real power flow through a critical interface, and
reactive power reserve. Possible contingencies such as a
line outage, loss of a generating unit or reactive power
source must be given due consideration.
2. Mechanism of voltage instability:
How and why does voltage instability take place?
What are the main factors leading to instability?
What are the voltage-weak areas?
What are the most effective ways to improve stability?

17. Prevention of voltage collapse
• Application of reactive power compensating
devices.
• control of network voltage and generator reactive
output
• Coordination of protections/controls
• Control of transformer tap changers
• Under voltage load shedding
• Operators' role