h.Abd.m.

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Prepared By: Humam A. Salih

M.Sc.Electrical Power Engineering

November, 2012

University of Kufa

College of Education

Teaching Development Center

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Voltage Stability of The Electrical Power System

The Aim of This lecture:Student acquisition of the concept &

types of voltage stability in the electrical

power system.

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The Behavioral Objectives of This Lecture:

1- Definition of voltage stability, voltage

instability & voltage collapse.2- Classification of voltage stability.

3- Discussion of voltage instability reasons.

4- How to prevent voltage collapse?

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The Main Contents:

1- Introduction

2- What is Voltage Stability?

3- Classification of Voltage Stability

4- The Causes of Voltage Instability

5- Prevention of Voltage Collapse

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Voltage stability is a problem in power

systems when they are heavily loaded,

faulted or have a shortage of reactive power.

The nature of voltage stability can be

analyzed by examining the production,

transmission and consumption of reactivepower.

1- Introduction:

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Voltage stability: it is a characteristic for a

power system to remain in a state of

equilibrium at normal operating conditions

and to restore an acceptable state of

equilibrium after a disturbance.

2- What is the voltage stability?

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Voltage instability: it happens when a

disturbance causes voltage to drop quickly or

drift downward and operators or automatic

system controls fail to halt the decay.


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Voltage collapse: it is the process by which

voltage instability leads to loss of voltage in a

significant part of the system.


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1- Large-disturbance voltage stability:

It refers to the systemsability to maintain

steady voltages following large

disturbances such as system faults, loss

of generation, etc.

3- Classification of voltage stability:

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2- Small-disturbance voltage stability:

It refers to the systemsability to maintain

steady voltages when subjected to small

disturbances such as incremental changes

in system load.

3- Classification of voltage stability:

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There are three major causes of voltage

instability:

1. Load dynamics. Loads are the main cause

of voltage instabilities, by:

increasing the power consumption due to

the natural evolution, or

due to some dynamic effects.

4- The Causes of Voltage Instability:

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The dynamic effects are represented by:

1. The Load Tap Changer (LTC) of the

transformers.

2. Thermostatic loads.

3. Induction motors.


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2. Transmission system. Each transmission

line or transformer, has a limited transfer

capability, and it depends on:

Transmission line impedance,

Load power factor,

The presence of SVC or Reactive

Compensation devices.


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3. Generation system. When the power

system flow increases, the transmission

system consumes more reactive power.

The generators must increase their

reactive power output to supply these

losses.


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Two protectors can be used to protect the

machines from overheating due to the

increment in the generated reactive power:

1. Over-Excitation Limiter (OEL).

2. Stator Current Limiter (SCL).

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5- Prevention of voltage collapse:

It can be avoided by considering two aspects:

1- System design measures:

reactive power compensating devices,

control of network voltage and generator

reactive output,

coordination of protection controls,

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5- Prevention of voltage collapse:

control of transformer tap changers,

under-voltage load shedding.

2- System operating measures:

stability margin

spinning reserve

operatorsaction

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References:

[1] B. Gao, G.K. Morison and P. Kundur, Voltage

Stability Evaluation Using Modal Analysis, IEEE

transactions on power Systems, Vol. 7, No. 4.

November 1992.

[2] Kundur P., Power System Stability and

Control,McGraw-Hill, New York, USA, 1994.

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References:

[3] Van Cutsem T. and Vournas C., Voltage

Stability of Electric Power Systems, Kluwer

academic publishers, Boston, USA 1998.

[4] P-A Lf, T. Smed, G. Andersson, D. J. Hill,

Fast Calculation of a Voltage Stability Index,

Transactions on Power Systems, Vol. 7, No. 1,

February 1992.

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