8/13/2019 h.Abd.m.
1/22
Prepared By: Humam A. Salih
M.Sc.Electrical Power Engineering
November, 2012
University of Kufa
College of Education
Teaching Development Center
8/13/2019 h.Abd.m.
2/22
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.
8/13/2019 h.Abd.m.
3/22
Voltage Stability of The Electrical Power System
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?
8/13/2019 h.Abd.m.
4/22
Voltage Stability of The Electrical Power System
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
8/13/2019 h.Abd.m.
5/22
Voltage Stability of The Electrical Power System
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:
8/13/2019 h.Abd.m.
6/22
Voltage Stability of The Electrical Power System
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?
8/13/2019 h.Abd.m.
7/22
Voltage Stability of The Electrical Power System
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.
2- What is the voltage stability?
8/13/2019 h.Abd.m.
8/22
Voltage Stability of The Electrical Power System
Voltage collapse: it is the process by which
voltage instability leads to loss of voltage in a
significant part of the system.
2- What is the voltage stability?
8/13/2019 h.Abd.m.
9/22
Voltage Stability of The Electrical Power System
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:
8/13/2019 h.Abd.m.
10/22
Voltage Stability of The Electrical Power System
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:
8/13/2019 h.Abd.m.
11/22
Voltage Stability of The Electrical Power System
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:
8/13/2019 h.Abd.m.
12/22
Voltage Stability of The Electrical Power System
The dynamic effects are represented by:
1. The Load Tap Changer (LTC) of the
transformers.
2. Thermostatic loads.
3. Induction motors.
4- The Causes of Voltage Instability:
8/13/2019 h.Abd.m.
13/22
Voltage Stability of The Electrical Power System
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.
4- The Causes of Voltage Instability:
8/13/2019 h.Abd.m.
14/22
Voltage Stability of The Electrical Power System
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.
4- The Causes of Voltage Instability:
8/13/2019 h.Abd.m.
15/22
Voltage Stability of The Electrical Power System
4- The Causes of Voltage Instability:
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).
8/13/2019 h.Abd.m.
16/22
Voltage Stability of The Electrical Power System
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,
8/13/2019 h.Abd.m.
17/22
Voltage Stability of The Electrical Power System
5- Prevention of voltage collapse:
control of transformer tap changers,
under-voltage load shedding.
2- System operating measures:
stability margin
spinning reserve
operatorsaction
8/13/2019 h.Abd.m.
18/22
Voltage Stability of The Electrical Power System
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.
8/13/2019 h.Abd.m.
19/22
Voltage Stability of The Electrical Power System
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.
8/13/2019 h.Abd.m.
20/22
Voltage Stability of The Electrical Power System
.
. .
8/13/2019 h.Abd.m.
21/22
Voltage Stability of The Electrical Power System
.
..
26112012
8/13/2019 h.Abd.m.
22/22