Bài giảng Chức năng các hệ thống truyền tải và phân phối điện năng - Chương 7: Ổn định trong hệ thống điện - Võ Ngọc Điều

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Chương 7
ỔN ĐỊNH TRONG HỆ THỐNG 
ĐIỆN
CHỨC NĂNG CÁC HỆ THỐNG 
TRUYỀN TẢI VÀ PHÂN PHỐI 
ĐIỆN NĂNG
Võ Ngọc Điều
Bộ môn Hệ Thống Điện
Email: vndieu@gmail.com
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Introduction
- At present the demand for electricity is rising phenomenally. 
- This persistent demand is leading to operation of the power 
system at its limit.
- On top of this the need for reliable, stable and quality power is 
also on the rise due to electric power sensitive industries like 
information technology, communication, electronics etc.
- In this scenario, meeting the electric power demand is not the 
only criteria but also it is the responsibility of the power system 
engineers to provide a stable and quality power to the 
consumers. 
- These issues highlight the necessity of understanding the 
power system stability
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Basic Concepts and Definitions of Power System Stability
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.
- The disturbances mentioned in the definition could be faults, 
load changes, generator outages, line outages, voltage collapse 
or some combination of these. 
- Power system stability can be broadly classified into rotor 
angle, voltage and frequency stability. Each of these three 
stabilities can be further classified into large disturbance or 
small disturbance, short term or long term. 
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Classification of power system stability
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Rotor angle stability
It is the ability of the system to remain in synchronism when 
subjected to a disturbance.
The rotor angle of a generator depends on the balance between 
the electromagnetic torque due to the generator electrical power 
output and mechanical torque due to the input mechanical 
power through a prime mover. 
Remaining in synchronism means that all the generators 
electromagnetic torque is exactly balanced by the mechanical 
torque.
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Rotor angle stability
If in some generator the balance between electromagnetic and 
mechanical torque is disturbed, due to disturbances in the 
system, then this will lead to oscillations in the rotor angle. 
Rotor angle stability is further classified into small disturbance 
angle stability and large disturbance angle stability.
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Small-disturbance or small-signal angle stability
It is the ability of the system to remain in synchronism when 
subjected to small disturbances. 
If a disturbance is small enough so that the nonlinear power 
system can be approximated as a linear system, then the study 
of rotor angle stability of that particular system is called as 
small-disturbance angle stability analysis. 
Small disturbances can be small load changes like switching on 
or off of small loads, line tripping, small generators tripping etc. 
Due to small disturbances there can be two types of instability: 
non-oscillatory instability and oscillatory instability.
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Small-disturbance or small-signal angle stability
In non-oscillatory instability the rotor angle of a generator 
keeps on increasing due to a small disturbance and in case of 
oscillatory instability the rotor angle oscillates with increasing 
magnitude.
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Large-disturbance or transient angle stability
It is the ability of the system to remain in synchronism when 
subjected to large disturbances. 
Large disturbances can be faults, switching on or off of large 
loads, large generators tripping etc. 
When a power system is subjected to large disturbances they 
will lead to large excursions of generator rotor angles. 
Since there are large rotor angle changes the power system 
cannot be approximated by a linear representation like in the 
case of small-disturbance stability. 
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Large-disturbance or transient angle stability
The time domain of interest in case of large-disturbance as well 
as small-disturbance angle stability is any where between 0.1-
10 s. 
Due to this reason small and large-disturbance angle stability 
are considered to be short term phenomenon. 
It has to be noted here that though in some literature “dynamic 
stability” is used in place of transient stability, only transient 
stability has to be used.
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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. 
Unlike angle stability, voltage stability can also be a long term 
phenomenon. 
In case voltage fluctuations occur due to fast acting devices like 
induction motors, power electronic drive, HVDC etc then the 
time frame for understanding the stability is in the range of 10-
20 s and hence can be treated as short term phenomenon. 
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Voltage stability
On the other hand if voltage variations are due to slow change 
in load, over loading of lines, generators hitting reactive power 
limits, tap changing transformers etc then time frame for 
voltage stability can stretch from 1 minute to several minutes.
The main difference between voltage stability and angle 
stability is that voltage stability depends on the balance of 
reactive power demand and generation in the system where as 
the angle stability mainly depends on the balance between real 
power generation and demand.
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Frequency stability
It refers to the ability of a power system to maintain steady 
frequency following a severe disturbance between generation 
and load. 
It depends on the ability to restore equilibrium between system 
generation and load, with minimum loss of load. 
Frequency instability may lead to sustained frequency swings 
leading to tripping of generating units or loads. 
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Frequency stability
During frequency excursions, the characteristic times of the 
processes and devices that are activated will range from 
fraction of seconds like under frequency control to several 
minutes, corresponding to the response of devices such as 
prime mover and hence frequency stability may be a short-term 
phenomenon or a long-term phenomenon.
Though, stability is classified into rotor angle, voltage and 
frequency stability they need not be independent isolated 
events. 
A voltage collapse at a bus can lead to large excursions in rotor 
angle and frequency. Similarly, large frequency deviations can 
lead to large changes in voltage magnitude.
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Frequency stability
Each component of the power system i.e. prime mover, 
generator rotor, generator stator, transformers, transmission 
lines, load, controlling devices and protection systems should 
be mathematically represented to assess the rotor angle, 
voltage and frequency stability through appropriate analysis 
tools.
In fact entire power system can be represented by a set of 
Differential Algebraic Equations (DAE) through which system 
stability can be analyzed. 
In the next few Chapters we will be concentrating on power 
system components modeling for stability analysis.
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