Classification Of Power System Stability

Contents
 Power System Stability Overview
 Power System Stability: A Proposed Definition
 Need of Stability Classification
 Classification of stability
 Power System Stability Classification
 Rotor Angle Stability
 Voltage Stability
 Frequency Stability
 Rotor Angle Stability vs. Voltage Stability
 References
2

Power System Stability Overview
 Power system is defined as a network of one or more
generating units, loads and power transmission lines
including the associated equipments connected to it.
 The stability of a power system is its ability to develop
restoring forces equal to or greater than the disturbing
forces to maintain the state of equilibrium.
 Power system stability problem gets more pronounced in
case of interconnection of large power networks.
3

Power System Stability
A Proposed Definition
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 system variables
bounded so that practically the entire system remains
intact.
4

Need of Stability Classification
 Stability analysis is easier. Also it leads to proper and
effective understanding of different power system
instabilities.
 Key factors that leads to instability can be easily
identified.
 Methods can be devised for improving power system
stability.
5

Classification of stability
Power system
stability
6
Classification is based on the following considerations:
physical nature of the resulting instability
size of the disturbance considered
processes, and the time span involved

 Ability to regain normal and stable operation after being subjected
to gradual or slow change in the load.
 Concerned with upper loading of machine before losing
synchronism.
 Load is assume to be applied at a rate which is slow.
 System is Analysis by the set of linear equation.
 Action of Voltage regulators and turbine governers are not included.
7

 Ability to regain normal and stable operation after being
subjected to sudden & large changes in the load.
 Losses-generator excitation, transmission, switching
operations and faults.
 Linearization of system equation is not permitted.
 Studied on the basis of swing.
 Action of Voltage regulators and turbine governer are not
included.
8

Same as steady state stability
Included action of turbine governers and voltage
regulators.
Study time is 4-10 sec
9

Power System Stability Classification
 Rotor angle stability.
 Small disturbance angle stability.
 Transient stability.
 Voltage stability.
 Small disturbance voltage stability.
 Large disturbance voltage stability.
 Frequency stability.
 Short term frequency stability.
 Long term frequency stability.
10

Stability Classification at a Glance
11

Rotor Angle Stability
 Rotor angle stability refers to the ability of synchronous
machines of an interconnected power system to remain
in synchronism after being subjected to a disturbance.
 Rotor angle instability occurs due to angular swings of
some generators leading to their loss of synchronism
with other generators.
12

contd...
 Depends on the ability to maintain/restore
equilibrium between electromagnetic torque and
mechanical torque of each synchronous machine.
 At equilibrium, Input mechanical torque equals
output electromagnetic torque of each generator. In
case of any disturbance the above equality doesn’t
hold leading to acceleration/ deceleration of rotors of
machines.
13

Rotor Angle Stability Classification
 Small Disturbance Rotor Angle Stability:
 It is the ability of the power system to maintain
synchronism under small disturbances.
 Disturbances are considered to be sufficiently small
such that the linearization of system equations is
permissible for purposes of analysis.
 The time frame of interest in small-disturbance stability
studies is of the order of 10 to 20 seconds following a
disturbance.
14

 Large Disturbance Rotor Angle Stability:
 It is the ability of the power system to maintain
synchronism under a severe disturbance, such as
a short circuit on a transmission line.
 Disturbances are large so that the linearization of
system equations is not permissible for purposes
of analysis.
 The time frame of interest in small-disturbance
stability studies is of the order of 3 to 5 seconds
following a disturbance.
15

Voltage Stability
 Voltage stability refers to the ability of a power system
to maintain steady voltages at all buses in the system
after being subjected to a disturbance from a given
initial operating condition.
 A system is voltage instable if for atleast one bus in the
system, the voltage magnitude decreases as reactive
power injection is increased.
 Voltage instability results in progressive fall or rise of
voltages of some buses.
16

Contd...
 Large scale effect of voltage instability leads to Voltage
collapse. It is a process by which the sequence of events
accompanying voltage instability leads to a blackout or
abnormally low voltages in a significant part of the
power system.
 The driving force for voltage instability is usually the
loads.
 Voltage stability problems is also experienced at
terminals of HVDC links connected to weak ac systems.
17

 Small Disturbance Voltage Stability:
 Small-disturbance voltage stability refers to the
system’s ability to maintain steady voltages when
subjected to small disturbances such as
incremental changes in system load.
 A combination of both linear and non-linear
techniques are used for analysis.
18
Voltage Stability Classification

 Large Disturbance Voltage Stability:
 Large-disturbance voltage stability refers to the
system’s ability to maintain steady voltages
following large disturbances such as system
faults, loss of generation, or circuit
contingencies.
 The study period of interest may extend from a
few seconds to tens of minutes.
19

Frequency Stability
 Frequency stability refers to the ability of a power system
to maintain steady frequency following a severe system
upset resulting in a significant imbalance between
generation and load.
 Frequency instability leads to tripping of generating units
and/or loads.
 Frequency stability may be a short-term phenomenon or
a long-term phenomenon.
20

Rotor Angle Stability vs.
Voltage Stability
 Rotor angle stability is basically a generator stability
while voltage stability means load stability.
 Rotor angle stability is mainly interlinked to real
power transfer whereas voltage stability is mainly
related to reactive power transfer.
21

Conclusion
Power system is always required normal and stable
operation at rated operating condition & it’s also
required improvement of stability .
Stability of power system is improved by using shunt
& series capacitors, governing system and Facts
controllers.
Reduce transmission losses.
Power is generated by renewable energy sources and
ceate a power park.
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Classification Of Power System Stability

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