Basics of Stability
Content
• Introduction
• Stability – Definition
• Classification
• Steady State Stability
• Dynamic Stability and
• Transient Stability
Introduction
• Power system engineering forms a vast and major portion
of electrical engineering studies.
• It is mainly concerned with the production of electrical
power and its transmission from the sending end to the
receiving end as per consumer requirements, incurring
minimum amount of losses.
• The power at the consumer end is often subjected to changes
due to the variation of load or due to disturbances induced
within the length of transmission line.
• For this reason the term power system stability is of utmost
importance
Power System Stability
• “The stability of an interconnected power system is its
ability to return to normal or stable operation after having
been subjected to some form of disturbance.”
• Conversely, instability means a condition denoting loss of
synchronism or falling out of step.
• Stability considerations have been recognized as an essential
part of power system planning for a long time.
• In the power plants, several synchronous generators with
different voltage ratings are connected to the bus terminals
having the same frequency and phase sequence as the
generators, while the consumer ends are feeded directly from
those bus terminals.
• And therefore for stable operation, it is important for the bus to
be well synchronized with the generators over the entire
duration of transmission, and for this reason the power system
stability is also referred to as synchronous stability and is
defined as the ability of the system to return to synchronism
after having undergone some disturbance due to switching on
and off of load or due to line transience.
• To understand stability well, another factor that is to be taken
into consideration is the stability limit of the system.
• The stability limit defines the maximum power permissible to
flow through a particular point or a part of the system during
which it is subjected to line disturbances or faulty flow of
power.
• Having understood these terminologies related to power
system stability let us now look into the different types of
stability.
Classification of stability problems
• The synchronous stability of a power system can be of several
types depending upon the nature of disturbance, and for the
purpose of successful analysis it can be classified into the
following 3 types as shown below:
- Steady state stability.
- Transient stability.
- Dynamic stability.
Steady State Stability of a Power System
• The steady state stability of a power system is defined as the
ability of the system to bring itself back to its stable configuration
following a small disturbance in the network (like normal load
fluctuation or action of automatic voltage regulator).
• It can be considered only during a very gradual and very small
power change.
• In case the power flow through the circuit exceeds the maximum
power permissible, then there are chances that a particular
machine or a group of machines will cease to operate in
synchronism, and result in yet more disturbances.
• In such a situation, the steady state limit of the system is said to
have reached or in other words the steady state stability limit of a
system refers to the maximum amount of power that is
permissible through the system without loss of its steady state
stability.
Dynamic Stability of a Power System
• Dynamic instability is more probable than steady state instability.
• Small disturbances are continually occurring in a power system (variations in
loadings, changes in turbine speeds, etc.) which are small enough not to
cause the system to lose synchronism but do excite the system into the state
of natural oscillations.
• The system is said to be dynamically stable if the oscillations do not acquire
more than certain amplitude and die out quickly.
• In a dynamically unstable system, the oscillation amplitude is large and these
persist for a long time (i.e., the system is underdamped). This kind of
instability behavior constitutes a serious threat to system security and creates
very difficult operating conditions.
• Dynamic stability can be significantly improved through the use of power
system stabilizers. Dynamic system study has to be carried out for 5.10 s and
sometimes up to 30s
.
Transient Stability of a Power System
• Transient stability of a power system refers to the ability of the
system to reach a stable condition following a large disturbance
in the network condition.
• In all cases related to large changes in the system like sudden
application or removal of load, switching operations, line faults
or loss due to excitation the transient stability of the system
comes into play.
• It infact deals in the ability of the system to retain synchronism
following a disturbance sustaining for a reasonably long period of
time.
• And the maximum power that is permissible to flow through the
network without loss of stability following a sustained period of
disturbance is referred to as the transient stability of the system.
• Going beyond that maximum permissible value for power flow,
the system would temporarily be rendered as unstable.
