Module 2 : Equipment and Stability Constraints in System Operation
Lecture 4 : Introduction
Objectives
In this lecture you will learn the following
What are the nature of constraints faced in power system operation ?
What are the various types of equipment constraints ?
Power System Operating Constraints
Can you use the 50 Hz power supply at your home to drive a 100 kW load? Obviously, the answer is no!
The reason is that the power supply in your house is not designed to handle such power. If you tried to connect
a motor of that rating, you would find that the fuse (or miniature circuit breaker) will trip and disconnect the
power supply. If it did not do so, the wiring would be damaged.
In a similar fashion, a power system is designed to handle several load demand scenarios. This design is done
much in advance (planning stages) based on the expected demand, while keeping some reserve "margins" for
situations in which one or more equipment is out of service. A planner will not over-design (i.e., have
unreasonably large margins) since it will cost a lot of money.
However during operation, it is not possible to requisition or install equipment at short notice. Therefore, an
operator is forced to ensure that the system is operated within the existing design constraints.
Nature of constraints
There are two types of constraints which limit the capability of a power system:
a) Equipment Constraints: An equipment must be operated within the specified ratings otherwise it may
result in damage. Examples of such ratings are the maximum current handling capability of a conductor, the
maximum voltage across an insulator before it breaks down etc. Equipment like generators may have a
relatively large number of constraints. An equipment which is designed to have a larger capability is also
costlier (e.g. a higher current ability will require one to use thicker conductors). Therefore, system and
equipment designers do not over-design an equipment. Under abnormal or unforeseen situations, an
equipment may get overloaded. If the overloading exceeds limits, the equipment is tripped out by protection
systems.
b) Stability Constraints: A power system may not be able to cater to power flows beyond a certain point
due to stability constraints. An unstable system is a one which cannot withstand disturbances, i.e., it may not
settle to an equilibrium although a post-disturbance equilibrium condition may exist. This is due to the basic
physical characteristics which define the behavior under transient conditions. Improvement of stability may
require system reinforcement (like adding new transmission lines) and/or improving/augmenting existing
automatic controllers. Inability to come to an equilibrium may eventually lead to equipment constraints being
violated too. This will cause operation of protection systems.
Loss of equipment due to stability or equipment constraints may take the system into an emergency or in-
extremis state wherein interconnected operation may become unviable.
Therefore, it is important to characterize the capability of the system to handle load power demands and power
flows in a transmission network without violation of the above constraints.
Lecture 4 : Introduction
Objectives
In this lecture you will learn the following
What are the nature of constraints faced in power system operation ?
What are the various types of equipment constraints ?
Power System Operating Constraints
Can you use the 50 Hz power supply at your home to drive a 100 kW load? Obviously, the answer is no!
The reason is that the power supply in your house is not designed to handle such power. If you tried to connect
a motor of that rating, you would find that the fuse (or miniature circuit breaker) will trip and disconnect the
power supply. If it did not do so, the wiring would be damaged.
In a similar fashion, a power system is designed to handle several load demand scenarios. This design is done
much in advance (planning stages) based on the expected demand, while keeping some reserve "margins" for
situations in which one or more equipment is out of service. A planner will not over-design (i.e., have
unreasonably large margins) since it will cost a lot of money.
However during operation, it is not possible to requisition or install equipment at short notice. Therefore, an
operator is forced to ensure that the system is operated within the existing design constraints.
Nature of constraints
There are two types of constraints which limit the capability of a power system:
a) Equipment Constraints: An equipment must be operated within the specified ratings otherwise it may
result in damage. Examples of such ratings are the maximum current handling capability of a conductor, the
maximum voltage across an insulator before it breaks down etc. Equipment like generators may have a
relatively large number of constraints. An equipment which is designed to have a larger capability is also
costlier (e.g. a higher current ability will require one to use thicker conductors). Therefore, system and
equipment designers do not over-design an equipment. Under abnormal or unforeseen situations, an
equipment may get overloaded. If the overloading exceeds limits, the equipment is tripped out by protection
systems.
b) Stability Constraints: A power system may not be able to cater to power flows beyond a certain point
due to stability constraints. An unstable system is a one which cannot withstand disturbances, i.e., it may not
settle to an equilibrium although a post-disturbance equilibrium condition may exist. This is due to the basic
physical characteristics which define the behavior under transient conditions. Improvement of stability may
require system reinforcement (like adding new transmission lines) and/or improving/augmenting existing
automatic controllers. Inability to come to an equilibrium may eventually lead to equipment constraints being
violated too. This will cause operation of protection systems.
Loss of equipment due to stability or equipment constraints may take the system into an emergency or in-
extremis state wherein interconnected operation may become unviable.
Therefore, it is important to characterize the capability of the system to handle load power demands and power
flows in a transmission network without violation of the above constraints.
