Module 1: Introduction
Lecture 3 : Operating States
Objectives
In this lecture you will learn the following
Operational Objectives of a Power System.
Hierarchical Control in Power Systems.
Issues of ownership and co-ordination.
Definition of various operating states, nature of various control actions and their significance.
An overview of what you will learn in this course.
Operational Objectives
Most people give little thought to the source of power that comes out of the electrical outlet. And why should
they? The American electric power system was designed as the ultimate plug and play convenience, seemingly
as dependable as the sun rising in the morning. " - Thomas Overbye, Re-engineering the Electric Grid, American
Scientist, 2000, Vol. 88, Iss. 3.
Before we discuss issues in control of a power system, it is important to understand the objectives.
The operational objectives of a power system are:
Continual matching of load demand (plug and play!): Electrical energy cannot be conveniently stored in
sufficient quantities. Therefore a readily available reserve of generation should be available and
controlled. However in many developing countries, there are shortages of generation resulting in load
curtailment.
The power system should provide for a certain level of reliability and quality (frequency and magnitude of
voltages should lie in a narrow, pre-defined range).
The electrical energy should be at low cost and have a low environmental impact.
Hierarchy of controls in a power system
Does a power system require continuous monitoring and control?
Yes. Like many other engineering systems, a power system operator has to continuously monitor the health of a
power system and perform control actions when necessary. A power system has a large number of automatic
controllers too. Manual actions may be necessary to supplement these controllers. In fact, there is a well defined
hierarchy of controls which are deployed.
Equipments like generators have a wide array of automatic feedback control systems which not only serve to
implement real and reactive power scheduling commands, but also regulate key parameters like voltage &
frequency, prevent thermal overloads, over-speed etc. Most of the controllers are decentralised, i.e., they use
locally available feedback and control a locally accessible quantity. An example is generator terminal voltage
regulation by controlling its field voltage. Parameters of some decentralised controllers require system wide co-
ordination (e.g. governors which control generator speed).
The reference values of some regulators
, can be set by other slow acting closed
loop controllers. Some references are
also set or can be changed by system or
power plant operators for optimal or
secure system operation.
A large power system may consist of
autonomous areas ("control areas")
which are controlled independently and
exchange pre-decided amounts of power
with other areas through interconnecting
tie lines or DC links. However during
transients and abnormal conditions, they
are expected to act in co-ordination with
each other.
In the figure given on the right, we
illustrate the various controls in a power
system (this includes some important
controllers but not all controllers).
(click on figure to enlarge)
Ownership and Co-ordination
We have seen in the previous slide that the reference of many controllers ('scheduling') are set by an energy
control center.
The scheduling of references may have economic and technical consequences. For example, increasing the
scheduled power of a costly generator while reducing that of a cheap one, will increase the cost of power.
Moreover, large power plows beyond transmission system capability may endanger inter-connected system
operation (we shall see why later in the course).
The question which then arises is : Who directs the whole show? In the previous slide, we assumed all directives
emanate from an "energy control center" or an omnipotent "system operator". However, considering that
ownership of generation, transmission and distribution systems maybe with several different entities, there may
not be one energy control center, but several of them obeying a certain hierarchy with a strictly defined
authority. Thus, ownership of power system components is an important issue in grid co-ordination.
A typical power system consists of several "control areas". In India, a control area can be a state. Most of the
transmission and generation in an area is usually owned by state electricity boards. They have an obligation to
serve customers in their respective areas and have complete authority over all activities in generation,
transmission and distribution in their domain of operation ("vertically integrated utilities").
Ownership and Co-ordination (con td...)
Inter-state and some other major transmission links may be owned by another independent entity (e.g. Power Grid
Corporation of India). At present, the vertically integrated utilities (state electricity boards) can import or export a
pre-decided amount of power from neighboring states or generators owned by other entities like National Thermal
Power Corporation or independent power producers.
However, this ownership structure is gradually changing. The functions and ownership of the hitherto vertical
integrated utilities (like state electricity boards) are being divided by forming separate companies (GENeration
COmpanies (GENCOs), a TRANSmission COmpany(TRANSCO) and DIStribution COmpanies(DISCOs) ). While
transmission will remain a natural monopoly, generation and distribution can be open to competition with private
Lecture 3 : Operating States
Objectives
In this lecture you will learn the following
Operational Objectives of a Power System.
Hierarchical Control in Power Systems.
Issues of ownership and co-ordination.
Definition of various operating states, nature of various control actions and their significance.
An overview of what you will learn in this course.
Operational Objectives
Most people give little thought to the source of power that comes out of the electrical outlet. And why should
they? The American electric power system was designed as the ultimate plug and play convenience, seemingly
as dependable as the sun rising in the morning. " - Thomas Overbye, Re-engineering the Electric Grid, American
Scientist, 2000, Vol. 88, Iss. 3.
Before we discuss issues in control of a power system, it is important to understand the objectives.
The operational objectives of a power system are:
Continual matching of load demand (plug and play!): Electrical energy cannot be conveniently stored in
sufficient quantities. Therefore a readily available reserve of generation should be available and
controlled. However in many developing countries, there are shortages of generation resulting in load
curtailment.
The power system should provide for a certain level of reliability and quality (frequency and magnitude of
voltages should lie in a narrow, pre-defined range).
The electrical energy should be at low cost and have a low environmental impact.
Hierarchy of controls in a power system
Does a power system require continuous monitoring and control?
Yes. Like many other engineering systems, a power system operator has to continuously monitor the health of a
power system and perform control actions when necessary. A power system has a large number of automatic
controllers too. Manual actions may be necessary to supplement these controllers. In fact, there is a well defined
hierarchy of controls which are deployed.
Equipments like generators have a wide array of automatic feedback control systems which not only serve to
implement real and reactive power scheduling commands, but also regulate key parameters like voltage &
frequency, prevent thermal overloads, over-speed etc. Most of the controllers are decentralised, i.e., they use
locally available feedback and control a locally accessible quantity. An example is generator terminal voltage
regulation by controlling its field voltage. Parameters of some decentralised controllers require system wide co-
ordination (e.g. governors which control generator speed).
The reference values of some regulators
, can be set by other slow acting closed
loop controllers. Some references are
also set or can be changed by system or
power plant operators for optimal or
secure system operation.
A large power system may consist of
autonomous areas ("control areas")
which are controlled independently and
exchange pre-decided amounts of power
with other areas through interconnecting
tie lines or DC links. However during
transients and abnormal conditions, they
are expected to act in co-ordination with
each other.
In the figure given on the right, we
illustrate the various controls in a power
system (this includes some important
controllers but not all controllers).
(click on figure to enlarge)
Ownership and Co-ordination
We have seen in the previous slide that the reference of many controllers ('scheduling') are set by an energy
control center.
The scheduling of references may have economic and technical consequences. For example, increasing the
scheduled power of a costly generator while reducing that of a cheap one, will increase the cost of power.
Moreover, large power plows beyond transmission system capability may endanger inter-connected system
operation (we shall see why later in the course).
The question which then arises is : Who directs the whole show? In the previous slide, we assumed all directives
emanate from an "energy control center" or an omnipotent "system operator". However, considering that
ownership of generation, transmission and distribution systems maybe with several different entities, there may
not be one energy control center, but several of them obeying a certain hierarchy with a strictly defined
authority. Thus, ownership of power system components is an important issue in grid co-ordination.
A typical power system consists of several "control areas". In India, a control area can be a state. Most of the
transmission and generation in an area is usually owned by state electricity boards. They have an obligation to
serve customers in their respective areas and have complete authority over all activities in generation,
transmission and distribution in their domain of operation ("vertically integrated utilities").
Ownership and Co-ordination (con td...)
Inter-state and some other major transmission links may be owned by another independent entity (e.g. Power Grid
Corporation of India). At present, the vertically integrated utilities (state electricity boards) can import or export a
pre-decided amount of power from neighboring states or generators owned by other entities like National Thermal
Power Corporation or independent power producers.
However, this ownership structure is gradually changing. The functions and ownership of the hitherto vertical
integrated utilities (like state electricity boards) are being divided by forming separate companies (GENeration
COmpanies (GENCOs), a TRANSmission COmpany(TRANSCO) and DIStribution COmpanies(DISCOs) ). While
transmission will remain a natural monopoly, generation and distribution can be open to competition with private
