Chapter 1
Lecture 1: Introduction
May 26, 2024
Debre Markos, Ethiopia
Presentation Outlines
Computers in Power Systems
Computer Tasks
Automatic Generation Control
Generation Scheduling
Security Assessment
Optimal Power Flow (Contingency Analysis)
Transmission System Development
Interactive Power System Analysis
1
Computers in Power System
The main frame computers are used in power system
calculations
Power flow,
Stability,
Short circuit and similar studies.
Special computers are use with software and
hardware interface
2
Computer Tasks
In order to increase processing requirements, the main
computer tasks involved in the management of electrical
energy systems are as follows.
Automatic generation control (AGC).
Supervisory control and data acquisition (SCADA).
Generation scheduling
Network analysis
3
Power system control
Operation and control action Time period
4
….Cont’d
A properly designed and operated power system must meet the
following requirements:
The system must have adequate capability to meet the continuously
varying active and reactive power demand of system load.
The system should be designed and operated so as to supply electrical
energy at minimum cost and with minimum adverse ecological impact.
The electrical power supplied to the consumers must meet certain
minimum quality standards with respect to the following:
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Overall view of generation control
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AGC
• AGC(Automatic Generation Control)
–Is a mechanism used to compute the required
changes or parameters to optimize the operation of
generators
–The command signal may be a raise or lower
generation output
–Remote control and telemetry is used
7
….Cont’d
During normal operation the following four tasks can be
identified with the purpose of AGC:
Matching of system generation and system load.
Reducing the system frequency deviations to zero.
Distributing the total system generation among the various
control areas to comply
Distributing the individual area generation among its
generating sources so as to minimize operating costs
8
….Cont’d
Task-1: is met by governor
speed control.
Task-2: require supplementary
controls coming from the other
control centers.
Task-3: regulation function, or
load-frequency control and
Task-4: Economic dispatch
function of AGC.
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SCADA for the following tasks:
Data acquisition
Information display
Supervisory control
Alarm processing
Information storage and reports
Sequence of events acquisition
Data calculations
Remote terminal unit processing
11
….Cont’d
Major SCADA component
SCADA Master Terminal Unit (MTU): The server that acts as SCADA
system
RTU/PLCs (remote terminal unit) : remote telemetry data
acquisition units located at remote stations
IED (intelligent electronic devices) smart sensors/actuators with
intelligence to acquire data, process it, and communicate
HMI (human-machine interface) : software to provide for
visualization and interaction with SCADA
9
….Cont’d
The controls provided in generating units consist of prime mover control
and excitation controls as shown in Figure. The controls are also called
as Load frequency control (LFC) and automatic voltage control (AVC).
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Load frequency control
In LFC, two feedback loops namely, primary and
secondary loops.
The primary LFC loop senses the generator speed and
accordingly controls the turbine input.
The secondary LFC loop which senses the system
frequency and tie-time power, fine tunes the frequency
back to the nominal value.
Those loop used to eliminate frequency error.
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Automatic voltage control
In AVC, the bus voltage is measured and compared to a
reference.
The resulting error voltage is then amplified and applied to
the excitation control system.
The output of the exciter controls to the generator field
current.
An increase in the reactive power load of the generator
causes the terminal voltage to decrease and this results in
generation of voltage error signal.
The amplified error signal then increases the exciter field
current which in turn increases the exciter terminal voltage.
This increases the generator field current, which results in
an increase in the generated emf.
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Economic dispatch
Economic operation and planning of electric energy generating system has
been accorded the power system operators.
Power systems need to be operated economically to make electrical energy
cost-effective to the consumer and profitable for the provider.
The operational economics that deals with power generation and delivery
can be divided into two sub-problems
One dealing with minimum cost of power generation and
Other dealing with delivery of power with minimum power loss.
The problem of minimum production cost is solved using economic
dispatch. 14
Security Assessment
The system operating conditions are classified into five states:
Normal,
Alert,
Emergency,
Extremis
Restorative
15
….Cont’d
Normal state:
All the system variables are within the normal range with no equipment being overloaded.
The system is in a secure state with both ‘equality’ (total system generation equals total system
load) and ‘inequality’(bus voltages and equipment currents within the limits)
Alert state:
If the security level of the system falls below some specified threshold, the
system then enters the alert state and is termed as “insecure”.
The system variables are still within limits. This state may be brought about
by a single contingency, large increase in system load or adverse weather
conditions.
16
….Cont’d
Preventive control steps taken to restore generation or to eliminate disturbance can help in restoring the
system to the normal state.
If these restorative steps do not succeed, the system remains in the alert state. Occurrence of a
contingency with the system already in alert state, may cause overloading of equipment's and the
system may enter emergency state.
If the disturbance is very severe, the system may enter into extremis state directly from alert state.
Emergency state: If the preventive controls fail or if a severe disturbance occurs, the system
enters emergency state. The transition to this state can occur either from normal state or alert state. In this
state the balance between generation and load is still maintained (equality constraints still satisfied) and the
system remains in synchronism.
• Some components are however overloaded(some inequality constraints violated).
• Failure of these components results in system disintegration.
• Emergency control actions like disconnection of faulted section, re-routing of power excitation control, fast
valving, and load curtailment have to be taken. 17
….Cont’d
• In-extremis state: If the emergency control actions fail when the
system is in emergency state, then the system enters into in-extremis
state.
• The system starts to disintegrate into sections or islands. Some of
these islands may still have sufficient generation to meet the load.
• The components are overloaded and the active power balance is also
disrupted.
• Overloaded generators start tripping leading to cascade outages and
possible ‘blackout’.
• Control actions, such as load shedding and controlled system
operation are taken to save as much of the system as possible from a
widespread blackout.
• Restorative state: The restorative state represents a condition in
which control action is being taken to restart the tripped generators
and restore the interconnections.
• The system transition can be either to normal or alert state depending
on system conditions. 18
….Cont’d
The sequence of events that result in system transition
from normal to in-extremis state may take from few
seconds to several minutes.
Bringing the system back to normal through the
restorative state is an extremely time consuming process
and may last for hours or may be days.
A large generator may require many hours from restart
to synchronization.
The switched off loads can be picked up gradually and
resynchronization of operating islands to the grids is
also a time consuming process.
The control actions may be initiated from the central
energy control Centre either through operators or
automatically 19
Unit Commitment
The total load in the power system varies throughout a day
and its value also changes with the day of the week and
season.
Hence, it is not economical to run all the units available all
the time.
Thus, the problem of unit commitment is to determine in
advance, the start and the shut down sequence of the
available generators such that the load demand is met and the
cost of generation is minimum.
20
Maintenance scheduling
Preventive maintenance has to be carried out on power system
components to ensure that they continue to operate efficiently and
reliably.
Generators are usually put on maintenance once every year. Their
maintenance has to be so scheduled such that the available
generation is sufficient to meet the system load demand.
The problem of maintenance scheduling deals with the
sequencing of generator maintenance such that sufficient
generation is always available to meet the load demand and the
cost of maintenances and cost of lost generation is minimum.
21
System planning
To meet ever increasing load demand, either new power systems
have to be built or the existing power systems are expanded by
adding new generators and transmission lines.
Many analyses must be performed to design and study the
performance of the system and plan expansion.
To study the system feasibility and performance, the following
analyses need to be carried out:
(a) Load flow analysis
(b) Fault analysis/short circuit studies
(c) Stability studies
(d) Contingency analysis 22
Thank You!
&
Any Query?
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