Course Title: Power System Engineering

Course No: EE-3113

Course Teacher
Nibir Mondol
Assistant Professor
Department of ESE , KUET.
 Reference Books

1.Principles of Power System- V.K.Mehta and Rohit Mehta .

 Syllabus
Load flow : P.U. method of performance calculation, P.U. impedance of three winding
transformers, and Power flow in simple systems, voltage and frequency controls.
Fault analysis: Symmetrical three phase faults on synchronous machine, Unsymmetrical
Faults: Single line to ground fault, line to line fault, double line to ground fault.

Circuit breakers : Circuit breakers’ types, ratings, constructions, and selections. Arc
extinction, recovery voltage.

Fuse: Constructions, characteristics, and applications of commercially available fuses.

Types, construction, operating principle of over current relays. Sub-station, Lightning
arrestors, surge absorbers, ground wire, generators grounding.

Introduction to Smart grid: Definition, key functions of smart grid, smart grid’s control
elements and their operations, smart grid’s communications and cyber security.
Class
Course
Course Title
Plan Related Books
No

construction, operating principle of over current relays. Chapter 21 ,497 page

Chapter 23,521 page
Lightning arrestors, Chapter 24,552 page
surge absorbers, Chapter 25,569 page
ground wire, Chapter 26 ,586 page
generators grounding
Principles of Power
System- V.K.Mehta and
Rohit Mehta .
 Relay
A Relay is a simple electromechanical switch. Normal switches to close or open a circuit
manually, a Relay is also a switch that connects or disconnects two circuits. But instead of a
manual operation, a relay uses an electrical signal to control an electromagnet, which in turn
connects or disconnects another circuit automatically.
Relays can be of different types like electromechanical, solid state. Electromechanical
relays are frequently used. Let us see the internal parts of this relay before knowing about it
working. Although many different types of relay were present, their working is same.
Every electromechanical relay consists of
• Electromagnet
• Mechanically movable contact
• Switching points and
• Spring
Protective Relays
 How a Relay Works?

The following figure shows a simplified working of a relay.

• Relay works on the principle of electromagnetic induction.

• When the electromagnet is applied with some current, it induces a magnetic field around it.
• Above image shows working of the relay. A switch is used to apply DC current to the load.
• In the relay, Copper coil and the iron core acts as electromagnet.
• When the coil is applied with DC current, it starts attracting the contact as shown. This is called energizing
of relay.
• When the supply is removed it retrieves back to the original position. This is called De energizing of relay.

There are also such relays, whose contacts are initially closed and opened when there is supply i.e. exactly to
opposite to the above shown relay.
Control Input or Coil Terminals: Control input terminals are two input terminals of a relay that controls
its switching mechanism.
A low power source is connected to these terminals to activate and deactivate the relay. The source can
be AC or DC depending on the type of the relay.
COM or Common Terminal: COM refers to the common terminal of the relay. This is the output
terminal of the relay where one end of the load circuit is connected.
This terminal is internally connected with either of the other two terminals depending on the state of
the relay.
NO Terminal: NO or Normally Open terminal is also a load terminal of a relay which remains open
when the relay is not active.
The NO terminal becomes closed with the COM terminal when the relay activates.
NC Terminal: NC or Normally Closed terminal is the other load terminal of a relay. This terminal is
normally connected with COM terminal of the relay when there is no control input.
When relay activates, the NC terminal disconnects from the COM terminal and stays open until the
relay is deactivated
 How are relays classified?
In addition to the functional categories, relays may be classified by input, operating
principle or structure and performance characteristics.

Input characteristics : Current. Voltage. Power. Frequency. Pressure.

Temperature. Flow. Vibration.

Operating principle or structure characteristics: Percentage. Multi-restraint. Product.

Solid state. Electro-mechanical . Thermal.

Performance characteristics: Inverse and definite time overcurrent. Directional

overcurrent. Distance. Under voltage or overvoltage. Ground or phase. High or
slow speed. Phase comparison. Directional comparison. Segregated phase.
Relays can be divided into five functional categories.

• Protective relays: Protective relays are one of the critical components of the electrical power
grid that serve to detect defective equipment or other dangerous or intolerable conditions and can
either initiate or permit switching or simply provide an alarm to provide a safer, more reliable
delivery system.
• Monitoring relays: Verify conditions on the power system or in the protection system
• Programming relays: Establish or detect electrical sequences
• Regulating relays: Activate when an operating parameter deviated from predetermined limits
• Auxiliary relays: Operating in response to the opening or closing of the operating circuit to
supplement another relay or device. These include timers, sealing units, lock-out relays, closing
relays, trip relays, etc
 Relay Applications

Typical application areas of the relays include

Lighting control systems

Telecommunication
Industrial process controllers
Traffic control
Motor drives control
Protection systems of electrical power system
Computer interfaces
Automotive
Home appliances
 Protective Relays
In a power system consisting of generators, transformers, transmission and
distribution circuits, it is inevitable that sooner or later some failure will occur
somewhere in the system.
When a failure occurs on any part of the system, it must be quickly detected and
disconnected from the system.
There are two principal reasons for it.
Firstly, if the fault is not cleared quickly, it may cause unnecessary interruption of
service to the customers.
Secondly, rapid disconnection of faulted apparatus limits the amount of damage
to it and prevents the effects of fault from spreading into the system.
The relays detect the fault and supply information to the circuit breaker which
performs the function of circuit interruption.
 Working principle of Protective Relays
A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to
isolate the defective element from the rest of the system.
The relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical
quantities which are different under normal and fault conditions. The electrical quantities which may
change under fault conditions are voltage, current, frequency and phase angle. Through the changes in one
or more of these quantities, the faults signal their presence, type and location to the protective relays.
Having detected the fault, the relay operates to close the trip circuit of the breaker. This results in the
opening of the breaker and disconnection of the faulty circuit.
The relay circuit connections can be divided into three parts viz.

(i) First part is the primary winding of a current transformer (C.T.) which is
connected in series with the line to be protected.
(ii) Second part consists of secondary winding of C.T. and the relay operating coil.

(iii) Third part is the tripping circuit which may be either a.c. or d.c. It consists of a
source of supply, the trip coil of the circuit breaker and the relay stationary contacts.

When a short circuit occurs at point F on the transmission line, the current flowing in the line
increases to an enormous value. This results in a heavy current flow through the relay coil, causing
the relay to operate by closing its contacts. This in turn closes the trip circuit of the breaker, making
the circuit breaker open and isolating the faulty section from the rest of the system. In this way, the
relay ensures the safety of the circuit equipment from damage and normal working of the healthy
portion of the system.
 Requirements of Protective Relaying
The principal function of protective relaying is to cause the prompt removal from
service of any element of the power system when it starts to operate in an abnormal
manner or interfere with the effective operation of the rest of the system.

(i) selectivity
(ii) speed
(iii) sensitivity
(iv) Reliability
(v) simplicity
(vi) economy
• Selectivity: It is the ability of the protective system to select correctly that part of the system in trouble
and disconnect the faulty part without disturbing the rest of the system.
• Speed: The relay system should disconnect the faulty section as fast as possible for the
following reasons :
(a) Electrical apparatus may be damaged if they are made to carry the fault currents for a long time.
(b) A failure on the system leads to a great reduction in the system voltage. If the faulty section is not disconnected quickly, then the
low voltage created by the fault may shut down consumers’ motors and the generators on the system may become unstable.
(c) The high speed relay system decreases the possibility of development of one type of fault into the other more severe type
• Sensitivity: It is the ability of the relay system to operate with low value of actuating quantity
• Reliability: It is the ability of the relay system to operate under the pre-determined conditions. Without
reliability, the protection would be rendered largely ineffective and could even become a liability
• Simplicity: The relaying system should be simple so that it can be easily maintained. Reliability is
closely related to simplicity. The simpler the protection scheme, the greater will be its reliability.
• Economy.: The most important factor in the choice of a particular protection scheme is the economic
aspect. Sometimes it is economically unjustified to use an ideal scheme of protection and a compromise
method has to be adopted. As a rule, the protective gear should not cost more than 5% of total cost.
However, when the apparatus to be protected is of utmost importance (e.g. generator, main transmission
line etc.), economic considerations are often subordinated to reliability.
 Basic Relays

Most of the relays in service on electric power system today are of electro-mechanical type.They work
on the following two main operating principles
(i) Electromagnetic attraction (ii) Electromagnetic induction

Electromagnetic attraction
Electromagnetic attraction relays operate by virtue of an armature being attracted to the poles of an
electromagnet or a plunger being drawn into a solenoid. Such relays may be actuated by d.c. or a.c.
quantities. The important types of electromagnetic attraction relays are :
(i) Attracted armature type relay.
(ii) Solenoid type relay.
(iii) Balanced beam type relay.
 Attracted armature type relay.
Attracted armature type relay. Fig. shows the schematic arrangement of an attracted armature type
relay. It consists of a laminated electromagnet M carrying a coil C and a pivoted laminated armature.
The armature is balanced by a counterweight and carries a pair of spring contact fingers at its
free end. Under normal operating conditions, the current through the relay coil C is such that
counterweight holds the armature in the position shown. However, when a short-circuit occurs, the
current through the relay coil increases sufficiently and the relay armature is attracted upwards. The
contacts on the relay armature bridge a pair of stationary contacts attached to the relay frame. This
completes the trip circuit which results in the opening of the circuit breaker and, therefore, in the
disconnection of the faulty circuit.
The minimum current at which the relay armature is attracted to close the trip circuit is called pickup
current. It is a usual practice to provide a number of tapping's on the relay coil so that the number of
turns in use and hence the setting value at which the relay operates can be varied.
 Solenoid type relay
shows the schematic arrangement of a solenoid type relay. It consists of a solenoid and movable iron
plunger arranged as shown. Under normal operating conditions, the current through the relay coil C is
such that it holds the plunger by gravity or spring in the position shown. However, on the occurrence
of a fault, the current through the relay coil becomes more than the pickup value, causing the plunger
to be attracted to the solenoid. The upward movement of the plunger closes the trip circuit, thus
opening the circuit breaker and disconnecting the faulty circuit.
 Balanced beam type relay.

Fig. shows the schematic arrangement of a balanced beam type relay. It consists of an iron armature
fastened to a balance beam. Under normal operating conditions, the current through the relay coil is
such that the beam is held in the horizontal position by the spring. However, when a fault occurs, the
current through the relay coil becomes greater than the pickup value and the beam is attracted to
close the trip circuit. This causes the opening of the circuit breaker to isolate the faulty circuit.
Functional Relay Types
(i) Induction type overcurrent relays
(ii) Induction type reverse power relays
(iii) Distance relays
(iv) Differential relays
(v) Translay scheme
 Types of Protec Types of Protection
Primary Protection.: It is the protection scheme which is designed to protect the component parts of the power
system. Thus referring to Fig, each line has an overcurrent
relay that protects the line. If a fault occurs on any line, it will be cleared by its relay and
circuit breaker. This forms the primary or main protection and serves as the first line of
defence. The service record of primary relaying is very high with well over ninety percent of all operations
being correct. However, sometimes faults are not cleared by primary relay system because of trouble within the
relay, wiring system or breaker. Under such conditions, back-up protection does the required job
Back-up protection. It is the second line of defence in case of failure of the primary
protection. It is designed to operate with sufficient time delay so that primary
relaying will be given enough time to function if it is able to. Thus referring to Fig.,
relay A provides back-up protection for each of the four lines. If a line fault is not
cleared by its relay and breaker, the relay A on the group breaker will operate after a
definite time delay and clear the entire group of lines. It is evident that when back-
up relaying functions, a larger part is disconnected than when primary relaying
functions correctly. Therefore, greater emphasis should be placed on the better
maintenance of primary relaying
 Protection of Alternators and Transformers

The modern electric power system consists of several elements e.g.

alternators,
transformers,
station bus-bars,
transmission lines and other equipment.

It is desirable and necessary to protect each element from a variety of

fault conditions which may occur sooner or later
 Protection of Alternators

(i) failure of prime-mover

(ii) failure of field
(iii) Overcurrent
(iv) over speed
(v) overvoltage
(vi) unbalanced loading
(vii) stator winding faults
 Protection of Transformers

(i) open circuits

(ii) overheating
(iii) winding short-circuits e.g. earth-faults, phase-to-phase faults and
inter-turn faults.
 Protection of Bus bars and Lines

(i) Differential protection

(ii) Fault bus protection
 Protection Against Over voltage

Voltage Surge :
A sudden rise in voltage for a very short duration on the power system is known as
a voltage surge or transient voltage.

Transients or surges are of temporary nature and exist for a very short duration (a few hundred μs)
but they cause over voltages on the power system. They originate from switching and from other
causes but by far the most important transients are those caused by lightning striking a transmission
line.
When lightning strikes a line, the surge rushes along the line, just as a flood of water rushes along a
narrow valley when the retaining wall of a reservoir at its head suddenly gives way.
In most of the cases, such surges may cause the line insulators (near the point where lightning has
struck) to flash over and may also damage the nearby transformers, generators or other equipment
connected to the line if the equipment is not suitably protected.
Causes of Voltage Surge

1. Internal causes
(i) Switching surges
(ii) Insulation failure
(iii) Arcing ground
(iv) Resonance

2. External causes i.e. lightning