Electromagnetic Protection (PART 2)

LENDI INSTITUTE OF ENGINEERING AND TECHNOLOGY
Jonnada, Andhra Pradesh- 535005
UNIT -II
Electromagnetic Protection (PART II)
Presented by,
Dr. Rohit Babu, Associate Professor
Department of Electrical and Electronics Engineering

LENDI INSTITUTE OF ENGINEERING AND TECHNOLOGY
Jonnada, Andhra Pradesh- 535005
(Part 1) Relay connection – Balanced beam type attracted armature relay -
induction disc and induction cup relays–Torque equation – (Part 2) Relays
classification–Instantaneous– DMT and IDMT types– Applications of relays:
Over current and under voltage relays– Directional relays– Differential
relays and percentage differential relays– Universal torque equation–
Distance relays: Impedance– Reactance– Mho and offset mho relays–
Characteristics of distance relays and comparison.

Relays classification
CLASSIFICATION OF PROTECTIVE RELAYS
1. Classification of Protective Relays Based on Technology
Protective relays can be broadly classified into the following three categories, depending on the
technology they use for their construction and operation.
i. Electromechanical relays
ii. Static relays
iii. Numerical relays

i. Electromechanical Relays
• Electromechanical relays are further classified into two categories, i.e.,
(i) Electromagnetic relays, and
(ii) Thermal relays.
• Electromagnetic relays work on the principle of either electromagnetic attraction or
electromagnetic induction.
• Thermal relays utilise the electrothermal effect of the actuating current for their operation.
• First of all, electromagnetic relays working on the principle of electromagnetic attraction were
developed.
• These relays were called attracted armature-type electromagnetic relays.
• This type of relay operates through an armature which is attracted to an electromagnet or through
a plunger drawn into a solenoid.

ii. Static Relays
• Static relays contain electronic circuitry which may include transistors, ICs, diodes and other
electronic components.
• A static relay containing a slave relay is a semi-static relay.
• A relay using a thyristor circuit is a wholly static relay.
• Static relays possess the advantages of having low burden on the CT and VT, fast operation,
absence of mechanical inertia and contact trouble, long life and less maintenance.
• Static relays have proved to be superior to electromechanical relays and they are being used for the
protection of important lines, power stations and sub-stations.
• Static relays are treated as an addition to the family of relays.
• Electromechanical relays continue to be in use because of their simplicity and low cost.

iii. Numerical Relays
• Numerical relays acquire the sequential samples of the ac quantities in numeric (digital) data form
through the data acquisition system, and process the data numerically using an algorithm to
calculate the fault discriminants and make trip decisions.
• They are based on numerical (digital) devices, e.g., microprocessors, microcontrollers, Digital
Signal Processors (DSPs), etc. At present microprocessor/microcontroller-based numerical relays
are widely used.
• Microprocessor/microcontroller- based relays are called numerical relays specifically.
• The term ‘digital relay’ was originally used to designate a previous-generation relay with analog
measurement circuits and digital coincidence time measurement (angle measurement) using
microprocessors.

2. Classification of Protective Relays Based on Speed of Operation
Protective relays can be generally classified by their speed of operation as follows:
i. Instantaneous relays
ii. Time-delay relays
iii. High-speed relays
iv. Ultra high-speed relays

3. Classification of Protective Relays Based on their Generation of Development
Relays can be classified into the following categories, depending on generation of their
development.
i. First-generation relays: Electromechanical relays
ii. Second-generation relays: Static relays
iii. Third-generation relays: Numerical relays.

4. Classification of Protective Relays Based on their Function
Protective relays can be classified into the following categories, depending on the duty they are
required to perform:
i. Overcurrent relays
ii. Undervoltage relays
iii. Impedance relays
iv. Underfrequency relays
v. Directional relays

5 Classification of Protective Relays as Comparators
Protective relays are basically comparators which must be able to carry out addition, subtraction,
multiplication or division of some scalar or some phasor quantities and make comparisons of the
input quantities as desired. Based upon this principle, the protective relays can be classified as
comparators into the following categories.
i. Single-input comparator
ii. Dual-input comparator
iii. Multi-input comparator

Single-input Comparator
These relays have only one input signal and are also known as level detectors. An example of this
type of relay is an over current relay. They have several drawbacks such as (i) they are non-
directional, (ii) they are not reliable, and (iii) they fail to attain the desired reliability.
Dual-input Comparator
These relays have two input signals. The typical examples of such type of relays are distance relays
and differential relays.
Multi-input Comparator
Multi-input comparators have more than two input signals and are used for the realization of
special characteristics other than straight lines or circle. These comparators are also of two types,
i.e., (i) multi-input phase comparator, and (ii) multi-input amplitude comparator.

Instantaneous– DMT and IDMT types
Instantaneous relay: An instantaneous relay has no intentional time delay in its operation. It
operates in 0.1 second. Sometimes the terms high set or high speed relays are also used for the
relays which have operating times less than 0.1 second.
Inverse time relay: A relay in which the operating time is inversely proportional to the magnitude
of the operating current.
Definite time relay: A relay in which the operating time is independent of the magnitude of the
actuating current.
Inverse Definite Minimum Time (IDMT) Relay: A relay which gives an inverse time characteristic
at lower values of the operating current and definite time characteristic at higher values of the
operating current.

Instantaneous– DMT and IDMT types:
Overcurrent Protection
Introduction
• A protective relay which operates when the load current exceeds a preset value, is called an
overcurrent relay.
• The value of the preset current above which the relay operates is known as its pick-up value.
• These relays are used for the protection of distribution lines, large motors, power equipment,
industrial systems, etc.
• A scheme which incorporates overcurrent relays for the protection of an element of a power
system, is known as an overcurrent protection scheme or overcurrent protection.
• An overcurrent protection scheme may include one or more overcurrent relays.
• At present, electromechanical relays are widely used for overcurrent protection.

Time-current Characteristics
The name assigned to an overcurrent relay indicates its time-current characteristic as describe
below:
Fig. Definite-time and inverse-time characteristics
of overcurrent relays
1. Definite-time Overcurrent Relay
• A definite-time overcurrent relay operates after a
predetermined time when the current exceeds its
pick-up value.
• The operating time is constant, irrespective of the
magnitude of the current above the pick-up value.

Fig. Definite-time and inverse-time characteristics
of overcurrent relays
2. Instantaneous Overcurrent Relay
• An instantaneous relay operates in a definite time
when the current exceeds its pick-up value.
• There is no intentional time-delay.
• It operates in 0.1s or less.
3. Inverse-time Overcurrent Relay
• An inverse-time overcurrent relay operates when
the current exceeds its pick-up value.
• The operating time depends on the magnitude of
the operating current.
• The operating time decreases as the current
increases.

General Characteristics of an Inverse Time Relay
In this type of relays, the time of operation depends
upon the magnitude of actuating quantity.
The inverse time relay, where the actuating quantity
is current, is known as inverse current relay.
In this type of relay, the inverse time is achieved by
attaching some mechanical accessories in the relay.

4. Inverse Definite Minimum Time Overcurrent (I.D.M.T)
Relay
• This type of a relay gives an inverse-time current characteristic
at lower values of the fault current and definite-time
characteristic at higher values of the fault current.
• For values of plug setting multiplier between 10 and 20, the
characteristic tends to become a straight line, i.e. towards the
definite time characteristic.
• I.D.M.T. relays are widely used for the protection of
distribution lines.

5. Very Inverse-time Overcurrent Relay
• It gives more inverse characteristic than that of a plain inverse
relay or the I.D.M.T. relay.
• It gives better selectivity than the I.D.M.T. characteristic.
• Its recommended standard time-current characteristic is given
by
• The general expression for time-current characteristic of
overcurrent relays is given by
**The value of n for very inverse
characteristic may lie between
1.02 and 2.

6. Extremely Inverse-time Overcurrent Relay
• It gives a time-current characteristic more inverse than that of
the very inverse and I.D.M.T. relays.
• When I.D.M.T. and very inverse relays fail in selectivity,
extremely inverse relays are employed.
• I.D.M.T. relays are not suitable to be graded with fuses.
• Enclosed fuses have time-current characteristics according to
the law
An extremely inverse relay is used for the protection of machines
against overheating. The heating characteristics of machines and other
apparatus is also governed by the law I^2*t = K.
Also, used for the protection of alternators, power transformers,
earthing transformers, expensive cables, railways trolley wires, etc.

7. Special Characteristics
• Overcurrent relays, having their time-current characteristics steeper than those of extremely
inverse relays are required for certain industrial applications. These relays have time-current
characteristic I^n = K with n = 2.
• To protect rectifier transformers, a highly inverse characteristic of I^8*t = K is required.
• Enclosed fuses have a time-current characteristic of I^3.5*t = K.
• A static relay or microprocessor- base relay can be designed to give I^3.5*t = K characteristic,
suitable to be graded with fuses.

8. Method of Defining Shape of Time-current Characteristics
The general expression for time-current characteristics is given by
The approximate expression is
For definite-time characteristic, the value of n is equal to 0. According to the British Standard, the
following are the important characteristics of overcurrent relays.
(i) I.D.M.T.:
(ii) Very inverse:
(iii) Extremely inverse:

Application of Relays:
Overcurrent Relays & Under voltage Relays
Overcurrent Relay
The overcurrent relay is defined as the relay, which operates only when the value of the current is
greater than the relay setting time.
Depending on the time of operation the overcurrent relay is categorized into following types.
 Instantaneous Overcurrent relay
 Inverse time Overcurrent Relay
 Definite Time Overcurrent Relay
 Inverse Definite Time Overcurrent Relay
 Very Inverse Definite Time Overcurrent Relay
 Extremely Inverse Definite Time Overcurrent Relay

Overcurrent Relays & Undervoltage Relays
Undervoltage Relay (UVR)
• A relay that has contacts that operate when the voltage drops below a set voltage.
• Undervoltage relays are used for protection against voltage drops, to detect short-circuit faults,
etc.
• Undervoltage occurs when the average voltage of a three-phase power system drops below
intended levels, and is sometimes referred to as a brown-out.
• Undervoltage conditions are usually be caused by undersized or overloaded utility and facility
transformers.
• A three-phase monitor relay, with undervoltage protection, can shutdown equipment when
undervoltage occurs preventing damage.

• Under voltage fault protection is used to protect the alternator/generator/transformer winding
from low voltage operation.
• If the voltage drops by 35% to 70% then the UVR activates and trips the breaker and keeps it
until the supply voltage reaches 85% of rated voltage.
Under Voltage Protection Working Principle 27
Principle of Under voltage protection
Three number of potential transformer normally installed
in the generator LAVT panel (lighting arrester voltage
transformer).
They detect the voltage across the generator in real time.

Under voltage relay setting:
Stage1: 90% of the rated voltage trip command to grid circuit breaker.
Stage2: 85% of the rated voltage, trip command to Generator circuit breaker.
ANSI code for under voltage protection: 27

Directional Relays
• This is also a special type of over current relay with a
directional features.
• This directional over current relay employs the
principle of actuation of the relay, when the fault
current flows into the relay in a particular direction.
• If the power flow is in the opposite direction, the
relay will not operate.
• The directional over current relay recognizes the
direction in which fault occurs, relative to the location
of the relay.
• Reverse power flow relays with directional features,
not only senses the direction flow, but also measures
magnitude of power flow.

Directional Relays
Directional Relay Connections
• Directional overcurrent relays are a combination of
directional and overcurrent relay units in the same
enclosing case.
• Directional Control is a design feature that is highly
desirable for this type of a relay.
• With this feature, an overcurrent unit is inoperative, no
matter how large the current may be, unless the
contacts of the directional unit are closed.
• When the lag coil is open, no operating torque is
developed in the overcurrent unit.

Directional Relays
Directional Relay Connections
• Each relay is energized by current from its respective phase and voltage.
• One of the methods of such connections is 30o connection and other is 90o connection.
30o Connection phasor Diagram
The relay will develop maximum torque when its
current and voltage are in phase.

Directional Relays
90o Connection Phasor Diagram
The relay is designated to develop maximum
torque when the relay current leads the voltage
by 45o.

Directional Relays
Constructional Details and Operation of Non Directional over Current Relay (Wattmeter Type)
It has a metallic disc free to rotate between the poles of two electromagnets (EM).
• The spindle of this disc carries a moving contact which
bridges two fixed contacts when the disc rotates
through an angle, which is adjustable between 0 degree
to 360 degree.
• The relay time from name plate curve is to be
multiplied by time multiplier setting.

Directional Relays
Constructional Details and Operation of Non Directional Over Current Relay (Wattmeter Type)
• A directional over current relay operates when the
current exceeds a specified value in a specified direction.
• It contains two relaying units, over current units and the
other a directional unit.
• For directional unit, the secondary winding of the over
current (relay) unit is kept open (AB).
• When the directional unit operates, it closes the open
contacts of the secondary winding of the relay may be
either wattmeter or shaded pole type.

Directional Relays
Shaded Pole Type Directional Over Current Relay
The fig. shows how an induction
disc type over current relay with
split pole i.e., shaded pole magnet
having in addition a directional
unit consisting of a capacitance C
or resistance capacitance RC
circuit works as a directional
relay.
• The main flux is split into two fluxes displaced in time and space
with the help of a shaded ring.
• The air gap flux if shaded pole lags behind the non-shaded pole
flux.

Differential relays and percentage differential relays
• A differential relay is defined as the relay that operates when the phasor difference of two or
more similar electrical quantities exceeds a predetermined amount.
This means that for a differential relay, it should have:
i. Two or more similar electrical quantities and
ii. These quantities should have phase displacement (normally approximately 180°) for the
operation of the relay.
• Differential protection is generally unit protection.
• The protected zone is exactly determined by the location of CTs and PTs.
• The phasor difference is achieved by suitable connections of secondaries of CTs or PTs.

Types of Differential Relays:
1. Current Differential Relays:
In current differential relay two current transformers are
fitted on the either side of the equipment to be protected.
The secondary circuits of CTs are connected in series in
such a way that they carry secondary CT current in same
direction.

Differential Protection for 3-Phase Circuits:
• A three-phase circuit is only necessary, as before, that all the CTs have the same ratio, and that they
be connected so that the differential relay carries no current when the total current entering the
circuit is equal to that current leaving the circuit.
• During normal operating conditions the three secondary currents of CTs are balanced and no
current flows through the relay coil.

A. Difficulties Associated with Differential Protection
1. Difference in Length of Pilot Wires:
The power system element under protection and CTs are located at different places and normally it
is not possible to connect the relay operating coil to the equipotential points.
2. CT Ratio Errors during Short Circuits:
The CTs used may have almost equal ratio at normal currents, but during short-circuit conditions,
the primary currents are unduly large and the ratio errors of CTs on either side differ.

B. Saturation of Magnetic Circuits of CTs under Short-Circuit Condition:
The differential relay of the type explained above is likely to operate inaccurately with heavy
through (i.e., external) faults. The relay may lose its stability for through faults.
C. Magnetizing Current Inrush at the Switching Instant:
When the power transformer is connected to the supply, a large current (about 6 to 10 times full-load
current) inrush takes place. The differential relay operates due to such inrush current, though the
transformer has no fault.
D. Tap-Changing:
Transformer transformation ratio is changed whenever the taps are changed. Due to this CT ratios do
not match with the new-tap settings and result in flow of current in pilot wires even during healthy
condition.

2. Biased or Percentage-Differential Relay
• The percentage differential relay is
defined as the relay that operates on the
phase difference of two or more similar
electrical quantities.
• It is the advanced form of differential
protection relay.
• The only difference between them is the
restraining coil.

Working of Percentage Differential Relay
• The torque due to the restraining coil prevents
the closing of the trip circuit while the torque
due to the operating coil tends to close the trip
circuit contacts.
• Under normal operating conditions and through
load condition the torque developed by the
restraining coil is greater than the operating coil
torque.

The differential current required to operate this relay is a variable quantity, owing to the effect of the
restraining coil. The differential current in the operating coil is proportional to (I1 – I2) and the
equivalent current in the restraining coil is proportional to [(I1 + I2)/2] as the operating coil is
connected to the mid-point of the restraining coil.
The torque developed by the operating coil is proportional to the ampere-turns i.e.,
T0 ∝ (I1 – I2) N0
where N0 is the number of turns on the operating coil.
The torque due to restraining coil
T ∝ [(I1 + I2)/2]N
where, N is the number of turns on the restraining coil. For external faults both I1 and I2 increase and
thereby the restraining torque increases which prevents the mal-operation.

Operating characteristic of the Percentage Differential Relay
• The operating characteristic of the relay is shown in
the figure below.
• The graph shows that the ratio of their operating
current and restraining current is fixed percentage.
• This relay is also called the biased differential relay
because the restraining coil is also called a bias coil
as it provides additional flux.

Types of Percentage Differential Relay
They are of two types:
i. Three terminal system application of the percentage differential relay.
ii. Induction Type biased Differential Relay.

i. Three Terminal System Application
• This relay can be applied to the element
having more than two terminals.
• Each of the three terminals has the same
number of turns, and each of these coils
develops a torque which is independent of
each other.
• Their torques are added arithmetically.

ii. Induction Type Biased Differential Relay
• This relay consists pivoted disc, which moves
in the air gaps of two electromagnets.
• The portion of each pole is fitted with a
copper ring.
• This ring can further move from, into or out of
the pole.
The advantages of such a relay over the beam type are:
(i) The induction element is not susceptible to operation due to transients and
(ii) A slight time delay can be obtained and the biasing feature can be finally adjusted merely by changing
the position of the shading rings on either or both elements.

3. Voltage Balance Differential Relay
• Another class of relays is the voltage balance differential relays, which are preferred for the
feeder protection.
• In this arrangement, two similar current transformers are connected at either end of the system
element under protection (such as a feeder) by means of pilot wires.
• The relays are connected in series with the pilot wires, one at each end.
• The relative polarity of the CTs is such that there is no current through the relay under normal
operating conditions and through fault conditions.
• The CTs used in such protection should be such that they should induce voltages in the
secondary linearly with respect to the current.
• Since the magnitude of fault current is very large, in order that the voltage should be a linear
function of such large currents the CTs should be air-cored.

Electromagnetic Protection (PART 2)

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