In this slide, we have discussed relays and the types of relays.

1. RELAY

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•A relay is an automatic device by means of
which an electrical circuit is indirectly controlled
(opened or closed) and is governed by a change
in the same or another electrical circuit.
A protective relay is an automatic device which
detects an abnormal condition in an electrical
circuit and causes a circuit breaker to isolate the
faulty element of the system. In some cases it
may give an alarm or visible indication to alert
operator.
INTRODUCTION

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FUNCTIONS OF PROTECTIVE RELAYING
• To sound an alarm, so that the operator may take
some corrective action and/or to close the trip
circuit of CB so as to disconnect a component
during an abnormal fault condition such as
overload, under voltage, temperature etc.
• To disconnect the faulty parts as quickly as
possible so as to minimize the damage to the
faulty part.

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REQUIREMENTS OF PROTECTIVE RELAYING
• SPEED
• SENSITIVITY
• SELECTIVITY
• SIMPLICITY: easy handling and maintenance
• RELIABILTY:remain in-operative other than
fault.
• ECONOMY

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Electromagnetic
relays
Such relays operate on the electromagnetic
principle i.e.
an electromagnet attracts magnetic moving part or
a force is exerted or a current carrying conductor
when placed in the magnetic field or a force is
produced by the principle of induction, etc.
Moving iron, moving coil, attracted armature,
induction disc and induction cup type relays come
under this group of relays.
Static relays These are solid state relays and employ
semiconductor diodes, transistors thyristors, logic
gates, Ics, etc. The measuring circuit is a static
circuit and there are no moving parts. In some
static relays, a slave relay which is a d.c. polarized
relay is used as the tripping device.
Microprocessor
BASED RELAY
A microprocessor can be used to perform all
functions of a relay. It can measure the electrical
quantities, makes comparisons, perform
computations, and send tripping signals. It can
realize all sorts of relay in characteristics, even
irregular curves which cannot be realized
by electromagnetic or static relays easily.
CLASSIFICATION OF RELAYS

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7. 7

8. 8

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10. 10

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DESCRIPTION OF E E RELAYS
CDG
• “C” Stands for operating quantity i.e. current Second Letter
“D” Stands for Movement of the relay component i.e. Disc.
• “G” Stands for application i.e. for general use
•
DTH
• “D” Stands for differential quantity.
• “T” Stands for transistorized.
• “H” Stands for Harmonic restraint.

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First letter – operating quantity :
A – Phase Angle comparison
B – Balanced current
C – Current (amperes)
D – Differential
E – Direction
F – Frequency
L – Directional current
K – Rate of rise of current
O – Oil, Pressure
R – Reactive VA
S – Slip Frequency
T - Temperature
V – Potential (volts)
W –Watts (power)
P – Polyphase VA
X - Reactance
M – Manual

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Second letter – Movement :
A – Attracted armature
B – Buchholz
C – Induction cup.
D – Induction Disc.
G – Galvanometer (moving oil)
I – Transactor
M – Magnet (Polarised)
R - Rectifier
S – Synchronous motor
T - Transistor
W – Weight (gravity)
P - Plug
J – Mixed types

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Third Letter – Application :
A – Auxiliary M – Semaphore
B – Testing N – Negative sequence
C – Carrier or counting O – Out of step
D – Directional P – Potential failure
E – Earth ( Ground) Q - Alarm
F – Flag and alarm indicator R - Reclosing
G – General or generator S - Synchronising
H – Harmonic restraint T – Time or transformer
I – Interlocked or industrial U – Definite time
J – Tripping V - Voltage
JE – Tripping (Elect. Reset) W – Pilot wire
JH – Tripping (Head-reset) WA - Interposing
JS – Tripping (self-reset) WJ - Interpreting
JC – Control X - Supervisory
K – Check alarm Y – Flashback (Backfire)
L – Load limiting ZS – Zero Sequence.

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Fourth Letter :-
M – Special variations.
First Figure Indicates the number of units in the relay essential to
its operation not including seal – in auxiliary units.
Second Figure Indicates a particular characteristic of on a group of
similar relays e.g. CDG 11. CDG12, CDG13 and CD 14 are all
inverse time overcurrent relays but with difference characteristic
curves.

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