Protection relays

1. Power System Protection
Lecture (2)
Protective Relays

2. • All the relays consist of one element or more which
get energized and actuated by electrical quantities of
the circuit.
• Intelligent device sense and detect abnormal
condition in the system using measuring actuation
quantity.
• Protection relays can be classified in accordance with
the function which they carry out, their construction,
the incoming signal and the type of protection.

3. Classification base on function:
Auxiliary
Protection.
Monitoring.
Control.
Classification base Construction:
Electromagnetic.
Solid state
Digital
Numeric.
Nonelectric (thermal, pressure ......etc.).
Classification of Relays

4. Classification base Incoming signal:
Current.
Voltage.
Frequency.
Temperature.
Pressure.
Velocity.
Classification base Type of protection
Over current.
Directional over current.
Distance.
Over voltage.
Differential.
Reverse power.
Classification of Relays

6. Electromagnetic relays
• These relays were the earliest forms of relay used
for the protection of power systems.
• They work on the principle of a mechanical force
causing operation of a relay contact in response to
a stimulus.
• The mechanical force is generated through current
flow in one or more windings on a magnetic core
or cores.
• The two types of Electromagnetic relays are:
1. Electromagnetic Attraction Type relays
2. Electromagnetic Induction Type relays

7. Attracted Armature Type
When the current in the circuit exceed the setting limit, the
armature get attracted by the magnetic force produced by
undesired current

8. Operating principles

9. Plunger type relay
• In this relay, the plunger or iron core moves into a solenoid and
operation of relay depend on the movement of the plunger.
• The force of attraction is equal to K1I2 - K2, where Κ1 depends
upon the number of turns on the operating solenoid, the air gap,
the effective area and the reluctance of the magnetic circuit.
• K2 is the restraining force, usually produced by a spring.
• In order to control the value at which the relay starts to operate,
the restraining tension of the spring or the resistance of the
solenoid circuit can be varied, thus modifying the restricting force

12. Electromagnetic Induction Type relays
These relays work on principles of electromagnetic induction.
1. Shaded Induction Disc Relay
• In this type of relay, a metal disc is allowed to rotate between
two electromagnets. The torque is developed by interaction
of the flux of one the magnets and the eddy current induced
in the disc by the other.
• Effect of shading ring is to produce flux in shaded portion of
the magnet (1) which is displaced in phase from flux in
remaining portion (2).
• The flux 1 produce E1 lagging at 90. The E.M.F E1 produce
lagging current I1 beyond E1 by small angle. The interaction
between I1 and 2 produce the torque which is proportional
to 2I1cos. Where I1cos is components of I1 in phase with
2.

14. 2. Wattmetric-type relay
• It consists of an E-shaped electromagnet and a U-shaped
electromagnet with disc free to rotate in between.
• The E-shaped magnet produce flux 1 and the U-shaped
produce flux 2.
• Torque is produced by interaction between flux and eddy
current in the disc (flux 1 and 2).
• The relay coil is tapped and the setting current is selected by
take desired number of turns of the coil.
• The time/current characteristic of induction disc relay is
inverse characteristic (the time reduce as current increase).
• The current setting can change by taken suitable number of
turns.

16. 3. Induction-Cup relay
• The relay has two or four electromagnet in stator and
energized by relay coils.
• The rotor consists of metallic cylindrical cup. The rotor is free
to rotate in the gap in between.
• The eddy currents are produce in metallic cup and then
interact with flux produce by stator magnets and produce a
torque.

17. Solid State Relays
• Based on solid state electronic component that provides a
similar function to an electromechanical relay but does not
have any moving components.
• Their design is based on the use of analogue electronic
devices instead of coils and magnets to create the relay
characteristic.
• Early versions used discrete devices such as transistors and
diodes in conjunction with resistors, capacitors, inductors,
etc., but advances in electronics enabled the use of linear and
digital integrated circuits in later versions for signal processing
and implementation of logic functions.

19. Digital Relay
The operation based on microcomputer controlled relay.

20. Operation:
 The relay applies A/D (analog/digital) conversion processes to
the incoming the voltages and currents from CT and VT.
 The relay analyzes the A/D converter output to extract the
magnitude of the incoming quantity (RMS value) using Fourier
transform concept.
 The digital relay is capable of analyzing whether the relay should
trip or restrain from tripping based on current and/or voltage
magnitude (and angle in some applications).
Features
 The relay has some form of advanced event recording.
 The relay has an extensive collection of settings, and these
settings are transferred to the relay via an interface with a PC
(personal computer), and this same PC interface is used to
collect event reports from the relay.
 the relay is multifunction