1. Agenda
Introduction of Protective Relays
Electrical System Protection with
Protective Relays
Conclusion
2. What are Relays?
Relays are electrical
switches that open or close
another circuit under
certain conditions.
3. Relay Purpose
Isolate controlling circuit from controlled circuit.
Control high voltage system with low voltage.
Control high current system with low current.
Logic Functions
4. Relay Types
Electromagnetic Relays (EMRs)
EMRs consist of an input coil that's wound to accept a
particular voltage signal, plus a set of one or more
contacts that rely on an armature (or lever) activated
by the energized coil to open or close an electrical
circuit.
Solid-state Relays (SSRs)
SSRs use semiconductor output instead of mechanical
contacts to switch the circuit. The output device is
optically-coupled to an LED light source inside the
relay. The relay is turned on by energizing this LED,
usually with low-voltage DC power.
Microprocessor Based Relays
Use microprocessor for switching mechanism.
Commonly used in power system monitoring and
protection.
7. Advantages/Disadvantages
Electromagnetic Relays (EMRs)
Simplicity
Not expensive
Mechanical Wear
Solid-state Relays (SSRs)
No Mechanical movements
Faster than EMR
No sparking between contacts
Microprocessor-based Relay
Much higher precision and more reliable and durable.
Improve the reliability and power quality of electrical
power systems before, during and after faults occur.
Capable of both digital and analog I/O.
Higher cost
8. Why A System Needs Protection?
There is no ‘fault free’ system.
It is neither practical nor economical to
build a ‘fault free’ system.
Electrical system shall tolerate certain
degree of faults.
Usually faults are caused by breakdown of
insulation due to various reasons: system
aging, lighting, etc.
10. Advantages for Using Protective
Relays
Detect system failures when they occur
and isolate the faulted section from the
remaining of the system.
Mitigating the effects of failures after they
occur. Minimize risk of fire, danger to
personal and other high voltage systems.
12. Protective Devices Comparison
Circuit Breakers V.S. Relays
Relays are like human brain; circuit
breakers are like human muscle.
Relays ‘make decisions’ based on settings.
Relays send signals to circuit breakers.
Based the sending signals circuit breakers
will open/close.
13. Protective Devices Comparison
Fuses V.S. Relays
Relays have different settings and can be
set based on protection requirements.
Relays can be reset.
Fuses only have one specific characteristic
for a individual type.
Fuses cannot be reset but replaced if they
blow.
14. Protection and Relay Schemes
Motor Protection
Transformer Protection
Generator Protection
17. Motor Protection
Timed Overload Protection
Timed Overload Definition:
Continuously operate motor above its
rated value will cause thermal damage to
the motor.
18. Thermal Overload Relays
Use bimetallic strips to open/close
relay contacts when temperature
exceeds/drops to certain level.
Require certain reaction time
Inverse time/current relationship
24. Motor Protection
Stalling
Some Definitions…
Motor Stalling:
It happens when motor circuits are
energized, but motor rotor is not
rotating. It is also called locked rotor.
Effects: this will result in excessive
currents flow given the same load. This
will cause thermal damage to the
motor winding and insulation.
25. Motor Protection
Stalling
Similar types of relays that are used
for motor timed overload protection
could be used for motor stalling
protection.
26. Motor Protection
Single Phase and Phase Unbalance
Some definitions…
Single Phase:
three-phase motors are subject to loss
of one of the three phases from the
power distribution system.
27. Motor Protection
Single Phase and Phase Unbalance
Some definitions…
Phase Unbalance:
In a balanced system the three line-
neutral voltages are equal in magnitude
and are 120 degrees out of phase with
each other. Otherwise, the system is
unbalanced.
28. Motor Protection
Single Phase and Phase Unbalance
These conditions will cause
Motor winding overheating
Excessive vibrations
Cause motor
insulation/winding/bearing damage
29. Motor Protection
Single Phase and Phase Unbalance
These conditions will cause
Motor winding overheating
Excessive vibrations
Cause motor
insulation/winding/bearing damage
33. Transformer Protection
Gas Monitoring Relays:
These relays will sense any amount of gas
inside the transformer. A tiny little
amount of gas will cause transformer
explosion.
Temperature Monitoring Relays:
These relays are used to monitor the
winding temperature of the transformer
and prevent overheating.
39. Generator Protection
Phase Unbalance
Some Definitions..
Negative Sequence:
The direction of rotation of a negative
sequence is opposite to what is
obtained when the positive sequence
are applied.
Negative sequence unbalance factor:
Factor= V-/V+ or I-/I+
41. Conclusion
Relays control output circuits of a
much higher power.
Safety is increased
Protective relays are essential for
keeping faults in the system
isolated and keep equipment from
being damaged.