Presented by
KALOLIA ALAP R

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What are Relays?
 Relays are electrical switches that open or close another circuit under certain conditions.
 Relay Purpose
 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.
How a Relay Works
 Sold-State Relay
 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
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.
 Electrical Faults
 majority are phase-to-ground faults
 phase-to-phase
 phase-phase-phase
 double-phase-to-ground
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.
 Protective Devices Comparison
 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.
 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.
 Protection and Relay Schemes
 Motor Protection
 Timed Overload
 Locked Rotor
 Single Phase and Phase Unbalance
 Other
 Motor Protection
Timed Overload
Solution:
 Thermal overload relays
 Plunger-type relays
 Induction-type relays

Motor Protection
Timed Overload Protection
Timed Overload Definition:
Continuously operate motor above its
rated value will cause thermal damage to
the motor.
 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
 Thermal Overload Relays
 Plunger-type Relays
 Fast reaction time
 Use timer for time delay
 Such as oil dash pot.
 Inverse time/current relationship
 Plunger-Type Relays
 Induction-type Relays
 Most frequently used when AC power presents
 Change taps to adjust time delay
 Induction-Type Relays
 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.
 Motor Protection
Stalling
 Similar types of relays that are used for motor timed overload protection could be used for motor stalling protection.
 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.
 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.
 Motor Protection
Single Phase and Phase Unbalance
These conditions will cause
 Motor winding overheating
 Excessive vibrations
 Cause motor insulation/winding/bearing damage
 Motor Protection
Single Phase and Phase Unbalance
These conditions will cause
 Motor winding overheating
 Excessive vibrations
 Cause motor insulation/winding/bearing damage
 Motor Protection
Single Phase and Phase Unbalance
 Motor Protection
Other
 Instantaneous Overcurrent
 Differential Relays
 Undervoltage
 Electromagnetic Relays
 Ground Fault
 Differential Relays
 Transformer Protection
 Gas and Temperature Monitoring
 Differential and Ground Fault Protection
 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.
 Transformer Protection
Ground Fault
 For a wye connection, ground fault can be detected from the grounded neutral wire.
 Transformer Protection
Ground Fault and Differential Relay
 Generator Protection
 Differential and Ground Fault Protection
 Phase Unbalance
 Generator Protection
Differential and Ground Fault
 Generator Protection
Phase Unbalance
Some Definitions..
 Negative Sequence
 Voltage example:
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+
 Generator Protection
Phase Unbalance
 Negative Sequence Relay will constantly measure and compare the magnitude and direction of the current.
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.