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Protection and Relay Schemes
Chris FraserAmanda Chen Wang
Group#4
October 5, 2005
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Agenda
Introduction of Protective Relays
Electrical System Protection with Protective Relays
Conclusion
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What are Relays?
Relays are electrical switches that open or close another circuit under certain conditions.
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Relay Purpose
Isolate controlling circuit from controlled circuit. Control high voltage system with low voltage. Control high current system with low current. Logic Functions
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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.
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How a Relay Works
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Sold-State Relay
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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
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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.
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Electrical Faults
majority are phase-to-ground faults phase-to-phase phase-phase-phase double-phase-to-ground
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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.
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Protective Devices Comparison
Relays Circuit Breakers Fuses
Acquisition Detection
Activation Actuation
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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.
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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.
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Protection and Relay Schemes
Motor Protection Transformer Protection Generator Protection
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Motor Protection
Timed Overload Locked Rotor Single Phase and Phase Unbalance Other
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Motor Protection Timed Overload
Solution: Thermal overload relays
Plunger-type relays
Induction-type relays
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Motor ProtectionTimed Overload Protection
Timed Overload Definition: Continuously operate motor above itsrated value will cause thermal damage tothe motor.
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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
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Thermal Overload Relays
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Plunger-type Relays
Fast reaction time Use timer for time delay
Such as oil dash pot. Inverse time/current relationship
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Plunger-Type Relays
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Induction-type Relays
Most frequently used when AC power presents
Change taps to adjust time delay
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Induction-Type Relays
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Motor ProtectionStalling
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.
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Motor ProtectionStalling
Similar types of relays that are used for motor timed overload protection could be used for motor stalling protection.
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Motor ProtectionSingle 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.
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Motor ProtectionSingle 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.
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Motor ProtectionSingle Phase and Phase Unbalance
These conditions will cause Motor winding overheating Excessive vibrations Cause motor
insulation/winding/bearing damage
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Motor ProtectionSingle Phase and Phase Unbalance
These conditions will cause Motor winding overheating Excessive vibrations Cause motor
insulation/winding/bearing damage
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Motor ProtectionSingle Phase and Phase Unbalance
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Motor ProtectionOther
Instantaneous Overcurrent Differential Relays
Undervoltage Electromagnetic Relays
Ground Fault Differential Relays
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Transformer Protection
Gas and Temperature Monitoring Differential and Ground Fault
Protection
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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.
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Transformer ProtectionGround Fault
For a wye connection, ground fault can be detected from the grounded neutral wire.
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Transformer ProtectionGround Fault and Differential Relay
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Generator Protection
Differential and Ground Fault Protection
Phase Unbalance
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Generator ProtectionDifferential and Ground Fault
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Generator ProtectionPhase Unbalance
Some Definitions.. Negative Sequence
Voltage example:
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Generator ProtectionPhase 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+
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Generator ProtectionPhase Unbalance
Negative Sequence Relay will constantly measure and compare the magnitude and direction of the current.
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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.
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Reference:
IEEE Red Book Ontario Power Generation Training
Course (Electrical Equipment) www.howstuffworks.com
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