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8/10/2019 Protection & Relay Scheme
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Protection and Relay Schemes
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Agenda
Introduction of Protective Relays
Electrical System Protection withProtective Relays
Conclusion
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What are Relays?
Relays are electricalswitches that open or closeanother circuit undercertain conditions.
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Advantages for Using Protective
Relays
Detect system failures when they occurand isolate the faulted section from theremaining of the system.
Mitigating the effects of failures after theyoccur. Minimize risk of fire, danger topersonal and other high voltage systems.
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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 morecontacts that rely on an armature (or lever) activatedby the energized coil to open or close an electrical
circuit. Solid-state Relays (SSRs)
SSRs use semiconductor output instead of mechanicalcontacts to switch the circuit. The output device isoptically-coupled to an LED light source inside therelay. 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 andprotection.
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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 tobuild a fault free system.
Electrical system shall tolerate certaindegree of faults.
Usually faults are caused by breakdown ofinsulation 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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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; circuitbreakers are like human muscle.
Relays make decisions based on settings.
Relays send signals to circuit breakers.Based the sending signals circuit breakerswill open/close.
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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 gasinside the transformer. A tiny little
amount of gas will cause transformerexplosion.
Temperature Monitoring Relays:
These relays are used to monitor thewinding temperature of the transformerand prevent overheating.
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Transformer Protection
Ground Fault
For a wye connection, ground faultcan be detected from the groundedneutral wire.
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Transformer Protection
Ground Fault and Differential Relay
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Conclusion
Relays control output circuits of amuch higher power.
Safety is increased Protective relays are essential for
keeping faults in the systemisolated and keep equipment from
being damaged.
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Reference:
IEEE Red Book (IEEERecommendedPractice for Electric PowerDistribution for Industrial Plants)
Principles of power system by V.KMehta
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