protection & relay scheme

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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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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Ground Fault

For a wye connection, ground faultcan be detected from the groundedneutral wire.


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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

protection & relay scheme

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