protection and relay schemes(1).ppt

8/22/2019 Protection and Relay Schemes(1).ppt

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PROTECTIONAND RELAY SCHEMES

Muhammad Ali Shujaat

MSEE01123023

July 5, 2013


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AGENDA

Introduction of Protective Relays

Electrical System Protection with Protective Relays

Conclusion


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WHATARE 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 anarmature (or lever) activated by the energized coil to open orclose 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 anLED light source inside the relay. The relay is turned on byenergizing 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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HOWA 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 offaults.

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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ADVANTAGESFOR 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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PROTECTIONAND 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 PROTECTION

TIMED OVERLOAD PROTECTION

Timed Overload Definition:

Continuously operate motor above its

rated 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 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.


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MOTOR PROTECTION

STALLING

Similar types of relays that are used for motor

timed overload protection could be used for motor

stalling protection.


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MOTOR PROTECTION

SINGLE PHASEAND 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 PROTECTION


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 PROTECTION


These conditions will cause

Motor winding overheating

Excessive vibrations

Cause motor insulation/winding/bearing damage


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MOTOR PROTECTION


These conditions will cause

Motor winding overheating

Excessive vibrations

Cause motor insulation/winding/bearing damage


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MOTOR PROTECTION



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MOTOR PROTECTION

OTHER

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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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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GROUND FAULT

For a wye connection, ground fault can be detected

from the grounded neutral wire.


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GROUND FAULTAND DIFFERENTIAL RELAY


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GENERATOR PROTECTION

Differential and Ground Fault Protection

Phase Unbalance


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DIFFERENTIALAND GROUND FAULT


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PHASE UNBALANCE

Some Definitions..

Negative Sequence

Voltage example:


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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+


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PHASE 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 higherpower.

Safety is increased

Protective relays are essential for keeping faultsin the system isolated and keep equipment frombeing damaged.

protection and relay schemes(1).ppt

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