Protection and Relay Schemes

Chris FraserAmanda Chen Wang

Group#4

October 5, 2005

Agenda

Introduction of Protective Relays

Electrical System Protection with Protective Relays

Conclusion

What are Relays?

Relays are electrical switches that open or close another circuit under certain conditions.

Relay Purpose

Isolate controlling circuit from controlled circuit. Control high voltage system with low voltage. Control high current system with low current. Logic Functions

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

Relays Circuit Breakers Fuses

Acquisition Detection

Activation Actuation

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 Transformer Protection Generator Protection

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 ProtectionTimed Overload Protection

Timed Overload Definition: Continuously operate motor above itsrated value will cause thermal damage tothe 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 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.

Motor ProtectionStalling

Similar types of relays that are used for motor timed overload protection could be used for motor stalling protection.

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.

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.

Motor ProtectionSingle Phase and Phase Unbalance

These conditions will cause Motor winding overheating Excessive vibrations Cause motor

insulation/winding/bearing damage

Motor ProtectionSingle Phase and Phase Unbalance

These conditions will cause Motor winding overheating Excessive vibrations Cause motor

insulation/winding/bearing damage

Motor ProtectionSingle Phase and Phase Unbalance

Motor ProtectionOther

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 ProtectionGround Fault

For a wye connection, ground fault can be detected from the grounded neutral wire.

Transformer ProtectionGround Fault and Differential Relay

Generator Protection

Differential and Ground Fault Protection

Phase Unbalance

Generator ProtectionDifferential and Ground Fault

Generator ProtectionPhase Unbalance

Some Definitions.. Negative Sequence

Voltage example:

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+

Generator ProtectionPhase 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.

Reference:

IEEE Red Book Ontario Power Generation Training

Course (Electrical Equipment) www.howstuffworks.com