Protection and Relay Schemes.ppt

Protection and Relay Schemes

Chris FraserAmanda Chen WangGroup#4

October 5, 2005

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

Relay TypesElectromagnetic 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 RelaysUse microprocessor for switching mechanism. Commonly used in power system monitoring and protection.

How a Relay Works

Sold-State Relay

Advantages/DisadvantagesElectromagnetic Relays (EMRs)SimplicityNot expensiveMechanical WearSolid-state Relays (SSRs)No Mechanical movementsFaster than EMRNo sparking between contactsMicroprocessor-based RelayMuch 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 Faultsmajority are phase-to-ground faultsphase-to-phase phase-phase-phase double-phase-to-ground

Advantages for Using Protective RelaysDetect 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

RelaysCircuit BreakersFusesAcquisitionDetectionActivationActuation

Protective Devices ComparisonCircuit Breakers V.S. RelaysRelays 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 ComparisonFuses V.S. RelaysRelays 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 SchemesMotor ProtectionTransformer ProtectionGenerator Protection

Motor ProtectionTimed OverloadLocked RotorSingle Phase and Phase UnbalanceOther

Motor Protection Timed Overload Solution:Thermal overload relays

Plunger-type relays

Induction-type relays

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

Thermal Overload RelaysUse bimetallic strips to open/close relay contacts when temperature exceeds/drops to certain level.Require certain reaction timeInverse time/current relationship

Thermal Overload Relays

Plunger-type RelaysFast reaction timeUse timer for time delaySuch as oil dash pot.Inverse time/current relationship

Plunger-Type Relays

Induction-type RelaysMost frequently used when AC power presentsChange taps to adjust time delay

Induction-Type Relays

Motor ProtectionStallingSome DefinitionsMotor 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 ProtectionStallingSimilar types of relays that are used for motor timed overload protection could be used for motor stalling protection.

Motor ProtectionSingle Phase and Phase UnbalanceSome definitionsSingle Phase:three-phase motors are subject to loss of one of the three phases from the power distribution system.

Motor ProtectionSingle Phase and Phase UnbalanceSome definitionsPhase 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 UnbalanceThese conditions will cause Motor winding overheating Excessive vibrationsCause motor insulation/winding/bearing damage

Motor ProtectionSingle Phase and Phase UnbalanceThese conditions will cause Motor winding overheating Excessive vibrationsCause motor insulation/winding/bearing damage

Motor ProtectionSingle Phase and Phase Unbalance

Motor ProtectionOtherInstantaneous OvercurrentDifferential RelaysUndervoltageElectromagnetic RelaysGround FaultDifferential Relays

Transformer ProtectionGas and Temperature MonitoringDifferential and Ground Fault Protection

Transformer ProtectionGas 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 FaultFor a wye connection, ground fault can be detected from the grounded neutral wire.

Transformer ProtectionGround Fault and Differential Relay

Generator ProtectionDifferential and Ground Fault ProtectionPhase Unbalance

Generator ProtectionDifferential and Ground Fault

Generator ProtectionPhase UnbalanceSome Definitions..Negative SequenceVoltage example:

Generator ProtectionPhase UnbalanceSome 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 UnbalanceNegative Sequence Relay will constantly measure and compare the magnitude and direction of the current.

ConclusionRelays control output circuits of a much higher power.Safety is increasedProtective relays are essential for keeping faults in the system isolated and keep equipment from being damaged.

Reference:IEEE Red BookOntario Power Generation Training Course (Electrical Equipment)www.howstuffworks.com

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