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Protection and Relay Schemes

Protection and Relay Schemes

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Protection and Relay Schemes

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  1. Protection and Relay Schemes Chris Fraser Amanda Chen Wang Group#4 October 5, 2005

  2. Agenda • Introduction of Protective Relays • Electrical System Protection with Protective Relays • Conclusion

  3. What are Relays? • Relays are electrical switches that open or close another circuit under certain conditions.

  4. Relay Purpose • Isolate controlling circuit from controlled circuit. • Control high voltage system with low voltage. • Control high current system with low current. • Logic Functions

  5. 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.

  6. How a Relay Works

  7. Sold-State Relay

  8. 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

  9. 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.

  10. Electrical Faults • majority are phase-to-ground faults • phase-to-phase • phase-phase-phase • double-phase-to-ground

  11. 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.

  12. Protective Devices Comparison

  13. 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.

  14. 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.

  15. Protection and Relay Schemes • Motor Protection • Transformer Protection • Generator Protection

  16. Motor Protection • Timed Overload • Locked Rotor • Single Phase and Phase Unbalance • Other

  17. Motor Protection Timed Overload Solution: • Thermal overload relays • Plunger-type relays • Induction-type relays

  18. Timed Overload Definition: Continuously operate motor above its rated value will cause thermal damage to the motor. Motor ProtectionTimed Overload Protection

  19. 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

  20. Thermal Overload Relays

  21. Plunger-type Relays • Fast reaction time • Use timer for time delay • Such as oil dash pot. • Inverse time/current relationship

  22. Plunger-Type Relays

  23. Induction-type Relays • Most frequently used when AC power presents • Change taps to adjust time delay

  24. Induction-Type Relays

  25. 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.

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

  27. 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.

  28. 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.

  29. Motor ProtectionSingle Phase and Phase Unbalance These conditions will cause • Motor winding overheating • Excessive vibrations • Cause motor insulation/winding/bearing damage

  30. Motor ProtectionSingle Phase and Phase Unbalance These conditions will cause • Motor winding overheating • Excessive vibrations • Cause motor insulation/winding/bearing damage

  31. Motor ProtectionSingle Phase and Phase Unbalance

  32. Motor ProtectionOther • Instantaneous Overcurrent • Differential Relays • Undervoltage • Electromagnetic Relays • Ground Fault • Differential Relays

  33. Transformer Protection • Gas and Temperature Monitoring • Differential and Ground Fault Protection

  34. 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.

  35. Transformer ProtectionGround Fault • For a wye connection, ground fault can be detected from the grounded neutral wire.

  36. Transformer ProtectionGround Fault and Differential Relay

  37. Generator Protection • Differential and Ground Fault Protection • Phase Unbalance

  38. Generator ProtectionDifferential and Ground Fault

  39. Generator ProtectionPhase Unbalance Some Definitions.. • Negative Sequence • Voltage example:

  40. 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+

  41. Generator ProtectionPhase Unbalance • Negative Sequence Relay will constantly measure and compare the magnitude and direction of the current.

  42. 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.

  43. Reference: • IEEE Red Book • Ontario Power Generation Training Course (Electrical Equipment) • www.howstuffworks.com