1. Agenda Introduction of Protective Relays
Electrical System Protection with Protective Relays
Conclusion

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

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

4. Relay Types Electromagnetic Relays (EMRs) Solid-state Relays (SSRs)
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.

5. How a Relay Works

6. Sold-State Relay

7. 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

8. 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.

9. Electrical Faults majority are phase-to-ground faults phase-to-phase
phase-phase-phase
double-phase-to-ground

10. 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.

11. Protective Devices Comparison
Relays
Circuit Breakers
Fuses
Acquisition
Detection
Activation
Actuation

12. 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.

13. 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.

14. Protection and Relay Schemes
Motor Protection
Transformer Protection
Generator Protection

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

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

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

18. 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

19. Thermal Overload Relays

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

21. Plunger-Type Relays

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

23. Induction-Type Relays

24. 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.

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

26. Motor Protection Single 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.

27. Motor Protection Single 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.

28. Motor Protection Single Phase and Phase Unbalance
These conditions will cause
Motor winding overheating
Excessive vibrations
Cause motor insulation/winding/bearing damage

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

30. Motor Protection Single Phase and Phase Unbalance

31. Motor Protection Other
Instantaneous Overcurrent
Differential Relays
Undervoltage
Electromagnetic Relays
Ground Fault

32. Transformer Protection
Gas and Temperature Monitoring
Differential and Ground Fault Protection

33. 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.

34. Transformer Protection Ground Fault
For a wye connection, ground fault can be detected from the grounded neutral wire.

35. Transformer Protection Ground Fault and Differential Relay

36. Generator Protection Differential and Ground Fault Protection
Phase Unbalance

37. Generator Protection Differential and Ground Fault

38. Generator Protection Phase Unbalance
Some Definitions..
Negative Sequence
Voltage example:

39. Generator Protection 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+

40. Generator Protection Phase Unbalance
Negative Sequence Relay will constantly measure and compare the magnitude and direction of the current.

41. 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.