1. UNIT II
APPARATUS PROTECTION

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

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

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

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

6. 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

7. Thermal Overload Relays

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

9. Plunger-Type Relays

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

11. Induction-Type Relays

12. 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.

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

14. 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.

15. 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.

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

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

18. Motor Protection Single Phase and Phase Unbalance

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

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

21. 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.

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

23. Transformer Protection Ground Fault and Differential Relay

24. Generator Protection Differential and Ground Fault Protection
Phase Unbalance

25. Generator Protection Differential and Ground Fault

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

27. 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+

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

29. Conclusion
Protective relays are essential for keeping faults in the system isolated and keep equipment from being damaged.