UNIT II APPARATUS PROTECTION

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

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

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.

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.

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

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

Motor Protection Single Phase and Phase Unbalance

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

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

Transformer Protection Ground Fault and Differential Relay

Generator Protection Differential and Ground Fault Protection Phase Unbalance

Generator Protection Differential and Ground Fault

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

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+

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

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