Page 253 - Practical Power System and Protective Relays Commissioning

250 Practical Power System and Protective Relays Commissioning

Subchapter 18.8

Motor Protection

18.8.1 INTRODUCTION

18.8.2 Different Types of Motor Protections
1. Instantaneous earth fault protection
Motors with powers greater than 50 HP, supplied from an earthed
system, should be protected against earth faults to reduce fault damage
and accident risk, especially motors not protected by differential
protection.
The rotor is also protected against earth faults.
2. Differential protections
These are normally fitted in machines of 1000 HP and above.
Transverse differential protection can be used against interturn faults,
where the stator windings are divided into two or more circuits.
3. Overloading and stalling protection
Thermal relays are employed for overloading, also a separate stalling
relay is employed for motor stalling conditions.
4. Instantaneous high-set overcurrent protection
This can be included with thermal overload relays.
5. Unbalance protection
Unbalanced or negative-phase sequence protection should be used as
rotor heating due to unbalanced currents being a function of the negative-
sequence component of line currents.
When a motor is stalled due to loss of one phase, the heating is con-
centrated in one part of the rotor and the instantaneous negative-sequence
unit can give complete protection.
6. Protection against restoration of supply
Synchronous machines must be protected against this condition
because it can go out of step with supply after an interruption. A sensitive
underfrequency relay is used for this condition.
Induction motors are protected against this condition by no-voltage
release on the starter because the motor terminal voltage falls rapidly on
loss of supply.
7. Reverse-phase sequence protection
A reverse-phase sequence and undervoltage relay can be used to
detect this condition.
8. Bearing failure protection
Failure of a bearing can cause motor stalling. A defective bearing is
indicated by temperature rise and vibration and a small rise in motor