Generator Apnormal Conditions :
- Faults in the Winding .
- Loss of Excitation.
- Motoring of Generator.
- Over speed.
- Unbalanced Operation.
- Out of Synchronization.
- External Faults.
- Faults in the Winding :
(i) Inter phase Fault.
(ii) Inter turn Fault in Stator.
(iii) Stator Earth Fault.
*Rotor Faults :
(i) Rotor Eatth Fault.
(ii) Inter turn Fault in Rotor.
- Inter phase Short Circuit :
It’s a Short Circuit between Parts of Different Phases of the Winding.
It can be Protected Using Differential Protection, Which has High Speed of Operation.
- Inter turn Faults:
It’s a Short Circuit between Turns of The Same Phase, it can be Protected Using Residual Voltage Method.
A Voltage Transformer is connected between each phase Terminal and The Neutral, The Secondary Terminals are connected in Open Delta to a Voltage Relay.
During The Normal Conditions :
Vres = V12 + V23 + V31 .= Zero.
At Short Circuit between Turns, Vres doesn’t equal Zero, then Relay operates.
The Relay shouldn’t operate for Earth Fault, The Earth Fault causes Voltages of Third Harmonics so, a Low Pass Filter LPF is used to prevent The 3rd Harmonic Current.
- Stator Earth Fault :
The Protection against Earth Fault is affected by The Grounding Method of the Generator, so a Separate Additional Earth Fault Protection is Necessary in Addition to The Differential Protection, Because, The Differential Protection provides Earth Fault Protection to about 85% of the Generator Winding.
Where: The % of Winding Unprotected = (Io*Rg / Vph).
Io = The Relay Operating Current in The Primary Side of the Current Transformer.
Rg = The Grounding Resistance.
Vph = The Line to Neutral Voltage.
(vi) Rotor Earth Fault :
The Field System isn’t connected to Earth, So That a Single Earth Fault doesn’t give Rise to any Fault Current, But a Second Earth Fault will cause short Circuit current and produce Force on the Rotor, Bearing, and Shaft, It can be protected by The Following Method:
A High Resistance is Connected across the Rotor Circuit, The Center Point of This Resistance is connected to Earth Through a Sensitive Relay, Which detects The Earth Fault for most of the Rotor Circuit except the Center Point of The Rotor.
AC Voltage Source is connected between the Field Circuit and Earth Through an Over Voltage Relay, a Single Earth Fault in The Rotor Circuit completes The Circuit and Then The Fault is sensed by The Voltage Relay.
- Loss of Excitation :
When a Synchronous Generator loses its Excitation (Field), and If it’s connected in Parallel with Other Units, it can draw The Magnetization Curve from The Bus Bars and operate as an Induction Generator.
It can be protected Using:
*Under Current Relay connected in Field Circuit.
*Distance Relay connected to A.C Voltage & Currents of the Generator Terminals.
(3) Motoring of The Generator:
When the Input to The Turbine is stopped, The Generator will operate as a Synchronous Motor, it can be protected by Using Power Reverse Protection.
(4) Over Load Protection:
Over Load Protection is Used to Back up Protection for Bus Bar or Feeder Faults than to protect the Machine Directly.
It can be protected Using Thermal Relay which is used to monitor the Stator Winding Temperature and Signal an Alarm If a Certain Limit is exceeded.
(5) Over Heating Protection:
Over Heating may be due to Loss of Cooling System or Lubricating Oil, it may Damage The Mechanical System and Bearings.
It can be protected Using Thermocouple System to monitor The Temperature an signal an Alarm if it’s Necessary.