18-08-2012, 11:07 AM
Generator Protection
08-Generator.ppt (Size: 1.55 MB / Downloads: 108)
Generator Protection
The extent and types of protection specified will depend on the following factors :-
Type of prime mover and generator construction
MW and voltage ratings
Mode of operation
Method of connection to the power system
Method of earthing
Generator Protection Requirements
To detect faults on the generator
To protection generator from the effects of abnormal power system operating conditions
To isolate generator from system faults not cleared remotely
Action required depends upon the nature of the fault.
Usual to segregate protection functions into :
Urgent
Non-urgent
Alarm
Stator Earth Fault Protection
Fault caused by failure of stator winding insulation
Leads to burning of machine core
welding of laminations
Rebuilding of machine core can be a very expensive process
Earth fault protection is therefore a principal feature of any generator protection package
Method of Earthing (1)
Machine stator windings are surrounded by a mass of earthed metal
Most probable result of stator winding insulation failure is a phase-earth fault
Desirable to earth neutral point of generator to prevent dangerous transient over voltages during arcing earth faults
Several methods of earthing are in use
Damage resulting from a stator earth fault will depend upon the earthing arrangement
Method of Earthing (3)
Desirable to limit earth fault current :
limits damage
reduces possibility of developing into phase-phase
fault
Degree to which fault current is limited must take into account :
detection of earth faults as near as possible to the neutral point
ease of discrimination with system earth fault protection (directly
connected machines)
Method of Earthing : Limitation of Earth Fault Current (1)
Less than 5A :
Discrimination not required can limit current to very low value.
20A :
Used on oil and gas platforms.
Limits power supply disturbance, but still enables grading of
up to 3 zones.
Method of Earthing : Limitation of Earth Fault Current (2)
100A :
As for 20A, but higher current allows better discrimination and sensitivity.
Generator Full Load Current (1200A max) :
Most popular. Used for ease of fault detection and discrimination.
Residual connection of CTs can be used, BUT can result in serious core damage.
Stator Earth Fault Protection and Protection Against Earth Faults on Generator Connections
Depending on the Generator arrangement this can be provided by :-
Time delayed overcurrent protection
Time delayed earth fault protection
Sensitive earth fault protection
Neutral displacement voltage relay
Neutral displacement voltage detection by overcurrent
relay
High impedance restricted earth fault protection
High impedance differential protection
Biased differential protection
Directional earth fault protection
100% stator earth fault protection
Stator Earth Fault Protection
Directly Connected Generators :
Earthed Generator : Earth fault relay must be time delayed for
co-ordination with other earth fault protection on the power system.
Unearthed Generators : Other generators connected in parallel
will generally be unearthed.
Protection is restricted to faults on the generator, grading with power
system earth fault protection is not required. A high impedance instantaneous
relay can be used (Balanced Earth Fault protection).
Stator Earth Fault Protection (1)
Generators connected via step-up transformer (resistance earthed) :
Instantaneous protection (50N) :
System earth faults ARE not seen by generator earth fault protection instantaneous relay may be used.
Set to 10% of resistor rating (avoids operation due to transient surges passed through generator transformer inter winding capacitance).
Advantage : Fast
Stator Earth Fault Protection (2)
Time delayed protection (51N) :
Time delay prevents operation on transient surges.
A more sensitive current setting may be used.
Set to 5% of resistor rating.
Advantage : Sensitive
On large machines considered worthwhile to use both instantaneous and time delayed.
Stator Earth Fault Protection
100% Stator Earth Fault Protection :
Standard relays only cover 95% of winding.
Probability of fault occurring in end 5% is low.
On large machines 100% stator earth fault protection may be required.
Two methods :
* Low Frequency Injection
* Third Harmonic Voltage Measurement
100% Stator Earth Fault Protection
For Large Machines Only
Two methods :-
Low frequency injection Third harmonic voltage - various
Low Frequency Injection
Stator Phase-Phase Fault Protection (1)
Phase-phase faults caused by :
Insulation failure
Flashover in terminal box
Majority of phase-phase faults begin as earth faults.
High fault current causes rapid damage
fast protection required.
Stator Phase-Phase Fault Protection (2)
Single Generator
Use time delayed over current.
CTs must be in neutral side to cover winding faults.
Small solidly earthed machines – over current also provides degree of earth fault protection.
Over current is often only protection applied to small machines.
Stator Phase-Phase Fault Protection (3)
Larger Machines, Parallel Operation
Require Differential Protection
Type types :
High impedance - most common
Biased (low impedance) - used for generator - generator transformer sets
Class X CTs required.
Stator Phase-Phase Fault Protection
Previous methods require access to winding neutral end
Small machines :
Star connection made inside machine
Winding neutral ends are not brought out
If high speed protection required, restricted earth fault scheme should be used
Stator Interturn Fault Protection (1)
Longitudinal differential system does not detect interturn faults
Interturn fault protection not commonly provided because :
Fault rare
Even if interturn fault occurs, will develop into earth fault
Possible that serious damage can occur before fault is detected
Stator Interturn Fault Protection (3)
Transverse Differential Protection
(Double Wound Machines) :
Prime Mover Failure (1)
Isolated Generators :
Machine slows down and stops. Other protection initiates shut down.
Parallel Sets :
System supplies power - generator operates as a motor.
Seriousness depends on type of drive.
Steam Turbine Sets :
Steam acts as a coolant.
Loss of steam causes overheating.
Turbulence in trapped steam causes distortion of turbine blades.
Motoring power 0.5% to 6% rated.
Condensing turbines, rate of heating slow. Loss of steam instantly recognised.
Prime Mover Failure (2)
Diesel Driven Sets :
Prime mover failure due to mechanical fault.
Serious mechanical damage if allowed to persist.
Motoring power from 35% rated for stiff machine, to 5% rated for run in machine.
Gas Turbines :
Motoring power 100% rated for single shaft machine, 10% to 15% rated for double shaft.
Hydro Sets :
Mechanical precautions taken if water level drops.
Low head types - erosion and cavitation of runner can occur.
Additional protection may be required.
Prime Mover Failure (3)
Reverse Power Protection :
Reverse power measuring relays used where protection required.
Single phase relay is sufficient as prime mover failure results in balanced conditions.
Sensitive settings required - metering class CTs required for accuracy.