Generator Protection – Various Functions
Generating units are the source of the power system and their security
against any adverse conditions is most important in the system. The generator
protection must ensure a fast and selective detection of any fault in order to
minimize their dangerous effects.
Protection of passive elements like transmission lines and transformers is
relatively simple which involves isolation of faulty element from the system,
whereas protection of generators involves tripping of generator field breaker,
generator breaker and turbine.
Generator Protections are broadly classified into three types.
CLASS – A : This covers all electrical protections for faults within the
generating unit in which generator field breaker, generator
breaker and turbine should be tripped.
CLASS – B: this covers all mechanical protections of the turbine in
which turbine will be tripped first and following this
generator will trip on reverse power / low forward power
protections.
CLASS – C: This covers electrical protection for faults in the system in
which generator will be unloaded by tripping of generator
breaker only. The unit will come to house load operation
and the UAT will be in service. Various protections of this
class are:
i) 220 KV (HV side of Generator Transformer) busbar
protection.
ii) Generator Transformer HV side breaker pole
discrepancy.
iii) Generator negative phase sequence protection
iv) Generator Transformer over current / Earth fault
protection
v) Reverse power protection without turbine trip.
Generator Differential Protection (87 G):
Covering the stator winding for phase to phase faults due to breakdown
of insulation between stator phase windings.
This relay is not sensitive for single line to earth faults as the earth fault
current is limited due to the high neutral earthing resistance.
Generator & Generator Transformer Overall Differential Protection
(87O):
Besides generator differential and generator transformer differential, an
overall differential relay can be provided between generator neutral side
CTs and generator transformer Hv side CTs (and HV side CTs of UAT if
provided) covering both generator and generator transformer.
The principle of operation of above relay is similar to any differential
relay and it is also termed as unit differential relay.
Backup impedance Protection (21G):
This operates for phase faults in the unit, in the HV yard or in the
adjacent transmission lines, with a suitable time delay. It operates as a
backup when the corresponding main protection fails.
In A.P. System the reach is set as 120% of generator transformer with a
time delay of about 1.0 to 1.5 Sec
Voltage restrained over current protection (51 / 27 G):
This will operate when the fault current from the generator terminals
becomes low due to excitation system characteristic with under voltage
criteria.
It operates as a backup protection for system faults with suitable time
delay.
Generator overloads protection (51G):
It is used as an additional check of the stator winding temperature high
protection. The relay can be connected
For alarm with a setting of 110%.
For trip with a setting of 125% with due time delay
Generator Stator Earth Fault Protection (64G):
The high neutral earthing resistance arrangement limits the generator earth
fault current, minimizing the damage to core laminations. Although a single
phase earth fault is not critical, it requires clearance within a short time due
to:
i) It may develop into a phase to phase fault
ii) If a second earth fault occurs the current is not longer limited by
the earthing resistor.
iii) Fire may result from earth fault arc.
Fundamental Frequency 50HZ – 0-95% EARTH FAULT
Third harmonic Frequency (3 X NORMAL F) 150HZ – 95% -
100% EARTH FAULT
a) 95% stator earth fault protection (64G1):
It is an over voltage relay monitoring the voltage developed across the
secondary of the neutral grounding transformer in case of ground faults.
It covers generator, LV winding of generator transformer and HV winding of
UAT.
A pickup voltage setting of 5% is adopted with a time delay setting of about
1.0 Sec. For all machines of ratings 10 MVA and above this shall be
provided.
b) 100% stator earth fault protection (64G2):
This is a 3rd harmonic U/V relay.
It protects 100% of stator winding.
During the machine running condition there will be certain third
harmonic voltage at neutral side of the generator.
This 3rd harmonic voltage will come down when a stator earth fault
occurs causing this relay to operate.
This shall have voltage check or current check unit, to prevent faulty
operation of the relay at generator stand still or during the machine
running down period.
Generator Residual Voltage status.( CHECKED BY WPCL 135 MW
PLANT)
Generator residual voltage at 3000 RPM before excitation for O.C.C
recorded 796 volt at generator terminals and secondary 6.34VOLT
recorded.
Due to unbalance loading third harmonic voltage will increase
100% STATOR EARTH FAULT PROTECTION 3rd HARMONIC (27TN)
M 3425 relay( beck with relay – RS SMITA PLANT) directly harmonic
Voltage when generator is running of the rated speed.
The 3rd harmonic voltage is normally lower when the generator is not
loaded. However the 3rd harmonic voltage shall also be measured
when the generator is fully loaded.
Recommended setting: (Set 50% of minimum value of 3rd Harmonic
voltage measured.)
Pick up = 0.3V
Time delay = 125 cycles (2.5 sec)
