GENERATOR PROTECTION SCHEME
CLASS A1 CLASS A2
Gen. rotor earth fault (64F2) Stator O/C during start (50S ABC)
100% stator earth fault (64A) Stator E/F during start (64 C)
GT restricted earth fault (64 GT) Stator backup E/F (64 B)
UT restricted earth fault LV A (64 UT A) GT backup O/C (50/51 GT)
UT restricted earth fault LV B (64 UT B) GT backup E/F (51 N GT)
Gen. differential (87 G) Gen. backup impedance (21G – 1 ABC)
Gen. interturn (87 IT) Gen. field fail with U/V (27/40G)
GT overall differential (87 GT) UT backup O/C (51 UT ABC)
UT differential (87 UT A/B/C) UT backup E/F LV-A (51N LV-A)
Reverse power (37 G) UT backup E/F LV-B (51N LV-B)
GT buchholz, OLTC oil surge, fire (30 A/G/D) LBB protection (50 Z)
UT buchholz, fire (30 A/D)
Excitation O/C stage – 2
Rotor + & - ve over voltage
Excitation 48 V DC fail
More than 3 bridge fails (3/4 logic)
CLASS B CLASS C
Gen. field failure without U/V (40 G) Gen. backup impedance stage – 2(21G – 2)
Gen. negative phase sequence (46 G/GT) Gen. pole slip (78G)
Gen. over frequency (81 – 3) Gen. under frequency (81 – 1 / 2)
GT over fluxing protection (99 GT) GT backup earth fault (51N GT)
GT oil temp / winding temp high (30 C/E)
UT oil temp / winding temp high (30 C/E)
Low forward power (32 B/A)
Turbine process parameter trip (86 BG)
Excitation transformer temp high
Manual channel fails
Excitation transformer O/C stage – 1
Regulation under test
• State class – B process side trip parameters.
Sl Parameter Normal Value Low Value High Value Trip Value
1. Reactor trip + 200 milli sec
2. Reheater steam Pr. High 5.4 kg/cm2 c 5.75 kg/cm2
3. Exhaust hood steams temp. 93°C 149°C
4. Lub. oil Pressure low [0.35 kg/cm2
5. Relay oil pressure low 21 kg/cm2 17.38 kg/cm2 [ 3.5 kg/cm2
6. Trust bearing <P high !9.114 kg/cm2
7. Condenser vacuum low 696.5 mm Hg 660 mm Hg 559 mm Hg
8. Stator water cond. High 5 μ Mho / cm 13.3μ Mho 20 μ Mho
9. Stator water flow low 30 M3 / hr 21 M3 / hr 17 M3 / hr
10. Boiler level high 2/3 trip
• What are the manual trips required from the generator side?
Quantity 1st ann. Action/2nd ann. Action
Bearing babbitt temp. high 75°C 80°C >80°C manual trip
Bearing outlet oil temp. high 60°C 65°C >65°C manual trip
Generator seal oil inlet temp 45°C >45°C manual trip
Presence of liquid in Gen. Manual trip
DM water outlet temp 85°C Unload >85°C Rundown trip
Stator winding temp high 75°C Unload >75°C Rundown trip
Hot gas temp high 75°C Unload >75°C Rundown trip
Stator core temp high 95°C Unload >105°C Rundown trip
Rotor winding temp high 110°C Unload >110°C Rundown trip
Temp of cold hydrogen gas 55°C Unload >55°C Rundown trip
Temp of inlet water to gas coolers 37-48°C Unload >48°C Rundown trip
Temp of inlet water to stator winding 44-48°C Unload >48°C Rundown trip
Generator seal oil outlet temp 65°C >65°C manual trip
Purity of hydrogen in casing <97% <95% <95% manual trip
*Unload – Decreasing load to a lower value manually
*Rundown – Reducing load to no-load condition (manually/automatic)
• Why boiler level high trip has been provided in turbine?
In condition of boiler level high moisture contents in the steam will rise and rise in
moisture content is harmful to turbine.
• What are the characteristics of protection system?
CHARACTERISTICS OF PROTECTIVE SYSTEM
Protective relaying is an important requirement in power generation, transmission and
distribution, which identifies the exact location of the fault and give command for
isolating the faulty portion very close to the fault by sensing variations in electrical
quantities for ensuring safe operation. The protective relay should have the following
characteristics:
a) Reliability
The protective relay should operate positively and isolate the faulty portion of the
power system as and when required.
b) Selectivity
Protection is arranged in zone, which should cover the power system completely,
having no part unprotected. When a fault occurs the protection is required to select
and trip the only the nearest circuit breaker.
c) Stability
This term, applied to protection on distinct from power network, refers to the ability of
the system to remain inert to all load conditions and fault external to the relevant zone.
d) Speed
The function of automatic protection is to isolate fault from the power system in a very
much shorter time than could be isolated manually, even with great deal of
supervision.
e) Sensitivity
Sensitivity is a term frequently used when referring to the minimum operating limit of
a complete protective system. A protective system is said to be sensitive, if the
primary operating current is low.
• What are the working principles of generator main protections?
GENERATOR START UP PROTECTIONS
SUPPLEMENTARY PROTECTION OF GENERATOR
The generator is normally expected to run rated speed before excitation power is applied
by closing the field breaker. However the residual magnetism in the field circuit may
provide small voltage build up even when the machine is run upto its rated speed without
excitation. At this stage fault if any in the generator stator circuit may not be sensed by
the regular protection, as must of the relays are having higher current ranges. Hence
separate protection (Phase & Ground) are provided with low current ranges.
a) PHASE OVER CURRENT PROTECTION
The CT current is stepped down by an internal CT and converted to voltage signal. The
signal is compared with the internal reference. The protection is interlocked with the
auxiliary relay for the generator transformer breaker closed position to ensure that the
protection is inoperable when the machine is synchronized to grid.
b) GROUND FAULT PROTECTION DURING START UP
The generator neutral current as measure in series with the resistance of the secondary of
the earthing transformer is fed to the relay through CT. CT current is converted to a
voltage. This is compared with the internal resistance references. This protection also
interlocked with generator breaker position to ensure that the protection is inoperable
when the machine is connected to grid.
OTHER PROTECTIONS
a) STATOR EARTH FAULT PROTECTION (64A, 64B, 64C)
The conventional unit type generator has the neutral earthed through a resistance loaded
distribution type transformer. For a single ground fault near the neutral end of the
winding, there will be proportionately less voltage available to drive the current through
the ground, resulting in a lower fault current and lower neutral bus displacement voltage.
Low magnitude of fundamental ground current may flow under normal conditions,
possibly due to generator winding imbalance or due to fault on HV side of generator
transformer or on the secondary of generator PT. Under these conditions, the generator
should not be removed from service. To allow for these low magnitude earth fault
current, trip setting of the overvoltage ground relay are set to detect neutral displacement
voltage in excess of 5-10% of the phased neutral voltage.
If an earth fault occurs and undetected because of its location or otherwise, the probability
of second earth fault occurring is much greater. The second earth fault may result from
insulation deterioration caused by transient over voltage due to erratic, low current,
unstable arcing at first fault point. The second point may yield current of larger
magnitude.
A 100% stator earth fault protection is designed to detect earth fault occurring in the
region of the machine windings close to the neutral end. Composite static modular relay
that gives 100% earth fault protection of the machine, whose neutral is directly earthed. It
works on the principle of monitoring the neutral side and the line side of the component
of third harmonic voltage produced by the generator in service.
OPERATING PRINCIPLE
Alternating Current generator in service produces a certain magnitude of third harmonic
voltage in their winding. However no third harmonic voltage appear across the star/delta
connected generator, though there will be a certain magnitude of third harmonic voltage
between each phase and ground of the machine output. This voltage in case of machine
earth through high impedance can cause the flow of third harmonic current between the
ground and the neutral. In fact under normal healthy operating condition the third
harmonic voltage generated in the machine is shared between the phase to ground
capacity impedance at the machine terminal and neutral to ground impedance at the
machine neutral.
The figure-1 shows the third harmonic voltage distribution during normal working
conditions.
V3 = Generated third harmonic voltage.
VL3 = Third harmonic voltage at machine line end.
VN3 = Third harmonic voltage at machine neutral end
V3
VN3 VL3
Fig (1)
Whenever fault occurs at the point (Figure-2) say F on the machine winding, the voltage
distribution VN3 / VL3 undergoes a change from that during the running condition. In the
extreme case of a fault occurring on the machine neutral, the VN3 becomes zero and VL3
=V3. Similarly when the fault occurs on the phase terminal, VN3 become equal to V3.
The change in 3rd harmonic voltage will sense the relay and trip the generator.
N Line
Fault
V3
VN3
Faulty
VL3 Healthy
VN3 VL3 Faulty
Healthy
Fig (2) 3rd harmonic voltage distribution during healthy and faulty condition.
Figure-3 shows the VN3 Vs VL3 plot under healthy condition, it is clear that in order to
remain stable under healthy condition, the relay should restrain within the two lines L1 &
L2. The slopes of two lines are suitably set to ensure stability.
Line 1
Fault on neutral Healthy condition
VL3 Line 2
Fault on phase
VN3
The fault scheme of main generator is having first relay 64A, covers 100% of the stator
winding, the 2nd relay 64B covers 0-90% of stator winding from phase terminals. The 3rd
relay 64C used for the protection of stator earth fault during start-up.
Variation of neutral and line side
3rd harmonic voltage at load
b) GENERATOR UNBALANCE PROTECTION (46)
Negative phase sequence current in the stator of generator due to unbalance load, fault,
induces double frequency eddy currents in the rotor. This current if allowed to persist,
can cause serious over heating. The unbalance protection relay disconnects the machine
before such excess over heat. In order to avoid unnecessary tripping of the machine, the
time characteristics of the relay should match the heating characteristics of the machine.
The neg. phase sequence current creates magnetic flux wave in the air gap, which induces
current in the rotor body iron. These currents with twice rated frequency tend to flow in
the non-magnetic rotor wedges and retaining rings. Heating occurs in these areas due to
watt loss and quickly raises the temp.
DESCRIPTION
Figure-1 shows the block diagram of the unbalance protection relay. The input from the
CT which are connected in the each phase of the generator supply (Fig-2) are fed to a
negative sequence filter (Fig-3) which gives an a.c. output voltage proportional to the
negative sequence current. This voltage is rectified, smoothened and fed to the squaring
unit of the main measuring element, the time delay circuit and the alarm unit.
The output of the squaring circuit is proportional to the square of the input voltage and is
applied directly to the main timing circuit to give the required relation ship between I2t
and relay operating time (t).
The voltage upto, which the timing capacitor charge depends upon the voltage, applied
from the squaring circuit. This means that even when the negative current is less than the
relay setting, the timer circuit will partially charges and reduces the relay operating time
when the current exceeds the setting value.
When the output exceeds the reference voltage it provides one of the input to a 2-input
AND gate. The other input comes from the 0.3-sec timer, which is activated by the timer
starter unit when the relay setting exceeds the relay setting. When the both inputs to the
AND gate are present the relay will operate and trip the generator from fault.
OPERATING PRINCIPLE
The negative sequence filter shown in Figure-2 is connected in delta to eliminate the
effect of zero sequence currents. A fourth auxiliary transformer is provided to get a phase
shift of 180o Ic – A in figure–3. Vector diagram of both positive and negative sequence
current in the filter are shown in figure-4&5. It can be seen that the output produced
when negative sequence current is present, but zero when the current are of positive
sequence.
c) GENERATOR FIELD FAILURE PROTECTION (40)
Loss of field supply to a synchronous generator can be caused by a fault in the excitation
circuit or by incorrect opening of field breaker. On loss of field, the machine operates as
an induction generator excited by the reactive power drawn from the system to which it
connected. This could result in instability of power in the system and overheating the
rotor.
One parameter which changes significantly when the machine is subject to severe loss of
excitation is the impedance measured at the terminals and it move into the negative
reactance area. The relay is set to detect this abnormal operating condition using its
circular impedance characteristics, which lies in the negative reactance area.
OPERATION
Figure-1 shows the fundamental block diagram of the relay vector V and I are voltage
and current input to relay terminal. The input to the relay current circuit is through a CT
(T1), which is tapped on the both the primary, and the secondary windings to give a
course (K3) and medium reach (K2) setting of the relay. The relay characteristic angle is
continuously variable from 45o to 75o lagging by means of a potentiometer (Q). The
forward reach of the relay (Z) is continuously variable by means of potentiometer (K1) in
the voltage-restrained circuit of mixing transformer (T3).
Output vector S2 proportional to the vector V ± I Z of the voltage mixing transformer (T2)
forms the second input signal of the phase angle comparator. The comparator is a 2-input
block average comparator and operates by comparing the signal vector S1 & S2. The
output of the comparator is fed into a squaring amplifier whose output switches ON for a
positive input and OFF for a negative input. The output waveforms of the amplifier are
varying mark/space square wave, mark/space being equal for 90o-phase angle difference
between two inputs. The squared output is averaged by an auxiliary element set to just to
operate for an equal mark/space ratio. The current build up in the inductive auxiliary coil
to reach the operate level only if the ON period are longer than the OFF period. The L/R
ratio of the auxiliary coil and pick up level are accurately set. The output auxiliary relay
then picks up if the phase angle between the signal vector S1 & S2 are 90o or more as
shown in figure-2. Fig-3 shows the typical circuit connection for field failure protection
of generator.
d) GENERATOR POLE SLIPPING PROTECTION (78)
Sudden occurrence in the electrical grid such as rapid load changes, short circuit
interruptions, which destroy the equilibrium of the energy balance are usually followed by
oscillations. If the system stability is retained, the stationary stage will take over. If the
oscillations are not stable, a loss of synchronism of one or more machine will result. If
the angular displacement of the rotor exceeds the stable limit, the rotor will slip a pole
pitch. Pole slip occurs and excitation is maintained the machine will oscillate strongly on
reactive and active power side.
This relay operates on the principle of measuring impedance course on R-X diagram and
operates to trip on pole slipping condition. The scheme consists of two numbers angle
impedance relay and a timer to distinguish between pole slipping and power swing
blocking condition. When gen. Losses synchronism the resulting high current picks and
off freq. Operation can cause winding stresses, pulsating torque and mechanical
resonance that have potential of damaging the Turbine Generator.
X
Blinder Directional
Load Area
Q2 Q1 R
Operate Restrain
B Operate
A
Generator pole slipping protection
e) GENERATOR DIFFERENTIAL PROTECTION (87G)
This is a high-speed differential protection, the relay of high impedance is provided for
this protection. The high impedance principle is used for thorough fault stability even
under current transformer saturation.
This protection has an operating time of 25 millisecond at 5 time’s current setting. A
non- linear resistance is connected across the relay to limit the over voltage during
internal fault.
This protection covers phase to phase and 3-phase faults. It does not cover phase to
ground fault as the ground fault current is limited to a very low value. This protection
energizes Class-A trip.
f) GENERATOR INTER TURN DIFFERENTIAL PROTECTION: (87 GI)
This protection is by means of a differential current relay connected across crossconnected
CT on the two parallel winding of each of the phase of the generator as shown
in figure-2. The relay which is used for t he protection is of high impedance circulating
current type with an operating speed of 25 millisecond at 5 times the current setting. A
non-linear resistance is connected across the relay to limit the over voltage during the
internal fault. This protection energizes Class-A trip.
PRINCIPLE OF OPERATION (DIFFERENTIAL)
Fig-3 shows the simplified diagram of differential current protection of generator
winding, the CT’s of both end of the generator winding will sense the current which is
flowing through the stator winding. During normal balanced condition the current vector
I1 & I2 are equal and opposite so the resultant forces experiences in the coil of the relay R
is zero.
When the fault ‘F’ occurs on the stator winding, the differential current will be sensed by
the CT and these differential current passes through the operating coil of the relay which
gives trip signal to the circuit breaker of the generator.
Ground To load
Fault
I1 I2
I1 I3 I2
I1 + I2 = 0 Normal condition
I1 + I2 = I3 Faulty condition
GENERATOR BACK UP PROTECTIONS
a) UNDER FREQUENCY PROTECTION (81)
The U/F limitations however are less restrictive than the limitations on the turbine. A
turbine blade is designed to have its natural frequencies sufficiently displaced from rated
speed and multiples of N (speed) to avoid a mechanical resonant condition that could
result in excessive mechanical Stresses in blades
This is a three stage under frequency protection, which consists of a time delay unit and 3
timer. The three stages of frequencies are ranging from 47 to 50 Hz. The timer which
gives the cumulative operating time of turbine during under frequency which calls for
turbine inspection/maintenance as per the design formula.
(48.5-F) t < 3.
Where F is the frequency,
t is the timer duration in seconds.
From the above formula, it can be seen that the turbine can be operable at 48.5 Hz
continuously at rated load. The cumulative timer which gives alarm in Data acquisition
system then call for turbine inspection.
OPERATING PRINCIPLE:
The operating principle of the relay is the comparison of the incoming frequency with that
of a pre-set value of time derived from the oscillator of the relay.
The incoming frequency signal is connected to an input circuit, which then drive an
impulse generator to produce pulse at the beginning of each period of the input voltage.
The preset time interval is obtained from an oscillator and counter, adjustment is achieved
using selector switches, which drives the decoder circuit.
A comparator compares the two-time interval and this triggers an adjustable timer, which
then operate the output voltage. An under voltage detector inhibits the relay when the
incoming signal drops below the preset value.
b) OVER FREQUENCY PROTECTION (81)
Generator over frequency protection is provided to limit the over speeding of turbine,
which leads to greater vibration due to resonance. The over speeding and vibration leads
to mechanical damage of turbine bearings and blades. This protection schemes also
similar to under frequency. The preset time of over frequency operation is more than the
preset time of under frequency protection.
c) GENERATOR OVER VOLTAGE ALARM (59)
This protection give time delayed alarm for continuous operation of the generator at more than
permissible voltage of AVR failure or during manual control of excitation.
d) GENERATOR ANTIMOTORING PROTECTION (32)
Motoring results from low prime mover input to generator. While generator is still in line. When this
input is less than no load losses deficiency is supplied by absorbing real power from the system. Since
the field excitation should remain same, The same reactive power would flow as before the motoring and
generator will operate as a synchronous motor driving the turbine. Generator will not be harmed by this
action but turbine can be harmed through over heating. It is detected by low forward power relay.
EXCITATION SYSTEM PROTECTIONS
The generator is provided with static excitation, which obtains the necessary excitation
power from the excitation transformer, which rectifies and feed the AC power through
controlled rectifier circuits.
a) EXCITATION TRANSFORMER OVER CURRENT PROTECTION:
Time delayed over current protection with instantaneous high set unit is provided for the
short circuit protection of the excitation transformer, which trips the field breaker by
energizing class-B trip.
b) ROTOR OVER VOLTAGE PROTECTION:
This protection is envisaged to limit over voltage occurring in the field circuit during
excitation of the field an air gap arrestor with a series resistor is connected across the
field. On overvoltage the gap flasher over and the arrestor connects the resistor directly
across the field.
This over voltage is not due to the field forcing. Field forcing will happen only when PT
actual voltage value comes down due to the PT fuse drop or due to any other reason. At
that time PT voltage is 110 V – drop. That is actual voltage value is less and field forced
to increase the voltage. Field forcing value is twice the actual value after looking the
system healthiness. Means in some earth faults in the grid, the voltages may come down
to 110 kV and PT will sense this voltage as the generator is synchronised with the grid.
This will force the field of the generator to match the generator actual voltage. If the fault
not cleared the generator will trip after some time delay. This is generator field forcing.
But in some grid disturbances or power swing conditions the stator and rotor voltage and
current changes. This will induce some voltage in rotor. This protection is used to protect
machine from this type of over voltage.
c) ROTOR 1ST EARTH FAULT PROTECTION
A single earth fault is not in itself dangerous since it does not cause fault current, but a
second earth fault effectively short circuits parts or all of the field system and the
unbalancing of the magnetic forces causes. That force may be sufficient to spring the
shaft and make it eccentric. If the condition were allowed to persist, however it might
lead to severe mechanical damage.
The method of detecting rotor first earth fault using the principle of negative biasing,
where by an earth fault anywhere in the field circuit can be detected. The dc injection
supply establishes a small bias on the alternator field circuit so that all points are negative
with respect to earth.
The rectified output of the supply provides a biasing potential of approximately 65V.
This is connected with a positive terminal to earth and negative terminal to the positive
terminal of the field circuit through a relay. When the fault occurs, the current flows
through the relay coil which intern operate the circuit breaker. This relay will not operate
on auxiliary supply failed condition, during that time the relay will give annunciation in
main control room.
d) ROTOR 2ND EARTH FAULT (64F)
While the machine is continuous in service with one earth fault, appearance of 2nd earth
fault will severely affect the magnetic balance in the air gap and result in rotor distortion
and severe damage. Hence it is advisable that the machine taken out of service as early as
possible after appearance of 1st earth fault. However, to take care of the situation of 2nd
earth fault appearing immediately after 1st stator earth fault before the machine is taken
out, 2nd rotor earth fault protection is provided. This protection system normally
disconnect the field effect and has top be switched ON when 1st earth fault appears.
The scheme consists of a bridge circuit which to be balanced manually with the 1st rotor
earth fault in the machine. This balance is disturbed when the 2nd earth fault appears and
the bridge null deflector initiate tripping of the circuit.
It can be seen in the below diagram the protection of the field winding on either side of
the first earth fault and the balancing potentiometer forms a dc bridge with 64F2 (Relay)
connected across the pair of opposite modes.
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CLASS A1 CLASS A2
Gen. rotor earth fault (64F2) Stator O/C during start (50S ABC)
100% stator earth fault (64A) Stator E/F during start (64 C)
GT restricted earth fault (64 GT) Stator backup E/F (64 B)
UT restricted earth fault LV A (64 UT A) GT backup O/C (50/51 GT)
UT restricted earth fault LV B (64 UT B) GT backup E/F (51 N GT)
Gen. differential (87 G) Gen. backup impedance (21G – 1 ABC)
Gen. interturn (87 IT) Gen. field fail with U/V (27/40G)
GT overall differential (87 GT) UT backup O/C (51 UT ABC)
UT differential (87 UT A/B/C) UT backup E/F LV-A (51N LV-A)
Reverse power (37 G) UT backup E/F LV-B (51N LV-B)
GT buchholz, OLTC oil surge, fire (30 A/G/D) LBB protection (50 Z)
UT buchholz, fire (30 A/D)
Excitation O/C stage – 2
Rotor + & - ve over voltage
Excitation 48 V DC fail
More than 3 bridge fails (3/4 logic)
CLASS B CLASS C
Gen. field failure without U/V (40 G) Gen. backup impedance stage – 2(21G – 2)
Gen. negative phase sequence (46 G/GT) Gen. pole slip (78G)
Gen. over frequency (81 – 3) Gen. under frequency (81 – 1 / 2)
GT over fluxing protection (99 GT) GT backup earth fault (51N GT)
GT oil temp / winding temp high (30 C/E)
UT oil temp / winding temp high (30 C/E)
Low forward power (32 B/A)
Turbine process parameter trip (86 BG)
Excitation transformer temp high
Manual channel fails
Excitation transformer O/C stage – 1
Regulation under test
• State class – B process side trip parameters.
Sl Parameter Normal Value Low Value High Value Trip Value
1. Reactor trip + 200 milli sec
2. Reheater steam Pr. High 5.4 kg/cm2 c 5.75 kg/cm2
3. Exhaust hood steams temp. 93°C 149°C
4. Lub. oil Pressure low [0.35 kg/cm2
5. Relay oil pressure low 21 kg/cm2 17.38 kg/cm2 [ 3.5 kg/cm2
6. Trust bearing <P high !9.114 kg/cm2
7. Condenser vacuum low 696.5 mm Hg 660 mm Hg 559 mm Hg
8. Stator water cond. High 5 μ Mho / cm 13.3μ Mho 20 μ Mho
9. Stator water flow low 30 M3 / hr 21 M3 / hr 17 M3 / hr
10. Boiler level high 2/3 trip
• What are the manual trips required from the generator side?
Quantity 1st ann. Action/2nd ann. Action
Bearing babbitt temp. high 75°C 80°C >80°C manual trip
Bearing outlet oil temp. high 60°C 65°C >65°C manual trip
Generator seal oil inlet temp 45°C >45°C manual trip
Presence of liquid in Gen. Manual trip
DM water outlet temp 85°C Unload >85°C Rundown trip
Stator winding temp high 75°C Unload >75°C Rundown trip
Hot gas temp high 75°C Unload >75°C Rundown trip
Stator core temp high 95°C Unload >105°C Rundown trip
Rotor winding temp high 110°C Unload >110°C Rundown trip
Temp of cold hydrogen gas 55°C Unload >55°C Rundown trip
Temp of inlet water to gas coolers 37-48°C Unload >48°C Rundown trip
Temp of inlet water to stator winding 44-48°C Unload >48°C Rundown trip
Generator seal oil outlet temp 65°C >65°C manual trip
Purity of hydrogen in casing <97% <95% <95% manual trip
*Unload – Decreasing load to a lower value manually
*Rundown – Reducing load to no-load condition (manually/automatic)
• Why boiler level high trip has been provided in turbine?
In condition of boiler level high moisture contents in the steam will rise and rise in
moisture content is harmful to turbine.
• What are the characteristics of protection system?
CHARACTERISTICS OF PROTECTIVE SYSTEM
Protective relaying is an important requirement in power generation, transmission and
distribution, which identifies the exact location of the fault and give command for
isolating the faulty portion very close to the fault by sensing variations in electrical
quantities for ensuring safe operation. The protective relay should have the following
characteristics:
a) Reliability
The protective relay should operate positively and isolate the faulty portion of the
power system as and when required.
b) Selectivity
Protection is arranged in zone, which should cover the power system completely,
having no part unprotected. When a fault occurs the protection is required to select
and trip the only the nearest circuit breaker.
c) Stability
This term, applied to protection on distinct from power network, refers to the ability of
the system to remain inert to all load conditions and fault external to the relevant zone.
d) Speed
The function of automatic protection is to isolate fault from the power system in a very
much shorter time than could be isolated manually, even with great deal of
supervision.
e) Sensitivity
Sensitivity is a term frequently used when referring to the minimum operating limit of
a complete protective system. A protective system is said to be sensitive, if the
primary operating current is low.
• What are the working principles of generator main protections?
GENERATOR START UP PROTECTIONS
SUPPLEMENTARY PROTECTION OF GENERATOR
The generator is normally expected to run rated speed before excitation power is applied
by closing the field breaker. However the residual magnetism in the field circuit may
provide small voltage build up even when the machine is run upto its rated speed without
excitation. At this stage fault if any in the generator stator circuit may not be sensed by
the regular protection, as must of the relays are having higher current ranges. Hence
separate protection (Phase & Ground) are provided with low current ranges.
a) PHASE OVER CURRENT PROTECTION
The CT current is stepped down by an internal CT and converted to voltage signal. The
signal is compared with the internal reference. The protection is interlocked with the
auxiliary relay for the generator transformer breaker closed position to ensure that the
protection is inoperable when the machine is synchronized to grid.
b) GROUND FAULT PROTECTION DURING START UP
The generator neutral current as measure in series with the resistance of the secondary of
the earthing transformer is fed to the relay through CT. CT current is converted to a
voltage. This is compared with the internal resistance references. This protection also
interlocked with generator breaker position to ensure that the protection is inoperable
when the machine is connected to grid.
OTHER PROTECTIONS
a) STATOR EARTH FAULT PROTECTION (64A, 64B, 64C)
The conventional unit type generator has the neutral earthed through a resistance loaded
distribution type transformer. For a single ground fault near the neutral end of the
winding, there will be proportionately less voltage available to drive the current through
the ground, resulting in a lower fault current and lower neutral bus displacement voltage.
Low magnitude of fundamental ground current may flow under normal conditions,
possibly due to generator winding imbalance or due to fault on HV side of generator
transformer or on the secondary of generator PT. Under these conditions, the generator
should not be removed from service. To allow for these low magnitude earth fault
current, trip setting of the overvoltage ground relay are set to detect neutral displacement
voltage in excess of 5-10% of the phased neutral voltage.
If an earth fault occurs and undetected because of its location or otherwise, the probability
of second earth fault occurring is much greater. The second earth fault may result from
insulation deterioration caused by transient over voltage due to erratic, low current,
unstable arcing at first fault point. The second point may yield current of larger
magnitude.
A 100% stator earth fault protection is designed to detect earth fault occurring in the
region of the machine windings close to the neutral end. Composite static modular relay
that gives 100% earth fault protection of the machine, whose neutral is directly earthed. It
works on the principle of monitoring the neutral side and the line side of the component
of third harmonic voltage produced by the generator in service.
OPERATING PRINCIPLE
Alternating Current generator in service produces a certain magnitude of third harmonic
voltage in their winding. However no third harmonic voltage appear across the star/delta
connected generator, though there will be a certain magnitude of third harmonic voltage
between each phase and ground of the machine output. This voltage in case of machine
earth through high impedance can cause the flow of third harmonic current between the
ground and the neutral. In fact under normal healthy operating condition the third
harmonic voltage generated in the machine is shared between the phase to ground
capacity impedance at the machine terminal and neutral to ground impedance at the
machine neutral.
The figure-1 shows the third harmonic voltage distribution during normal working
conditions.
V3 = Generated third harmonic voltage.
VL3 = Third harmonic voltage at machine line end.
VN3 = Third harmonic voltage at machine neutral end
V3
VN3 VL3
Fig (1)
Whenever fault occurs at the point (Figure-2) say F on the machine winding, the voltage
distribution VN3 / VL3 undergoes a change from that during the running condition. In the
extreme case of a fault occurring on the machine neutral, the VN3 becomes zero and VL3
=V3. Similarly when the fault occurs on the phase terminal, VN3 become equal to V3.
The change in 3rd harmonic voltage will sense the relay and trip the generator.
N Line
Fault
V3
VN3
Faulty
VL3 Healthy
VN3 VL3 Faulty
Healthy
Fig (2) 3rd harmonic voltage distribution during healthy and faulty condition.
Figure-3 shows the VN3 Vs VL3 plot under healthy condition, it is clear that in order to
remain stable under healthy condition, the relay should restrain within the two lines L1 &
L2. The slopes of two lines are suitably set to ensure stability.
Line 1
Fault on neutral Healthy condition
VL3 Line 2
Fault on phase
VN3
The fault scheme of main generator is having first relay 64A, covers 100% of the stator
winding, the 2nd relay 64B covers 0-90% of stator winding from phase terminals. The 3rd
relay 64C used for the protection of stator earth fault during start-up.
Variation of neutral and line side
3rd harmonic voltage at load
b) GENERATOR UNBALANCE PROTECTION (46)
Negative phase sequence current in the stator of generator due to unbalance load, fault,
induces double frequency eddy currents in the rotor. This current if allowed to persist,
can cause serious over heating. The unbalance protection relay disconnects the machine
before such excess over heat. In order to avoid unnecessary tripping of the machine, the
time characteristics of the relay should match the heating characteristics of the machine.
The neg. phase sequence current creates magnetic flux wave in the air gap, which induces
current in the rotor body iron. These currents with twice rated frequency tend to flow in
the non-magnetic rotor wedges and retaining rings. Heating occurs in these areas due to
watt loss and quickly raises the temp.
DESCRIPTION
Figure-1 shows the block diagram of the unbalance protection relay. The input from the
CT which are connected in the each phase of the generator supply (Fig-2) are fed to a
negative sequence filter (Fig-3) which gives an a.c. output voltage proportional to the
negative sequence current. This voltage is rectified, smoothened and fed to the squaring
unit of the main measuring element, the time delay circuit and the alarm unit.
The output of the squaring circuit is proportional to the square of the input voltage and is
applied directly to the main timing circuit to give the required relation ship between I2t
and relay operating time (t).
The voltage upto, which the timing capacitor charge depends upon the voltage, applied
from the squaring circuit. This means that even when the negative current is less than the
relay setting, the timer circuit will partially charges and reduces the relay operating time
when the current exceeds the setting value.
When the output exceeds the reference voltage it provides one of the input to a 2-input
AND gate. The other input comes from the 0.3-sec timer, which is activated by the timer
starter unit when the relay setting exceeds the relay setting. When the both inputs to the
AND gate are present the relay will operate and trip the generator from fault.
OPERATING PRINCIPLE
The negative sequence filter shown in Figure-2 is connected in delta to eliminate the
effect of zero sequence currents. A fourth auxiliary transformer is provided to get a phase
shift of 180o Ic – A in figure–3. Vector diagram of both positive and negative sequence
current in the filter are shown in figure-4&5. It can be seen that the output produced
when negative sequence current is present, but zero when the current are of positive
sequence.
c) GENERATOR FIELD FAILURE PROTECTION (40)
Loss of field supply to a synchronous generator can be caused by a fault in the excitation
circuit or by incorrect opening of field breaker. On loss of field, the machine operates as
an induction generator excited by the reactive power drawn from the system to which it
connected. This could result in instability of power in the system and overheating the
rotor.
One parameter which changes significantly when the machine is subject to severe loss of
excitation is the impedance measured at the terminals and it move into the negative
reactance area. The relay is set to detect this abnormal operating condition using its
circular impedance characteristics, which lies in the negative reactance area.
OPERATION
Figure-1 shows the fundamental block diagram of the relay vector V and I are voltage
and current input to relay terminal. The input to the relay current circuit is through a CT
(T1), which is tapped on the both the primary, and the secondary windings to give a
course (K3) and medium reach (K2) setting of the relay. The relay characteristic angle is
continuously variable from 45o to 75o lagging by means of a potentiometer (Q). The
forward reach of the relay (Z) is continuously variable by means of potentiometer (K1) in
the voltage-restrained circuit of mixing transformer (T3).
Output vector S2 proportional to the vector V ± I Z of the voltage mixing transformer (T2)
forms the second input signal of the phase angle comparator. The comparator is a 2-input
block average comparator and operates by comparing the signal vector S1 & S2. The
output of the comparator is fed into a squaring amplifier whose output switches ON for a
positive input and OFF for a negative input. The output waveforms of the amplifier are
varying mark/space square wave, mark/space being equal for 90o-phase angle difference
between two inputs. The squared output is averaged by an auxiliary element set to just to
operate for an equal mark/space ratio. The current build up in the inductive auxiliary coil
to reach the operate level only if the ON period are longer than the OFF period. The L/R
ratio of the auxiliary coil and pick up level are accurately set. The output auxiliary relay
then picks up if the phase angle between the signal vector S1 & S2 are 90o or more as
shown in figure-2. Fig-3 shows the typical circuit connection for field failure protection
of generator.
d) GENERATOR POLE SLIPPING PROTECTION (78)
Sudden occurrence in the electrical grid such as rapid load changes, short circuit
interruptions, which destroy the equilibrium of the energy balance are usually followed by
oscillations. If the system stability is retained, the stationary stage will take over. If the
oscillations are not stable, a loss of synchronism of one or more machine will result. If
the angular displacement of the rotor exceeds the stable limit, the rotor will slip a pole
pitch. Pole slip occurs and excitation is maintained the machine will oscillate strongly on
reactive and active power side.
This relay operates on the principle of measuring impedance course on R-X diagram and
operates to trip on pole slipping condition. The scheme consists of two numbers angle
impedance relay and a timer to distinguish between pole slipping and power swing
blocking condition. When gen. Losses synchronism the resulting high current picks and
off freq. Operation can cause winding stresses, pulsating torque and mechanical
resonance that have potential of damaging the Turbine Generator.
X
Blinder Directional
Load Area
Q2 Q1 R
Operate Restrain
B Operate
A
Generator pole slipping protection
e) GENERATOR DIFFERENTIAL PROTECTION (87G)
This is a high-speed differential protection, the relay of high impedance is provided for
this protection. The high impedance principle is used for thorough fault stability even
under current transformer saturation.
This protection has an operating time of 25 millisecond at 5 time’s current setting. A
non- linear resistance is connected across the relay to limit the over voltage during
internal fault.
This protection covers phase to phase and 3-phase faults. It does not cover phase to
ground fault as the ground fault current is limited to a very low value. This protection
energizes Class-A trip.
f) GENERATOR INTER TURN DIFFERENTIAL PROTECTION: (87 GI)
This protection is by means of a differential current relay connected across crossconnected
CT on the two parallel winding of each of the phase of the generator as shown
in figure-2. The relay which is used for t he protection is of high impedance circulating
current type with an operating speed of 25 millisecond at 5 times the current setting. A
non-linear resistance is connected across the relay to limit the over voltage during the
internal fault. This protection energizes Class-A trip.
PRINCIPLE OF OPERATION (DIFFERENTIAL)
Fig-3 shows the simplified diagram of differential current protection of generator
winding, the CT’s of both end of the generator winding will sense the current which is
flowing through the stator winding. During normal balanced condition the current vector
I1 & I2 are equal and opposite so the resultant forces experiences in the coil of the relay R
is zero.
When the fault ‘F’ occurs on the stator winding, the differential current will be sensed by
the CT and these differential current passes through the operating coil of the relay which
gives trip signal to the circuit breaker of the generator.
Ground To load
Fault
I1 I2
I1 I3 I2
I1 + I2 = 0 Normal condition
I1 + I2 = I3 Faulty condition
GENERATOR BACK UP PROTECTIONS
a) UNDER FREQUENCY PROTECTION (81)
The U/F limitations however are less restrictive than the limitations on the turbine. A
turbine blade is designed to have its natural frequencies sufficiently displaced from rated
speed and multiples of N (speed) to avoid a mechanical resonant condition that could
result in excessive mechanical Stresses in blades
This is a three stage under frequency protection, which consists of a time delay unit and 3
timer. The three stages of frequencies are ranging from 47 to 50 Hz. The timer which
gives the cumulative operating time of turbine during under frequency which calls for
turbine inspection/maintenance as per the design formula.
(48.5-F) t < 3.
Where F is the frequency,
t is the timer duration in seconds.
From the above formula, it can be seen that the turbine can be operable at 48.5 Hz
continuously at rated load. The cumulative timer which gives alarm in Data acquisition
system then call for turbine inspection.
OPERATING PRINCIPLE:
The operating principle of the relay is the comparison of the incoming frequency with that
of a pre-set value of time derived from the oscillator of the relay.
The incoming frequency signal is connected to an input circuit, which then drive an
impulse generator to produce pulse at the beginning of each period of the input voltage.
The preset time interval is obtained from an oscillator and counter, adjustment is achieved
using selector switches, which drives the decoder circuit.
A comparator compares the two-time interval and this triggers an adjustable timer, which
then operate the output voltage. An under voltage detector inhibits the relay when the
incoming signal drops below the preset value.
b) OVER FREQUENCY PROTECTION (81)
Generator over frequency protection is provided to limit the over speeding of turbine,
which leads to greater vibration due to resonance. The over speeding and vibration leads
to mechanical damage of turbine bearings and blades. This protection schemes also
similar to under frequency. The preset time of over frequency operation is more than the
preset time of under frequency protection.
c) GENERATOR OVER VOLTAGE ALARM (59)
This protection give time delayed alarm for continuous operation of the generator at more than
permissible voltage of AVR failure or during manual control of excitation.
d) GENERATOR ANTIMOTORING PROTECTION (32)
Motoring results from low prime mover input to generator. While generator is still in line. When this
input is less than no load losses deficiency is supplied by absorbing real power from the system. Since
the field excitation should remain same, The same reactive power would flow as before the motoring and
generator will operate as a synchronous motor driving the turbine. Generator will not be harmed by this
action but turbine can be harmed through over heating. It is detected by low forward power relay.
EXCITATION SYSTEM PROTECTIONS
The generator is provided with static excitation, which obtains the necessary excitation
power from the excitation transformer, which rectifies and feed the AC power through
controlled rectifier circuits.
a) EXCITATION TRANSFORMER OVER CURRENT PROTECTION:
Time delayed over current protection with instantaneous high set unit is provided for the
short circuit protection of the excitation transformer, which trips the field breaker by
energizing class-B trip.
b) ROTOR OVER VOLTAGE PROTECTION:
This protection is envisaged to limit over voltage occurring in the field circuit during
excitation of the field an air gap arrestor with a series resistor is connected across the
field. On overvoltage the gap flasher over and the arrestor connects the resistor directly
across the field.
This over voltage is not due to the field forcing. Field forcing will happen only when PT
actual voltage value comes down due to the PT fuse drop or due to any other reason. At
that time PT voltage is 110 V – drop. That is actual voltage value is less and field forced
to increase the voltage. Field forcing value is twice the actual value after looking the
system healthiness. Means in some earth faults in the grid, the voltages may come down
to 110 kV and PT will sense this voltage as the generator is synchronised with the grid.
This will force the field of the generator to match the generator actual voltage. If the fault
not cleared the generator will trip after some time delay. This is generator field forcing.
But in some grid disturbances or power swing conditions the stator and rotor voltage and
current changes. This will induce some voltage in rotor. This protection is used to protect
machine from this type of over voltage.
c) ROTOR 1ST EARTH FAULT PROTECTION
A single earth fault is not in itself dangerous since it does not cause fault current, but a
second earth fault effectively short circuits parts or all of the field system and the
unbalancing of the magnetic forces causes. That force may be sufficient to spring the
shaft and make it eccentric. If the condition were allowed to persist, however it might
lead to severe mechanical damage.
The method of detecting rotor first earth fault using the principle of negative biasing,
where by an earth fault anywhere in the field circuit can be detected. The dc injection
supply establishes a small bias on the alternator field circuit so that all points are negative
with respect to earth.
The rectified output of the supply provides a biasing potential of approximately 65V.
This is connected with a positive terminal to earth and negative terminal to the positive
terminal of the field circuit through a relay. When the fault occurs, the current flows
through the relay coil which intern operate the circuit breaker. This relay will not operate
on auxiliary supply failed condition, during that time the relay will give annunciation in
main control room.
d) ROTOR 2ND EARTH FAULT (64F)
While the machine is continuous in service with one earth fault, appearance of 2nd earth
fault will severely affect the magnetic balance in the air gap and result in rotor distortion
and severe damage. Hence it is advisable that the machine taken out of service as early as
possible after appearance of 1st earth fault. However, to take care of the situation of 2nd
earth fault appearing immediately after 1st stator earth fault before the machine is taken
out, 2nd rotor earth fault protection is provided. This protection system normally
disconnect the field effect and has top be switched ON when 1st earth fault appears.
The scheme consists of a bridge circuit which to be balanced manually with the 1st rotor
earth fault in the machine. This balance is disturbed when the 2nd earth fault appears and
the bridge null deflector initiate tripping of the circuit.
It can be seen in the below diagram the protection of the field winding on either side of
the first earth fault and the balancing potentiometer forms a dc bridge with 64F2 (Relay)
connected across the pair of opposite modes.
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