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Electrical Protections Presentation Transcript
1. Protections
2. Electrical Protections
3. Protections - 2
4.THW-210: Elect. Protection
5.THW 210: Elect. Protection - 2
6. THW-210: Elect. Protection - 3
Back up protection for generator main breaker failure shall be provided.
Operation at a frequency below 47.5 or above 51.5 Hz allowed only for a total of 2 Hrs. in entire life of set. (Turbine)
Reverse power relay to be set at 0.5% of rated output with a time delay of 3 - 10 seconds.
7. THW: Tripping Scheme Protection TT GCB FB A
Back up protection for generator main breaker failure shall be provided.
Operation at a frequency below 47.5 or above 51.5 Hz allowed only for a total of 2 Hrs. in entire life of set. (Turbine)
Reverse power relay to be set at 0.5% of rated output with a time delay of 3 - 10 seconds.
7. THW: Tripping Scheme Protection TT GCB FB A
8.THW: Mechanical Protections
9. Mechanical Protections - 2
10.Liquid :If CW, isolate cooler and unload.
in LLI If seal oil, rectify; / Trip B manual.
If DM water, check H2 pressure - it may be lower than specified.
Humidity: Measure on line humidity in H2 using a Hygrometer with ceramic moisture sensor. Mitchell Instruments Ltd. UK have a similar hygrometer used in Simhadri.
11. TARI TGs: Elect. Protection Settings
in LLI If seal oil, rectify; / Trip B manual.
If DM water, check H2 pressure - it may be lower than specified.
Humidity: Measure on line humidity in H2 using a Hygrometer with ceramic moisture sensor. Mitchell Instruments Ltd. UK have a similar hygrometer used in Simhadri.
11. TARI TGs: Elect. Protection Settings
12.Rotor earth fault: Alarm at IR <80 Kohm De-excite and auto trip if IR < 5 Kohm. Relay time: approximately: 1 sec delay.
Under excitation: Alarm on reaching steady state limit. If excitation is totally lost, instant tripping.
With brushless exciter, recommended to introduce a 2nd stator criterion covering range of permeance values 1/Xd & 1/Xd’ and to provide for instant tripping when this criterion is exceeded.
Under excitation: Alarm on reaching steady state limit. If excitation is totally lost, instant tripping.
With brushless exciter, recommended to introduce a 2nd stator criterion covering range of permeance values 1/Xd & 1/Xd’ and to provide for instant tripping when this criterion is exceeded.
13.TARI TGs: Protection Settings - 3
14.TARI TGs: Protection Settings - 4
Over voltage:Advisable to provide surge diverters for TG also - to be connected between phases & earth. Design value: 1.2 - 1.4 Un : Allows 50 Hz over voltage on load rejections.
50 Hz spark over voltage: Approx. 2 Un. Test voltage for stator wdg being 2Un+1 Impulse spark over < 4 Un.
15.Surge diverters to be explosion proof or other constructional measures be taken to avoid danger to persons or near by components in case of over voltages.
Under frequency with voltage variation.
Over voltage:Advisable to provide surge diverters for TG also - to be connected between phases & earth. Design value: 1.2 - 1.4 Un : Allows 50 Hz over voltage on load rejections.
50 Hz spark over voltage: Approx. 2 Un. Test voltage for stator wdg being 2Un+1 Impulse spark over < 4 Un.
15.Surge diverters to be explosion proof or other constructional measures be taken to avoid danger to persons or near by components in case of over voltages.
Under frequency with voltage variation.
16. Stator Earth Fault
Type of earthing and earth fault current value decide the relay.
For resistance grounded TG, definite time or IDMT ground over current relay is used. For solid earth, an IDMT relay is preferred.
For solidly / resist. grounded TGs, standby E/F relay operated off a CT is used.
Type of earthing and earth fault current value decide the relay.
For resistance grounded TG, definite time or IDMT ground over current relay is used. For solid earth, an IDMT relay is preferred.
For solidly / resist. grounded TGs, standby E/F relay operated off a CT is used.
17.Standby E/F relay backs up the differential or restricted E/F protection when provided, against internal earth faults.
For resistance grounded TG, with low E/F in-feeds, lower ratio neutral CT can be selected, achieving optimum sensitivity.
18. Induction Type IDMT Relay
For resistance grounded TG, with low E/F in-feeds, lower ratio neutral CT can be selected, achieving optimum sensitivity.
18. Induction Type IDMT Relay
19. Over-current Relay Induction Type
20. Reverse Power Relay
When power flows normally, fluxes in windings tend to rotate disc away from trip contact. When it flows in reverse direction, torque is in opposite direction and trip contacts close.
Relay is made sensitive by having a very light control spring.
When power flows normally, fluxes in windings tend to rotate disc away from trip contact. When it flows in reverse direction, torque is in opposite direction and trip contacts close.
Relay is made sensitive by having a very light control spring.
21.Stator Earth Fault - 3
IDMT Type Relay 64S
IDMT Type Relay 64S
22.Stator Earth Fault - 4
Relay 64S For High Impedance Earth
Relay 64S For High Impedance Earth
23.Earth fault relay is normally used along with follower timer for a sensitive setting or 2 stage protection using 2 relays used.
1st stage with sensitive setting ~ 5% with a follower timer and second with coarser setting say 10% with instantaneous trip.
They cover 90-95% of winding. For 100%, sub harmonic voltage injection or 3rd harmonic voltage comparison based relay is used.
1st stage with sensitive setting ~ 5% with a follower timer and second with coarser setting say 10% with instantaneous trip.
They cover 90-95% of winding. For 100%, sub harmonic voltage injection or 3rd harmonic voltage comparison based relay is used.
24.Phase to ground fault, depending upon fault location, increases elect. stresses on unaffected winding. Probability of 2nd ground fault increases.
Serious damage may result, if a fault occurs near to neutral and is then followed by a 2nd fault higher up in same phase.
Serious damage may result, if a fault occurs near to neutral and is then followed by a 2nd fault higher up in same phase.
25.This 2nd fault may result from insulation deterioration caused by transient over- -voltages due to erratic, low current unstable arcing of the 1st fault.
2nd fault may result in high currents.
To cover entire winding with earth fault protection, three methods are used:
1. Measurement of 3rd harmonic at generator neutral voltage.
2nd fault may result in high currents.
To cover entire winding with earth fault protection, three methods are used:
1. Measurement of 3rd harmonic at generator neutral voltage.
26.2. Deliberate displacement of neutral potential with respect to ground, by injecting a voltage, at a sub-multiple of power frequency.
3. Comparison of 3rd harmonic voltages generated at both: neutral and line ends of the winding.
English Electric uses 3rd method.
3rd harmonic voltage builds up across capacitiv impedance of phase to ground
3. Comparison of 3rd harmonic voltages generated at both: neutral and line ends of the winding.
English Electric uses 3rd method.
3rd harmonic voltage builds up across capacitiv impedance of phase to ground
27.Due to ground impedance, neutral shall also have 3rd harmonic voltage to earth.
Actual 3rd harmonic voltage is 1- 3 % of rated voltage at no load. At loads, it can be 0.5 to 2.5 times the no load value. However ‘VN3 / VL3’ remains constant.
When fault occurs at point F, VN3 / VL3 ratio changes. Difference of VN3 & VL3 as a % of V3, is sensed by relay PVMM.
Actual 3rd harmonic voltage is 1- 3 % of rated voltage at no load. At loads, it can be 0.5 to 2.5 times the no load value. However ‘VN3 / VL3’ remains constant.
When fault occurs at point F, VN3 / VL3 ratio changes. Difference of VN3 & VL3 as a % of V3, is sensed by relay PVMM.
28.There is a blind zone when the ratio VN3 / VL3 remains same, in healthy as well as,in fault condition. Relay shall not sense the fault.
This is taken care of by the usual 95% winding coverage relay tuned to 50 Hz.
Relay stability depends upon availability of both neutral and line voltages. Later can disappear if VT fuse blows.
This is taken care of by the usual 95% winding coverage relay tuned to 50 Hz.
Relay stability depends upon availability of both neutral and line voltages. Later can disappear if VT fuse blows.
29.VT fuse health is monitored by a circuit consisting of a reed relay which gets energized if the fuse fails.
30. Stator Ground Faults - 7 SAMPTH & PRATAPKUMAR, ENGLISH ELECTRIC
31.PVMM Relay Inputs
32.Generators have bar type stator winding. Possibility of inter-turn faults is ‘remote’.
Damages to insulation did occur because of metallic objects falling/left on overhang
Subsequent overheating/ burning created carbon smoke that lead to inter-turn fault. This developed into earth fault later.
All THW sets are provided with inter-turn protection. At Parli it is set at 400A,1sec, whereas in Bhusawal at 250 A, 0.25 sec.
Damages to insulation did occur because of metallic objects falling/left on overhang
Subsequent overheating/ burning created carbon smoke that lead to inter-turn fault. This developed into earth fault later.
All THW sets are provided with inter-turn protection. At Parli it is set at 400A,1sec, whereas in Bhusawal at 250 A, 0.25 sec.
33.Generator Differential
It is a unit protection, covering both phase and earth faults within machine. Protection zone is defined by CTs on neutral and line sides of stator winding.
High impedance type protection, off CTs having identical ratio and rating used.
CTs are low reactance type class PS with minimum turns ratio error and of identical magnetizing characteristics.
It is a unit protection, covering both phase and earth faults within machine. Protection zone is defined by CTs on neutral and line sides of stator winding.
High impedance type protection, off CTs having identical ratio and rating used.
CTs are low reactance type class PS with minimum turns ratio error and of identical magnetizing characteristics.
34.The relays are either current or voltage calibrated & tuned to system frequency to ensure stability on through faults in presence of 3rd harmonic currents and transient DC offsets in fault current.
35.Stator Wdg.
36.Generator Differential -
Associated CTs will see current inrush into Generator for an internal fault. This results in high peak voltage across relay and CT secondary pilots.
This value may exceed 3 KV, so it is customary to use nonlinear resistors (Metrosits) across relay branch to limit such voltages within limits.
37. Generator Differential - 8 Biased Differential Relay
Associated CTs will see current inrush into Generator for an internal fault. This results in high peak voltage across relay and CT secondary pilots.
This value may exceed 3 KV, so it is customary to use nonlinear resistors (Metrosits) across relay branch to limit such voltages within limits.
37. Generator Differential - 8 Biased Differential Relay
38.Unbalanced loads in TG cause negative sequence currents to flow. These create synchronous field in reverse direction.
This field produces 2f (100Hz) currents to flow on rotor surface & intense heat.
Negative sequence relay give alarm if I2 increases preset value and trip the m/c if I2²•t exceeds limit. Alarm is set at 80% of I2²•t trip value and instant trip at 100%
This field produces 2f (100Hz) currents to flow on rotor surface & intense heat.
Negative sequence relay give alarm if I2 increases preset value and trip the m/c if I2²•t exceeds limit. Alarm is set at 80% of I2²•t trip value and instant trip at 100%
39.Negative Sequence Relay ‘46’
40. Rotor Earth Fault
First or single rotor earth fault is detected based on DC injection principle. This method requires access to field circuit.
For brushless exciters often instrument slip rings are provided to which relay can be connected.
Relay is time delayed and mostly made to initiate alarm. It can detect fault even during machine standstill condition.
First or single rotor earth fault is detected based on DC injection principle. This method requires access to field circuit.
For brushless exciters often instrument slip rings are provided to which relay can be connected.
Relay is time delayed and mostly made to initiate alarm. It can detect fault even during machine standstill condition.
41.If first earth fault appears, it is essential to protect rotor from second earth fault damage which is severe.
This is brought in service in steps using 4 position selector switch. Relay is based on disturbed bridge balance which appears on first fault. By adjusting on a potentiometer, bridge is balanced. 2nd fault shall flow current thro’ 64R2 to trip.
This is brought in service in steps using 4 position selector switch. Relay is based on disturbed bridge balance which appears on first fault. By adjusting on a potentiometer, bridge is balanced. 2nd fault shall flow current thro’ 64R2 to trip.
42. Field Failure Protection SH Y K PANDHARIPANDE, NASIK JULY 1999
43.Under Current Relay
44.Mho Type Relay
Offset MHO relay monitors impedance at TG terminals. Without field, machine draws reactive current from system, the terminal impedance shifts from1st to 4th quadrant on the R - X plane and settles within the relay characteristic.
Field failure protections are used with 1.5- 2 sec delay to ensure transient free operation.
Offset MHO relay monitors impedance at TG terminals. Without field, machine draws reactive current from system, the terminal impedance shifts from1st to 4th quadrant on the R - X plane and settles within the relay characteristic.
Field failure protections are used with 1.5- 2 sec delay to ensure transient free operation.
45.For large TGs, it is supplemented by under voltage relay, which overrides time delay.
This ensures stability when field failure occurs say at full load & accompanied by drop in stator voltage. System is not strong enough to support VAR needs of the generator.
This ensures stability when field failure occurs say at full load & accompanied by drop in stator voltage. System is not strong enough to support VAR needs of the generator.
46.Back Up Protection
It is provided for tripping, in case system faults are not cleared. For close up faults, AVR may not be able to boost voltage and hence low fault current levels obtained. Voltage controlled over-current relays are used.
The relay is designed to become more sensitive with voltage reduction and operates positively, even if fault current is less than rated.
It is provided for tripping, in case system faults are not cleared. For close up faults, AVR may not be able to boost voltage and hence low fault current levels obtained. Voltage controlled over-current relays are used.
The relay is designed to become more sensitive with voltage reduction and operates positively, even if fault current is less than rated.
47.Voltage controlled over-current relays are applied for directly connected m/cs. Relay characteristic shifts from over- current to fault when input voltage falls below preset level.
The relay is time
coordinated with
the down stream
back up protections.
The relay is time
coordinated with
the down stream
back up protections.
48.For unit connected generators, single step offset MHO relay is used for back up impedance protection along with a follower timer.
Relay is set to cover longest emanating line from station bus bars. Effect of in-feeds from parallel generator is also taken into account while setting the relay to the extent permitted by load.
Relay is set to cover longest emanating line from station bus bars. Effect of in-feeds from parallel generator is also taken into account while setting the relay to the extent permitted by load.
49.Since relay is connected at TG voltage level but measures line impedance(thro’ GT impedance),input voltage from VT is phase corrected by providing ? Y VT to compensate phase shift due to ? Y GT
Back up imped. relay caters for ph to ph or 3 ph faults on the line. E/F back up is provided by standby E/F relay 51N operated off neutral CT on GT side.
Back up imped. relay caters for ph to ph or 3 ph faults on the line. E/F back up is provided by standby E/F relay 51N operated off neutral CT on GT side.
50. Anti Motoring Protection
For steam turbine sets, motoring power is about 0.5 - 6% of rated. Lower for condensing & higher for back pressure. Gas turbines may draw 10 - 15% rated.
Reverse power relay with time delay is used. While sensitive relay with about 0.5% power setting is required for STG, coarser setting of about 3% for gas turbine/engine driven sets is used.
For steam turbine sets, motoring power is about 0.5 - 6% of rated. Lower for condensing & higher for back pressure. Gas turbines may draw 10 - 15% rated.
Reverse power relay with time delay is used. While sensitive relay with about 0.5% power setting is required for STG, coarser setting of about 3% for gas turbine/engine driven sets is used.
51.Over Voltage Protection
Over voltages may occur due to sudden load throw off and consequent turbine over-speeding. Although AVR controls voltages and speed governors control speed, back up may be required.
Usually definite time over voltage relay is used. The relay should have high drop off /pick up ratio and preferably be of continuously adjustable setting.
Over voltages may occur due to sudden load throw off and consequent turbine over-speeding. Although AVR controls voltages and speed governors control speed, back up may be required.
Usually definite time over voltage relay is used. The relay should have high drop off /pick up ratio and preferably be of continuously adjustable setting.
52. Frequency Protection
Multi stage under frequency schemes are applied. Cumulative timers along with under frequency relays are used to initiate alarms, to isolate machine for a shut down if cumulative operation exceeds limits.Over frequency relays are used as a back up to mechanical over-speed protection.
Under / over frequency relays are time delayed to prevent transient operation.
Multi stage under frequency schemes are applied. Cumulative timers along with under frequency relays are used to initiate alarms, to isolate machine for a shut down if cumulative operation exceeds limits.Over frequency relays are used as a back up to mechanical over-speed protection.
Under / over frequency relays are time delayed to prevent transient operation.
53.Over Fluxing Protection
54. Out Of Step - Pole Slipping
Prolonged fault clearing time, low system voltage, weak field condition or some line switching operation may cause pole to slip
Rotor oscillations cause variations in V, I, PF & torque reversals. Loss of excitation protection can not be relied upon under all system conditions. Separate out of step protection is provided.
Impedance as measured at TG terminals changes most during pole slipping.
Prolonged fault clearing time, low system voltage, weak field condition or some line switching operation may cause pole to slip
Rotor oscillations cause variations in V, I, PF & torque reversals. Loss of excitation protection can not be relied upon under all system conditions. Separate out of step protection is provided.
Impedance as measured at TG terminals changes most during pole slipping.
55 Pole Slipping - 2
56.Pole Slipping
57. Dead Machine Protection
TG is protected at standstill or on barring gear, from accidental energisation.
A high speed protection involving current detection in all three phases trips EHV breaker. Supplemented with under voltage relays, protection is coordinated to prevent mal-operation for close in faults.
TG is protected at standstill or on barring gear, from accidental energisation.
A high speed protection involving current detection in all three phases trips EHV breaker. Supplemented with under voltage relays, protection is coordinated to prevent mal-operation for close in faults.
58.Numerical Protection
Microprocessor based protection is user friendly with configurable software-based tripping matrix. It has also the following:
Continuous self monitoring; facility to communicate with station control & with remote load dispatch control, reducing cabling.
Events recording;flexible relay settings & reduction in CT / VT burden are possible
Microprocessor based protection is user friendly with configurable software-based tripping matrix. It has also the following:
Continuous self monitoring; facility to communicate with station control & with remote load dispatch control, reducing cabling.
Events recording;flexible relay settings & reduction in CT / VT burden are possible
59.It has a library of protection functions that make it easy to apply, replacing large no. of discrete relays and reduction in panels
Protection functions in numerical relays are defined by software,resulting in better algorithms for individual functions and capability to adapt to changed operating conditions.
Comprehensive multi-function MP-based generator protection relay is developed.
Protection functions in numerical relays are defined by software,resulting in better algorithms for individual functions and capability to adapt to changed operating conditions.
Comprehensive multi-function MP-based generator protection relay is developed.
60.Tripping Modes
61.Class B is applied where elec. isolation of TG can be delayed. ST trips immediately. Tripping of TG,UAT, field circuit breaker is interlocked with low forward power relay. This avoids over-speeding of TG.Mode ‘Class D’ is provided for GTG which involves tripping of GCB & excitation only. GT is not tripped. It can spin at no load w/o overspeeding. Cl A trip affects GT life.
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