Alternatorprotection Rev 01
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ALTERNATOR PROTECTION
Large MW Generator Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Problems that demand Protection • • • • • • • • • • • • • • • •
Stator electrical faults Overload Overvoltage Unbalanced loading Over fluxing Inadvertent energisation Rotor electrical faults Loss of excitation Loss of synchronism Failure of prime mover Lubrication oil failure Over speeding Rotor distortion Difference in expansion between rotating and stationary parts Excessive vibration Core lamination faults
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Electrical Fault Protection • Stator Earth Fault – Using Overcurrent relay – Using Overvoltage relay – Using Third Harmonic voltage – By low frequency voltage injection • Phase-Phase Fault – Differential Protection • Inter Turn Fault – Overcurrent protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Earth Fault Protection using Overcurrent Relay
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Earth Fault Protection using Overvoltage Relay
Vph
95%
Protection is achieved against the faults occurred from 0% to 95% of the winding WLSA-IND/GKT/2006
0%
ALTERNATOR PROTECTION
Restricted Earth Fault Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Phase to Phase fault protection using Differential protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Generator Differential Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Inter-turn fault protection • Effect: Unnecessary heating & Unbalanced Flux distribution
• If Generator is with Parallel wdg, - Opposed connected Diff. Relay can be used
• If Generator is without access to parallel winding.– Third Harmonic Measurement can be used
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Inter-turn Fault Protection-Using Inst over current relays
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Inter-turn Fault Protection-Using 3rd Harmonic measurement
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Loss of excitation • Cause : – Short or Open-circuit of the exciter. – A failure of automatic voltage regulator – An operator error under manual control – Opening the excitation switch by mistake
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Loss of excitation • Effects – Falling out of synchronism – Runs as Induction Generator drawing Reactive power from grid causing deficiency in grid reactive power. – Local hot spots in stator and rotor – Due to super synchronous speed >105% of Synchronous speed, rotor gets damaged. – Damage to rotor insulation WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Behavior of Generator impedance during Loss of excitation Swing Curves and Loss of Synchronism locus
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Protection against Loss of excitation Loss of Excitation Protection using impedance relays
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Rotor Earth Fault Protection • To avoid unbalanced flux distribution and excessive vibration. • Current Balance relay can be used to detect.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Rotor Earth Fault Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Rotor Earth Fault Protection- Operating principle An ac voltage stepped down to 48 Volts from 100 V or 230 V is set up via coupling capacitors to the rotor circuit towards earth This ac voltage forms a small charging current to flow via the coupling capacitor, resistances of the brushes and earth capacitance of field circuit, depending upon the value of earth capacitanceCE the value of this current varies. The value of this current depends upon the value of earth capacitance,CE which is a few mA during normal no fault operating condition If an E/F arises in the field circuit this current increases and this current is amplified up to 10 times by the CT T2 A sensitive definite time O/C relay is used to measure this current , and an alarm/trip is generated from the relay The relay selected should be insensitive to harmonics since there will be considerable harmonics especially with thyristor excitation and rotating rectifier system WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Over Voltage Protection • Cause: Power frequency switching Over voltages & High frequency Lightning Strokes. • Effect: Stator Insulation failure. Time delayed over voltage protection relay can be used
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Negative Phase Sequence Current
• Cause: – Unbalance loading of the three phases of a generator results in NPS current generation. (Negative Sequence Current is given by: I2 = Ia + a*Ib + a2 *Ic a = 1 120o a2 =1 240o It is clear that for any Unbalance loading I2 will be present.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Negative Phase Sequence Current • Effects: – Associated negative-sequence component in the stator current induces currents at double the power system frequency in the rotor – Temperature rise mainly in the rotor retaining rings
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Negative Phase Sequence Protection • Typical negative phase sequence current withstand of cylindrical rotor generators:
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Negative Phase Sequence Protection • Modern numerical relays derive the negative sequence current level by calculation. • For protection a true thermal replica approach is followed. With trip time t
WLSA-IND/GKT/2006
Trip time t for a typical relay element characteristic:
ALTERNATOR PROTECTION
Reverse Power Protection • This is also known as Protection against motoring of Generator • Cause: – Supply of energy (steam, water or gas) to the primover fails. • Effects: – In case of steam turbine: overheating of turbine blades. – In case of Diesel generator turbine: Explosion may occur. WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Reverse Power Protection • Generator Reverse Power problems:
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Reverse Power Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Pole Slipping Protection • Requirements – To avoid cascade tripping. • Concept – Tracking of system impedance. • Problems – Tracking the transient nature of System Impedance. • Solution – use Blinder Relay.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Differential Protection for Generator transformer
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Overall Differential Protection for TG,GT & Unit transformer
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Overfluxing in Generator transformer • Cause: – Increase in the voltage – Decrease in the frequency • Basic governing equation for both the cases: V Φ*f
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Overfluxing in Generator transformer • Effects: – Saturation of Core – Heating of iron core – Flux leaks into other parts of Transformer, producing eddy currents, which in-turn causes more heat in windings and structural parts
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Protection against Overfluxing in Generator transformer • Measure of degree of overfluxing of the transformer is the Ratio of Saturation flux BS to rated flux BN
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Types of Protection • Class A Protection – Operation Sequence – Gen Breaker Trips – Gen Field Breaker Trips – Incomers of UAT’s are tripped – Tie Breaker between Aux. Station Bus & Aux. Unit Bus are closed. – Boiler Trips – Prime Mover Trips – “Class A” Annunciation will appear. WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Types of Protection • Class B Protection - Operating Sequence – Boiler Trips – Turbine Trips – Class A protection will operate through low forward power relay. – “Class B Trip” Annunciation will appear.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Types of Protection •
Class C Protection – Trips Generator Breaker only.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Class A Protection • • • • • • •
Differential Protection Stator Earth Fault Protection Interturn Fault Protection Rotor Second Earth Fault Protection Over Voltage Protection Reverse Power Protection Pole Slipping Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Class B Protection • Negative Phase Sequence Protection • Field Failure Protection • Back Up Impedance Protection • Under Frequency Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Class C Protection • Generator Transformer Back up Over Current • Generator Transformer Back up E/F Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Effect of Neutral Grounding through transformer ► With a step down transformer of turns ratio n:1 • The resistance size will be reduced by a factor of (1/n2) • Current Transformer size will be reduced by a factor of (1/n) • Electrical isolation is provided for the protection equipment. WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
NGR Calculations • Earth fault current is limited to three different values: – Rated current – 200A-400A (low impedance earthing) – 10A-20A (high impedance earthing) • For limiting Earth fault current to Ifr the NGR required is given by: Vph
NGR =
WLSA-IND/GKT/2006
Ifr
ALTERNATOR PROTECTION
NGR Calculations • When it is earthed through Transformer of turns ratio = n:1 Then, Resistance required is
NGR’
WLSA-IND/GKT/2006
=
Vph n2 x Ifr
ALTERNATOR PROTECTION
Thank You
WLSA-IND/GKT/2006
Large MW Generator Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Problems that demand Protection • • • • • • • • • • • • • • • •
Stator electrical faults Overload Overvoltage Unbalanced loading Over fluxing Inadvertent energisation Rotor electrical faults Loss of excitation Loss of synchronism Failure of prime mover Lubrication oil failure Over speeding Rotor distortion Difference in expansion between rotating and stationary parts Excessive vibration Core lamination faults
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Electrical Fault Protection • Stator Earth Fault – Using Overcurrent relay – Using Overvoltage relay – Using Third Harmonic voltage – By low frequency voltage injection • Phase-Phase Fault – Differential Protection • Inter Turn Fault – Overcurrent protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Earth Fault Protection using Overcurrent Relay
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Earth Fault Protection using Overvoltage Relay
Vph
95%
Protection is achieved against the faults occurred from 0% to 95% of the winding WLSA-IND/GKT/2006
0%
ALTERNATOR PROTECTION
Restricted Earth Fault Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Phase to Phase fault protection using Differential protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Generator Differential Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Inter-turn fault protection • Effect: Unnecessary heating & Unbalanced Flux distribution
• If Generator is with Parallel wdg, - Opposed connected Diff. Relay can be used
• If Generator is without access to parallel winding.– Third Harmonic Measurement can be used
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Inter-turn Fault Protection-Using Inst over current relays
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Inter-turn Fault Protection-Using 3rd Harmonic measurement
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Loss of excitation • Cause : – Short or Open-circuit of the exciter. – A failure of automatic voltage regulator – An operator error under manual control – Opening the excitation switch by mistake
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Loss of excitation • Effects – Falling out of synchronism – Runs as Induction Generator drawing Reactive power from grid causing deficiency in grid reactive power. – Local hot spots in stator and rotor – Due to super synchronous speed >105% of Synchronous speed, rotor gets damaged. – Damage to rotor insulation WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Behavior of Generator impedance during Loss of excitation Swing Curves and Loss of Synchronism locus
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Protection against Loss of excitation Loss of Excitation Protection using impedance relays
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Rotor Earth Fault Protection • To avoid unbalanced flux distribution and excessive vibration. • Current Balance relay can be used to detect.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Rotor Earth Fault Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Rotor Earth Fault Protection- Operating principle An ac voltage stepped down to 48 Volts from 100 V or 230 V is set up via coupling capacitors to the rotor circuit towards earth This ac voltage forms a small charging current to flow via the coupling capacitor, resistances of the brushes and earth capacitance of field circuit, depending upon the value of earth capacitanceCE the value of this current varies. The value of this current depends upon the value of earth capacitance,CE which is a few mA during normal no fault operating condition If an E/F arises in the field circuit this current increases and this current is amplified up to 10 times by the CT T2 A sensitive definite time O/C relay is used to measure this current , and an alarm/trip is generated from the relay The relay selected should be insensitive to harmonics since there will be considerable harmonics especially with thyristor excitation and rotating rectifier system WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Over Voltage Protection • Cause: Power frequency switching Over voltages & High frequency Lightning Strokes. • Effect: Stator Insulation failure. Time delayed over voltage protection relay can be used
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Negative Phase Sequence Current
• Cause: – Unbalance loading of the three phases of a generator results in NPS current generation. (Negative Sequence Current is given by: I2 = Ia + a*Ib + a2 *Ic a = 1 120o a2 =1 240o It is clear that for any Unbalance loading I2 will be present.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Negative Phase Sequence Current • Effects: – Associated negative-sequence component in the stator current induces currents at double the power system frequency in the rotor – Temperature rise mainly in the rotor retaining rings
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Negative Phase Sequence Protection • Typical negative phase sequence current withstand of cylindrical rotor generators:
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Negative Phase Sequence Protection • Modern numerical relays derive the negative sequence current level by calculation. • For protection a true thermal replica approach is followed. With trip time t
WLSA-IND/GKT/2006
Trip time t for a typical relay element characteristic:
ALTERNATOR PROTECTION
Reverse Power Protection • This is also known as Protection against motoring of Generator • Cause: – Supply of energy (steam, water or gas) to the primover fails. • Effects: – In case of steam turbine: overheating of turbine blades. – In case of Diesel generator turbine: Explosion may occur. WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Reverse Power Protection • Generator Reverse Power problems:
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Reverse Power Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Pole Slipping Protection • Requirements – To avoid cascade tripping. • Concept – Tracking of system impedance. • Problems – Tracking the transient nature of System Impedance. • Solution – use Blinder Relay.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Differential Protection for Generator transformer
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Overall Differential Protection for TG,GT & Unit transformer
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Overfluxing in Generator transformer • Cause: – Increase in the voltage – Decrease in the frequency • Basic governing equation for both the cases: V Φ*f
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Overfluxing in Generator transformer • Effects: – Saturation of Core – Heating of iron core – Flux leaks into other parts of Transformer, producing eddy currents, which in-turn causes more heat in windings and structural parts
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Protection against Overfluxing in Generator transformer • Measure of degree of overfluxing of the transformer is the Ratio of Saturation flux BS to rated flux BN
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Types of Protection • Class A Protection – Operation Sequence – Gen Breaker Trips – Gen Field Breaker Trips – Incomers of UAT’s are tripped – Tie Breaker between Aux. Station Bus & Aux. Unit Bus are closed. – Boiler Trips – Prime Mover Trips – “Class A” Annunciation will appear. WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Types of Protection • Class B Protection - Operating Sequence – Boiler Trips – Turbine Trips – Class A protection will operate through low forward power relay. – “Class B Trip” Annunciation will appear.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Types of Protection •
Class C Protection – Trips Generator Breaker only.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Class A Protection • • • • • • •
Differential Protection Stator Earth Fault Protection Interturn Fault Protection Rotor Second Earth Fault Protection Over Voltage Protection Reverse Power Protection Pole Slipping Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Class B Protection • Negative Phase Sequence Protection • Field Failure Protection • Back Up Impedance Protection • Under Frequency Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Class C Protection • Generator Transformer Back up Over Current • Generator Transformer Back up E/F Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Effect of Neutral Grounding through transformer ► With a step down transformer of turns ratio n:1 • The resistance size will be reduced by a factor of (1/n2) • Current Transformer size will be reduced by a factor of (1/n) • Electrical isolation is provided for the protection equipment. WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
NGR Calculations • Earth fault current is limited to three different values: – Rated current – 200A-400A (low impedance earthing) – 10A-20A (high impedance earthing) • For limiting Earth fault current to Ifr the NGR required is given by: Vph
NGR =
WLSA-IND/GKT/2006
Ifr
ALTERNATOR PROTECTION
NGR Calculations • When it is earthed through Transformer of turns ratio = n:1 Then, Resistance required is
NGR’
WLSA-IND/GKT/2006
=
Vph n2 x Ifr
ALTERNATOR PROTECTION
Thank You
WLSA-IND/GKT/2006
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