1/29/2013
th
School Generation Track Overview Lecture
Generator Design, Connections, and Grounding
1
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Generator Main Components • Stator – Core lamination – Winding
• Rotor – Shaft – Poles – Slip rings
Stator Core
Source: www.alstom.com/power/fossil/gas/
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Stato r (Cor e + Win din g) Winding Connections Core Lamination
Winding (Roebel bars)
Typi cal Types o f Generator Wind in gs Stator Windin g: Random-W ound Coils
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Typi cal Types o f Generator Wind in gs Stator Winding : Form-W ound Coils
Typi cal Types o f Generator Wind in gs Stator Windin g: Roebe l Bars
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Roebel Bars I nside S tator S lot
Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, Maughan Engineering Consultants
Stator W indi ng Combin atio ns Typi cal for T wo- and Four -Pole Machines
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Series C onnection of Roebe l Bars
Source:www.ansaldoenergia.com/Hydro_Gallery.asp
Rotor
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Classifica tion o f Synchronou s Generators
Rotor design
Cylindrical rotor Salient-pole rotor
Cooling: Stator and rotor
Direct
connection to dc source
Indirect Brushless
Rotor Design
Salient-Pole Rotor
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Two -Pole Roun d Rot or
Source: www.alstom.com
Sali ent Pole R ot or
Source:www.ansaldoenergia.com/Hydro_Gallery.asp
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Stator Winding Cool ing Indirectly Cooled
Directly Cooled
Cooling Ducts, Water Cooled Bar
Rotor Winding Cooling Indirectly Cooled
Directly Cooled
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Field Windi ng Connection to DC S ource Bru sh Type
Field Windi ng Connection to DC S ource Brushless
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Generator Station Arr angements Generator-T ransfo rm er Unit
Generating Station Arr angements Directly Connected Ge nerator
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Synchronous Generator Grounding IEEE C62.92.2-1989 • Resonant roundin
Petersen Coil
• Ungrounded neutral • High-resistance grounding • Low-resistance grounding • Low-reactance grounding • Effective grounding
Increasing Ground Fault Current
Why Grou nd the Ne utr al?
• Minimize damage for internal ground faults • Limit mechanical stress for external ground faults
• Allow for ground fault detection • Ability to coordinate generator protection with other equipment requirements
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Ungroun ded Ne utr al
• No intentional connection to round • Maximum ground fault current higher than for resonant grounding • Excessive transient overvoltages may result
High-R esis tance G rou ndi ng
distribution transformer • Resistor value selected to limit transient overvoltages • Maximum single-phase-to-ground fault current: 5–15 A
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Low -Resistance G rou ndi ng
• Limit ground fault current to hundreds of amperes to allow operation of selective (differential) relays • Low temporary/transient overvoltages
Effective Grounding
• A low-impedance ground connection where: X0 / X1 3 and R0 / X1 1 • roun au curren s g • Low temporary overvoltages during phaseto-ground faults
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Generator Capability Curves
Defin ing Genera tor Capabilit y • Curve provided by the generator manufacturer • Defines the generator operating limits during steady s ae con ons • Assumes generator is connected to an infinite bus • Limits are influenced by: – Terminal voltage – Coolant – Generator construction
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Generator Capabilit y Curve for a Round Rotor Generator
Generator Capability Curve for a Sali ent Pole Generator
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Capability Curve Construction
Phasor Diagra m – Round R otor G enera tor Xd P V I cos( ) E 0 sin( ) Xd I cos( )
I
V
V
E0
Xd V C
Xd
0
s n
cos
( BC ) V I cos( )
φ
E0
P
Q V I sin( ) 0
Xd I
V
cos
s n
(( E cos( )) V ) V I sin( ) 0
V A
I
B
Q
Xd V Xd
( AB ) V I sin( )
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Powe r Ang le Cha racteristi c P
Opera tion with Constant Active Powe r and Variable Excit ation C
C’’
C’
Xd I Xd I E 0 I
Xd I
E0
E0
P
V B’’
A
I
Q
Q
B’
B
Q Xd 1.6 V 1.00 I 1 36.87 E 0 2.3433.15
I
I 1.6 60 E 0 3.46621.7 I 1.1345
E 0 1.3178.5
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Powe r Ang le Cha racteristi c E 0 2.3433.15
P
E 0 3.46621.7 E 0 1.3178.5
V-Curves I ( p.u )
cos
cap.
cos
inductive
E 0 (p.u.)
Excitation Current
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Opera tio n wit h Constant Apparent Power and V ariable E xci tatio n
E0 Xd I
A
Xd 1.6
B
I
V 1.00 I 1 36.87
Opera tio n wit h Cons tant E xcit ation and Variable Act iv e Pow er E0 ti m i L tiy il b ta S .r o e h T
Xd I C
E0 I Xd I
V A
B
I
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Capability Curve – Round Rotor it im L tiy li b ta S .r
V (( E 0 cos( )) V ) V I sin( ) Xd E0 0
Q
o e h T
)r e w o P l a e R ( P
Xd V E 0 sin( ) V I cos( ) Xd E0 0
P 0
Xd 1.6
max. Q (Reactive Power)
Q
- VV Xd
V 1.0
0.625
Generator Fault Prot ection
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Generator Fault Prot ection • Stator phase faults • Stator ground faults • Field ground faults • External faults (backup protection)
Stato r Phase Faul t Prot ectio n • Phase fault protection – Percentage differential – High-impedance differential – Self-balancing differential
• Turn-to-turn fault protection – Split-phase differential – Split-phase self-balancing
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Phase Fault Protection Perc entage Diff erenti al
Dual-Slope Characteristic
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Phase Fault Protection High-Impeda nce Differential
O
O
O
Phase Fault Protection Self-Balancin g Diff erenti al
http://www.polycastinternational.com/old_cat/pdfs/Section4/Section4-Part2.pdf
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Stator Winding Coils with Mult iple Turn s
Tur n-to-Tur n Faul t Prot ectio n Split -Phase Self-Balancin g
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Tur n-to-Tur n Faul t Prot ectio n Split-Phase Percentage Differential
Stator Groun d Fault Prot ection • High-impedance-grounded generators – Neutral fundamental-frequency overvoltage – Third-harmonic undervoltage or differential – Low-frequency injection
• Low-impedance-grounded generators – Ground overcurrent – Ground directional overcurrent – Restricted earth fault (REF) protection
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Grou nd Fault in a Unit -Conn ecte d Generator
High-Impedance G ro un ded Ge nerator Neutr al Fundamental O vervol tage
Fault Location/ % of Winding
Volt age V
F1 / 3%
3% • Vnom 3 Vnom 85%• 3
F2 / 85%
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Genera tor – Flux Distributio n in Air Ga p
Total Flux Fundamental Harmonics Generator – Flux Distribution in Air Gap
High-Impedance G ro un ded Ge nerator Neutral Third-Harmonic Undervoltage GSU
F1
(3) V
R
59GN
Full Load No Load
VN3
OR(2)
27TN
No Load VN3
VP3
No Fault
VP3
Fault at F1
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High-Impedance G ro un ded Ge nerator Third-Harmonic Differential GSU
(3)
V
R
(3)
59GN
k •VP 3 VN 3
+ –
Pickup Setting Third-Harmonic Differential Element
Generator Windi ng An alys is • Generator data – – 216 slots
• Winding pitch – Full pitch = 216/18 = 12 slots – Actual pitch = 128 – 120 = 8 slots – Actual pitch / full pitch = 8/12 = 2/3
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Full -Pitc h Winding
2/3 Pitch Win din g Remov es Thir d Harmo nic
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High-Impedance G ro un ded Ge nerator Low-Frequency Injection GSU
(3)
OR(2)
I R
59GN
V
64S
Coupling Filter
Low-Frequency Voltage Injector
Protection Measurements
100% Stato r Grou nd Fault Prot ectio n Elements Coverage
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Low-Impedance-Grounded Generator Ground Overcurrent and Directional Overcurrent
Low-Impedance-Grounded Generator Ground Differential
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Low-Impedance-Grounded Generator Self-Balancin g Ground Differenti al
Zero-Sequence CTs f d .p z y b _ b d 1 6 5 8 2 4 p a v /1 e ifl $ / 5 6 7 a 2 6 0 0 6 4 3 7 5 2 3 8 1 4 5 6 7 3 3 2 1 0 b e a a e b / y la p s i d y itr v f/ s n . 5 3 2 t o c s t/ o c /ls a b lo /g m o c . b b .a 5 0 w w w / :/ ttp h
Zero-sequence CT
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Field Ground Prot ecti on
Field Ground Prot ecti on • Types of rotors • Winding failure mechanisms • Importance of field ground protection • Field ground detection methods • Switched-DC injection principle of operation • Shaft grounding brushes
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Sali ent Pole R ot or
Source:www.ansaldoenergia.com/Hydro_Gallery.asp
A Round Rotor Being Milled
Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, MaughanEngineering Consultants
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Round Rotor – End Turns
Source: Main Generator Rotor Maintenance – Lessons Learned - EPRI
Source: Main Generator Rotor Maintenance – Lessons Learned - EPRI
Two -Pole Roun d Rot or
Source: www.alstom.com
36
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Two -Pole Roun d Rot or
Source: www.alstom.com
Two -Pole Roun d Rot or
Source: www.alstom.com
37
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Round Rotor S lot — Cross S ection Coil Slot Wedge e a n ng
ng
Creepage Block
Retaining Ring
Insulation Copper Winding Winding Short
Winding Ground Turn Insulation
End Windings
Winding Ground
Slot Armor
Field Windi ng Failur e Mechanism s in Round Rotors • Thermal deterioration • Thermal cycling • Abrasion • Pollution •
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Salient Pol e Cross Sectio n Pole Body
Winding Turn Turn Insulation Winding Ground Windin Short Insulation Pole Collar * Strip-On-Edge
Field Windi ng Failur e Mechanism s i n Salient Pole Rot or s • Thermal deterioration • Abrasive particles • Pollution • Repetitive voltage surges •
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Import ance of Fie ld Groun d Detection • Presence of a single point-to-ground in field winding circuit does not affect the operation of the generator • Second point-to-ground can cause severe damage to machine – Excessive vibration – Rotor steel and / or copper melting
Rotor Groun d Detection Methods • Voltage divider • DC injection • AC injection • Switched-DC injection
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Voltage Divider Field Breaker
+
Rotor and Field Winding
R3 R2
Exciter
Brushes R1 –
Sensitive Detector
Grounding Brush
DC Injecti on Field Breaker
Rotor and Field Winding
+ Exciter
Brushes –
Sensitive Detector +
roun ng Brush
DC Supply –
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AC Injection Field Breaker
Rotor and Field Winding
+ Brushes Exciter –
Sensitive Detector roun ng Brush AC Supply
Switched-DC Injection Method Field Breaker
Rotor and Field Winding
+ Brushes
Exciter –
R1
Grounding Brush
R2 Measured Voltage Rs
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Switched DC Injection Princ iple of Operation Voscp
VDC
+
Voscn Vrs
–
Rx
R
Cfg
Vosc R Measured Voltage (Vrs)
Vrs
Rs
V
Shaft G round ing with Carbon Brush
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Shaft G roundi ng with Wire Bristle Brush
Source: SOHRE Turbomachinery, Inc. (www.sohreturbo.com)
Genera tor Abno rmal Opera tio n Protection
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Genera tor Abno rmal Opera tio n Protection • Thermal
• Overvoltage
• Current unbalance
• Abnormal frequency
• Loss-of-field • Motoring • Overexcitation
• Out-of-step • energization • Backup
Stator Thermal Protecti on Generators With Temperature Sensors
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Stator Thermal Protecti on Generators Withou t Tempera tur e Senso rs
T ln
I2 I2 2 I 2 k I NOM
Curr ent Unbalance Ca us es • Single-phase transformers • Untransposed transmission lines • Unbalanced loads • Unbalanced system faults • Open phases
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Generator Current Unbalance Produces negative-sequence currents that: – Cause magnetic flux that rotates in opposition to rotor – Induce double-frequency currents in the rotor
Rotor-I ndu ced C urr ents
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Negativ e-Sequ ence Curr ent Damage
Negativ e-Sequ ence C ur rent Capabil it y Continuous Type of Generator
I2
Max %
Salient pole (C50.12-2005) Connected amortisseur windings
10
Unconnected amortisseur windings
5
Cylindrical rotor (C50.13-2005) Indirectlycooled
10
Directlyc ooled,to350MVA
8
351to1250MVA 1251 to 1600 MVA
8– (MVA– 350)/300 5
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Negativ e-Sequ ence C ur rent Capabil it y Short Time I 22t K 2 I22t
Type of Generator Salientpole(C37.102-2006)
Max %
40
Synchronous condenser (C37.102-2006)
30
Cylindrical rotor (C50.13-2005) Indirectl cooled
30
Directlycooled,to800MVA
10
Directly cooled, 801 to 1600 MVA
→
Negativ e-Sequ ence C ur rent Capabil it y Short Time
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NegativeSequence Overcurrent
T
K2
I2
2
NOM
Common Ca uses of L oss o f Fie ld • Accidental field breaker tripping • Field open circuit • Field short circuit • Voltage regulator failure • Loss of field to the main exciter • Loss of ac supply to the excitation system
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Effects of L oss o f Fie ld • Rotor temperature increases because of edd currents • Stator temperature increases because of high reactive power draw • Pulsating torques may occur • Power system may experience voltage collapse or lose steady-state stability
Negativ e-Sequ ence Curr ent Caused Damp er Win di ng Damage
Damper Windings
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LOF Prot ectio n Using a Mho Ele ment
LOF Protection Using NegativeOffset Mho Eleme nt s
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LOF Protection Using Negative - and Positive-Offset Mho Elements
Zone 2 S etting Cons iderations
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Possi bl e Pri me Mover Damage From Ge nera tor Motorin g • • Hydraulic turbine blade cavitation • Gas turbine gear damage unburned fuel
Small Re verse Power Flo w Can Caus e Damage T
ical values of reverse ower re uired to spin a generator at synchronous speed Steam turbines
0.5–3%
Hydro turbines
0.2–2+%
Diesel engines Gast urbines
5–25% 50+%
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Directional Power Element Q
32P1 32P2
P
P1 P2
Overe xcit ation Protection
V
•
f NOM NOM
• Overexcitation occurs when 1.05
V/f
exceeds
• Causes enerator heatin • Volts/hertz (24) protection should trip generator
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Core D amage d du e to Ove rexci tatio n
Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, Maughan Engineering Consultants
Core D amage d du e to Ove rexci tatio n
Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, Maughan Engineering Consultants
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Overe xcit ation Protection Dual-Leve l, Definit e Tim e Chara cteri sti c
Overe xcit ation Protection Inverse- and De fini te Time Cha racte rist ics
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Overvolt age Prot ection • Overvoltage most frequently occurs in y roe ec r c genera ors • Overvoltage protection (59): – Instantaneous element set at 130–150 percent of rated voltage – Time-delayed element set at approximately 110 percent of rated voltage
Abnormal Frequency Protection
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Possi bl e Damage Fro m Out-of-Ste p Gene rator Operati on • • Damage to shaft resulting from pulsating torques • High stator core temperatures • Thermal stress in the step-up transformer
Single-Blinder Out-of-Step Scheme
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Double-Blinder Out-of-Step Scheme
Generator Inadvertent Energization • Common causes: human errors, control , • The generator starts as an induction motor • High currents induced in the rotor cause rapid heating • High stator current
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Inadve rtent Ene rgiza tion Protection Logic
Log ic f or Comb ined Br eaker- Failu re and B reaker-F lashover Prot ection
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Backup Protection Directly Connected Ge nerator
Generator With Step-Up Transf orm er
Vol tage- Rest rained Overc ur rent Element Pickup Curr ent
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Mho Distance Element Characteristic
Synchro nism -Check Element
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Power System Disturbance Caused by an O ut-of-S ynch ron ism Clos e
Nominal Current: 10560 A Voltage: 6.5 kV
Poss ibl e Damagin g Effects During Synchro nizing
• Bearing damage • Loosened stator windings • Loosened stator laminations
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IEEE Generator Synch ro ni zing Limits rea erc os ngang e
–
Generator-side voltage relative to system
100% to 105%
Frequency difference
+/–0.067 Hz
Source: IEEE Std. C50.12 and C50.13
Issues Affecting Generator Synchronizing • Voltage ratio differences • Voltage angle differences • Voltage, angle, and slip limits
Synchronism Check relay
Synchronism Check relay
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Synchro nis m-Check Logic Ove rvi ew
66
