32 Hands-on Relay School: Generator Design, Connections, And Grounding

2/3/2015

32nd Hands-On Relay School

Generation Track Overview Lecture

Generator Design, Connections, and Grounding

1

2/3/2015

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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Stator (Core + Winding) Winding Connections Core Lamination

Winding (Roebel bars)

Typical Types of Generator Windings Stator Winding: Random-Wound Coils

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Typical Types of Generator Windings Stator Winding: Form-Wound Coils

Typical Types of Generator Windings Stator Winding: Roebel Bars

4

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Roebel Bars Inside Stator Slot

Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, Maughan Engineering Consultants

Stator Winding Combinations Typical for Two- and Four-Pole Machines

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Series Connection of Roebel Bars

Series connection

Source:www.ansaldoenergia.com/Hydro_Gallery.asp

Rotor

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Classification of Synchronous Generators Synchronous Generator Classification Cylindrical rotor Rotor design Salient-pole rotor Direct Cooling: Stator and rotor Indirect Field winding Brush connection to dc Brushless source

Rotor Design

Salient-Pole Rotor

Cylindrical Rotor

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Two-Pole Round Rotor

Source: www.alstom.com

Salient Pole Rotor

Source:www.ansaldoenergia.com/Hydro_Gallery.asp

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Stator Winding Cooling Indirectly Cooled

Directly Cooled

Cooling Ducts, Water Cooled Bar

Rotor Winding Cooling Indirectly Cooled

Directly Cooled

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Field Winding Connection to DC Source Brush Type

Field Winding Connection to DC Source Brushless

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Generator Station Arrangements Generator-Transformer Unit

Generating Station Arrangements Directly Connected Generator

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Synchronous Generator Grounding IEEE C62.92.2-1989 • Resonant grounding (Petersen Coil) • Ungrounded neutral • High-resistance grounding • Low-resistance grounding • Low-reactance grounding • Effective grounding

Increasing Ground Fault Current

Why Ground the Neutral?

• Minimize damage for internal ground faults • Limit mechanical stress for external ground faults • Limit temporary/transient overvoltages • Allow for ground fault detection • Ability to coordinate generator protection with other equipment requirements

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Ungrounded Neutral

• No intentional connection to ground • Maximum ground fault current higher than for resonant grounding • Excessive transient overvoltages may result

High-Resistance Grounding

• Low value resistor connected to secondary of 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 Grounding

• 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 • Ground fault current is high • Low temporary overvoltages during phaseto-ground faults

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Generator Capability Curves

Defining Generator Capability • Curve provided by the generator manufacturer • Defines the generator operating limits during steady state conditions • Assumes generator is connected to an infinite bus • Limits are influenced by: – Terminal voltage – Coolant – Generator construction

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Generator Capability Curve for a Round Rotor Generator

Generator Capability Curve for a Salient Pole Generator

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Capability Curve Construction

Phasor Diagram – Round Rotor Generator Xd

P  V  I  cos( ) E 0  sin( )  Xd  I  cos( )

I

V  E 0  sin( )  V  I  cos( ) Xd V  ( BC )  V  I  cos( ) Xd

V

E0

C φ

E0

P Xd  I



V



A

I

B

Q

Q  V  I  sin( ) ( E 0  cos( ))  V  Xd  I  sin( ) V  (( E 0  cos( ))  V )  V  I  sin( ) Xd V  ( AB)  V  I  sin( ) Xd

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Power Angle Characteristic P



Operation with Constant Active Power and Variable Excitation C

C’’

Xd  I

Xd  I  E 0 

I 

C’

Xd  I 

E0 

E0

P

 V B’’ 

A

I

 Q

I

Q

B’

B

 Q Xd  1.6 V  1.00 I  1  36.87 E 0  2.3433.15 I   1.6  60 E 0  3.46621.7 I   1.1345 E 0  1.3178.5

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Power Angle Characteristic P

E 0  2.3433.15 E 0  3.46621.7 E 0  1.3178.5



V-Curves I ( p.u )

cos   cap.

cos   inductive

E 0 (p.u.)

 Excitation Current

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Operation with Constant Apparent Power and Variable Excitation C

E0 Xd  I



V



Xd  1.6

A

B

I

V  1.00 I  1  36.87

Operation with Constant Excitation and Variable Active Power Theor. Stability Limit

E0 

Xd  I  C

E0

I Xd  I



V



A

B

I

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Theor. Stability Limit

Capability Curve – Round Rotor

P (Real Power)

V  (( E 0  cos( ))  V )  V  I  sin( ) Xd E0  0 - VV Q Xd V  E 0  sin( )  V  I  cos( ) Xd E0  0 P0



 max.

Q

Xd  1.6 Q (Reactive Power) V  1.0

- VV  0.625 Xd

Generator Fault Protection

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Generator Fault Protection • Stator phase faults • Stator ground faults • Field ground faults • External faults (backup protection)

Stator Phase Fault Protection • 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 Percentage Differential

Dual-Slope Characteristic

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Phase Fault Protection High-Impedance Differential

O

O

O

Phase Fault Protection Self-Balancing Differential

http://www.polycastinternational.com/old_cat/pdfs/Section4/Section4-Part2.pdf

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Stator Winding Coils with Multiple Turns

Turn-to-Turn Fault Protection Split-Phase Self-Balancing

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Turn-to-Turn Fault Protection Split-Phase Percentage Differential

Stator Ground Fault Protection • 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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Ground Fault in a Unit-Connected Generator XC1 T G

XG1

XT1

XS1

S XG2

XC2

XS2 XT2

3R XC0 XG0

XS0 XT0

High-Impedance Grounded Generator Neutral Fundamental Overvoltage

Fault Location/ % of Winding

Voltage V

F1 / 3%

Vnom 3 Vnom 85% • 3

F2 / 85%

3% •

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Generator – Flux Distribution in Air Gap

Total Flux Fundamental Harmonics Generator – Flux Distribution in Air Gap

High-Impedance Grounded Generator Neutral Third-Harmonic Undervoltage GSU

F1

V

R

(3) 59GN

OR (2)

27TN

Full Load Full Load No Load

VN3

No Load VN3 VP3

VP3

No Fault

Fault at F1

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High-Impedance Grounded Generator Third-Harmonic Differential

GSU

(3) (3)

V

R

59GN

VN3

VP3

k • VP3  VN 3 Pickup Setting

59THD

+ –

Third-Harmonic Differential Element

Generator Winding Analysis • Generator data – 18 poles – 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-Pitch Winding

2/3 Pitch Winding Removes Third Harmonic

30

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High-Impedance Grounded Generator Low-Frequency Injection GSU

(3)

OR (2)

I R

59GN

V 64S Coupling Filter

Low-Frequency Voltage Injector

Protection Measurements

100% Stator Ground Fault Protection Elements Coverage

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Low-Impedance-Grounded Generator Ground Overcurrent and Directional Overcurrent

Low-Impedance-Grounded Generator Ground Differential

32

http://www05.abb.com/global/scot/scot235.nsf/veritydisplay/beaaeb0123376541832573460062a765/$file/1vap428561-db_byz.pdf

2/3/2015

Low-Impedance-Grounded Generator Self-Balancing Ground Differential

Zero-Sequence CTs

Zero-sequence CT

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Field Ground Protection

Field Ground Protection • Types of rotors • Winding failure mechanisms • Importance of field ground protection • Field ground detection methods • Switched-DC injection principle of operation • Shaft grounding brushes

34

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Salient Pole Rotor

Source:www.ansaldoenergia.com/Hydro_Gallery.asp

A Round Rotor Being Milled

Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, Maughan Engineering Consultants

35

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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 Round Rotor

Source: www.alstom.com

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Two-Pole Round Rotor

Source: www.alstom.com

Two-Pole Round Rotor

Source: www.alstom.com

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Round Rotor Slot — Cross Section Coil Slot Wedge Retaining Ring Creepage Block Insulation

Retaining Ring

Copper Winding Winding Short Winding Ground Turn Insulation

End Windings

Winding Ground

Slot Armor

Field Winding Failure Mechanisms in Round Rotors • Thermal deterioration • Thermal cycling • Abrasion • Pollution • Repetitive voltage surges

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Salient Pole Cross Section Pole Body Pole Collar Winding Turn Turn Insulation Winding Ground Pole Body Insulation

Winding Short

Pole Collar * Strip-On-Edge

Field Winding Failure Mechanisms in Salient Pole Rotors • Thermal deterioration • Abrasive particles • Pollution • Repetitive voltage surges • Centrifugal forces

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Importance of Field Ground 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 Ground Detection Methods • Voltage divider • DC injection • AC injection • Switched-DC injection

40

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Voltage Divider Field Breaker

Rotor and Field Winding

R3

+ R2

Exciter

Brushes

R1

–

Sensitive Detector

Grounding Brush

DC Injection Field Breaker

Rotor and Field Winding

+ Exciter

Brushes –

Sensitive Detector + DC Supply

Grounding Brush

–

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AC Injection Field Breaker

Rotor and Field Winding

+ Brushes

Exciter –

Sensitive Detector

Grounding Brush

AC Supply

Switched-DC Injection Method Field Breaker

Rotor and Field Winding

+ Brushes

Exciter –

R1

Grounding Brush

R2 Rs

Measured Voltage

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Switched DC Injection Principle of Operation Voscp VDC

+

Voscn

–

Vrs

Rx

R

Cfg

Vosc R Measured Voltage (Vrs)

Vrs

Rs

V

Shaft Grounding with Carbon Brush

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Shaft Grounding with Wire Bristle Brush

Source: SOHRE Turbomachinery, Inc. (www.sohreturbo.com)

Generator Abnormal Operation Protection

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Generator Abnormal Operation Protection • Thermal

• Overvoltage

• Current unbalance

• Abnormal frequency

• Loss-of-field

• Out-of-step

• Motoring

• Inadvertent energization

• Overexcitation

• Backup

Stator Thermal Protection Generators With Temperature Sensors

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Stator Thermal Protection Generators Without Temperature Sensors

  I 2  I P2 T   ln  2 2  I  k I  NOM   

Current Unbalance Causes • 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-Induced Currents

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Negative-Sequence Current Damage

Negative-Sequence Current Capability Continuous Type of Generator

I2 Max %

Salient pole (C50.12-2005) Connected amortisseur windings

10

Unconnected amortisseur windings

5

Cylindrical rotor (C50.13-2005) Indirectly cooled

10

Directly cooled, to 350 MVA

8

351 to 1250 MVA

8 – (MVA – 350) / 300

1251 to 1600 MVA

5

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Negative-Sequence Current Capability Short Time I 22t  K 2 Type of Generator

I22t Max %

Salient pole (C37.102-2006)

40

Synchronous condenser (C37.102-2006)

30

Cylindrical rotor (C50.13-2005) Indirectly cooled

30

Directly cooled, to 800 MVA

10

Directly cooled, 801 to 1600 MVA

→

Negative-Sequence Current Capability Short Time

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NegativeSequence Overcurrent Protection T

K2  I2  I   NOM 

2

Common Causes of Loss of Field • 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 Loss of Field • Rotor temperature increases because of eddy 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

Negative-Sequence Current Caused Damper Winding Damage

Damper Windings

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LOF Protection Using a Mho Element

LOF Protection Using NegativeOffset Mho Elements

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LOF Protection Using Negative- and Positive-Offset Mho Elements

Zone 2 Setting Considerations

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Possible Prime Mover Damage From Generator Motoring • Steam turbine blade overheating • Hydraulic turbine blade cavitation • Gas turbine gear damage • Diesel engine explosion danger from unburned fuel

Small Reverse Power Flow Can Cause Damage Typical values of reverse power required to spin a generator at synchronous speed Steam turbines Hydro turbines Diesel engines Gas turbines

0.5–3% 0.2–2+% 5–25% 50+%

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Directional Power Element Q 32P1 32P2

P

P1 P2

Overexcitation Protection 

V f NOM • f VNOM

• Overexcitation occurs when V/f exceeds 1.05 • Causes generator heating • Volts/hertz (24) protection should trip generator

55

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Core Damaged due to Overexcitation

Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, Maughan Engineering Consultants

Core Damaged due to Overexcitation

Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, Maughan Engineering Consultants

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Overexcitation Protection Dual-Level, Definite Time Characteristic

Overexcitation Protection Inverse- and Definite Time Characteristics

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Overvoltage Protection • Overvoltage most frequently occurs in hydroelectric generators • 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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Possible Damage From Out-of-Step Generator Operation • Mechanical stress in the machine windings • 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 circuit failures, and breaker flashovers • The generator starts as an induction motor • High currents induced in the rotor cause rapid heating • High stator current

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Inadvertent Energization Protection Logic

Logic for Combined Breaker-Failure and Breaker-Flashover Protection

61

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Backup Protection Directly Connected Generator

Generator With Step-Up Transformer

Voltage-Restrained Overcurrent Element Pickup Current

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Mho Distance Element Characteristic

Synchronism-Check Element

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Power System Disturbance Caused by an Out-of-Synchronism Close

Nominal Current: 10560 A Voltage: 6.5 kV

Possible Damaging Effects During Synchronizing

• • • •

Shaft damage due to torque Bearing damage Loosened stator windings Loosened stator laminations

64

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IEEE Generator Synchronizing Limits Breaker closing angle

+/–10°

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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Synchronism-Check Logic Overview

66