generator overview

8/20/2019 Generator Overview

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th -

School

Generation Track

Overview Lecture

Generator Design, Connections, and

Grounding


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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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Stator (Core + Winding)

Core Lamination

Winding Connections

Winding (Roebel bars)

Typical Types of Generator Windings

Stator Winding: Random-Wound Coils


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Typical Types of Generator WindingsStator Winding: Form-Wound Coils

Typical Types of Generator Windings

Stator Winding: Roebel Bars


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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

Source:www.ansaldoenergia.com/Hydro_Gallery.asp

Rotor


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Classification of Synchronous

Generators

Rotor designCylindrical rotor

Salient-pole rotor

Cooling: Stator and

rotor

Direct

Indirect

connection to dc

source

Brushless

Rotor Design

Salient-Pole Rotor


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Two-Pole Round Rotor

Source: www.alstom.com

Salient Pole Rotor



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Stator Winding Cooling

Directly CooledIndirectly Cooled

Cooling Ducts,

Water Cooled Bar

Rotor Winding Cooling

Directly CooledIndirectly 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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• Resonant roundin Petersen Coil

IEEE C62.92.2-1989

Synchronous Generator Grounding

• Ungrounded neutral

• High-resistance grounding

• Low-resistance grounding

• Low-reactance grounding

• Effective groundingIncreasing Ground

Fault Current

Why Ground the Neutral?

• Minimize damage for internal ground faults

• Limit mechanical stress for external ground faults

• Allow for ground fault detection

• Ability to coordinate generator protection withother equipment requirements


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Ungrounded Neutral

• No intentional connection to round

• Maximum ground fault current higher than forresonant grounding

• Excessive transient overvoltages may result

High-Resistance Grounding

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 connectionwhere: X0 / X1 3 and R0 / X1 1

• roun au curren s g

• Low temporary overvoltages during phase-to-ground faults


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Generator Capability Curves

Defining Generator Capability• Curve provided by the generator manufacturer

• Defines the generator operating limits during steadys a e con ons

• 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

)cos()sin(

)cos(

0

V

I Xd E

I V P

0 E

Xd

I

V

)cos()(

coss n0

I V BC Xd

V

Xd

)sin( I V Q

φ

C

0 E

P

)sin()(

)sin()))cos(((

s ncos

0

0

I V AB Xd

V

I V V E Xd

V

A B

V

I Xd

Q I


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Power Angle Characteristic

P

Operation with Constant Active Power

and Variable Excitation

C

I Xd

C’C’’

A B

0 E

V

Q

I 0 E 0 E

I Xd I Xd

B’B’’

P

I

I

Q

Q

4513.1

606.1

87.361

00.1

6.1

I

I

I

V

Xd

5.7831.1

7.21466.3

15.3334.2

0

0

0

E

E

E


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Power Angle Characteristic

5.7831.1

7.21466.315.3334.2

0

0

0

E

E E P

V-Curves

).( u p I

(p.u.)0 E

CurrentExcitation

inductivecos cap.cos


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Operation with Constant Apparent

Power and Variable Excitation

0 E

I Xd

A B

I

87.361

00.1

6.1

I

V

Xd

Operation with Constant Excitation

and Variable Active Power

0 E

C

0 E

I Xd

I T h e o r .

S t a b i l i t y

L i m i t

A B

V

I Xd

I


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Capability Curve – Round Rotor

E

I V V E Xd

V

-

0

)sin()))cos(((

0

0

t a b i l i t y

L i m

i t

0P

0

)cos()sin(

0

0

E

I V E Xd

V

Xd Q

T h e o r .

S

P ( R e a

l P o w e r )

max.

625.0VV-

Q

Xd

0.1

6.1

V

Xd

Q (Reactive Power)

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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High-Impedance Differential

O O O


Self-Balancing Differential

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


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Stator Winding Coils with Mult iple 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

High-Impedance Grounded Generator

Neutral Fundamental Overvol tage

Fault Location/

% of Winding

Voltage V

F1 / 3%

F2 / 85%

3%•3

Vnom

85%•3

Vnom


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Generator – Flux Distribution in Air Gap

Total Flux

Fundamental

Harmonics

Generator – Flux Distribution in Air Gap

Neutral Third-Harmonic Undervoltage

F1

GSU


59GNRV(3) OR (2)

27TN

Full Load

No LoadVN3

No Fault

VP3

No Load

VP3

VN3

Fault at F1


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Third-Harmonic DifferentialGSU


59GNRV

(3)

(3)

–+

Pickup Setting

• 3 3k VP VN

Third-Harmonic Differential Element

Generator Winding Analysis

• 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-Pitch Winding

2/3 Pitch Winding

Removes Third Harmonic


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Low-Frequency InjectionGSU


(3) OR (2)

I

59GNR 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


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Low-Impedance-Grounded Generator

Self-Balancing Ground Differential

Zero-Sequence CTs

5 / $ f i l e / 1 v a p

4 2 8 5 6 1 - d

b_

b y z . p

d f

r i t y d i s p

l a y

/ b e a a e

b 0 1 2 3 3 7 6 5 4 1 8 3 2 5 7 3 4 6 0 0 6 2 a

7 6

h t t p : /

/ w w w

0 5

. a b b

. c o m

/ g l o b a

l / s c o

t / s c o

t 2 3 5

. n s

f / v

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


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Salient Pole Rotor


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




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Round Rotor Slot — Cross Section

Coil Slot

Wedge

Copper Winding

Creepage BlockRetaining Ringe a n ng ng

Insulation

Winding Short

Slot Armor

Turn InsulationEnd Windings

Winding GroundWinding Ground

Field Winding Failure Mechanisms inRound Rotors

• Thermal deterioration

• Thermal cycling

• Abrasion

• Pollution

•


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Salient Pole Cross Section

Pole Body

Turn Insulation

Winding Turn

Winding Ground

Windin Short

Insulation

Pole Collar

* Strip-On-Edge

Field Winding Failure Mechanisms inSalient Pole Rotors

• Thermal deterioration

• Abrasive particles

• Pollution

• Repetitive voltage surges

•


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Importance of Field Ground

Detection

• Presence of a single point-to-ground in fieldwinding circuit does not affect the operation ofthe generator

• Second point-to-ground can cause severedamage to machine

– Excessive vibration

– Rotor steel and / or copper melting

Rotor Ground Detection Methods

• Voltage divider

• DC injection

• AC injection

• Switched-DC injection


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Voltage Divider

Field Breaker Rotor and Field Winding

+

–

Exciter BrushesR1

R2

R3

Grounding BrushSensitive Detector

DC Injection

+


–

Exciter Brushes

Sensitive Detector

roun ngBrush

DC Supply

+

–


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AC Injection

+


–

Exciter Brushes

Sensitive Detector roun ng

Brush

AC Supply

Switched-DC Injection Method

+


–

Exciter

Grounding

BrushR1

Brushes

Measured Voltage

R2

Rs


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Switched DC Injection Princ iple of Operation

VDC

Voscp

R

R

Cfg

Rx

+

–Vrs

Voscn

Vosc

Measured Voltage (Vrs)

Rs

V

Vrs

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

• Loss-of-field

• Abnormalfrequency

• Out-of-step

•• Motoring

• Overexcitation

energization

• Backup

Stator Thermal Protection

Generators With Temperature Sensors


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Stator Thermal Protection

Generators Without Temperature Sensors

2 2 I I

22

ln NOM

T I k I

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 Capabil ity

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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Short Time

22 2 I t K


Type of Generator I22t Max %

Salient pole (C37.102-2006) 40

Synchronous condenser (C37.102-2006) 30

Cylindrical rotor (C50.13-2005)

Indirectl cooled 30

Directly cooled, to 800 MVA 10

Directly cooled, 801 to 1600 MVA →

Short Time



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Negative-

Sequence

Overcurrent

22

2

K T

I

NOM

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

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

Negative-Sequence Current Caused

Damper Winding Damage

Damper

Windings


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LOF Protection Using a Mho Element

LOF Protection Using Negative-

Offset 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

•

• Hydraulic turbine blade cavitation

• Gas turbine gear damage

unburned fuel

T ical values of reverse ower re uired to

Small Reverse Power Flow

Can Cause Damage

spin a generator at synchronous speed

Steam turbines 0.5–3%

Hydro turbines 0.2–2+%

Diesel engines 5–25%Gas turbines 50+%


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Directional Power Element

Q

P

32P1

32P2

P1

P2

Overexcitation Protection

• NOM f V

• Overexcitation occurs when V/f exceeds

1.05

• Causes enerator heatin

NOM

• Volts/hertz (24) protection should trip

generator


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Core Damaged due to Overexcitation


Core Damaged due to Overexcitation



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Dual-Level, Definite Time Characteristic


Inverse- and Defini te Time Characterist ics


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Overvoltage Protection

• Overvoltage most frequently occurs iny roe ec r c genera ors

• Overvoltage protection (59):

– Instantaneous element set at 130–150percent 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

•

• 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 causerapid heating

• High stator current


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Inadvertent Energization Protection

Logic

Logic for Combined Breaker-Failure

and Breaker-Flashover Protection


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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

• Bearing damage

• Loosened stator windings

• Loosened stator laminations


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IEEE Generator Synchronizing

Limits

rea er c os ng ang e –

Generator-side voltage

relative to system

100% to 105%

Source: IEEE Std. C50.12 and C50.13

Frequency difference +/–0.067 Hz

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

generator overview

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