generation track overview lecture - event …€¢ limit mechanical stress for external ground...

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34th Hands-On Relay 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 (Roebel bars)

Winding Connections

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

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Roebel Bars Inside Stator Slot

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

Stator Winding CombinationsTypical for Two- and Four-Pole Machines

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Series Connection of Roebel Bars

Source:www.ansaldoenergia.com/Hydro_Gallery.asp

Series connection

Rotor

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Classification of Synchronous Generators

Synchronous Generator Classification

Rotor designCylindrical rotor

Salient-pole rotor

Cooling: Stator and rotor

Direct

Indirect

Field winding connection to dc

source

Brush

Brushless

Rotor Design

Salient-Pole Rotor

Cylindrical 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 ArrangementsGenerator-Transformer Unit

Generating Station ArrangementsDirectly Connected Generator

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• Resonant grounding (Petersen Coil)

• Ungrounded neutral

• High-resistance grounding

• Low-resistance grounding

• Low-reactance grounding

• Effective grounding

IEEE C62.92.2-1989

Synchronous Generator Grounding

Increasing GroundFault 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 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 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

)cos()(

)cos()sin(

)cos()sin(

)cos(

0

0

IVBCXd

V

IVEXd

V

IXdE

IVP

)sin()(

)sin()))cos(((

)sin())cos((

)sin(

0

0

IVABXd

V

IVVEXd

V

IXdVE

IVQ

0E

Xd

I

V

φ

A B

C

0E

V

IXd

Q

P

I

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Power Angle Characteristic

P

Operation with Constant Active Power and Variable Excitation

A B

C

0E

V

IXd

Q

I

I

I 0 E 0 E

IXd IXd

B’B’’

C’C’’

P

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

7.21466.3

15.3334.2

0

0

0

E

E

E

P

V-Curves

(p.u.) 0E

).( upI

Current Excitation

inductive cos cap. cos

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Operation with Constant Apparent Power and Variable Excitation

A B

C

0E

V

IXd

I

87.361

00.1

6.1

I

V

Xd

Operation with Constant Excitation and Variable Active Power

A B

C

0E

V

IXd

I

0 E

IXd

I The

or. S

tabi

lity

Lim

it

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Capability Curve – Round Rotor

max.

0 P

0

)cos()sin(

0

0

E

IVEXd

V

Xd

E

IVVEXd

V

VV - Q

0

)sin()))cos(((

0

0

625.0VV -

Q

Xd

The

or. S

tabi

lity

Lim

it

0.1

6.1

V

Xd

P (

Rea

l Pow

er)

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 ProtectionPercentage Differential

Dual-Slope Characteristic

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O O O

Phase Fault ProtectionHigh-Impedance Differential

Phase Fault ProtectionSelf-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 ProtectionSplit-Phase Self-Balancing

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Turn-to-Turn Fault ProtectionSplit-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

G

T XG1

XG2

3R

XG0

XC0

XC2

XC1

XT1

XT2

XT0

XS0

XS2

XS1

S

High-Impedance Grounded GeneratorNeutral Fundamental Overvoltage

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

FundamentalHarmonics

Generator – Flux Distribution in Air Gap

Neutral Third-Harmonic Undervoltage

59GNRV

F1

GSU

Full LoadNo LoadVN3

No Fault

VP3

Full Load

No Load

VP3

VN3

Fault at F1

(3) OR (2)27TN

High-Impedance Grounded Generator

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Third-Harmonic Differential

59GNRV

GSU

(3)(3)

–+

Pickup Setting

• 3 3k VP VN

VN3 VP3 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 WindingRemoves Third Harmonic

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Low-Frequency Injection

59GNR

GSU

(3) OR (2)

V

I

64S

Coupling Filter

Low-Frequency Voltage Injector

Protection Measurements


100% Stator Ground Fault ProtectionElements Coverage

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Low-Impedance-Grounded GeneratorGround Overcurrent and Directional Overcurrent

Low-Impedance-Grounded GeneratorGround Differential

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Low-Impedance-Grounded GeneratorSelf-Balancing Ground Differential

Zero-Sequence CTs

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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


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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

Slot Armor

Copper Winding

Creepage Block

Coil SlotWedge

Turn InsulationEnd Windings

Retaining RingRetaining Ring Insulation

Winding Short

Winding GroundWinding Ground

Field Winding Failure Mechanisms in Round Rotors

• Thermal deterioration

• Thermal cycling

• Abrasion

• Pollution

• Repetitive voltage surges

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Salient Pole Cross Section

Turn Insulation

Winding Turn

Pole Body

Pole BodyInsulation

Pole Collar

Pole Collar

* Strip-On-Edge

Winding Ground

Winding Short

Field Winding Failure Mechanisms inSalient 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

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Voltage Divider

+

–

Exciter

Field Breaker

Grounding Brush

Rotor and Field Winding

Brushes

Sensitive Detector

R1

R2

R3

DC Injection

+

–

Exciter

Field Breaker

GroundingBrush


Brushes

Sensitive Detector

DC Supply+

–

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AC Injection

+

–

Exciter

Field Breaker

GroundingBrush


Brushes

Sensitive Detector

AC Supply

Switched-DC Injection Method

+

–

Exciter

Field Breaker

GroundingBrush


Measured Voltage

R1

R2

Rs

Brushes

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Switched DC Injection Principle of Operation

Measured Voltage (Vrs)

R

R

Rs

Cfg

Rx

+

–

VDC

V

Vrs

Vrs

Voscp

Voscn

Vosc

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

• Currentunbalance

• Loss-of-field

• Motoring

• Overexcitation

• Overvoltage

• Abnormal frequency

• Out-of-step

• Inadvertent energization

• Backup

Stator Thermal ProtectionGenerators With Temperature Sensors

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Stator Thermal ProtectionGenerators Without Temperature Sensors

2 2

22 ln

P

NOM

I IT

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 CapabilityContinuous

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

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 →

22 2I t K

Negative-Sequence Current Capability

Short Time

Negative-Sequence Current Capability

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Negative-Sequence

Overcurrent Protection

22

2

NOM

KT

II

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 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 DamageFrom Generator Motoring

• Steam turbine blade overheating

• Hydraulic turbine blade cavitation

• Gas turbine gear damage

• Diesel engine explosion danger from unburned fuel

Typical values of reverse power required to spin a generator at synchronous speed

Small Reverse Power FlowCan Cause Damage

Steam turbines 0.5–3%

Hydro turbines 0.2–2+%

Diesel engines 5–25%

Gas turbines 50+%

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Directional Power ElementQ

P

32P1

32P2

P1

P2

Overexcitation Protection

• Overexcitation occurs when V/f exceeds 1.05

• Causes generator heating

• Volts/hertz (24) protection should trip generator

• NOM

NOM

fV

f V

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Core Damaged due to Overexcitation


Core Damaged due to Overexcitation


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Overexcitation ProtectionDual-Level, Definite Time Characteristic

Overexcitation ProtectionInverse- 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

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Backup ProtectionDirectly Connected Generator

Generator With Step-Up Transformer

Voltage-Restrained OvercurrentElement Pickup Current

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Mho Distance Element Characteristic

Synchronism-Check Element

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Nominal Current: 10560 A

Voltage: 6.5 kV

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

Possible Damaging EffectsDuring Synchronizing

• Shaft damage due to torque

• Bearing damage

• Loosened stator windings

• Loosened stator laminations

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

IEEE Generator Synchronizing Limits

Breaker closing angle +/–10°

Generator-side voltage relative to system

100% to 105%

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

generation track overview lecture - event …€¢ limit mechanical stress for external ground...

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