2013 ARIPPA Technical Symposium
Turbine Generator Testing
and
On-Site Repairs
David Branton, CEO
Turbine Generator Maintenance, Inc.
Generator Assessments
There has been a lot of attention on the importance of periodic generator
maintenance inspections, but how can a plant anticipate and plan for repairs to
minimize the impact of correcting a problem during a maintenance outage?
This presentation discusses the following:
• Designing a long-term generator maintenance program
•
•
•
•
Why Should a Generator Be Assessed
What Parts of a Generator Should Be Assessed
When Should a Generator Be Assessed
How Should a Generator Be Assessed
• Common Generator Aging Mechanisms
• Performing Successful Generator Rotor Repairs On-Site
Designing a Long-Term Maintenance Plan
Requirements:
• Maximum Unit Availability
• No Unplanned Outages
• Predictable Maintenance Costs
Qualitative Tools:
• Running Assessments
• Periodic Minor Inspections Based on Outage Window
• Objective Based Major Inspections
Power plant operations are usually more aware of boiler, BOP, and turbine
maintenance and largely ignore the generator. By developing a multi-year
assessment and maintenance program, unplanned outages can be mostly prevented.
Critical Areas for Inspection
and Monitoring - Stator
Critical Areas for Inspection
and Monitoring - Rotor
Why Should a Generator Be Assessed
Aging Mechanisms: “Insulation in service is exposed to high temperature, high voltage, vibration,
and other mechanical forces as well as some adverse environmental conditions. These various factors
act together and individually to wear out or age the insulation.”
Thermal Aging:
Electrical Aging:
Mechanical Aging:
Environmental Aging:
Core Insulation:
Inadequate Cooling
General Over-Heating
Localized Over-Heating
Burnout at High Temp
Stator/Rotor Insulation:
Continuous High Temp
Differential Expansion
Thermal Cycling
Girth Cracking
Scarf-Joint
Loosening of End Winding
Loosening of Coils in Slots
Core Insulation:
Under-Excitation
Over-Excitation
Manufacturing Defects
Ground Fault in Core Slots
Stator Insulation:
Electrical Discharges
Surface Tracking
Moisture Absorption
System Surge Voltages
Unbalanced Supply Voltages
Rotor Insulation:
Transient Over-Voltages
Static Excitation Transients
Surface Tracking
Moisture Absorption
Core Insulation:
Core Looseness & Fretting
Back Iron Over-Heating
Stator Insulation:
120 Hz Bar Vibration Forces
Electromagnetic Forces on End
Windings
Abrasive Materials
Rotor Insulation:
Centrifugal Forces
Abrasive Materials
Operation on Turning Gear
Core Insulation:
Water Absorption
Chemical Contamination
Stator/Rotor Insulation:
Water Absorption
Chemical Contamination
Handbook To Assess The Insulation Condition Of Large Rotating Machines
Authors I.M. Culbert, H. Dhirani, G.C. Stone
Generator Maintenance Issues
Common Issues
 Low insulation resistance (rotor and stator) caused by
moisture/dirt
 Partial Discharge and Spark Gap
 Bar vibration caused by loose wedges
 Rotor winding ground and turn shorts
 Stator end winding and circuit ring looseness
 Collector ring and brush issues
Generator Maintenance Issues
Less Common resulting in Major Unavailability
 Stator winding ground faults caused by vibration,
ground wall failure
 Stator core failures
 Multi-Turn rotor shorts or grounds
 Bushing failures
 Iso-Phase failures
Generator Assessments
Running Assessments:
• Observe – Changes in Electrical/Mechanical Noise, RTD Temperatures, H2 Purity
and Usage, Voltage and Current Levels and Balance, Bearing Oil Usage, arcing….
• Monitor – Stator RTD Temperature, Generator Vibration, End Winding
Resonance, Partial Discharge Activity, Flux Probe Readings, Field Ground System,
etc…
• Trend – Partial Discharges, Temperatures, Excitation Current,Vibration Levels,
etc…
Outage Assessments:
• Inspect – Visual, Borescopic, Boresonic, Measure, ….
• Test – Stator Winding, Stator Core, Rotor, and Exciter Insulation Systems
• Examine – Non-Destructive Examination of Critical Components
• Trend – All Available Historical Assessment Data Against Current Findings
Generator Assessments
Minor Inspection: Performed in Conjunction with Turbine Borescope, Auxiliary Inspection.
Plan for a Maximum 1-2 Day Outage
Unit Condition: Generator Assembled but Inspection Access Covers Removed
Inspections/tests performed:
• Borescope Inspection of Turbine Generator Stator Windings, Turbine Generator Rotor Windings and Blocking,
Stator Core Step Iron and Fingerplates (All as Accessible)
• Turbine Generator Stator Winding Assessment: IR, PI, and Copper Resistance
• Turbine Generator Rotor Winding Assessment: IR, PI, Copper Resistance, and Impedance
• Turbine Generator Exciter Assessment: Visual Inspection, Insulation Resistance Tests
Benefits
• Reveals pending faults in winding of the rotor and armature
• Reveals deterioration of coil insulation caused by corona and/or end winding vibration
• Establishes a “base line” for evaluating future inspections and input for establishing planned major maintenance
activities
Final Report
• Standardized format provided electronically (cd or electronic mail)
• Photographs of critical areas in the stator and rotor
• Dielectric absorption graph and analysis of insulation integrity
• Summary of all insulation and winding resistance measurements taken
Generator Assessments
Moderate Inspection: Performed in Conjunction with Turbine Valve and Bearing Inspection.
Plan for a Maximum 3 Day Outage
Unit Condition: Turbine Generator Rotor Installed But Top Half of End Brackets Removed
Inspections/tests performed:
• Visual and Borescope Inspection of Turbine Generator Stator Windings, Turbine Generator Rotor Windings and Blocking,
Stator Core Step Iron, Main Iron, and Fingerplates (all as accessible)
• Resistance Thermal Detector and Heater Tests
• Turbine Generator Stator Assessment:
o Insulation Resistance and Polarization Index
o Copper Resistance
o Power Factor and Tip-up
o Visual Corona Inspection, as Accessible, to Determine Areas and Severity of Corona Activity
o Step Voltage Test (D.C. Step Voltage / Dielectric Absorption and/or High Potential) to Determine Insulation
Integrity of Armature Windings
o End Winding Vibration Test (End Winding Basket Frequency Examination)
o Core Tightness Examination
• Turbine Generator Rotor Assessment:
o Insulation Resistance and Polarization Index
o Copper Resistance
o Impedance
• Turbine Generator Exciter Assessment:
o Visual Inspection
o Insulation Resistance Tests
o Collector Ring Integrity
Generator Assessments
Moderate Inspection: Performed in Conjunction with Turbine Valve and Bearing Inspection.
Plan for a Maximum 3 Day Outage
Unit Condition: Turbine Generator Rotor Installed But Top Half of End Brackets Removed
Benefits
• Reveals pending faults in winding of the rotor and armature
• Reveals deterioration of coil insulation caused by corona or end winding vibration
• Establishes a “base line” for evaluating future inspections and input for establishing planned major maintenance activities
• Additional data on armature winding insulation integrity to more accurately access life and future maintenance requirements
• Assessment of “end winding” stability (resonance conditions can be immediately corrected to avoid premature winding deterioration)
• Record of core tightness
Final Report
• Standardized format provided electronically (cd or electronic mail)
• Photographs of critical areas in the stator winding, core, and rotor
• Dielectric absorption graph and analysis of insulation integrity
• Summary of all insulation and winding resistance measurements taken
• Data sheets showing location and levels of corona activity
• Plots showing frequency distribution of end winding basket core tightness record
• Any anomalies reported immediately with the Final Report submitted within 10 days of inspection
Generator Assessments
Major Inspection: Performed in Conjunction with a Major Turbine Outage.
Plan for a Maximum 7 Day Outage
Unit Condition: Turbine Generator Completely Dismantled with the Rotor Removed
Inspections/tests performed:
• Visual Inspection of Turbine Generator Stator Windings, Turbine Generator Rotor Windings and Blocking, Stator Core
• Borescope inspection of rotor winding and blocking (as accessible)
• Resistance Thermal Detector and Heater Tests
• Turbine Generator Stator Assessment:
• Insulation Resistance and Polarization Index
• Copper Resistance
• Power Factor and Tip-up
• Visual Corona Inspection, as Accessible, to Determine Areas and Severity of Corona Activity
• Step Voltage Test (D.C. Step Voltage / Dielectric Absorption and/or High Potential) to Determine
Insulation Integrity of Armature Windings
• End Winding Vibration test (End Winding Basket Frequency Examination)
• Core Tightness Examination
• El CID Core Test
• Wedge Tightness Detection Mapping
Generator Assessments
Major Inspection: Performed in Conjunction with a Major Turbine Outage.
Plan for a Maximum 7 Day Outage
Unit Condition: Turbine Generator Completely Dismantled with the Rotor Removed
Inspections/Tests Performed:
• Turbine Generator Rotor Assessment:
• Insulation Resistance and Polarization Index Test
• Copper Resistance Measurement
• AC Impedance Test
• Pole Balance Test
• Retaining Ring NDE inspection (based on OEM requirements)
• Turbine Generator Exciter Assessment:
• Visual Inspection
• Insulation Resistance Tests
• Collector Ring Integrity
Generator Assessments
Major Inspection: Performed in Conjunction with a Major Turbine Outage.
Plan for a Maximum 7 Day Outage
Unit Condition: Turbine Generator Completely Dismantled with the Rotor Removed
Benefits
• Reveals pending faults in winding of the rotor and armature
• Reveals deterioration of coil insulation caused by corona or end winding vibration
• Establishes a “base line” for evaluating future inspections and input for establishing planned major maintenance activities
• Additional data on armature winding insulation integrity to more accurately access life and future maintenance requirements
• Assessment of “end winding” stability (resonance conditions can be immediately corrected to avoid premature winding deterioration)
• Record of core tightness and stator lamination condition
• Record of rotor retaining ring, wedges, and rotor body condition
Final Report
• Standardized format provided electronically (cd or electronic mail)
• Photographs of critical areas in the stator winding, core, and rotor
• Dielectric absorption graph and analysis of insulation integrity
• Summary of all insulation and winding resistance measurements taken
• Data sheets showing location and levels of corona activity
• Plots showing frequency distribution of end winding basket core tightness record
• Core tightness record
Generator Life Expectancy
General Electric
Mitsubishi uses an operating hours
versus start-stop cycle relationship
to estimate the remaining life in a
stator winding. Once the estimated
life reaches 40% or less, they
recommend a stator rewind.
GER 4223
Generator Running Assessments
Generator maintenance & reliability
issues can be largely predicted by
monitoring and trending these
parameters:
• Partial Discharge Levels
• Flux Probe Data
• Over or Under Excitation Excursions
• Generator Rotor and End Winding
Vibration
• Stator RTD Temperatures
Running Assessments
Generator Rotor Windings
Shorted Turns in Pole A - Coil 5, Pole B - Coils 6 and 7
Magnetic Wedges in Coil 1 – note small signal
Repairs that can be done with rotor installed
during a turbine valve & bearing inspection
Outage Inspection
Repairs that can be done with rotor installed
during a turbine valve & bearing inspection
Outage Inspection
Repairs that can be done with rotor installed
during a turbine valve & bearing inspection
A “bump” test can be used to assure that
the end winding is out of resonance
Outage Inspection
Repairs that can be performed in
conjunction with turbine major inspection
Stator Rewedge
Outage Inspection
Repairs that can be performed in
conjunction with turbine major inspection
Rotor retaining ring removal
Outage Inspection
Repairs that can be performed in
conjunction with turbine major inspection
Outage Inspection
Rotor Rewind at the Customer’s Facility
Outage Inspection
Rotor Rewind at the Customer’s Facility
Outage Inspection
Rotor Rewind at the Customer’s Facility
Outage Inspection
Rotor Rewind at the Customer’s Facility
Outage Inspection
Rotor Rewind at the Customer’s Facility
Outage Inspection
Rotor Rewind at the Customer’s Facility
Outage Inspection
Summary & Conclusions
•
The same Running Condition Assessment principles apply to generators as turbines
•
Turbine Generator insulation systems are susceptible to various degrees of aging
based upon Electrical, Thermal, Mechanical, and Environmental mechanisms, which
work individually and/or in concert to reduce machine reliability .
•
Using RCA and performing Minor, Moderate, and/or Major Inspections, most forced
outages can be avoided with maintenance costs predicted and planned.
•
Turbine Generator Rotor rewinds can be performed at the power plant without
sacrificing safety or quality while minimizing outage schedule impact.
•
With respect to all on-site generator work, remember the TAP3: Tooling, Attitude,
People, Processes, Planning…
Thermal Aging: Thermal cycling,
especially with respect to the main
field windings of peaking units, can
lead
to
significant
physical
deformation of the associated coils.
Environmental Aging: Oil is both a
solvent as well as a lubricant. Internal oil
contamination can breakdown insulation, and
loosen the frictional-force blocks, ties, and
packing throughout a generator.
Mechanical / Electrical Aging: Loose stator
winding will vibrate within the stator slots. Fretting
against the stator core iron, corona suppression
materials will abrade, increasing PD activity.