Post on 26-May-2018
transcript
Rotor Winding Problems & Recommended Actions!
Bill Moore, P.E.
Director, Technical Services
National Electric Coil bmoore@national-electric-coil.com
614-488-1151
Generator User Group
1st Annual Conference
November 4-5, 2015
NV Energy
Las Vegas, NC
NEC 2015 – All Rights Reserved 2
Rotor Winding Problems
• High Vibration – Thermal Sensitivity / Variation w/ Load/Vars
• Trip / Forced Off-Line
– Open Circuit / Ground Fault
NEC 2015 – All Rights Reserved 3
High Vibration / Thermal Sensitivity
– Shorted Turns • Turn insulation fails, migrates, turns touch / arc
• Coil with short runs cooler, rotor bends, excessive vibration
– Shorted Coils • Coils touch, Entire Coil Shorted,
• Rotor bends, excessive vibration
– Vent Passage Restriction • Insulation migrates, amortissuer migrates, wedge migrates, turn insulation
migrates – misalignment
• Coil that is blocked off runs hotter, bends the rotor, excessive vibration
– Inadequate Coil Clearance • Coil binding in slot or inadequate room to grow
• Coil is restricted, bends the rotor, excessive vibration
NEC 2015 – All Rights Reserved 4
Trip / Forced Off-Line
– Open Circuit • Copper winding separates
– Brazed joint, J-Strap or Pole to Pole Crossover, Top Turns
– Ground Fault - Ground insulation fails • Slot liner cracks or Liners Migrate – Path to Ground
• Retaining Ring Insulation is Penetrated
• Contamination and Arc Path to Ground
• Main Lead Insulation Fails
NEC 2015 – All Rights Reserved 9
Generator Rotor Forces
• Mechanical due to Rotation – up to 3600 rpm and back down to 0 rpm, plus occasional overspeeds – Force = ω x m x r2
– Compressive stress in the copper turns, but bending stresses in the J-straps, Terminal Studs and Pole Crossover, significant displacement between standstill and speed
• Electrical – DC, not that high when compared to the stator, but high mechanical stresses compound the problem – Turn insulation fails (short) or Ground insulation fails (ground)
• Thermal – rotor winding heats up due to DC current flow and operating temperatures – differential expansion between copper, forging, retaining rings, wedges – Friction, abrasion, wear, bending, compressive loads
NEC 2015 – All Rights Reserved 10
Shorted Turns
• High Vibration
– Shorted Turns
• Turn insulation fails, migrates, turns touch / arc
• Coil with short runs cooler
• Rotor bends
• Excessive vibration
– Shorted Coils
• Coils touch, Entire Coil Shorted,Top Turn Elongation, Blocking Issue, Rtr bends
– Vent passage restriction
• Insulation migrates, amortissuer migrates, wedge migrates, turn insulation migrates – misalignment
– Thermal sensitivity
• Lack of clearance, coil binding, short
NEC 2015 – All Rights Reserved 11 11
• Insulation fails (cracks,
migrates, moves), burr
• Foreign object
between turns (copper
shaving, braze alloy,
metal contaminant)
Shorted Turns
NEC 2015 – All Rights Reserved 12
Shorted Turn in Top #1 Slot
This Pole Cooler
This Pole Hotter
Short in these coils has no effect
NEC 2015 – All Rights Reserved 13
Shorted Turns
• GE Frame 7 & 9
• Defective turn
insulation adhesive
• Turn to turn insulation
is migrating out of the
end turns
• Multiple shorts occur
• TIL 1308 – Rotors
originally mfg or
rewound between
1988 and 1995
NEC 2015 – All Rights Reserved 14 14
• Arcing through turn
insulation in end
turns after four
years of service –
root cause
excessive moisture
• Courtesy Dave
Albright of
Generatortech,
“Generator Field
Winding Shorted
Turns: Moisture
Effects” EPRI 2003
Nashville
Shorted Turns
NEC 2015 – All Rights Reserved 15
Shorted Coils
• Severe Situation
• Top Turn elongation
caused by a poor slip plane
between top copper turns and
insulated retaining ring
• Thermal ratcheting pushes
turn out and shorts to other coil
• Shorted coils/turns run cooler
causing the rotor to bow and
affects the balance
• Can also limit the excitation
capability of the rotor
NEC 2015 – All Rights Reserved 16 16
Shorted Coils
• Would
anyone offer to
explain what
may have
happened
here?
NEC 2015 – All Rights Reserved 18
Shorts
What’s a User to Do?
• Prevention
– Control Environment
• Filtration during Operation, Dew Point Control (Moisture)
• FME during Outages
• Clean Room & FME during Rewind
– Ensure Proper Blocking and Taping – Rewinds
– Limit Starts if Possible
• Identification
– High Speed Balance and Electrical Testing after Rewinds
• Flux Probe, Impedance
– Flux Probe in Operation to Confirm & Monitor
– RSO Testing at Standstill
NEC 2015 – All Rights Reserved 19
Recommend High Speed Balance &
Full Electrical Testing @ Rated Speed
Flux Probe
NEC 2015 – All Rights Reserved 20
• Detects presence of
short at any speed
• Identifies Pole # and
Coil#
Flux Probe Test @ Speed
NEC 2015 – All Rights Reserved 21
Running Impedance Test
• Impedance is
proportional to # of
turns-remove a turn-
impedance drops
• A sudden change of
5% indicates a short
• Above 3000 rpm very
difficult to locate at
standstill
NEC 2015 – All Rights Reserved 22
New EPRI Report
• “Best Practices to Avoid Problems During Generator Rotor Rewinds”
• EPRI Report #3002004969
• Final Report to be Available in about 2 Weeks for EPRI members
• Likely will be a presentation about it in January at the EPRI meeting in Clearwater, Florida
NEC 2015 – All Rights Reserved 23
Vent Passage Restriction
• High Vibration – Shorted Turns
• Turn insulation fails, Coil with short runs cooler, rotor bends
• TIL, Prevent Foreign Contamination, Overlap joints, Flux Probe
– Shorted Coils
• Coils touch, Entire Coil Shorted,Top Turn Elongation, Blocking Issue, Rtr bends
• Slip Planes, Blocking
– Vent passage restriction
• Insulation migrates, amortissuer migrates, wedge migrates, turn insulation migrates – misalignment
– Thermal sensitivity
• Lack of clearance, coil binding, short
• Proper end turn axial clearances, even radial pressure.
NEC 2015 – All Rights Reserved 24
Vent Passage Restriction
Radial Slits through
all components:
Wedges, Top
Fillers, Copper
Turns, Bottom
Runner, Slot Liner –
Any one of them
can migrate
causing gas flow
restriction
NEC 2015 – All Rights Reserved 25
Blocked Ventilation Passages
Partially Blocked Ventilation Passage
This isn’t too bad – more than 50% is a concern
– Component
migration
– Vent passages
blocked
– Cooling not
balanced,
creates
thermal bow
• Can be Problem
in Radial Direct-
Cooled Machines
NEC 2015 – All Rights Reserved 26
Blocked Ventilation Passages
Partially Blocked Ventilation Passage
NEC 2015 – All Rights Reserved 27
Vent Passage Blockage
“Generator Rotor Thermal Sensitivity: Causes and Cures,”
Moore & Hollandsworth, IRMC, Orlando, 2010
This is Bad!!
NEC 2015 – All Rights Reserved 28
Blocked Vent Passages
What’s a User to Do?
• Prevention – Radially Vented Rotors - Alignment
– Verification with New Rotors and with Rewinds
• Visual inspection, Radial dowel rod insertion
• Ensure Strip Insulation Properly Applied
– Verify wedges are staked, Slot Liners are Locked, Top Fillers
can’t move
– Limit Starts if Possible
• Identification
– Visual Inspection when Rotor is Out
– Borescope/Robotic Inspection when Rotor is In
NEC 2015 – All Rights Reserved 30
Inadequate Coil Clearance
• High Vibration – Shorted Turns
• Turn insulation fails, Coil with short runs cooler, rotor bends
– Shorted Coils
• Coils touch, Entire Coil Shorted,Top Turn Elongation, Blocking Issue, Rtr bends
– Vent passage restriction
• Insulation migrates, amortissuer migrates, wedge migrates, turn insulation migrates – misalignment
– Inadequate Coil Clearance
• Coil binding in slot or inadequate room to grow
NEC 2015 – All Rights Reserved 31
Inadequate Coil Clearance
Rotor Unbalance Due to Balance Weight Restraint
Balance Holes in Centering Ring Not Closed
Balance Weight Screwed in Too Far – Restricted Blocking in One Area
NEC 2015 – All Rights Reserved 32
Restricted Coil Growth
Reference: GER 3809, “Generator Rotor Thermal Sensitivity — Theory
and Experience, “ Zawoysky & Genovese.
NEC 2015 – All Rights Reserved 33
End Turn Blocking Restrictions
Blocking Restrictions Preventing Even Coil Growth with Temperature
No! Bad Location!
End Clearances Should be Consistent
NEC 2015 – All Rights Reserved 35
Inadequate Coil Clearance
What’s a User to Do?
• Prevention
– Verification for New Rotors and Rewinds
• Ensure that all clearances are relatively equal between the last coil, last
row of blocks and centering rings/spindle mounted rings
• Tap Test Wedges to Ensure None are Overly Tight
• Identification
– Thermal Sensitivity / High Vibration / Varies with Load
– Measure clearance between last coil, block and centering
ring, if access is possible
• Corrective Action
– Limit VAR Loading / Reduce Field Current / Reduce Load
– Repair/Rewind and Restore Proper Clearances
NEC 2015 – All Rights Reserved 36
Open Circuit Condition
• Forced Off-Line
– Open Circuit • Copper winding separates
– Brazed Joint Fails
– J-Strap or Terminal Stud Fails
– Pole to Pole Crossover Fails
– Top Turn Cracks and Fails
– Ground Fault
• Ground insulation fails, slot liner cracks
• Prevent excessive radial press on liner
• Prevent axial migration of liner
• Ensure sufficient length of liner
• Prevent side abrasion of liner
• Ground fault detection
NEC 2015 – All Rights Reserved 37
Brazed Joint Failure
• Siemens TLRI/THRI Style
• Braze Lack of Fill
• Product Bulletin
NEC 2015 – All Rights Reserved 38
Terminal Stud & J-Strap Failures
• Terminal studs and J-straps -
susceptible to low cycle fatigue
stresses from starts
• Some designs can appear flexible,
but braze alloy not controlled and
flexibility taken away
• Some designs are solid with little
flexibility – cracks occur in high stress
concentration areas at bends
• Risk is high – causes open circuit
NEC 2015 – All Rights Reserved 39
Top Turn Cracking Failures
• Spindle mounted
retaining ring - coil end
turns unsupported
• Radial deflection - high
stress in top turns at
end of rotor body
• Low cycle fatigue
causes crack initiation
• Failure of top turns as
shown at right, bottom
NEC 2015 – All Rights Reserved 40
Pole Crossover Failures
Rigid!
Stiff!
No Flexible
Laminations
Butt
Braze
@Corner
NEC 2015 – All Rights Reserved 41
Omega (Ω) Shape Works!
• Omega shape, based on the
Greek letter Ω, has proven to work very well over the years
• Many different variations within the Omega basic shape
• “End Condition” is still Critical
NEC 2015 – All Rights Reserved 43
Open Circuit Failures
What’s a User to Do?
• Prevention
– Insist on Flexible Design that can Accommodate Cycling
– Insist on Physical Testing for Verification
– Change Out Before Failure Occurs
– Inspect / Change if Machine Changes to Cyclic Operation
• Identification
– Determine if Your Design is Susceptible & Know # of Starts
– Visual Inspection when Rotor is Out
– Borescope Inspection when Rotor is In
– Look for Cracking
NEC 2015 – All Rights Reserved 44
In-Situ Crossover Replacement
• Consider Replacement
of Failure Prone
Crossovers
• Some designs allow
In-Situ Replacement
as shown at Right
• No Need to Remove
Rotor
NEC 2015 – All Rights Reserved 45
Ground Faults
• Forced Off-Line
– Open Circuit
• Copper winding separates
– Brazed joint, J-Strap or Pole to Pole Crossover
– Proper Braze Joint Fill, Flexible J-Strap, Terminal Stud and Pole Crossover
– Ground Fault • Main Lead Ground Insulation Fails
• Slot Liner Cracking and Arc Path to Ground
• Contamination in End Turns – Arc to Ground
NEC 2015 – All Rights Reserved 47 47
J-Lead Arrangement
Rotor Forging
End Winding J-Lead
Radial Stud Bore Copper
Damage to forging in this area
NEC 2015 – All Rights Reserved 48
Slot Liner Crack - Potential for
Ground Fault
Crack in
Corner of
Liner
This
design
requires
special
radial
flexibility
and
support at
end
NEC 2015 – All Rights Reserved 49
Rotor Ground Fault
Ground
Fault –
Same
Design
with Slot
Liner
Cracking
NEC 2015 – All Rights Reserved 50
Burning in Block
across the 6 and 7 coil
Ground to Retaining Ring
Loose Rotor Blocking Caused Copper Particle Generation and Contamination
NEC 2015 – All Rights Reserved 51
• Arc path
through split
of retaining
ring
insulation.
• No damage to
retaining ring.
Ground to Retaining Ring
NEC 2015 – All Rights Reserved 52
Ground Fault Failures
What’s a User to Do? • Prevention
– Insulation Resistance Test on a Regular Basis, 500 vdc, PI
– Ensure ground detection is working – test regularly
– Install if you don’t have one
– Know # of Starts & Your Machine’s Susceptibility
• Pole Crossovers, Brazed Joint Separations, J-Straps – these all
can cause ground faults as well
• Identification
– Visual Inspection when Rotor is Out – Borescope Under RR
– Borescope Inspection when Rotor is In – Under RR
– Pole Crossover, Main Leads & J-Straps if Accessible
NEC 2015 – All Rights Reserved 53
The End
www.generatortechnicalforum.org
Visit The IGTC and Join International Generator Technical Community
It’s Free and Informative!