Rotor Winding Problems - ccj-online.com · Rotor Winding Problems • High Vibration ... Reference:...

Rotor Winding Problems & Recommended Actions!

Bill Moore, P.E.

Director, Technical Services

National Electric Coil [email protected]

614-488-1151

Generator User Group

1st Annual Conference

November 4-5, 2015

NV Energy

Las Vegas, NC

mailto:[email protected]





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


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


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


Generator Rotor


Typical Construction


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


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


Shorted Turn in Top #1 Slot

This Pole Cooler

This Pole Hotter

Short in these coils has no effect


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


• 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


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


Shorted Coils

• Would

anyone offer to

explain what

may have

happened

here?


Improved Block Design


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


Recommend High Speed Balance &

Full Electrical Testing @ Rated Speed

Flux Probe


• Detects presence of

short at any speed

• Identifies Pole # and

Coil#

Flux Probe Test @ Speed


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


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


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


• Slip Planes, Blocking



– Thermal sensitivity

• Lack of clearance, coil binding, short

• Proper end turn axial clearances, even radial pressure.


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


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


Blocked Ventilation Passages

Partially Blocked Ventilation Passage


Vent Passage Blockage

“Generator Rotor Thermal Sensitivity: Causes and Cures,”

Moore & Hollandsworth, IRMC, Orlando, 2010

This is Bad!!


Blocked Vent Passages


• 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


On-Site Inspection w/

Borescope


Inadequate Coil Clearance

• High Vibration – Shorted Turns

• Turn insulation fails, Coil with short runs cooler, rotor bends

– Shorted Coils




– Inadequate Coil Clearance

• Coil binding in slot or inadequate room to grow



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


Restricted Coil Growth

Reference: GER 3809, “Generator Rotor Thermal Sensitivity — Theory

and Experience, “ Zawoysky & Genovese.


End Turn Blocking Restrictions

Blocking Restrictions Preventing Even Coil Growth with Temperature

No! Bad Location!

End Clearances Should be Consistent


Coil Expansion Restriction




• 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


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


Brazed Joint Failure

• Siemens TLRI/THRI Style

• Braze Lack of Fill

• Product Bulletin


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


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


Pole Crossover Failures

Rigid!

Stiff!

No Flexible

Laminations

Butt

Braze

@Corner


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


Flexible Pole Crossovers that Work!


Open Circuit Failures


• 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


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


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


Ground Fault – J-Strap


J-Lead Arrangement

Rotor Forging

End Winding J-Lead

Radial Stud Bore Copper

Damage to forging in this area


Slot Liner Crack - Potential for

Ground Fault

Crack in

Corner of

Liner

This

design

requires

special

radial

flexibility

and

support at

end


Rotor Ground Fault

Ground

Fault –

Same

Design

with Slot

Liner

Cracking


Burning in Block

across the 6 and 7 coil

Ground to Retaining Ring

Loose Rotor Blocking Caused Copper Particle Generation and Contamination


• Arc path

through split

of retaining

ring

insulation.

• No damage to

retaining ring.

Ground to Retaining Ring


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


The End

www.generatortechnicalforum.org

Visit The IGTC and Join International Generator Technical Community

It’s Free and Informative!

http://www.generatortechnicalforum.org/

http://www.generatortechnicalforum.org/

Rotor Winding Problems - ccj-online.com · Rotor Winding Problems • High Vibration ... Reference:...

Documents

Transcript of Rotor Winding Problems - ccj-online.com · Rotor Winding Problems • High Vibration ... Reference:...