H
YD
R
O
G
EN
ER
AT
O
R
ED
IT
IO
N
Top Ten
Reasons
Generators
Fail™
NATIONAL ELECTRIC COIL
©1999-2017 National Electric Coil
Printed in the USA
800 King Avenue
Columbus, Ohio 43212 USA
Phone: +1 614-488-1151 • FAX: +1 614-488-8892
www.highvoltagecoils.com
www.national-electric-coil.com
Top Ten Reasons Generators Fail™
PROBLEM IDENTIFICATION & FAILURE PREVENTION METHODS
P OC KE T F LI P CHART
Pr o vi d i n g an e a s y r e fe r e n c e
for co mmo n g en e r a to r p r o b le m s
The information in this booklet is organized to assist
you in your generator maintenance program, so you
can effectively:
•
Determine what components to inspect.
•
Know potential failure modes and their common
symptoms.
•
Understand the short-term and long-term
consequences of failure.
•
Take recommended actions.
If you have questions about the information contained
in this booklet or would like to discuss generator
problems in greater detail, please contact:
Mark Crittenden
National Electric Coil
+1 614-488-1151 x125
mcrittenden@national-electric-coil.com
Printed in the USA
©1999-2017 National Electric Coil
Top Ten Reasons Generators Fail™
TA B L E O F C O N T E N T S
Great Ideas for Generator Maintenance
1.
Cracking of Rotor Mechanical Components
2.
Cracking of Stator Mechanical Components
3.
Failure of Rotor Pole to Pole Connectors
4.
Stator Core Buckling
5.
Stator Core Heating Damage
6.
Stator Core Lamination Migration & Shifting
7.
Stator Winding Faults
8.
Stator Slot Wedge Loosening & Migration
9.
Stator Winding Partial Discharge
10. Foreign Object Intrusion & Contamination
©1999-2017 National Electric Coil
Printed in the USA
Tip: Visit www.national-electric-coil.com or
www.highvoltagecoils.com to join and access NEC’s Online
Technical Library. Look for our Specialized Engineering
Solutions™ (SES™) and Technical Maintenance Memos™
(TMM™). They are great diagnostic resources! Already a
member? Please contact one of our lrepresentatives listed in
this book for copies of these relevant and helpful documents.
Cracking of Rotor Mechanical Components
Figure 1 shows large crack in the braking surface of a hydrogenerator rotor. Figure 2 shows a large crack in a dovetail of
a field pole.
Printed in the USA
©1999-2017 National Electric Coil
Cracking of Rotor
Mechanical Components
Prevention: Visually inspect for signs of distress. Look for
telltale lines in painted areas. Perform Magnetic Particle
Tests or Dye Penetrant Tests when components are available during an outage or refurbishment project. Crack
removal by grinding can be done but a thorough evaluation
and/or analysis should be incorporated. Weld repair of a
crack may be necessary to restore the component back to its
original function. It is important that proper preheat and post
heat operations are done to prevent residual stresses and
subsequent distortion in the component.
©1999-2017 National Electric Coil
5
Printed in the USA
Cracking of Rotor Mechanical Components
Problem: Fatigue-initiated cracks can occur in hydrogenerator
rotor mechanical components such as the rotor spider and
field poles. Rotor spider stresses due to the forces from rotation, gravity and thermal growth can initiate cracks in high
stress concentration areas. Other cracks can occur in older
cast steel rotors with poor material properties. Overspeed
events and associated braking can also be a contributor to
hydrogenerator rotor cracking. Likewise, field pole dovetails
experience high stresses to keep the weight of the field pole
and copper winding attached during rotation.
Cracking of Stator Mechanical Components
Figure 1 shows the back of the hydrogenerator stator at the
weld between the frame and the keybar support. Figure 2
shows a crack initiating from a tack welded core bolt nut.
Printed in the USA
©1999-2017 National Electric Coil
Cracking of Stator
Mechanical Components
Prevention: Don’t overlook visual inspection as a valuable tool. Perform Non-Destructive Evaluation (NDE) when
components are available during an outage or refurbishment.
Crack removal by grinding can be done, but not without a
thorough evaluation and/or analysis. Weld repair of a crack
may be necessary to restore the component back to its
original function. It is important that proper preheat and post
heat operations are done to prevent residual stresses and
subsequent distortion in the component.
Tip: NEC has been involved with multiple projects that required
repairs or redesigns of hydrogenerator frame elements. Contact
NEC for engineering at +1 (614) 488-1151 to discuss the analysis
process for re-design and the field service repair options.
Also, CEATI International maintains a well-referenced Guide for
Erection Tolerances and Shaft System Alignment. In 2008, a chapter on Modern Alignment Tools was added to the original five chapters. The former R&D program of the Canadian Electric Association,
CEATI began independent operation in 2001 as CEA Technologies
Inc. and in 2008 became known as CEATI International. This document and others can be purchased through the CEATI website,
www.ceati.com.
©1999-2017 National Electric Coil
7
Printed in the USA
Cracking of Stator Mechanical Components
Problem: Fatigue initiated cracks can occur in hydrogenerator
stator mechanical components such as the frame, including gussets and supports, as well as pressure plates used
for clamping. As machines age, distortion of the frame and
core often occurs. Temperature cycles, coupled with torsional
stresses, can initiate cracks at gussets and other support
areas where stress concentration factors are high.
Failure of Rotor Pole to Pole Connectors
Both figures show electrical arcing damage from a failed pole
to pole connector on a hydrogenerator rotor.
Printed in the USA
©1999-2017 National Electric Coil
Failure of Rotor
Pole to Pole Connectors
Prevention: First and foremost, always replace rotor pole to
pole connectors when field coils are refurbished as part of
a rewind project. Give strong consideration to a laminated
lead design, with a flexibility loop, if the original design does
not have these features. Ensure that the field poles are
seated tightly to avoid looseness, rocking and vibration of
the field pole that can lead to fatigue cracking of the connector. Measure the field winding resistance every outage
and trend over time. An increase in field winding resistance
could indicate a crack in the copper circuit. Visually inspect
hydrogenerator field leads looking for cracks – especially if
the design is solid copper with no flexibility loop.
Tip: The technical paper “Refurbishment Solutions for Vari-
ous Field Pole Problems” discusses fatigue of field pole
connectors, as well as other issues such as the bowing of
the rotor field poles, looseness of the field poles on the rotor,
and cracking of field pole damper bars. Contact us to obtain
a copy.
©1999-2017 National Electric Coil
9
Printed in the USA
Failure of Rotor Pole to Pole Connectors
Problem: Fatigue cracking of the rotor pole to pole connector is a common failure mode with hydrogenerators. These
components are susceptible to cyclic fatigue stresses with
each start / stop cycle of the machine. Forces due to rotation and thermal heating are contributors, but the design of
the connector, including the attachment, plays a key role.
Other factors include rotor rim distortion, field pole distortion,
and locking key tightness.
Stator Core Buckling
Figure 1 shows the typical “wave-like formations that can
be seen from the back of the core when buckling occurs.
Figure 2 shows core buckling at the core split from a different angle.
Printed in the USA
©1999-2017 National Electric Coil
Stator Core Buckling
Problem: Stator core buckling can occur on older machines
that have seen many years of service and repeated cycles.
Older cores with splits are particularly susceptible. Buckling
can initiate coil failures especially in the split areas as coil
insulation can be damaged due to the displacement of the
two core sections. Buckling can also occur if the core and
frame are not properly designed for relative thermal expansion. Some units are operated beyond or outside the design
capability curve, whether intentional or not, and this can
contribute to this problem.
Tip: VibrosystM’s Zoom® software can monitor the hydrogenerator air gap between the rotor and stator and detect
out of round situations. NEC’s SES™ 600 discusses what
makes an effective hydrogenerator stator core clamping
system and also potential improvement ideas.
Magazines such as Hydro Review and Hydro Review Worldwide, publish articles on various hydrogenerator problems.
PennWell’s Hydrovision International conference on hydrogeneration is an excellent source of technical information.
©1999-2017 National Electric Coil
11
Printed in the USA
Stator Core Buckling
Prevention: Visually inspect the stator core from the back to
tell if the laminations form a wave pattern. Keep good notes
and trend over time to see if the condition is worsening.
Check the torque on the core clamping bolts and make sure
they are tight. Be sure that the frame and core have a mechanism to allow relative thermal expansion. Once a buckling
situation starts to occur on a core, it is nearly impossible
to reverse, aside from a complete core restack. Likewise,
severe distortion of the core could involve re-rounding of the
core and frame.
Stator Core Heating Damage
Figure 1 shows core overheating damage due to a stator coil
failure. Figure 2 shows evidence of insulation deterioration
and overheating after removal of core laminations.
Printed in the USA
©1999-2017 National Electric Coil
Stator Core
Heating Damage
Problem: Stator core laminations can overheat to the
point of melting. Deterioration and/or wearing away of the
thin coating of insulation on each lamination can result in
the shorting of laminations. Shorts generate heat, which
further deteriorates local lamination insulation, leading to
a larger hot spot and eventual melting of the core. Core
overheating damage is also frequently caused by a failed
coil.
Tip: NEC’s SES™ 400H discusses hydrogenerator core
restacks. TMM™ 409 addresses stator core testing. Contact
us for a copy of the useful documents.
Check out magazines such as the International Water Power
& Dam Construction and The International Journal on Hydropower & Dams. They publish technical articles on generator
issues. Aqua-Media’s Hydro conference is an annual European conference offering technical presentations and papers
on hydrogenerators.
©1999-2017 National Electric Coil
13
Printed in the USA
Stator Core Heating Damage
Prevention: Maintaining the reliability of the stator winding by regular electrical testing is critical to prevent stator
core damage. The stator core must also be kept tight to
prevent vibration and wearing away of lamination insulation. Check the torque on the core bolts at major outage
intervals. Any time the rotor is removed, the core should
be tested by ELCID or Core Loop Test to identify any hot
spots that can be corrected. Repairs of overheated and
damaged cores can range from localized mica splitting
insertions, to partial core restacks, to full core replacements.
Stator Core Lamination Migration / Shifting
Figure 1 shows fatigue failure of the lamination tooth that
initiated a stator winding failure. Figure 2 shows shifted laminations at a core split that caused a core hot spot.
Printed in the USA
©1999-2017 National Electric Coil
Stator Core Lamination
Migration and Shifting
Prevention: The stator core must be kept tight to prevent vibration and wearing away of the insulation coating. Tightness can
be checked with a knife inserted between laminations, or torque
measurements on the core clamping bolts. Core hot spots can
be identified with an ELCID test (100 milliamp threshold action
investigation level) or a Core Loop Test (no hot spot greater than
10 deg. C. above the average core temperature for an older
core). Local hot spots can be repaired by insertion of mica splittings. More significant damage can be repaired by a partial or
full stator core restack. New, replacement cores are often specified along with a rewind. New cores can typically be stacked
continuously, eliminating the troublesome splits.
Tip: Two important core lamination test standards include (1)
ASTM A343 (Epstein test) a test that measures iron losses
in watts/lb. (2) ASTM 717 (Franklin test), used in testing teh
interlaminar resistance coating of the iron. Also take a look at
“Repair and Replacement Considerations for Old Hydrogenerator Stator Cores.” This technical paper, presents four case
studies ranging from repairs and a partial core restack to full
restack. One case study presents the results of a cost/benefit
analysis comparing laser-cut laminations versus die-cut laminations.Contact us for a copy.
©1999-2017 National Electric Coil
15
Printed in the USA
Stator Core Lamination Migration & Shifting
Problem: Stator core laminations that are loose can vibrate,
wearing the insulation coating off the lamination causing shorts
and hot spots. In severe cases of loose cores, or cases where
the compression loading of the core is not carried all the way to
the top of the tooth, vibration can be so extensive that the core
tooth tip fatigues and migrates into the coil insulation causing a
ground fault. Older units, especially those assembled in sections
with splits, can see significant distortion at the core splits. Core
splits can become localized hot spots that damage nearby coil
insulation.
Stator Winding Ground Fault
Figure 1 shows the location of failed coils removed from the
core. The unit can now be returned to service. The coil in
Figure 2 failed due to an upset in mica tape application during the original coil manufacturing stage.
Printed in the USA
©1999-2017 National Electric Coil
Stator Winding Faults
Problem: Stator winding faults can initiate for several
reasons. One reason is a failure in the turn insulation within
a multi-turn coil. Turn faults can occur due to transient disturbances, thermal aging and operational stresses. Failures in
the ground insulation can also take the unit offline. Aging of
both the turn and ground insulation can occur over time as
insulation breaks down from thermal cycles, possible over
temperature operation, mechanical vibration and electrical
stress. Upsets in the insulation, such as shown in Figure 2,
are usually caught during the winding installation phase.
Tip: It is often possible to isolate a failed hydrogenerator
stator coil from the rest of the winding to continue use of
the generator in a reduced capacity. NEC is able to predict
the capacity reduction and perform this isolation. Refer to
the following IEEE standards for electrical tests to evaluate the condition of new windings and older windings: IEEE
4, 43, 56, 95, 115, 286, 522, 1043 and 1553. International
standards include IEC 60034, 60085, 60216, 60505, 60727,
60792, and 60894, as well as the BSI’s EN 50209. Applicable ANSI standards include C50.10 and C50.12.
©1999-2017 National Electric Coil
17
Printed in the USA
Stator Winding Ground Fault
Prevention: Electrical testing of the winding is a key step
in reliability. Maintenance tests include measuring winding
resistance, insulation resistance and calculation of Polarization Index, as well as over-potential tests such as a DC
Leakage test, Ramp Test, or a low level HIPOT test. Visual
inspections at frequent intervals are a must to detect winding
deterioration. This would include partial discharge activity,
dusting, greasing, broken ties, migrated slot wedges and so
on. On rewinds, some companies eliminate the multi-turn
coil design to remove that particular failure mode.
Stator Slot Wedge Loosening and Migration
Figure 1 shows a dark, grease-like substance extruding
out from underneath the stator slot wedges. Figure 2
shows a migrated stator slot wedge, stopped by a figure-8
tie. The gap shows the extent of the wedge movement.
Printed in the USA
©1999-2017 National Electric Coil
Stator Slot Wedge
Loosening and Migration
Problem: Stator wedges are designed to keep the stator coil
tight in the slot. Loose wedges can allow vibration of the coil due
to electro-magnetic forces. This can lead to insulation erosion
and copper strand cracking. On vertical hydrogenerators, stator
slot wedges must be locked, otherwise they can migrate and
eventually fall out at the bottom of the machine.
Tip: Top ripple springs inserted underneath the stator slot wedge
are an excellent method of keeping the coils and wedges tight
in the slots. Figure 8 ties are commonly used to keep the slot
wedges locked. Avoid application of epoxy resin for the purpose
of locking in stator wedges. IEEE 1665 includes rewinds of
hydrogenerators.
©1999-2017 National Electric Coil
19
Printed in the USA
Stator Slot Wedge Loosening and Migration
Prevention: Stator wedge tightness can be evaluated on
hydrogenerators by measuring the deflection of the ripple spring
under the wedge or performing a tap test on the wedge. On
hydrogenerators, due to the difficulty and expense in removing
the massive rotor, one or two field poles can be removed for this
testing, if conditions warrant. A routine visual inspection looking
for migrated, loose or missing wedges at the top and bottom of
the stator core can provide early indications of problems.
Stator Winding Partial Discharge
Figure 1 shows partial discharge activity in the stress grading area. Figure 2 shows PD in the tight space between two
high voltage bus bars.
Printed in the USA
©1999-2017 National Electric Coil
Stator Winding
Partial Discharge
Prevention: The occurrence of PD on the stator winding is
highly influenced by the original winding design. Extensive
damage can be minimized, however, by routine maintenance
starting off with a good visual inspection and identification of
the problem areas before the condition becomes too serious.
These areas can be repaired using various combinations of
semi-conductive, gradient and epoxy paints, combined with
mica and tape, as needed. Regular PD testing can trend
levels of PD over time so that timely corrective action can be
taken.
Tip: Contact us for a copy of SES™ 407, which provides
a discussion of PD repair options. Consider installing a PD
monitoring sytem for long-term trending. IRIS Power is a major supplier of this type of monitoring equipment. They also
can provide typical PD values for your type of machine.
©1999-2017 National Electric Coil
21
Printed in the USA
Stator Winding Partial Discharge
Problem: Partial Discharge (PD) is very common on high
voltage air-cooled hydrogenerators. The transition area between the semi-conductive coating on the coil in the slot and
the gradient coating on the coil as it rounds the first bend out
of the core is a common place to see deterioration. Typically, deterioration in this area is indicative of a gradient coating
that is too high in resistance. Other common areas for PD to
occur is between high voltage coils, leads or busses that are
too closely spaced together.
Foreign Object Intrusion & Contamination
Figure 1 shows damage to the stator core iron by physical
impact from a foreign object. Figure 2 shows core and winding damage from a socket wrench left in the generator.
Printed in the USA
©1999-2017 National Electric Coil
Foreign Object Intrusion
& Contamination
Prevention: Regular maintenance including inspection,
cleaning and testing is the key. A check on the tightness
and locking of all hardware at more extended intervals is
recommended. A proper FME (Foreign Material Exclusion)
zone should be set up during inspection and maintenance
outages with careful inventory and control of all tools and
parts taken in and out of the unit. A final and thorough visual
inspection should be done at the end of each inspection and
maintenance outage to assure the unit is clean and ready for
service.
Tip: Contact us for a copy of the handout, “Contamination
Can Affect Generator Reliability” for an expanded discussion
of these critical issues.
©1999-2017 National Electric Coil
23
Printed in the USA
Foreign Object Intrusion & Contamination
Problem: Open, air-cooled hydrogenerators can be especially susceptible to foreign object intrusion and contamination. Foreign objects can include items such as a broken
pantleg washer, a tool left in the machine during a maintenance outage, a slot wedge that migrates out of the slot, or
a V-block pole support that moves into the air gap. These
objects can all have a devastating effect when impacting
the stator winding and core. Contamination can be caused
by the buildup of dirt and debris over time due to a lack of
cleaning, or it can be a sudden filling of the unit due to a
heavy rain or flooding situation.
Top Ten Reasons
Generators Fail™
A leading manufacturer of high-voltage
stator windings, National Electric Coil
(NEC) was founded in 1917. NEC specializes in high-voltage stator windings
and rotor field windings for large thermal
and hydro generators. Design expertise
includes stator diamond (multi-turn) coils
and Roebel bars, including inner-gas
and inner-liquid cooled bars. NEC manufactures stator windings utilizing both VPI and resin-rich (Bstage) insulation systems. NEC also manufactures all types of
generator rotor windings, including those with complex cooling
designs. All manufacturing is supported by ISO 9001-certified
quality management systems. For more information, visit
our websites or please contact:
International: Stephen Jeney at +1 614-580-9306 or
hvcoils@national-electric-coil.com
North America: Howard Moudy at +1 724-787-4967 or
sendinfo@national-electric-coil.com
NATIONAL ELECTRIC COIL
800 King Avenue
Columbus, Ohio 43212 USA
Phone: +1 614-488-1151 • FAX: +1 614-488-8892
www.highvoltagecoils.com
www.national-electric-coil.com
©1999-2017 National Electric Coil
Printed in the USA
This book belongs to: