mpowering kema 2011 - matelec group kema 2011_0.pdf · mpowering energy pfdprof.dr. rppr.p ... •...
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Power Transformers
Mpowering EnergyP f d R PP S tProf.dr. R.P.P. Smeets
KEMA, the Netherlands
Conference Title
KEMA• World market leader intesting and certificationf HV i tof HV equipment
• World’s largest high‐World s largest highpowerlaboratory (8400 MVA)
• Test laboratories in‐ Netherlands‐ Netherlands‐ USA‐ Czech Republic
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Conference Title
Short‐circuit stresses transformersShort circuit stresses transformers• Every transformer sees several full short‐circuits in its
service life (CIGRE WG A2.12)• Mechanical stresses to windings are very severe.• Deformation of winding can reduce cooling and lifetime.
• Verification of mechanical strength can be through:‐ Short‐circuit testing, most reliable, service simulation.Design review cost wise solution but less reliable‐ Design review, cost‐wise solution, but less reliable.
• CIGRE WG12.19: From all transformers that failed because of short‐circuit 1/3 passed 'design review' successfully but none underwent a short circuit testsuccessfully but none underwent a short‐circuit test.
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Conference Title
Short‐circuit testsShort circuit tests• From IEC 60076‐5 (2006)• Most important are the electro‐Most important are the electro
dynamicalforces due to short‐circuit current
• KEMA NL performs 70 ‐ 80 tests onKEMA NL performs 70 80 tests on powertransformers per year
• Criterion to pass is change of reactanceCriterion to pass is change of reactancedue to short‐circuit current passage
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Conference Title What transformer for h t i it t ti ?short‐circuit testing?
• Key feeding transformers at power plant sub‐stationsKey feeding transformers at power plant sub stations or huge load centers
• Strategic intertie transformers, three‐winding system transformers (tertiary), auto‐transformers
• Series of transformers, one taken out for a "type" testapproach for distribution transformers‐ approach for distribution transformers
• Transformers with axially split winding connections• Transformers in networks known for high faultTransformers in networks known for high fault incidence or high fault current levels‐ Track feeding transformers
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Conference Title
Electro‐dynamical short‐circuit forces(Overhead line 40 kA ‐ 0.2 s)
this is a high‐speed movie
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normal condition10
20[p
u] Irated
-10
0
Cur
rent
rated
-40 -20 0 20 40 60 80 100-20
400
200
300
400
[pu]
100
200
Forc
e
force due to Irated
-40 -20 0 20 40 60 80 1000
Time [ms]
symmetrical short‐circuit current
10
20u] I
Isc sym
-10
0
Cur
rent
[p Irated
Short‐circuit at voltage maximum
-40 -20 0 20 40 60 80 100-20
300
400
pu]
100
200
Forc
e [p force due to
Isc symforce due to
Irated
-40 -20 0 20 40 60 80 1000
Time [ms]
asymmetrical short‐circuit current
10
20u]
Isc asym
I
-10
0
Cur
rent
[pu Irated
Short‐circuit at voltage zero
-40 -20 0 20 40 60 80 100-20
g
300
400
pu]
force due toIsc asym
100
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Forc
e [p
force due to Irated
-40 -20 0 20 40 60 80 1000
Time [ms]
relationship current ‐ force
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20u]
Isc asym
-10
0
Cur
rent
[pu
Isc sym
Irated
-40 -20 0 20 40 60 80 100-20
300
400
u]
F ‐ Isc asym Pulsating force!
100
200
Forc
e [p
F ‐ Isc sym
F I-40 -20 0 20 40 60 80 1000
Time [ms]
F ‐ Irated
Conference Title
pulsating forces on windingp g g
this is a high‐speed movie
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Conference Title
Possible damage by radial forcesCircular winding
Free buckling
External bulgebuckling
Axial supporting strips Compressive force
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Conference Title
Possible damage by axial forcesTilting F
Normal position F
Tilted conductors
limits oil flowlimits oil flow
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Conference Title Transformer testing at KEMA
Matelec Certified 400 & 1000 kVA, 2010
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Conference Title
KEMA's experience withtesting power transformers
• available power 8400available power 8400 MVA 3‐phase 16 ‐ 60 Hz.
• Large transformerstested on board oftransporting vesseltransporting vessel.
• Fast protection throughthrough special breaker.Limits damage in caseof internal failure.of internal failure.
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Conference Title
test results transformers > 25 MVA• 157 times a large transformer > 25 MVA offered 1996 2010 (140 transformers in 15 years)1996 – 2010 (140 transformers in 15 years).
• largest tested object250 MVA 1‐phase and 440 MVA 3‐phase
• evaluation of performance by:‐ reactance measurement before and after test‐ visual inspection (possibly at manufacturer's site)p (p y )
104 no short‐circuit problem at test site, but further results (inspection, routine tests) may be unknown(inspection, routine tests) may be unknown
36 faced problem at first access 17 re‐tested succesfully initial failure rate 26%
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Conference Title
Transformers25O Transformers
> 25 MVA15
20
num
ber o
f tes
ts
initially not OK
initially OK
1996 ‐20100
5
10
n
45 initially not OK
01996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
year Rapid rise in demand of short‐circuit tests
Failure rate not
25
30
35
40
ansf
orm
ers
initially not OK
initially OK Failure rate not significantly depending on MVA or kV rating
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10
15
20
num
ber
of t
ra
g
025-50 50-100 100-200 >200
MVA (rated)
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Conference Title
70
test results transformers <= 3 MVA
40
50
60
70
NOK
OK• Period Jan 2009 ‐ Apr 2011
28 h N h l d
10
20
30 28 months, Netherlands• 130 transformers tested• 50 – 3000 kVA
0
50‐500 500 ‐ 1000 1000 ‐ 2000 2000 ‐ 3000 kVA
120
• 4.16 – 33 kV
• 82% passed sc test
80
100
120
NOK
OK
17%
82% passed sc test• 18% failed to pass
Mostly internal deformation.Impedance change too large
20
40
60
22%
Impedance change too large.
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0
dry oil
Conference Title
buckling in: MV tap winding HV winding HV winding
tilting HV axial shift spiralling split layer 19
Conference Title
Observed failure modes• Axial clamping system: Looseness of force axial clamping, of axial compression force, of axial supporting spacers of top and bottom insulating blocksaxial supporting spacers, of top and bottom insulating blocks
• Winding deformation:Axial shift of windings, buckling of windings, spiralling of windings (helical or layer winding)( y g)
• Cable leads:Mechanical movement (tapchanger to regulating windings)Deformed or broken leads, outward displacement of exit leads from inner windings broken exit leadsfrom inner windings, broken exit leads
• Insulation:Crushed and damaged conductor insulation, displacement of vertical oil‐duct spacersvertical oil duct spacersDielectric flashover across HV‐winding, displacement of pressboard insulation, Tank current due to damaged conductor insulation, B hi LV b hi
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• Bushings: LV‐bushings
Conference Title
ConclusionsConclusions• Short‐circuit testing is the most realistic tool in QA to
verify withstand of electro‐dynamical stresses of short‐circuit
• KEMA finds failure rate against short‐circuit stresses:KEMA finds failure rate against short circuit stresses:26% transmission transformers >= 25 MVA18% distribution transformers <= 3MVA
• No common failure mode, mostly winding deformation
• Reactance change (confirmed by visual inspection) as• Reactance change (confirmed by visual inspection), as prescribed in the standard is a very good evaluation tool
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