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

2

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.  

3

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

4

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

5

Conference Title

Electro‐dynamical short‐circuit forces(Overhead line 40 kA ‐ 0.2 s)

this is a high‐speed movie

6

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

200

Forc

e [p

force due to Irated

-40 -20 0 20 40 60 80 1000

Time [ms]

relationship current ‐ force

10

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

11

Conference Title

Possible damage by radial forcesCircular winding

Free buckling

External bulgebuckling

Axial supporting strips Compressive force

12

Conference Title

Possible damage by axial forcesTilting F

Normal position F

Tilted conductors

limits oil flowlimits oil flow

13

Conference Title Transformer testing at KEMA 

Matelec Certified 400 & 1000  kVA, 2010

14

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.

15

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%

16

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

5

10

15

20

num

ber

of t

ra

g

025-50 50-100 100-200 >200

MVA (rated)

17

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.

18

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

20

• 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 

21