transformer short-circuit testing -...
TRANSCRIPT
DNV GL © 2014
Ungraded
16 July 2016DNV GL © 2014
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Transformer short-circuit testing
KEMA Laboratories’ experience
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Shankar Subramany
IEEE Transformer Committee Meeting
Vancouver, October 2016
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Testing at KEMA, Netherlands
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High Power Laboratory, Arnhem
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high-voltage
laboratory
new UHV
synthetic
installation
MV test labextension of
generator hall
power
transformer
test site
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KEMA Lab expansion: prepared for UHV transformers
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2 new generators installed
UHV synthetic installation
testing of test
transformer 1/4
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Preparing an 800 kV transformer for a short-circuit test
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features of the test circuit
� source can be kept on floating
potential
� pre-set test method preferred
� 1,5 phase method preferred
� Very fast master breaker
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Power transfer source to test-object
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The 1.5 phase method results in similar mechanical stresses as full 3-phase testing at the critical instant of asymmetrical current peak.
Leading laboratories use the 1.5 or 3-phase method to realize internal stresses as in service
current in 1.5 phase test
current in 1.5 phase test
current in three-phase test
current in three-phase test
full asymm. current in one phase moment ofmaximum stress
I
½I
½I
Transformer under test
Make switch
Supply
1,5Uphase
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Up to 800 kV class transformers
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0
100
200
300
400
500
600
700
800
900
0 200 400 600 800 1000 1200
vo
lta
ge
(k
V)
power (MVA)
1 phase
1.5 phase
3 phase
tested
2008-2014
after extension
acceptance test of a new 400 kV test transformerExtension project: from 4 to 6 generators
from 6 to 10 short-circuit transformers
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Short-circuit testing in grid- and generator supplied labs
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must be very fast in order to prevent test
transformer from destruction at a defect
must be very accurate in order
to apply adequate asymmetry
special design: 6 ms
special design: 3 deg
switching
at high-
voltage
switching at
medium-
voltage
moderate inrush current
high inrush current
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Testing
� IEC 60076-5
� three-phase transformer tested with
9 short-circuit applications of 0.25 s
� Criterion is impedance change before → after short-circuit current application
� ∆X ≤ 1% for transformers ≥ 100 MVA +
Pass routine tests and visual inspection
after test
� Testing is not destructive
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Some discussions in the IEC Maintenance team
� Impedance change trend during testing
� Number of short-circuits in normal service life?
� Life of transformer after short-circuit tests – testing is not destructive
� How best to test the tertiary windings?
� When and how mock-up transformers can be made and tested?
� What design review can show and what it cannot?
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What to learn from test results?
� Possibility to assess 'winding status' through evolution of reactance
measurement
� Hidden damage to voltage regulation winding
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0 1 2 3 4 5 6
0
0
0
TAP 5 TAP 3 TAP 1Isc=82% Isc=100% Isc=118%
Phase U Phase V Phase Wunder test under test under test
new
Sho
rt c
ircu
it im
peda
nce
(0.
5%/d
iv)
0 1 2 3 4 5 6test number
Phase U Phase V Phase Wunder test under test under test
new
TAP 5 TAP 3 TAP 1Isc=90% Isc=100% Isc=110%
0
0
0
phase U
phase V
phase W
variation of reactance after short-circuit tests
systematic (design?) flaw particular (production?) flaw
1%
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Summary results
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
1 2 3 4 5 6 7 8 9 10 11
maximum taprated tap
maximum tap position
after test
0.0
0.2
0.4
0.6
0.8
1.0
test number
minimum tap position
rated tap position
phase 1
phase 2
phase 3
after test
phase 3
after test, SC impedance of winding excl. regulation << with regulation
increase of reactance is entirely due to regulation winding
stress on phase 1
stress on phase 2
stress on phase 3Increase of reactance of regulation winding >> 1%
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Results
� 1996 – 2015: 297 tests series ≥ 25 MVA
� upto to 440 MVA, 525 kV
� in 230 cases no problems
at short-circuit
� 67 cases a problem, mostly
reactance change too large
� 39 transformers re-tested
� 78 visual inspection,
7 failed in that stage
� initial failure rate 67/297 = 23%
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Test statistics ≥ 25MVA
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0
5
10
15
20
25
30
35
40
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015
num
ber
of te
sts
initially not OK initially OK
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0
10
20
30
40
50
60
70
80
90
100
110
20-100 100-200 200-300 300-400 >400
num
ber
of t
ests
kV (rated)
initially not OK
initially OK
0
10
20
30
40
50
60
70
80
90
25-50 50-100 100-200 >200
num
ber
of t
ests
MVA (rated)
initially not OK
initially OK
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significant higher failure rate at high-end of power and voltage
Test statistics ≥ 25MVA
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Visible failure modes
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Broken bushing
Oil spills
Oil spill with fire
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Does calculation cover all aspects of short-circuit withstand?
� Calculation method given in the Standard is only based on static forces and do not cover all
parts of the transformer. The dynamic behaviour is not considered
� Following design aspects are not/cannot be addressed fully:
– Materials are considered ideal, and do not take into account poor workmanship, inhomogeneities,
production issues
– Models consider axial and rotational symmetry, whereas failure frequently are observed at
inhomogeneity’s like transpositions, crossover locations, pitch of winding, leads etc.
– Do not take into account friction of turns in axial direction, magnetic phase to phase interaction
– cross overs of turns (inside the winding) transpositions of parallel conductors (inside the winding)
– exit leads of the windings (fixation to prevent movement and friction (wear of insulation) of exit lead)
– support of cleats and leads
– connections to OLTC
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– support of leads to bushings
– stability of the radial support of windings (for example spacers used during winding the coil (untreated,
dried, dried and oil impregnated)
– effect of varying densities of the different windings due to axial compressing forces
– dynamic movement of the oil
– Successive movement of the winding, core, clamping arrangements and leads due to accumulated stresses
during a series of short-circuit tests, leading to a failure after a certain number shots cannot be
simulated/calculated
– Many observed failure modes cannot be predicted by models, like oil spill, broken bushings, internal
winding, deformation of leads.
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Does calculation cover all aspects of short-circuit withstand?
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transposition between layers
windings are not
rotational symmetric
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The actual value is the calculated value of the transformer under review. The allowable value is the design criteria of the manufacturer (including all his experience, tests, statistics and safety margins) to meet the specification. The critical value of a certain parameter is a number, one only can determine this if there were failures. In the case of lack of test data on this parameter it is just an estimated guess.
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..this is what that needs to be avoided…
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SAFER, SMARTER, GREENER
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Thank you for your kind attention!
before test after test