ABB
2019-11-22
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Transformer Short Circuit
Withstand Capability
Evolution of Short Circuit Withstand Capability of
Transformers 1885-1950
§ Low Transformer impedance for minimum voltage regulation
§ Increased Transformer rating + parallel generators > Transformer
failure from SC currents - Short Circuit test done at site for failure
mode study
§ Protective function of impedance > Increased percentage impedance
§ 1911–C.P. Steinmetz – AIEE Paper “Mechanical forces from magnetic
fields”
§ 1925 &1935 – E. T. Norris –Short circuit Testing of 1.2MVA & 80MVA
Transformers in UK
§ 1940-Short Circuit withstand capability - Studies in UK (ERA) &
Germany
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Transformer Short Circuit
Withstand Capability
Evolution of Short Circuit Withstand Capability of
Transformers 1885-1950
§ Thermal heating and conductor stretching by hoop stress –failure
modes considered under short circuit currents
§ C57.1942 on Transformer – Permissible duration of short circuit
currents–2 sec for 4% impedance & 5 sec for 7% impedance
transformers
§ Design guide lines – Max current density 4A/mm2; hoop tensile stress
<60N/mm2
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Transformer Short Circuit
Withstand Capability
1950-1980
§ Growth of Power system with grid interconnections; Transformer size
reach 1000MVA by 1970
§ Failure from short circuit current major problem in France, Italy, US,
Soviet Union where interconnection through low impedance autotransformers
§ 1960s – 62% of transformer failures in US - from through fault current
§ Germany – high impedance (15-24%) two winding transformers for
interconnection & sub-transmission grid neutral grounded through
Peterson coil. Immunity from failures due to short circuit currents
§ India:
§ 1960s -Failures in three winding transformers
§ 1970s -Tertiary failure in 100MVA 220kV auto-transformers
§ 1970’s-Tertiary failure in small power transformers prevented by opening
out stabilizing tertiary; elimination of stabilizing tertiary in transformers up
to 100MVA
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Transformer Short Circuit
Withstand Capability
1950-1980
§ Fundamental study on short circuit forces generated:
§ Radial and dynamic axial forces
§ Dynamic stresses on winding parts & structures
§ Withstand strength estimation
§ 6 CIGRE sessions (1958,1962,1968,1972,1980,2000) and 7 colloquia
(1957,1961,1967,1971,1995,1999,2019) with short circuit withstand
capability as special subject
§ Digital computers (1950s) for calculation of winding leakage flux
distribution – Accuracy in short circuit force calculation and means for
minimizing forces by ampere turn balancing
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Transformer Short Circuit
Withstand Capability
1950-1980
§ Sophisticated magnetic flux field calculation methods based on
§
§
§
§
Method of images
Analytical methods (E Roth 1928, Rabins 1956)
Numerical methods (Finite Element Method)
Static force calculation methods well documented in CIGRE WG 12.04
Study Report of 1979 (Electra No.67,1979 Pages29-75)
§ Dynamic response to forces (due to inertia of conductor, friction,
elasticity of insulating material, internal solid viscous damping,
hydrodynamic effects of oil motion) against withstand capability
§ Researchers study by short circuit testing partial winding models,
simulating full scale transformer forces
(Study by Brown Boveri, GE, Hitachi, Soviet Union, UK)
§ Most factories had in house Power Lab for conducting above tests
§ 1966 – First book on the subject – Short Circuit strength of
Transformers - M. Waters, London, UK
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Transformer Short Circuit
Withstand Capability
1950-1980
Pioneers for short circuit withstand study
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UK
E. Billing
M. Waters
E.T.Norris
(ERA)
USA
R B Ayers
W J McNutt
M R Patel (GE)
P L Bellaschi
(Westinghouse)
L Rabins
Italy
G.Bertagnolli
L. Bergonzi
(ABB)
Netherland
A K Bos (SMIT)
Japan
K Hiraishi
Y Hori
K Okuyama
France
Y Tournier
(Alstom)
Austria
G Preininger
(Elin – VA Tech)
Sweden
E Stenkvist
L Torseke
Germany
E Pirktl
M Christoffel
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Transformer Short Circuit
Withstand Capability
1950-1980
§ 1957-1980 – Regular short circuit testing of Distribution Transformers
and a few large Power Transformers
Poland (1957)
Soviet Union
(1970-90)
US (1960-70)
France (1956)
India (1975)
Netherland
Italy (1970s)
China (1965)
§ Standards for Short Circuit testing Procedure
§ IEC 76-5 (1976) >> IEC 60076-5 ed3.0 -2006
§ ANSI / IEEE C57.12.90 a & b (1974 & 1978) >> ANSI / IEEE
C57.12.90-2015 (C57.12.90 had a part II “Guide for Short Circuit
Testing of Distribution and Power Transformers” )
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Transformer Short Circuit
Withstand Capability
1950-1980
§ Dedicated high power laboratories and on-line testing stations come
up in various parts of the world
§ EdF France, Les Renardiers Test lab – 1080/3MVA & 1650/3MVA
400kV Single phase transformers short circuit tested (1976-79)
1980-2000
Italy test 63 MVA 220kV (1980); short circuit testing of large
transformers introduced by ENEL & Soviet Union in 1980s; Turkey
(TEAS ) in 1990s
§ Manufacturers get benefitted from short circuit test data and failure
rate from SC test comes down
§ India short circuit tests first Power Transformer (13.5MVA 132kV Rail
Trackside) at CPRI Bhopal (1983) and China 8MVA (1984) at Xian HV
Apparatus Research Institute
§
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Transformer Short Circuit
Withstand Capability
1980-2000
§ Indian success story in improving short circuit withstand capability:
§ Indian Railway Track Side supply Transformers TELK 13.5 MVA (1983)
§ Nathpa Jhakri Power Corporation BHEL 102MVA single phase 420kV
(1999)
§ National Thermal Power Corporation ABB 315 MVA 765 KV single phase
GSU (2016)
§ ABB Canada test 775MVA 24/230kV 3 phase GSU at IREQ High
Power Test Station, Varennes (1997)
§ USSR tests 20 large transformers (1983-91) and 500 distribution
transformers (1960-1998) including 630MVA 500kV 3 phase Unit
(1991) at High Power Lab, Togliatti (Russia)
§ 1996 – Book by G Bertagnolli– Short Circuit Duty of Power
Transformers ed3.0 in 2008, ABB, Zurich
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Transformer Short Circuit
Withstand Capability
2000-2019
§ Extensive testing of large Power Transformers from India
§ IEC 60076-5 on Short Circuit Withstand Capability revised in 2006.
Three options to prove dynamic short circuit withstand capability
(Clause 4.2.1):
§ Direct test on a fully assembled unit (special test)
§ Theoretical evaluation by design review + manufacturing evaluation
§ Evaluation can be done:
§
§
Comparing with a similar transformer (as defined in Annexure B of standard)
that has passed short circuit test
Checking the stresses against short circuit strength as per manufacturer’s
design rules
§ Second option requires in-house or external experts
§ Current short circuit test failure rate in power labs - 20-30% (CIGRE
Science & Engineering No. 10-Feb-2018: Verification of short circuit
with stand capability of transformers – R P P Smeets, KEMA)
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Transformer Short Circuit
Withstand Capability
2000-2019
§ New on line High Power test station at Bina, India (2017). KEMA
upgrades lab to 15,000MVA (2016). Both labs can short circuit test
large rated power transformers up to 765kV class.
§ 2014 – ABB & Siemens conducts short circuit test on mock up
transformers in lieu of full short circuit test on 570MVA 420kV Single
Phase GSU. 1/3 - 1/4 axially scaled down model but radially same
dimensions as actual transformer to simulate the same forces as in
actual unit. Advantage - in addition to short circuit withstand, test
current can be increased to create failure to study weak areas and
safety margins.
§
2014 – Second book by G Bertagnolli – Power Transformers & Short
Circuits - Performance Evaluation, ABB Zurich
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Transformer Short Circuit
Withstand Capability
Present Scenario & Proposals - Users
§ Short Circuit Test a Special Test and not Type test
§ SC test is expensive – 20-30% of unit transformer cost and for large
transformers 40-70% + cost of risk, delay and extra factory tests
§ Limit the number of SC tests:
§ Rationalize and standardize ratings to groups so that only one
transformer rating needs to be tested and balance by theoretical
evaluation as per IEC
§ Only when ordered lot >10 units; to justify test charges
§ Validity of test report – minimum 10 years
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Transformer Short Circuit
Withstand Capability
Present Scenario & Proposals - Users
§ Building short circuit withstand capability through specifications:
§ Avoid stabilizing tertiary if system permits (YNyn) for 3 phase 3 limbed
construction
§ Select interwinding impedances in three winding units to limit short
circuit currents to reasonable levels
§ Radially self supporting windings and proper sequencing of
winding positions
§ IEC 60076-5 revision (ed4.0) expected in 2022
§ Include data as per Annexure B (IEC 60076-5 ) as part of SC test
report.
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Transformer Short Circuit
Withstand Capability
Present Scenario & Proposals - Manufacturers
§ Short Circuit withstand capability – 10% specifications + 30%
manufacturing + 70% design & material quality
§ Electrical and Mechanical design based on proven rules and software
validated by full scale short circuit tests
§ Design for minimum stress and maximum withstand strength – type
of windings, self supporting against buckling, sequence of
positioning, type and grade of materials
§ Not to compromise short circuit strength for energy efficiency &
cooling –conductor size
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Transformer Short Circuit
Withstand Capability
Present Scenario & Proposals - Manufacturers
§ Quality control – conductor yield strength, epoxy glue strength,
mechanical strength of insulation
§ Tightly wound windings, properly sized and stabilized by prestressing
§ Reduce manufacturing variation in winding dimensions
asymmetry and misalignment between the windings
and
§ Learn from short circuit testing and failures – document the same
through “cook” books and use it to validate the design rules and
systems.
§ Make all transformers as if going for short circuit test !!!
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