Fahad Muhammad, MS, PhD Candidate* Kostis Oikonomou, MS, PhD Candidate Maria Koliou, MS, PhD Candidate Kong, Dohwan, PhD Hwasung Roh, PhD

Transcription

1 Testing of Transformer Bushings and Lessons Learned Fahad Muhammad, MS, PhD Candidate* Kostis Oikonomou, MS, PhD Candidate Maria Koliou, MS, PhD Candidate Kong, Dohwan, PhD Hwasung Roh, PhD Andrei M Reinhorn, PE, PhD Andre Filiatrault, PEng, PhD Anshel Schiff, PhD** * Presenting Author ** Consultant 1

2 Introduction Transformer bushings are key component in power distribution and transmission Section view of 550kV Bushing PEER 1999/05

3 Qualification Issues Bushings are designed and qualified / tested according to best knowledge to date; however, failures during earthquakes still occur. Why? 3

4 Observed Modes of Failure/Damage Rocking of internal segment of bushings (insufficient moment capacity) Sliding segments (insufficient shear friction) Porcelain cracking Long term impact on prestressing - clamping forces Breakage due to impact of other parts UPPER-1 Porcelain Unit Rubber Gasket Bushing Mounting Flange Turret Top flange 4

5

6 Overview Experimental Investigation Fixed base vs flexible mount properties Excitation ti motions at base of bushings Failure modes and capacity of bushings Seismic demand at base bushings Basic Issues Identified in Investigation Suggested Qualification Method 6

7 Bushing Testing to 3.5g PGA 7

8 Experimental Investigation MCEER engaged in an investigation sponsored by: California Institute for Energy & Environment (CIEE) of California Energy Commission (CEC) & Bonneville Power Administration (BPA) to find some answers. Tested 9 bushings 230 kv and 550 kv Several porcelain and several composites Tested fixed base in special set-up Tested on support structure simulating transformer tank Tests performed at SEESL at University at Buffalo 8

9 Typical Physical Properties of Bushings Used in Experimental Study Bushing#1 Bushing#2 Bushing#3 Bushing#4a Bushing#4b Bushing#5 Bushing#6 Bushing#7 Bushing#8 Manufacturer G.E. G.E. G.E. HSP HSP Trench ABB ABB ABB Provider CERL CERL CERL LADWP LADWP Trench ABB ABB ABB Material of Insulator Porcelain Porcelain Porcelain Composite Composite Composite Porcelain Porcelain Porcelain Voltage Capacity (kv ) / Total Height (in) Length over mounting Flange (in) Bushings tested Length below mounting flange (in) Max Dia. Over mounting flange (in) Max Dia. Below Mounting flange (in) Diameter of mounting flange (in) Bolt Pattern of Mounting flange (in) 16 Φ 1 1/8 / Φ31 12 Φ 3/4 / Φ28 12 Φ 3/4 / Φ28 12 Φ 1 1/4 / Φ21 12 Φ 1 1/4 / Φ21 12 Φ 1 1/4 / Φ32 12 Φ 1 1/4 / Φ21 12 Φ 1 1/4 / Φ32 12 Φ 1 1/4 / Φ23 Total Weight (lbs) Location of CG (above flange) (in) Upper Bushing Weight (lbs) Location of Upper Bushing CG (in) Lower Bushing Weight (lbs) Location of Lower Bushing CG (in) Connection Housing Weight (lbs) Weight per unit length (lbs/in) FixedBase Frequency (Hz)??? As installed Frequency (Hz)* Top Pull Load Fixed Base (lbs) Top Displacement Fixed Base (in) Stiffness Fixed Base (lbs/in) Designation GE-500 -TypeU GE 500 TypeU GE 500 TypeU HSP HSP D004C_3 196w0800bz T550W2000UD T550Z3000SE Note: the Upper\Lower Bushing weight is calcualted per unit length the numbers in bold face were measured in the lab the columns marked in yellow indicted the bushings tested in summer 2009 TBC = To be completed * Frequencies provided for center mount 9

10 Bushings tested Typical Physical Properties of Bushings Used in Experimental Study Bushing#1 Bushing#2 Bushing#3 Bushing#4a Bushing#4b Bushing#5 Bushing#6 Bushing#7 Bushing#8 Manufacturer G.E. G.E. G.E. HSP HSP Trench ABB ABB ABB Provider CERL CERL CERL LADWP LADWP Trench ABB ABB ABB Material of Insulator Porcelain Porcelain Porcelain Composite Composite Composite Porcelain Porcelain Porcelain Voltage Capacity (kv ) / Designation GE-500 -TypeU GE 500 TypeU GE 500 TypeU HSP HSP D004C_3 196w0800bz T550W2000UD T550Z3000SE 10

11 Overview Experimental Investigation Fixed base vs flexible mount properties Excitation ti motions at base of bushings Failure modes and capacity of bushings Seismic demand at base bushings Basic Issues Identified in Investigation Suggested Qualification Method 11

12 Fixed base and shake table testing 12

13 Frequencies Fixed Base vs As Installed from Current Testing 13

14 Bushing Testing Support Structure Support structure Designed to uncouple horizontal & vertical motion Horizontal o rigid Vertical motion affected by mounting system Re-locatable mounting plate Edge distance can be considered Turret system 8 FT To consider various ways of bushing mount Tilted turret Vertical turret L 5 X 5 X 3/4 20 PL 127 X 127 X 3/4 8 FT See D1. ~ D4. on Sheet #8 See D1. ~ D4. on Sheet #8 See D1. ~ See D4. D5. on on Sheet Sheet #7 #8 See D5. on Sheet #8 8 FT TS 5 X 5 X 1/2 17 Table Extension Front View Top View w/ Mounting Plate Shake Table Tilted Vertical No turret turret / At / At / At center center 14

15 Alternative locations of bushings on transformers and on support structure 15

16 Bushing at Various Locations on Support Structure on Shake Table 16

17 As-Installed Frequencies of Bushing at Various Locations on Support Structure Results of frequency search tests Frequency (Hz) No Turret Vertical Turret Tilted Turret x y z x y z x y z Center Side Corner W2: X Freq at center w /o turret W4: Y Freq at center w /o turret W6: Z Freq at center w /o turret G 0.6 G 0.6 G Hertz Hertz Hertz a) Bushing at center w/o turret 17

18 As-Installed Frequencies of Bushing at Various Locations on Support Structure W8: X Freq at side w/o turret W10: Y Freq at side w/o turret 1 1 G 0.6 G Hertz W14: X Freq at corner w /o turret Hertz b) Bushing at side w/o turret W16: Y Freq at corner w /o turret 1 1 G 0.6 G Hertz Hertz c) Bushing at corner w/o turret 18

19 Comparison of acceleration demands on bushings "Fixed Base" and "As Installed" 4 g Bushing #5 Bushing #6 AI AI 0.25g RRS 0.5g RRS 1.0g RRS 1.5 Fixed Base 1 As installed Fixed Base As installed Hz 19

20 Overview Experimental Investigation Fixed base vs flexible mount properties Excitation ti motions at base of bushings Failure modes and capacity of bushings Seismic demand at base bushings Basic Issues Identified in Investigation Suggested Qualification Method 20

21 Tank Amplifications Cover corners Effect of tank flexibility incorporated Acceleration spectrum and amplification factor at the cover corner in the transversal direction Amplification >2 (~ 4.1 at tank frequency )

22 Tank Amplifications Base of bushings on tank cover Effect of tank flexibility incorporated Acceleration spectrum and amplification factor at the cover corner in the transversal direction Amplification >2 (~ at a-i bushing freq. ~ 4.1 at tank freq. )

23 Overview Experimental Investigation Fixed base vs flexible mount properties Excitation ti motions at base of bushings Failure modes and capacity of bushings Seismic demand at base bushings Basic Issues Identified in Investigation Suggested Qualification Method 23

24 Bushing 4B

25 Bushing 7

26 Failure Modes Porcelain slippage UPPER-1 Porcelain Unit Oil leakage UPPER-1 Porcelain Unit Rubber Gasket Rubber Gasket Bushing Mounting Flange Bushing Mounting Flange Turret Top flange Turret Top flange Rubber gasket extrusion UPPER-1 Porcelain Unit Porcelain break UPPER-1 Porcelain Unit Rubber Gasket Rubber Gasket Bushing Mounting Flange Bushing Mounting Flange Turret Top flange Turret Top flange 26

27 Failure Modes Failure of the bond material 27

28 Failure Modes Buckling of the internal tube 28

29 Failure Modes Failure of the flange 29

30 Overview Experimental Investigation Fixed base vs flexible mount properties Excitation ti motions at base of bushings Failure modes and capacity of bushings Seismic demand at base bushings Basic Issues Identified in Investigation Suggested Qualification Method 30

31 Dynamic or static capacity testing

32 Pull Using Actuators or Shake Table CABLE Shake Table Pull AQR=256 Tn= Load Function Bushing # AQR Tn 1200 Bushing #2

33 Failure that determined capacity 33

34 Overview Experimental Investigation Fixed base vs flexible mount properties Excitation ti motions at base of bushings Failure modes and capacity of bushings Seismic demand at base bushings Basic Issues Identified in Investigation Suggested Qualification Method 34

35 Basic Issues Determined in Investigation The response acceleration intensity is dependent on the as installed conditions. Testing of bushings using rigid stands may produce severely non-conservative accelerations thus underestimating the demand during the qualification testing 35

36 Comparison of acceleration demands on bushings "Fixed Base" and "As Installed" 4 g Bushing #5 Bushing #6 AI AI 0.25g RRS 0.5g RRS 1.0g RRS 1.5 Fixed Base 1 As installed Fixed Base As installed Hz 36

37 Basic Issues Determined in Investigation While the current standards require qualifications of bushings to double (x2) the base motion, the acceleration demands on the bushings installed on roof covers of transformer tank, are greater than double (>2) the spectral demands at the base. 37

38 Tank Amplifications Base of bushings on tank cover Effect of tank flexibility incorporated Acceleration spectrum and amplification factor at the cover corner in the transversal direction Amplification >2 (~ at a-i bushing freq. ~ 4.1 at tank freq. )

39 Bushing Testing to 3.5g PGA 39

40 Basic Issues Determined in Investigation The response acceleration distribution in bushings is not uniform and the demand at the base of bushing cannot be determined from the well known formulation: Weight * acceleration at CG ( * height of CG above support) for moment demand Such estimate can be severely non-conservative as shown next Force and connection moment demands are affected 40

41 Inertia Forces in Bushings Shears and Moments m o a o a z a a b o ab z H ( ) z H b b o m z m m m m b a b

42 Inertia Forces in Bushings Shears and Moments m o a o m b a b

43 Inertia Forces in Bushings Shears and Moments m o a o m b a b a a z a a z H b o b ( ) m z m m m z H b b o mha b o mo V B a b ao ab ao 6 mb 2 / 1 / 2 2 mh b a o m o M B ab / ao 1 ab / ao 3 12 mb

44 Inertia Forces in Bushings Shears and Moments 1) Case I: a b = a o and and above): V m Ha 1 b o M 1 2 mh b ao 2 2) Case II: 0 and : inv mha b o V1 V V mh b ao M1 M M ) Case III: a b =0 and m o = m b /2 : inv mha b o V1 V V mh b ao M1 M M a a z a a z H b o b z m z m b ( m b m o ) H mha m V b o o B ab ao ab ao 6 mb 2 / 1 / 2 2 mh b a o mo M B ab ao ab ao 12 mb / 1 / 3

45 FORCES AT BASE OF BUSH- INGS 230kV-P

46 Basic Issues: Strength Capacity of Bushings Strength capacity should be defined by the first failure mode The strength capacity should be defined in terms of shear force and overturning moment in critical sections Shear strength capacity may be dominated by the slip failure Moment capacity may be dominated by the clamping prestressing load and gasket opening, or by porcelain crushing, cracking Both strength may be dominated by creep in composite bushings (insulators) 46

47 Basic Issues: Strength Capacity of Bushings Can be determined through dynamic testing (fragility testing) static testing 47

48 Strength capacity by dynamic testing or static testing

49 Basic Issues Determined in Investigation Strength Capacity The acceleration should not be used for rating or quantifying i the suitability of bushing during qualification. Moments and shear force capacities at the base of the bushing (or in critical sections) should be used instead Strength Demand Dependent on installed conditions on transformer cover (frequency modification) Dependent on tank amplifications of cover motion (magnitude of motion) 49

50 Bushing 4 and 6

51 Concluding remarks Accelerations capacity at CG is an inaccurate quantity: <Proven through experiments> Center of inertia during response moves depending on the overall acceleration profile and mass distribution Depends strongly on the fixity conditions and flexibility of the transformer cover. Fixed base frequencies are 30% to 110%! higher than as installed frequencies (note: first time that we have fixed base frequencies reliably obtained) <Testing on a rigid base (or rigid support structure) it reduces the observed demand versus capacity unconservatively> Most appropriate engineering design and qualification parameters must be SHEAR and OVERTURNING MOMENT s demands d and capacities <Can be monitored during testing and can be validated analytically / numerically> 51

52 Concluding remarks (cont) Most appropriate engineering design and qualification parameters must be SHEAR and OVERTURNING MOMENT s demands and capacities <Can be monitored during testing and can be validated analytically / numerically> Damping does not have significant consequence varies 0.5% to 3.5%. Testing at 2% demand RRS seem adequate. Failure modes are characterized by combination of shear (slip) and moment (fracture) capacity at base connection. Can be measured during testing. Can be calculated by mechanics <Verified with tested bushings> Slip failure may be delayed with proper restrainers - retest necessary. 52

53 Night Life at UB Thank you for your attention! 53