AGENDA. Day-1. Day-2. Session 4 : Advanced features of SFRA interpretation Conclusion and review of training

Transcription

1 Detection of Faults In Power Transformers by SFRA Manager Services Aradhana Ray Omicron

2 AGENDA Day-1 Session 1 : Introduction Session 2 : Requirement for good measurement of SFRA Session 3: Basic principle of interpretation of SFRA result. Day-2 Session 4 : Advanced features of SFRA interpretation Conclusion and review of training OMICRON Page: 2

3 Session 1 1. Introduction OMICRON Page: 3

4 Introduction Effects of Electricity Market Deregulation Age / Loading Need of more sensitive equipment to detect internal defects BEFORE faults occur Maintenance / Outage OMICRON Page: 4

5 SFRA What is it? Reliable and sensitive Method to evaluate the mechanical and electrical integrity of transformers active part Measurement of the transfer function for every winding over a wide frequency range OMICRON Page: 5

6 Model of Transformer Winding OMICRON Page: 6

7 How does it work? C = C ~ * A/d 1/d Force OMICRON Page: 7

8 How does it work? C = C ~ * A/d 1/d Force OMICRON Page: 8

9 Effects of Geometrical Changes Geometrial Changes inside = Change in Complex Electrical Network = Change in the Freq. Response 'Fingerprint' OMICRON Page: 9

10 The SFRA measurement principle Spannung U1/U1 in V/V Transformator Spannung U2/U1 in V/V Zeit t in µs Input signal Erregungssignal (sine wave of (variable Frequenz) variable frequency) TF(f1) -3 Zeit t in µs Output signal Antwortsignal Spannung U/U1 in V/V ^ U1 U2 ^ ϕ(f1)/2πf -2 TF(f1) = ^ U2 U1 ^ -3 arc(tf(f1)) = ϕ(f1) db -10 Betragsfunktion TF U2/U1(f) Magnitude 1.000e e e e f/hz Zeit t in µs Phase Phasenfunktion arc(tf U2/U1(f)) TFU2/U1(f) in V/V arc(tfu2/u1(f)) in deg e e e e+005 f/hz N V f in MHz NV f in MHz OMICRON Page: 10

11 Impulse Frequency Response Analysis (IFRA) U (V) transformer I (A) Zeit (µs) input signal complex RLCM-Network Zeit (µs) response signal filtering, sampling, windowing, FFT, quotient db TF(f) Magnitude transfer function (magnitude) 1.000e e e e e+006 f/hz arc(tf(f)) Phase transfer function (phase) 1.000e e e e e+006 f/hz MHz frequency Frequenz f H1 H2 H2 H3 H3 H MHz H1 H2 H2 H3 H3 H1 frequency Frequenz f OMICRON Page: 11

12 An Example (Impulse-FRA!) Same Transformer tested with FRAnalyzer OMICRON Page: 12

13 Cigré Test Results Amplitude (db) Frequency (Hz) C D E F G H I OMICRON Page: 13

14 What can be concluded from this? 1. FRA-devices need a sufficient dynamic range (at least down to -90 db). 2. Impulse measurements are not able to "see" the lower frequency range, which is expressive for several types of failure. OMICRON Page: 14

15 To Sum Up Ability to compare multiple tests Import functionality for all instrument manufacturers test files Increased dynamic range minimum upto -90 db. OMICRON Page: 15

16 OMICRON Page: 16

17 Reason of SRFA When a short-circuit fault has occurred. When there is evidence of transportation problems. When a weakness of the winding mechanical pressure system is known (Old and Suspected unit). As a Quality check in manufacturing stage. OMICRON Page: 17

18 Session 1 SFRA Theory and Practice OMICRON Page: 18

19 SFRA Theory and Practice In a passive device there are three basic components: resistors capacitors inductors They each have a different response to an AC signal Their response is closely related to their geometry: both internal and in relation to other components OMICRON Page: 19

20 SFRA Theory and Practice Impedance of an ideal resistor, capacitor and inductor Im pedance, Ohms Capacitor: Frequency, Hz Resistance: flat response v. frequency Inductor: Increased impedance with increased frequency; dead short at low frequency Reduced impedance with increased frequency; open circuit at low frequency OMICRON Page: 20

21 SFRA Theory and Practice Parallel RLC Circuit: Dead short at low frequency Constant dbs down v. frequency Open circuit at high frequency Open circuit at low frequency Dead short at high frequency OMICRON Page: 21

22 SFRA Theory and Practice Impedance of the Parallel RLC Circuit: R affects db size of resonance 6 Resonant frequency depends only on L and C values Impedance, Ohms Frequency, Hz OMICRON Page: 22

23 SFRA Theory db s: as impedance increases, V out falls V in Impedance, Z V out Response in db s = 20 log 10 (V out /V in ) OMICRON Page: 23

24 SFRA Theory and Practice Response of the RLC Circuit: Capacitive climb back Inductive roll off Resonance OMICRON Page: 24

25 SFRA Theory and Practice Response of an ideal inductor 0 db down at low frequency means it looks like a dead short Inductive roll off Larger inductances start to roll off at lower frequencies OMICRON Page: 25

26 SFRA Theory and Practice Response of ideal capacitor Knee point depends on size of capacitor Low frequency response is like an open circuit Capacitive climb back 0 db down at high frequency like a dead short OMICRON Page: 26

27 SFRA Theory and Practice We get a resonance for an inductor-capacitor (LC) combination Changing L or changing C gives a new resonance L and C are dependent on geometry Changing R changes the size of the resonance OMICRON Page: 27

28 SFRA Theory and Practice HV Winding A real transformer has many inductance and capacitances LV Winding C H C H C H C T C T R H L H R H C HL C HL C HL R L C T L L R L C T L H L L C L C L C L Each LC pair gives a resonance Inter Winding OMICRON Page: 28

29 Transformer Equivalent Circuit PARAMETR effecting SFRA PLOT Capacitance between the neighboring turns of the same winding. Capacitance between the turns of different windings. Capacitance between the turns and the ground. Turns self-inductance. Turns mutual inductance. Conductor dc resistance. Resistance that accounts for dielectric losses in insulation. Resistance that accounts for eddy losses in conducting and magnetic components. OMICRON Page: 29

30 SFRA Theory and Practice OMICRON Page: 30

31 Two Port Networks and SFRA Measurements In an SFRA measurement, the port input and output voltages, Vin and Vout, are referenced to ground OMICRON Page: 31

32 1. Introduction SFRA is really a lot of measurements... Frequency in MHz Individual measurement made at each frequency of interest... OMICRON Page: 32

33 Session 1 COMPETING METHOD OMICRON Page: 33

34 Detecting mechanical failure in large transformers can be very challenging. Mechanical failure may have no effect on DGA results. The windings could be deformed without this being visible even by an internal inspection. A great effort to develop effective methods of detecting mechanical failure has been made in recent years, culminating in some success! OMICRON Page: 34

35 Mechanical failure might result from: Large electromagnetic forces on connections and windings resulting from a short circuit, or Similar incident where very large currents flowed. Mechanical failure might also result from transport damage. Mechanical failure might further be a consequence of aging of Transformer. OMICRON Page: 35

36 Eventual failure resulting from winding damage is usually : Damage to the conductor insulation resulting in a turn-to-turn fault. Other failure modes are possible, including: Core/frame circulating current Phase-to-earth fault (from distorted connections). OMICRON Page: 36

37 All transformers should be designed to withstand the forces arising from external short-circuits, but: Fault levels may change through the life of the transformer. Cumulative damage may reduce the strength of the transformer. OMICRON Page: 37

38 (continued) Transformers are rarely short-circuit tested and so manufacturer s design criteria are rarely checked and may be inadequate. Many transformers which are short-circuited tested fail. So Design or manufacturing defects may reduce the strength of the transformer OMICRON Page: 38

39 The following methods have been used to detect mechanical failure: DGA Winding capacitance and power factor Magnetizing current Leakage reactance Frequency response analysis OMICRON Page: 39

40 DGA is a sensitive method for detecting any dielectric or thermal faults which develop as a consequence of mechanical damage. However DGA cannot itself detect mechanical damage. OMICRON Page: 40

41 Works on the principal that mechanical damage often results in bulk winding movement changing winding capacitance. However the damage may be localized, only giving rise to a small capacitance change. In some cases there may be considerable movement, but little change to a measurable capacitance (esp. in autotransformers). Useful for confirmation, but less sensitive for incipient fault detection. OMICRON Page: 41

42 Works on the principal that any short-circuited turns will result in a large increase in magnetizing current on the affected phase. However by the time there are short-circuited turns, the transformer may be damaged beyond repair. Useful in distinguishing between severe and less severe cases. OMICRON Page: 42

43 OMICRON Page: 43

44 OMICRON Page: 44

45 OMICRON Page: 45

46 The impedance change may be very small (1%), so accurate measurements and reference results are essential. This method is widely used for short-circuit testing (see IEC ). OMICRON Page: 46

47 OMICRON Page: 47

48 OMICRON Page: 48

49 OMICRON Page: 49

50 Analysis First, or benchmark test, should be within 3% of Nameplate. Subsequent tests should be within 2% of benchmark. If all three phases on Per-Phase tests agree, it is likely that there is no significant winding deformation. OMICRON Page: 50

51 Results are useful as a primary detection method, with capacitance or leakage reactance, Magnetising current now reduced to confirmation. OMICRON Page: 51

52 db SFRA INTERPRETATION Low Frequency 1.000e e e e+005 Core Movement Open circuit Short circuit High Frequency Movement of main and tap f/hz winding leads or change in earth circuit of core N V Mid Frequency Axial, Radial Deformation within the main and tap windings OMICRON Page: 52

53 Detecting Failure LVI/FRA OMICRON Page: 53

54 Detecting Failure LVI/FRA OMICRON Page: 54

55 Detecting Failure LVI/FRA OMICRON Page: 55

56 Took oil sample shortly after removal from service: Acetylene 103ppm Ethylene 104ppm Hydrogen 198ppm Methane 65ppm Clear evidence of power arc inside main tank. OMICRON Page: 56

57 OMICRON Page: 57

58 OMICRON Page: 58

59 OMICRON Page: 59

60 DGA, magnetising currents and SFRA all indicate a serious fault power arc and shortcircuited turns on A phase. The transformer was scrapped. OMICRON Page: 60

61 OMICRON Page: 61

62 ! OMICRON Page: 62

63 Case 2 Capacitances and power factors OMICRON Page: 63

64 Case 2 Frequency Responses LV-N OMICRON Page: 64

65 Case 2 -Photograph of Damage OMICRON Page: 65

66 Case 2 -Photographs of Winding Damage OMICRON Page: 66

67 3.0 Transformer Relocation OMICRON Page: 67

68 3. Transformer impact. Is the blocking safe, are the clamps still tight? Did we hit a bridge? OMICRON Page: 68

69 Transformer transportation and relocation From the experiment carried out by GE and Westinghouse suggests that transformer experience greater acceleration than maximum recommended le vel during transportation OMICRON Page: 69

70 Transformer transportation and relocation Mode Recommended value Measured value Z-Axis (Vertical) X Axis (Direction of Travel) Y-Axis (Transverse) 2.0 G 4.0 G 2.5 G 4.0 G to 5.0 G 1.5 G -- OMICRON Page: 70

71 The Transformer Manufacturer Capacity Cooling Class Frequency High voltage Low voltage VA Tech, EBG, Linz Austria 133/177 MVA OA/FA 60 HZ 70.8 kv Wye 17.1 and 13.2 kv Delta OMICRON Page: 71

72 OMICRON Page: 72

73 Upon arrival it was discovered that this unit was damaged in freight. Investigation revealed capacitance values changed by 3.3% from factory tests, Significant differences in the frequency response analysis tests. Electronic impact recorders showed multiple hits with the maximum in excess of 6.5 G impacts during rail shipment. OMICRON Page: 73

74 Data from the digital impact recorder showed the time of the major impacts. Comparing this time to the information obtained from the railroads confirm that at the time of the major impact the transformer just arrived in a railroad switching yard in Belen New Mexico. OMICRON Page: 74

75 OMICRON Page: 75

76 Frequency response analysis (FRA) proved very helpful in confirming the shipping damage. Factory FRA tests were performed both with the transformer oil filled and drained for shipment. OMICRON Page: 76

77 OMICRON Page: 77

78 OMICRON Page: 78

79 OMICRON Page: 79

80 3. Unknown Impact Here another scrape was touched up with paint In this case, the impact recorder was incorrectly set and had no data... OMICRON Page: 80

81 Decision time - is the transformer OK? You can never have too many supervisors... SFRA showed good results - transformer OK OMICRON Page: 81

82 Transformer Relocation (Normal) Amplitude in db HV Tap 1 This Graph Contains 6 Traces: A, B and C Before and After Relocation Phase to Phase Variations: A to B to C Frequency in khz No Identifiable Differences After Relocation OMICRON Page: 82

83 Transformer transportation and relocation Winding deformation is an irreversible fault and the replacement of the damage winding may be the only remedy. OMICRON Page: 83

84 Session 1 27 August OMICRON Page: 84

85 The basic equation for the calculation of electromagnetic forces is: - F = L I B where, B is leakage field density vector, I is the current vector and L is the length of winding Radial force in x-direction due to axial leakage flux density and Axial force in y-direction due to radial leakage flux density, as shown in the figure 27 August OMICRON Page: 85

86 Radial forces The radial forces are acting outwards on the outer winding tending to stretch the conductor, producing a tensile stress (also called as hoop stress). The radial forces are acting inwards on the inner winding tending to collapse or crush it, producing a compressive stress. The average hoop stress can be considered as uniform over the entire disk winding. In a layer / helical winding, the average hoop stress is not uniform, i.e., it is highest for the innermost layers and it decreases towards the outer layers. 27 August OMICRON Page: 86

87 Axial forces The axial forces due to radial fringing leakage field at winding ends are directed towards the center of winding from both ends for uniform ampere-turn distribution in windings with equal heights (ideal condition). The compressive force is maximum at the center of the windings. Both the inner and outer windings experience compressive forces. Strict sizing and dimension control are required during processing and assembly of windings 27 August OMICRON Page: 87

88 Failure Modes due to Radial Forces Radial collapse of inner winding is common, whereas outwards bursting of outer winding generally does not take place. If a winding is tightly wound, conductors in radial direction can be assumed to have uniform tensile stress. Most of the space in radial direction is occupied with copper (except for the small paper covering on conductors): high stiffness to mass ratio. The natural frequency is much higher than the excitation frequency and hence chances of resonance are remote. The chances of winding failing with tensile hoop stress are unlikely if conductor with a certain minimum 0.2% proof strength is used. 27 August OMICRON Page: 88

89 Failure Modes due to Radial Forces contd. Windings subjected to compressive stresses: Inner winding conductors, which are subjected to the radial compressive load, may fail due to bending between supports or buckling. Forced buckling (1): occurs when the winding cylinder has significant stiffness as compared to winding conductors. Free buckling (2): this kind of failure occurs mostly with lower thickness of winding cylinders. An adequate number of winding supports need to be provided to give strength to the winding against the radial forces. (1) (2) 27 August OMICRON Page: 89

90 OMICRON Page: 90

91 Failure Modes due to Axial Forces. If a layer winding is not wound tightly, some conductors may just axially pass over the adjacent conductors, which may result into damage to conductor insulation and a eventual turn-to-turn fault. If winding is set into vibration under the action of axial forces, conductor insulation may get damaged due to relative movement between winding and axial insulation spacers. There could be deformation of end clamping structure and windings due to high axial end thrust. To maintain effective pressure on windings a clamping ring made of stiff insulating material (pre-compressed board or permawood) is used. There are two principal types of failures, viz. bending between blocks and tilting. 27 August OMICRON Page: 91

92 OMICRON Page: 92

93 Physical damage observed Core clamping bolt sheared off OMICRON Page: 93

94 Design issues Transformers should be designed to withstand through faults. However, transformers are rarely short-circuit tested because of the costs involved. So the short-circuit strengths of designs are not often checked. Many new transformers fail short circuit tests, so the shortcircuit strength of many designs must be suspect. OMICRON Page: 94

95 Design limitations It if difficult to take into account the effects of twisting forces, tap to tap faults in Design calculations and manufacturing process. Tap windings are difficult to design for short-circuit strength. OMICRON Page: 95

96 Specification Impedance 20 % 5 % Short-circuit current 5 p.u. 20 p.u. Short-circuit forces 1 : 16 OMICRON Page: 96

97 2.1 Short circuit Fault If we can measure: changes in the frequency response of a winding then we can detect winding movement. Bulge in winding caused by electrical fault A good repeatable SFRA measurement will detect this problem OMICRON Page: 97

98 2.1 Electrical fault close in. Protection relays responded, but did the windings on the nearby transformer move? OMICRON Page: 98

99 Session 2 Good Measurements OMICRON Page: 99

100 Range : up to 2 MHz We need the three R s: Resolution : 1.2% of mesurement frequency Repeatability Good Measurements : +/- 1 db repeatability : for different test leads, operators and test sets OMICRON Page: 100

101 Good Measurements Three LV Phases Three HV Phases Three Phases Interwinding Typical Transformer Results OMICRON Page: 101

102 Good Measurements Transformer should be disconnected from system Transformer should be in normal service condition (assembled, oil-filled) Make connections as determined by winding configuration. Phases not under test are left floating. OMICRON Page: 102

103 Simple Procedure Procedure: For safety ground bushing terminals before connecting test leads Check for good test lead contact at bushing terminals Check for good ground lead contact at bushing flanges ANY system may be used incorrectly to give a bad result! OMICRON Page: 103

104 Connections Test Lead M5100 Signal & Reference Leads Shield Ground OMICRON Page: 104

105 OMICRON Page: 105

106 Measurements Connections - look at each winding separately: HV - H1-H2, H2-H3, H3-H1 LV - X1-X0, X2-X0, X3-X0 Short Circuit - H1-H2, H2-H3, H3-H1 with X1-X2-X3 shorted Follow the direction of connection as per the procedure. Repeat at neutral and extreme tap positions. OMICRON Page: 106

107 OMICRON Page: 107

108 OMICRON Page: 108

109 Session 2 Typical Results OMICRON Page: 109

110 HV Wye Typical Responses Three phases of one transformer Low frequency responses follow expected form High frequency responses show phase-to-phase variation which is common OMICRON Page: 110

111 OMICRON Page: 111

112 SFRA Variation with Tap Position One phase of one transformer Main winding responses unchanged Tap winding responses show stepwise variation Tested at 9 tap positions OMICRON Page: 112

113 HV Delta - Initial Field Effect of Magnetization Same phase measured on arrival at site High frequency responses unchanged Low frequency responses show offset due to core magnetization Results are acceptable - but test repeated for confirmation OMICRON Page: 113

114 HV Delta - Final Field Effect of Magnetization Same phase measured after demagnetization High frequency responses remain unchanged Low frequency offset now removed Results are acceptable Some site noise at low db response is not unusual OMICRON Page: 114

115 Test Results - SFRA Short Circuit Normal test on HVs - LVs float Signal & Reference HV Winding LV Winding R small R small M5200 Core R high Test Model is relevant for LOW FREQUENCIES OMICRON Page: 115

116 Test Results - SFRA Short Circuit Normal test on HVs LVs Shorted Signal & Reference HV Winding LV Winding R small R small M5200 Core R high LV Short Test Model is relevant for LOW FREQUENCIES OMICRON Page: 116

117 Test Results - SFRA Short Circuit One phase of one transformer tested in open and in short circuit configurations Short circuit response shows expected inductive roll off Expected for Wye winding open circuit measurement OMICRON Page: 117

118 Session 4 SFRA INTERPRETATION OMICRON Page: 118

119 Equivalent circuit parameter of Transformer The SFRA frequency range is divided into three sections: High frequency, Middle frequency and Low frequency section OMICRON Page: 119

120 Low-frequency response Low frequency variation Core fault Shorted turns fault Open circuit or high impedance winding fault OMICRON Page: 120

121 Low-frequency response SFRA plot of a winding with all other winding left open-circuited Core parameters is dominant in the frequency region up to 1 khz. OMICRON Page: 121

122 SC SFRA plot of HV winding for the two sister unit HV Short circuit Plot for both the sister unit Winding inductance and resistance is dominant component up to 2kHZ. Red- With oil, Blue- Without oil OMICRON Page: 122

123 Open circuit SFRA plot of HV winding for the two sister unit OMICRON Page: 123

124 "#$" "# %& ' Transformer has tripped on Overcurrent protection Rating: 132 kv/11 kv, 10 MVA, 3-phase Transformer Winding configuration HV(Star) / LV(Star) Year of manufacturing SFRA was conducted first time after the over current tripping Three phase comparison of SFRA plot was performed OMICRON Page: 124

125 "#$" "# %& ' 3-phase HV winding Open circuit plot- After fault OMICRON Page: 125

126 "#$" "# %& ' Data analysis is performed on three phase comparison basis. SFRA plot of HV B- phase (middle phase) is different than the Outer phases. In case of B-Phase winding the first resonance around 300 Hz. is absent. OMICRON Page: 126

127 "#$" "# %& ' 3-phase LV winding open circuit plot- After fault OMICRON Page: 127

128 "#$" "# %& ' 3-phase HV winding Short Circuit plot- After fault OMICRON Page: 128

129 "#$" "# %& ' In low frequency the difference in db of B-phase winding is around 0.4 db OMICRON Page: 129

130 "#$" "# %& ' SFRA is able to identify that turn to turn fault is in LV winding. Turn to turn fault can be identified by other method like No load excitation current, Turns ratio and Magnetic balance test, But in which winding HV or LV winding is a question mark. SFRA is the test which can identify the winding and phase both, of the shorted turn fault. OMICRON Page: 130

131 "#$" "# ' # Transformer was tested for SFRA after it tripped on Differential protection. Rating: 132 kv/11 kv, 16 MVA, 3-phase Transformer Winding configuration HV(Star)/LV(Star) Year of manufacturing SFRA was conducted second time on the transformer after the incident of fault and tripping. First base data of SFRA was available in the past record. OMICRON Page: 131

132 "#$" "# ' # Before Fault After Fault Before Fault After Fault HV winding SFRA plot OMICRON Page: 132

133 "#$" "# ' # HV winding SFRA data analysis concluded that transformer has developed turn to turn fault in H3-H0 phase. It was confirmed in conventional ratio test also, where the ratio of H3-H0 was different than other two phases. OMICRON Page: 133

134 "#$" "# ' # Before Fault After Fault LV Winding SFRA Plot OMICRON Page: 134

135 "#$" "# ' # LV winding resistance is found to be 32 miliohms for all 3 phases instead of 12 miliohms (i.e. in healthy condition). Same was confirmed by SFRA also as all the three phase LV plot was showing high impedance of the winding at low frequency. On physical inspection damage of LV neutral connection inside the tank was found due to severe arcing on connecting joints. However, there is no damage to outside LV neutral bushing. OMICRON Page: 135

136 Case study 3- Usefulness of SFRA technique in confirming the fault location in a gassing transformer 250 MVA 220 KV / 110 KV / 33 KV auto-transformer showed increasing trend of dissolved gasses, since its commissioning. The DGA analysis indicated core fault(hot spot). After un-tanking, no abnormality in the windings or core was noticed on physical inspection. SFRA test showed marked deviation of response between the phases from 2KHZ to 10KHZ frequency. OMICRON Page: 136

137 CASE STUDY 3- Usefulness of SFRA technique in confirming the fault location in a gassing transformer On removing the top yoke laminations some burn marks on lamination were observed. The core repair was carried out and all tests including SFRA test were done satisfactorily. OMICRON Page: 137

138 SFRA records of 220kv/110kv/33kv ICT before repairs (in un-tanked condition) OMICRON Page: 138

139 Photographs of the damaged laminations OMICRON Page: 139

140 OMICRON Page: 140

141 220kv/110kv/33kv ICT OMICRON Page: 141

142 Mid-frequency response 1 khz. to 1 MHz. represent the main winding structure of transformer. Overall axial and Radial deformation of the HV, LV winding due to high short circuit system fault is detected well OMICRON Page: 142

143 It is recommended to have internal inspection of the tap changer parts and winding of the transformer. Transformer should not be charged in this state & The root cause of the deformation should be identified. Still transformer was energized and again similar tripping happen in the Y- phase with a major fault which was predicted as mechanically damaged winding by SFRA. and sent for the repair which was a very costly affair. OMICRON Page: 143

144 Mid-frequency response Mid - frequency variation Overall radial shift of the winding Axial shift of the winding OMICRON Page: 144

145 Case 3: Hoop Buckling Compressive Failure of Winding - also known as Hoop Buckling Expect increase in LC combination of winding bulk - seen as left shift of resonance at mid range Consequent increase in winding impedance - seen in short circuit results OMICRON Page: 145

146 Case 3: Hoop Buckling Example here of 28 MVA units One LV bad phase detected, after SFRA test OMICRON Page: 146

147 Case 3: Hoop Buckling Clear shift left of bad phase Small change in frequency and db OMICRON Page: 147

148 Case 3: Hoop Buckling Diagnosis is that suspect unit has one bad phase through hoop buckling Transformer winding is bent, but not broken Subsequent internal inspection revealed hoop buckling on the suspect phase, as expected and predicted OMICRON Page: 148

149 Case 3: Hoop Buckling Inspection showed hoop buckling on suspect phase - as predicted OMICRON Page: 149

150 Case 4: Axial Shift of a Winding Example uses reference results taken 7 years previously Need range up to 1 MHz Need repeatability to within 1dB Need confidence in results to avoid a false negative Failure mode is reduction in LC combination Resonances should shift right at higher frequencies as a consequence of LC reduction OMICRON Page: 150

151 Case 4: Axial Shift of a Winding Scattered blocking under a collapsed winding OMICRON Page: 151

152 Case 4: Axial Shift of a Winding 0 Results from one GOOD phase taken in 1994 and Amplitude in db No variation between Frequency traces in implies MHz /11/94 8/11/01 that THIS PHASE has little change Small db variations are acceptable: no LC variation implied OMICRON Page: 152

153 Case 4: Axial Shift of a Winding -10 Ratcliffe SGT3 T4681 AEI A phase N to LV Tap 15 0 Results from bad phase taken in 1994 and 2001 Amplitude in db Low frequency results, up to -70 over khz, are acceptable frequencies 0.5 of several resonances 1 Variation implies we have a substantial 22/11/94 8/11/01 problem Clear and consistent shift to higher Frequency in MHz OMICRON Page: 153

154 Case 4: Axial Shift of a Winding Amplitude in db Good SFRA repeatability allows confident diagnosis of bad phase Amplitude in db Good SFRA repeatability gives confidence in the integrity of the good phase Ratcliffe SGT3 T4681 AEI A phase N to LV Tap Frequency in MHz OMICRON Page: 154

155 Case 4: Axial Shift of a Winding Bad phase clearly identified SFRA results up to 1 MHz required SFRA results taken with different test sets by different test teams Repeatability is key OMICRON Page: 155

156 Case 5: Clamping Failure 750 MVA transmission transformer Close in fault caused center winding to jump and break the clamping Stress shield dropped on to winding, altering capacitance of that winding OMICRON Page: 156

157 Case 5: Clamping Failure Clamping beam broken Stress shield collapsed on to center phase OMICRON Page: 157

158 Case 5: Clamping Failure Three HV phase results shown together Center phase shows shift to lower frequency for first resonances Amplitude in db Low frequency results Transformer has show a problem expected further form investigation required Frequency in khz OMICRON Page: 158

159 Case 5: Clamping Failure Initial low frequency (< 2kHz) core related resonance unaffected Resonances usually associated with bulk capacitance of windings to ground are moved to lower frequencies OMICRON Page: 159

160 OMICRON Page: 160

161 OMICRON Page: 161

162 Capacitance and Power factor test data: OMICRON Page: 162

163 No load excitation current test data: OMICRON Page: 163

164 Mid-frequency response 30 MVA 138/69 kv Transformer was subjected to over current. In LV winding traces phase x1- x0 has shifted resonance in mid frequency area of 2 khz. to 200 khz. OMICRON Page: 164

165 High-frequency response In high frequencies region between 1MHz. to 2 MHz., transformer winding behaves as a purely capacitive element. OMICRON Page: 165

166 High-frequency response High frequency variation Core earthing related faults in the transformer Tap changer leads & Bushing leads related faults OMICRON Page: 166

167 High-frequency response Auto-transformer, 400/220kV, Very low IR between core and tank after movement of transformer from factory to site. Comparison of factory and site SFRA plot indicates significant shift was found in frequency range of 1 MHz. to 2 MHz for all 3-phase common and series winding. OMICRON Page: 167

168 High-frequency response OMICRON Page: 168

169 Case 6: Bushing Failure Transmission Transformer had a bushing failure No reference results available Did the failure move the windings? Sister unit available for reference OMICRON Page: 169

170 5 Water seeped in to this bushing causing it to fail catastrophically Bushing failure. Neutral bushing lifted from turret and is no longer vertical OMICRON Page: 170

171 But did the windings move because of the pressure wave in the oil? SFRA showed that the windings were in good shape. Bushing. OMICRON Page: 171

172 Case 6: Bushing Failure Minor variations in suspect unit at high frequencies may relate to some internal lead movement Variation is small - but needs further investigation Low frequency variation as expected between phases OMICRON Page: 172

173 Case 6: Bushing Failure Six traces here - suspect unit and sister unit Variation is still a concern - need to do a phase-by-phase comparison of two sister units OMICRON Page: 173

174 Case 6: Bushing Failure There are two traces here - suspect phase of suspect unit and same phase of sister transformer The two traces are clearly very similar - can just see one trace behind the other Sister unit has exactly the same variation - it must be design related as sister is known good No variation between traces implies this phase is OK, despite bushing failure OMICRON Page: 174

175 Case 6: Bushing Failure No reference results available Suspect phase does show some variation at high frequency Sister unit shows same variation - known to be a good transformer Consequently - SFRA variation is due to design and construction OMICRON Page: 175

176 8. Interpretation Strategies There is a hierarchy Best: Reference results Next: Sister Unit and SC results Third: Phase by phase Work with what you have. OMICRON Page: 176

177 8. Interpretation Experience: Certain Frequency Bands Indicate Different Problem Conditions < 2-10 khz: Core Deformation, Open Circuits, Shorted Turns & Residual Magnetism 400kHz to 2MHz: Movement of main and tap winding leads 2kHz to 20kHz: Bulk Winding Movement Relative to Each Other, clamping structure 20 khz to 400 khz: Deformation Within the main and tap windings OMICRON Page: 177

178 6. Interpretation Changes to the overall shape of graph. Creation of new resonance frequencies or elimination of existing resonance frequencies Large shifts in the existing resonant frequencies OMICRON Page: 178

179 CASE study- Effect of Residual magnetism SFRA data was analyzed for the 50 MVA, 145/66 kv (Star/star), 3-Phase, New transformer manufactured in 2006, which was subjected to the short circuit test as part of type test. There was variation in the open circuit plots before and after short circuit test in each phase. Short circuit plots are identical before and after the SC test OMICRON Page: 179

180 Open circuit Plot Before and after SC test OMICRON Page: 180

181 Short circuit Plot Before and after SC test Short circuit Plot Before and after SC test OMICRON Page: 181

182 CASE study- Effect of Residual magnetism Below 2 khz the open circuit HV SFRA,show differences before and after the S/C test which is due to core degradation or residual magnetism. To trace the reason, the test data was evaluated related to core issue like no load magnetization current and magnetic balance test before and after Short circuit test. OMICRON Page: 182

183 CASE study- Effect of Residual magnetism From the Magnetic Balance performed Before and After SC test it was clear that, when energizing on V phase, the balance of induced voltages on the outer phases is slightly better after the Short circuit test. The observation is same with respect to the SFRA plot up to 2 khz., when we compare the three phase plot before and after SC test. OMICRON Page: 183

184 CASE study- Effect of Residual magnetism After Short circuit test the trace has 'renormalised' what was a slightly unbalanced pattern due to remanent magnetism before the test. SFRA measurements at low frequencies and magnetic balance measurements will both be affected by remanent magnetism. Hence, in this case, the variation in SFRA plot below 2 khz is only to be expected due to change in the residual magnetism of the core during SC test. OMICRON Page: 184

185 Poor Ground Connection 500 MVA Factory results Bad ground may give a db difference Can be detected by Doble test set ground loop check Control of ground lead contact quality is VERY important Original in white Bad signal ground in pink Bad measurement ground in blue OMICRON Page: 185

186 Aim of Ground Connections Aim of any SFRA ground connection is to provide a local ground reference Usually done at the bushing flange There is stray impedance at the connection to the ground and from the connection loop Ensure low impedance connection to bushing flange to provide consistent reference OMICRON Page: 186

187 Poor Grounding Three single phase transformers measured for comparison One phase has poor ground connection during measurement Otherwise results were OK; it is important to make good connections OMICRON Page: 187

188 Test Results: Effect of Core Grounding Green With core ground Red Without core ground OMICRON Page: 188

189 Open Circuit Traces Open Circuit inside test specimen db Open Circuit within test leads F req u e nc y - H z OMICRON Page: 189

190 Comparision of SFRA plots of Sister units in aged condition OMICRON Page: 190

191 Sister unit There are two traces here - suspect phase of suspect unit and same phase of sister transformer The two traces are clearly very similar - can just see one trace behind the other OMICRON Page: 191

192 CONCLUSION SFRA is a valuable tool in transformer test and assessment. It is best to keep testing procedures simple for SFRA and to understand : Transformer state and test set up Why the transformer is being tested and what other results mean Analysis supported by evidence with the other diagnostics test and prediction OMICRON Page: 192

193 References: Dick, E. P. and Erven, C. C., "Transformer Diagnostic Testing By Frequency Response Analysis". IEEE Trans. on PAS, Vol. PAS-97, No. 6, Nov/Dec 1978, pp CIGRE Report of working group, WG A2.26 in October 2006, Mechanical condition assessment of transformer winding using Frequency response analysis Lapworth J. A. and Jarman P. N., "UK Experience of the Use of Frequency Response Analysis (FRA) for Detecting Winding Movement Faults in Large Power Transformers," CIGRE Transformers Colloquium, 2-4 June Transformer winding parameters affecting SFRA plot and fault classification in 6th Doble Power Forum Conference, India, November Doble SFRA User s Guide, Doble Engineering company, Watertown, Massachusetts, USA, 72A Rev. E 06/06. OMICRON Page: 193

194 Thank You. Any questions? Contact No OMICRON Page: 194