MAGNETIC BALANCE TEST – AN EFFECTIVE DIGNOSTIC TOOL FOR DETECTION OF SUBTLE FAULTS IN TRANSFORMERS
Vishal Mahire, DGM (Testing), EMCO Limited, Thane E-mail id:
[email protected] Abstract:
Magnetic Balance test is a most commonly used proactive test to detect faults in the core and/or the windings of the transformer at early stage of manufacturing at works. Although considered as the simplest test to conduct, sometimes it is difficult to interpret the results conclusively because of some unpredictable variations in test results. Invariably, for authenticating the analysis of results, we have to conduct some supplementary tests also. Nevertheless, at times we do not find suitable technical explanation for certain strange test results, leaving us clueless about the type of abnormality in the transformer. This paper discusses typical trends of induced voltages in un-excited limbs of transformer, observed during magnetic balance test and the measurement of magnetizing currents. It also attempts to give explanation for the observed trend such as lesser magnetizing current in the centre limb than the extreme limbs, the effect of delta connected windings on distribution of 3-ph magnetizing currents and the combined voltage magnitude in the un-excited limbs in excess of the applied voltage, etc. Through various case studies supported by data analysis, the author shares his extensive testing experiences about magnetic balance and magnetizing current tests and suggests possibilities of leveraging the knowledge for detection of various faults and abnormalities within the windings and magnetic core of the transformer. The case studies present step by step method for analysis of test results to help diagnosis and location of faults. Findings of the paper will help gain an insight in to the subtle quality problems in the windings and core during manufacturing of transformer or the in-service problems developed during operation of the transformers, and enrich reader’s knowledge encouraging optimum deployment of magnetic balance test and the complementary magnetizing current measurements, which are recognized as very effective tests to detect typical problems like: a) b) c) d)
Inter-turn shorting; Inter-strand shorting; Any external loops around the core; Abnormal magnetizing current due to unequal turns in winding sections connected in parallel; e) Wrong interleaving joints in windings.
The paper discusses several aspects of core and winding configurations with a view to address some of the most sought-after answers on the subject. Keywords: Magnetic balance test, Magnetizing current, Fault diagnosis.
Introduction:
Magnetic balance test (MBT) and measurement of magnetizing current is the simple test used to detect various faults in windings and core in the transformer. As the term suggests, Magnetic balance involves checking of balancing of flux distribution in the magnetic circuit of a 3 phase transformer. This test is however not applicable in case of 1-ph transformers as 3 wound limbs are not available for measurement of induced voltages. Although this test is not specified in any national or international standards, yet it is quite popular in India. Magnetic Balance Test:
This test is performed by application of single phase voltage to individual limb by turn and the voltages induced in other two limbs are measured as shown in the following diagram.
When we apply voltage to individual limb, and measure the voltages in other two limbs, ideal expected conditions are, 1) Φ1 = Φ2 + Φ3 where, Φ2 = 60 to 90% & Φ3 = 40 to 10 % (When voltage is applied to U ph) 2) Φ2 = Φ1 + Φ3 where, Φ1 = 50% & Φ3 = 50 % (When voltage is applied to V ph) 3) Φ3 = Φ2 + Φ1 where, Φ2 = 60 to 90% & Φ1 = 40 to 10 % (When voltage is applied to W ph) Here Φ1 α Vu; Φ2 α Vv; and Φ3 α Vw. This flux distribution condition is purely dependent on core geometry where reluctance path plays major role in flux distribution, for eg. If we excite U phase winding by application of 230 V, flux Φ1 is produced in U limb, due to less reluctance path 60 to 90 % flux tries to close loop through adjacent V limb and remaining flux travel through higher reluctance path of limb W (Normally 60 to 90% flux gets linked in V phase and remaining 40 to 10% through extreme phase). Similarly if we excite V limb, due to similar path of core reluctance of other two limbs, almost same amount of flux passes through both the extreme limbs. As the voltage is directly proportional to the flux, distribution of magnetic flux is evident during voltage measurement.
A typical example of measurements on LV winding of 45 MVA, 21/11.5 kV Transformer is given below (Refer Table:1), here underlined figures are the voltage applied terminals. Table:1 Measured Voltage in Volts 2U-2N 249 (100%) 125 (50.2%) 71 (28.5%)
2V-2N 178 (71.5%) 249 (100%) 178 (71.5%)
2W-2N 71 (28.5%) 124 (49.8%) 249 (100%)
Total Voltage induced in other 2 phases 100.0% 100.0% 100.0%
Magnetizing Current Phase U 158.0 mA V 118.6 mA W 158.3 mA
Remnant DC component in Core substantially affect test results of MBT, hence it is recommended to perform this test before resistance measurement test or after proper demagnetization of the core. When winding resistance measurement is performed using DC current source, in the process of obtaining steady resistance value by nullifying the inductive effect of winding, the core gets magnetized. Due to this magnetization, DC components are present in the core. This affects the flux distribution in the core and the results r esults obtained can be confusing. A typical example is given below for reference which was performed on a 25MVA, 220/11 kV Transformer a) Before DC test (conducted at after- connection stage- Refer Table:2) Table:2 Measured Voltage in Volts 2U-2N 226 (100%) 115 (50.8%) 56.7 (25.1%)
2V-2N 2W-2N 174 (76.9%) 55 (24.3%) 226 (100%) 111 (49.1%) 172.6 (76.3%) 226 (100%)
Total Voltage induced in other 2 phases 101.3 % 100.0 % 101.4 %
Magnetizing Current Phase U 135 mA V 99 mA W 136 mA
b) After DC winding resistance measurement test (conducted at pre-tanking stage- Refer Table:3) Table:3 Measured Voltage in Volts Total Voltage induced Magnetizing in other 2 phases Current 2U-2N 2V-2N 2W-2N Phase 233 (100%) 202 (86.7%) 32 (13.7%) 100.4% U 181 mA 151 (64.8%) 233 (100%) 80.1 (34.4%) 99.2% V 144 mA 54.7 (23.5%) 179 (76.8%) 233 (100%) 100.3% W 260 mA From the foregoing we can say that DC test affects both the test results (voltage distribution as well as magnetizing current trend) substantially. Hence it is advisable to perform this test before conducting any DC test on the transformer. In large rating transformers it is observed that ideal condition of magnetic balance is not achieved, if this test is performed from HV winding (normally HV winding is placed away from the core). If low voltage is applied to HV winding, its large no of turns results in poor V/T in the windings,
which results in development of poor flux and the test results obtained may not be reliable. Hence it is advisable to conduct this test from the winding closer to the Core in case of large rating transformers. There are some cases where as low as 4 to 5% voltage induction is measured in the extreme limb. In such cases the test results measured from LV winding should be considered for analysis. An example of measurements on HV winding of 100 MVA, 220/66/11 kV System Transformer is given in Table: 4 Table:4 Measured Voltage in Volts 1U-1N 224.7 (100%) 111.2 (49.7%) 8.6(3.83%)
1V-1N 1W-1N 214.4(95.4%) 8.80 (3.92%) 223.6 (100%) 112.4(50.3%) 215.2(95.9%) 224.3(100%)
Total Voltage induced in other 2 phases 99.3 % 100.0 % 99.8 %
Magnetizing Current Phase U V W
2.85 mA 2.23 mA 2.95 mA
During commissioning stage some people perform this test from all windings and at all tap positions. Practically there is no need to perform this test from different windings and at different tap positions, as when we apply voltage to the LV winding; every turn of each windings of the particular phase gets proportionate voltage induced in it. Moreover due to higher voltage induction in HV windings it is as good as performing this test at higher voltage level. If there is any problem with any of the turns of any winding, it will reflect in the test results due to flux choking in that particular limb. Although it is advisable to perform this test from LV side there are some exceptions to this concept, like in case of Furnace Transformers, Rectifier Transformers and Transformers with higher voltage ratio like Generator Transformers. In such cases, if MBT is performed from LV side then voltage induced in HV windings may exceed the safe limits due to higher ratios. Thus, safety aspect becomes more crucial when this test is performed at manufacturing stage or on transformer in un-tanked condition where HV line leads are in open condition. Moreover, in LVs of Furnace and Rectifier Transformers, handling of very high currents is also not possible. Sometimes very surprising test results are experienced in case of MBT test from delta connected tertiary windings in large rating autotransformers. In such cases the sum of voltages induced in other two windings exceeds the voltage applied normally by 5% to 10%. However, in some extreme cases this voltage even exceeds 30% to 35%. Experience shows that these transformers, having satisfactorily passed all the performance tests, serving in the field satisfactorily for several years. Hence this phenomenon is considered to be a normal in case of autotransformers. Results of the measurements carried out from Tertiary winding of 315MVA, 400/220/33 kV, YNa0d11 connected autotransformer are given in Table: 5 as example.
Table:5 Measured Voltage in Volts 3U- 3W 239.2(100%) 124.0(52.1%) 80.2(33.4%)
3V-3U 3W-3V 238.3(99.6%) 75.2(31.4%) 238.2(100%) 114.8(48.2% 236.2(99.2%) 238.1(100%)
Total Voltage induced in other 2 phases 131.1% 100.2% 132.9 %
Magnetizing Current Phase U V W
22.9 mA 15 15.7 mA 23.0 mA
Above measurements are carried out on the autotransformer at final stage where Transformer is in tanked condition with bushings mounted. In contrast to above the results performed on same transformer in un-tanked condition the sum of voltage induced induced in other two phases is found normal but the magnetizing current is higher by about 30% to 40% as shown in Table:6 below. Table:6 Measured Voltage in Volts 3U- 3W 249.0(100%)
3V-3U
3W-3V
216.0(86.7%)
35.1(14.1%)
125.0(50.0%)
250.0(100%)
125.0(50.0%)
37.2(14.9%)
215.0(86.0%)
250.0(100%)
Total Voltage induced in other 2 phases 100.8 % 100.0% 100.8%
Magnetizing Current Phase U 36.1 mA V 25.6 mA W 36.7 mA
When studied this phenomenon in case of 11 kV star connected tertiary or auxiliary windings, similar behavior is observed. A case is given below in Table:7, as an example for a 50/75/100MVA, 220/132 autotransformer, where 11kV is the auxiliary star-connected winding provided for works testing purposes. Table:7 Measured Voltage in Volts 3U- 3N 230 (100%) 113 (49.1%) 87 (37.8%)
3V-3N 222 (96.5%) 230 (100%) 223 (96.9%)
3W-3N 88 (38.3%) 117 (50.8%) 230 (100%)
Total Voltage induced in other 2 phases 134.8% 100.0% 134.8%
Magnetizing Current Phase U 106.5 mA V 61.0 mA W 104.5 mA
Measurement of Magnetizing Current: A) Magnetizing current measurement by application of Single phase voltage.
Practically it is seen that, magnetizing current is more sensitive to the abnormal conditions or faults than voltage. Hence it is always advisable to perform & analyze both the tests simultaneously.
In normal conditions, magnetizing current measured in centre limb is comparatively lesser than that of extreme limbs. This variation w.r.t. extreme limbs is of the order of 62 % to 80%. While performing this test from the outermost winding (mainly HV of large Power Transformers) this variation may be of the order of 82% to 88%. But this variation is quite normal and is a result of lesser reluctance path experienced by flux in centre limb. This phenomenon can be explained as under 1.
The reluctance (S) of a magnetic path is directly proportional to its length (l) and inversely proportional to its area (A). The reluctance is also inversely proportional to the absolute permeability (�) of the magnetic material. Thus, S = l / (�*A) The permeability of a magnetic medium is a measure of its ability to support magnetic flux and it is equal to the ratio of flux density (B) to magnetizing force (H) Thus, � = B / H ,
Hence, S = l / ((B/H) * A) i.e. S = (l*H) / (B*A) --------- (1) Also, m.m.f. (H) = (N * I) / l ……. (Where N = No of turns; I = Current; l = length of flux path)
Therefore, S=
S=
l * ((N * I) / l)) (B * A) (N * I) (B * A)
------------ (2)
Comparing equation (1) & (2), as N, B & A are constant, S α l and S α I Hence I α l (i.e. Magnetizing current is directly proportional to the length of the magnetic reluctance path) Thus we can conclude that, due to asymmetrical core geometry, more reluctance path is offered to the flux produced in extreme limbs which is mainly responsible for drawing higher magnetizing current than centre limb. B) Magnetizing current measurement by application of three phase voltage.
Measurement of magnetizing current by application of 3 phase voltage is normally carried out at site. It may be helpful to compare the trend of magnetizing current with its historical data. However, this method is less helpful in fault diagnosis than single phase method. The trend of measured magnetizing current in 3 ph. method is different for different winding connections. This is due to different mutual impedances in 3 phase 3-limb transformers, as these mutual impedances are function of number of turns & disposition of windings, winding connections within a phase and more importantly on dimensions and layout of the core. Some measurements are tabulated below in Table:9. Table:9 Transformer Rating Vector Voltage Magnetizing current in mA, Group applied from when 3ph. 415 V applied U Phase V Phase W Phase Measurements from Delta connected winding
160MVA, 220/132/33 kV 100 MVA, 220/66/11kV 100 MVA, 220/66/11kV 315 MVA, 400/220/33 kV 160 MVA, 220/66/11 KV 315 MVA, 230/16.5 kV
YNa0d11 YNynd11 YNynd11 YNa0d11 YNynd11 YNd11
Tert. - 33 kV TER-11 KV TER-11 KV TER-33 KV TER-11 KV LV - 16.5 kV
15.3 136.3 159.3 23.3 275.1 244.3
18.1 157.6 183 44 289 239.5
25.5 212.3 245.4 53 409 322.4
20 MVA, 110/33-11KV 160 MVA,220/132/11 KV
Dyn11 YNa0d1
HV-110KV TER-11 KV
1.53 181.4
1.05 136
0.94 100.4
Transformer Rating
Vector Group
Voltage applied from
Magnetizing current in mA, when 3ph. 415 V applied
Measurements from Star connected winding
10 MVA, 110/33-11KV Dyn11 LV-33 KV 4.99 3.45 4.81 31.5 MVA, 16.5/6.9 KV Dyn1 LV-6.9 KV 200.6 133.4 200.5 100 MVA, 220/66/11kV YNynd11 IV- 66 KV 10.4 6.63 10.83 100 MVA,220/132/11 KV YNa0d1 IV-132 KV 1.7 1.06 1.68 From the foregoing, the trend of 3ph magnetizing current can be summarized as under. a) If measurements are done from star connected winding, the trend is similar to the single phase measurement trend (i.e Iv-mag is lesser lesse r by 60-80% of Iu-mag & Iw-mag). b) If measurements are done from delta connected winding, one of the extreme phases draws more current than other 2 phases depending upon the delta connection of the winding. As observed, Yd1 & Dyn11 - U phase draws higher magnetizing current than V & W phase Yd11 & Dyn1 - W phase draws higher magnetizing current than V & U phase This is to be noted that for delta connected winding also, magnetic section corresponding to V phase required least magnetizing current, but the phasor addition of currents of two phases results into a condition that current in V phase equals the current of one of the extreme phases. This trend is observed at different excitations and confirmed that it remains same at any excitation. Use of this test for Fault Diagnosis:
This test is widely used by Indian end users as a powerful diagnostic tool for checking healthiness of transformer at site post fault occurrence events during operation. Manufacturers also use this test to ascertain healthiness of windings at different manufacturing stages. General understanding about this test is that, only inter-turn failures are detected by this test but some case studies discussed below confirm that this test can detect other faults like shorting of parallel conductors and presence of external metallic loop around the core limb. This implies the need for proper study of results of MBT test along along with the complementary magnetizing current measurement tests. Following are some case studies which prove this test as one of the powerful techniques to locate the abnormalities in the transformer windings. Case Study: 1
50 MVA, 220/33 kV, YNyn0 connected Power Transformer At pre-connection stage of manufacturing, following test results were measured as given in Table:10. Table:10 Measured Voltage in Volts Total Voltage induced Magnetizing in other 2 phases Current 2U-2N 2V-2N 2W-2N Phase 99.4% U 239.0 (100%) 4.6 (1.92%) 233.8 (87.8%) 23.9 mA 99.8% V 238 (100%) 119.0 (50.0%) 2.49 Amp 118.6 (49.8%) 99.5% W 233.4 (97.8%) 3.47 (1.45%) 238.3 (100%) 23.3 mA
Here voltage trend shows that there is very less voltage is induced in the limb V, i.e. very poor flux linkage with the winding turns in the limb V. In other terms we can say that flux is not able to enter the V limb and getting the path through W limb (as evident from voltage trend). Magnetizing current trend shows high current in V limb, which indicates there is a closed loop in the V limb which is acting as a localized load, drawing the load current from the source. The abnormally high magnetizing current drawn by the limb suggests that there is a inter turn short circuit fault in the winding. To locate this fault, circulating current was checked on the winding discs by clamp meter and gauss meter in which circulating current was sensed in uppermost disc of HV winding. Based on these findings, visual inspection of top disc was carried out in which it is found that main lead take off is having sharp bend and this bend touched the 2 nd disc. This further crushed it’s insulation during pressing activity and resulted in to short circuiting of the turns. Case Study: 2
40MVA, 132/33 kV, YNyn0, 3 Phase Power Transformer Results given in Table:11 are of MBT test conducted from LV side of the transformer before commencing the routine tests on the transformer Table:11 Measured Voltage in Volts 2U-2N 226 (100%)
2V-2N
2W-2N
196(86.7%)
30.4 (13.5%)
97(42.7%)
227(100%)
130(57.3%)
27(11.9%)
198(87.6%)
226(100%)
Total Voltage induced in other 2 phases 100.2% 100.0% 99.6%
Magnetizing Current Phase U 61.3 mA V 39.8 mA W 49.0 mA
Prior to this test, resistance measurement test was carried out at pre-tanking stage. Considering this fact above test results could have been accepted. However, still to confirm the healthiness, all other routine tests were performed on this transformer. While the transformer withstood the dielectric tests like separate source & induced over voltage withstand successfully, the no- load losses were found erratic and on higher side (almost 1.8 times the expected value). DGA in oil was conducted before & after routine tests, which showed no abnormality. To investigate it further, 1-phase loss measurement was carried out. Rated phase voltage applied from LV side to individual phases in which power loss value of U limb was found substantially higher as compared to other two limbs. With this it was concluded that there might be an inter-strand shorting in the winding, forming a closed loop within within the winding, winding, leading to circulating current with with consequent increase in the losses in U limb. To locate the fault the core coil assembly of the transformer was un-tanked. Crimping lug of the lead was cut and discontinuity was checked between the winding strands, in which both parallel strands of regulating winding were found shorted at ID side at one of the transpositions.
Case Study: 3
315 MVA, 400/220/33 kV, YNa0d11 connected, 3 Phase Autotransformer Test results given in Table:12 are the results when this test was conducted from Tertiary side of the transformer at pre-connection stage. For this test temporary delta connection of Tertiary winding was formed. Table:12 Measured Voltage in Volts
Total Voltage induced in other 2 phases
3U-3W 226.4(100%)
3V-3U
3W-3V
70(30.9%)
171.6(75.8%)
112.4(49.4%)
227.6(100%)
115.8(50.9%)
171.8(75.3%)
69.6(30.6%)
228.3(100%)
106.9 % 100.2% 105.7%
Magnetizing Current Phase U 50.8 mA V 90.3 mA W 50.8 mA
As evident in this case, there is no significant variation in absolute values of voltages & magnetizing currents, but the trend of magnetizing current is abnormal. These types of test results are tricky and confusing and need to be tackled carefully. As magnetizing current in V limb was higher, to analyze it further, discontinuity was test performed between individual strands of each winding of V limb. In this case, out of 15 parallel strands of CTC conductor, 2 strands in common winding were found f ound shorted at inner transposition due to scissoring action. This type of failure was detected at very early stage by MBT test due to which heavy rework was avoided. Similarly one more case of inter-strand shorting gave following test results tabulated in Table:13. In which LV CTC coil of 100 MVA, 220/66-33/ 11 kV, YNyn0d11 connected System transformer had a inter-strand failure (confirmed by discontinuity test) which was detected at pre-connection stage. Table:13 Measured Voltage in Volts
Total Voltage induced in other 2 phases
2U-2N 231.8 (100%)
2V-2N
2W-2N
186.5 (80.5%)
46.1(19.9)
148.8 (63.9%)
232.6 (100%)
86.5 (37.2)
62.1 (26.7%)
169.9 (73.1%)
232.5
(100%)
100.3% 101.2% 99.8%
Magnetizing Current Phase U 167.9 mA V 140.1 mA W 256.1 mA
The above Case Studies-2 & 3 give a different dimension to the pre-conceived notion that inter strand shorting can not be detected by this test and shorting of parallel conductors does not affect the performance of the transformer. From the above findings it is quite evident that although the voltage distribution does not get affected by such fault, yet the trend of magnetizing current is influenced substantially. This phenomenon can be explained as under. Difference in the magnitude of leakage flux linkage in in different strands produces different voltages in the strands of the conductor. The diagram below illustrates the phenomenon. Say, ‘Φ’ is the flux linkage with conductor at position 3, ‘Φ/2’ is with middle conductor at position 2 & ‘0’ is of the conductor at position 3.
The alternating leakage flux linkage experienced by different strands being different, different potential would be induced in these strands. In case of shorting of any of the strands at different potentials due to insulation damage would nullify the effect of transposition and cause flow of circulating current within the shorted loop of strands. This would eventually reflect in to higher magnetizing current. Further, when the transformer with this type of defect is charged at the rated voltage for noload loss measurements, we observe abnormally higher no-load loss as a result of the circulating currents, which may almost twice the expected value of no-load losses. This is already discussed in Case Study-.2 Case study : 4
2 x 8789 / 2 x 2 x 6214 KVA, 11 kV / 2 x 2 x 0.3986 kV, 3 Phase Rectifier transformer Following are the test results when this test was conducted from HV side of the rectifier transformer at post-connection stage. (Refer Table:14)
Table:14 Measured Voltage in Volts 1U-1V 230(100%)
1V-1W
1W-1U
184(80.0%)
45.1(19.6%)
33(14.4%)
229(100%)
201.6(88.03%)
13(5.68%)
216.5(94.7%)
228.6(100%)
Total Voltage induced in other 2 phases 99.6% 102.4% 100.4%
Magnetizing Current Phase U V W
565 mA
117 mA 123 mA
As there was no abnormality at pre-connection stage, possibility of direct inter-turn shorting or wrong interleaved joint was ruled out. Circulating currents measured in the winding with the use of guass meter and clamp on milli-ammeter did not show any abnormality. Since no problem in the winding was evident, decided to carry out the visual inspection of the core thoroughly. During core inspection it was discovered that a core banding metallic strap was
forming a closed loop around the core. The operator had missed to provide the isolating wooden block to one metal strap, which formed a metallic loop over the top yoke between U and V limbs. Case study: 5
100 MVA, 220/66/ 11 kV, YNyn0d11 connected, 3 Phase System transformer Following are the test results when this test was conducted from Tertiary side of the transformer at post-connection stage during manufacturing of the transformer. (Refer Table:15) Table:15 Measured Voltage in Volts Total Voltage induced Magnetizing in other 2 phases Current 3U-3W 3V-3U 3W-3V Phase 99.9% U 213(100%) 200.4(94.1%) 12.4(5.8%) 140 mA 100.0% V 213(100%) 185.7(8.7%) 27.4(12.8%) 130 mA 100.0% W 213.3(100%) 750 mA 54.3(25.5%) 159(74.5%) Test results indicated problem in the windings of W- phase. While checking the circulating current in the shorting leads of HV Regulating winding (comprising 4 parallel sections) circulating current was observed, which had resulted in higher higher magnetizing current in W phase. Parallel shorting leads were cut apart and MBT was repeated with satisfactory results. Further, to diagnose the fault, ratio test between HV and individual step of Regulating winding was performed in which one turn was found more than specified in one of the steps. This extra one turn was causing the potential difference between the parallel coils, resulting in circulating current. To rectify this problem this extra one turn was removed from the winding and MBT was repeated after all connections which found normal. Case study 6:
21.6 MVA, 132/27 kV, 1 Phase Traction transformer As discussed earlier, in single phase transformers, only magnetizing current can be measured, hence it becomes essential to compare magnetizing currents of such transformers with similar units. While testing this transformer at pre-connection stage phase angle was found higher during ratio measurement. Also magnetizing current was found 26mA from HV side as against 4mA value measured in earlier units. To locate the fault, presence of circulating current in the winding checked with the help of clamp on milli-ammeter, which indicated the circulating current in bottom most disc. To rule out possibility of shorting of bottom Static End Ring (SER), SER was physically checked, which was found healthy. All cross-overs in that zone were checked for any damage of conductor paper insulation due to scissoring action and these were also found intact. Later, Ratio measurement was carried out on individual strand of the HV coil which is interleaved disc winding. Test results indicated that out of two strands, one strand has less number of turns compared to other, which is a clear indication of one interleaved joint made wrongly (i.e. dummy turn brazed to dummy turn), thus forming a short circuited turn. Physically this fault was located at the winding start and could be repaired by interchanging the main and dummy conductor at the interleaved joint.
Case study 7:
167 MVA, 400/√ 3 / 220/√3 / 33 kV, 1 Phase Autotransformer At pre-connection stage, high magnetizing current of the order of 10A was measured at 230V when tested from Tertiary winding side as against 40mA measured earlier on similar units. To confirm further with the help of clamp on type milli-ammeter, circulating current checked through out the series winding axially. In series winding between disc no. 61 & 62 high circulating current was observed. While inspecting those discs it is observed that while making interleaved joint in the winding, unscrupulously the winder had brazed dummy conductor to dummy conductor, which resulted in a short circuited turn. This turn was acting as a localized load to the supply source and drawing high current. Conclusion:
Magnetic Balance test when supplemented with measurement of magnetizing current, proves to be a strong diagnostic test to detect Faults like a) Inter-turn shorting; b) Inter-strand shorting; c) Any external loops around the core; d) Abnormal magnetizing current due to unequal turns in winding sections connected in parallel; e) Wrong interleaving joints in windings. To obtain accuracy in the test results, it is recommended to perform this test before resistance measurement test or after proper demagnetization of the core as remnant DC component in Core substantially affect test results of MBT. Reference books: 1) Electronic Circuits – Fundamental & Applications by Mike Tooley 2) Transformer Engineering – by S. V. Kulkarni Acknowledgements: Author is thankful to the EMCO Management for granting permission to publish this paper.
