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Dissolved Gas Analysis (DGA) of Current Transformer (CT) oil – A reliable tool to identify manufacturing defects

A.K. Datta, S.C. Singh, S.K. Mishra and S. Suresh

Power Grid Corporation of India Limited ABSTRACT: Current Transformer (CT) and Capacitive Voltage Transformer (CVT) are important equipment in any electrical installation. The protection, metering and operation of the sub-stations are decided based on their inputs. In our past experience, it was found that many CTs and CVTs have failed during service and some time in a few months after the initial commissioning.

Present paper discusses about prevention of failure of CTs by carrying out DGA of CT oil as a standard practice, after commissioning and also in case of violation of CT parameter such as C & Tan-Delta with respect to commissioning value during service life of CT. It also covers two case studies--- first case is related to generation of gases within 2 – 3 months after commissioning in some of CTs supplied in batch of 48 Nos. and the second is about CT with three years of service life found with increase in Tan-Delta value. Paper covers various measurements carried out at sites and subsequent shifting of CTs to respective manufacturer’s works, for detailed testing including high voltage insulation tests. In this paper, it is established that the DGA of CT oil indicates a clear sign of incipient fault in CTs, if not taken care of same, premature failure of CT is unavoidable. Keywords: DGA - Dissolved Gas Analysis CT - Current Transformer CVT - Capacitive Voltage Transformer PD - Partial Discharge

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Nomenclature: Name Symbol Nitrogen N2 Oxygen 02 Hydrogen H2 Carbon monoxide CO Carbon dioxide CO2 Methane CH4 Ethane C2H6 Ethylene C2H4 Acetylene C2H2 INTRODUCTION: Power Grid Corporation of India (POWERGRID) is one of the largest 400 kV / 220 kV / 132 kV transmission utility in the world operating about 55000 circuit kms and 95 sub-stations having transformation capacity of 49500 MVA, with average availability of system on yearly basis more than 99%. POWERGRID is having stringent system for maintenance of high voltage switchyard equipment. All equipments are tested as per the standard pre-decided frequency and test results are analysed in detail, for identification of defects in the equipments. The population of 400 / 220 / 132 kV Current Transformers in POWERGRID network is approx. 3200, these include both Dead and Live tank design and majority of CTs are oil immersed. The paper discuss about the introduction of DGA test for CT oil as standard test apart from routine tests at site. Normally Tan delta & Capacitance, Thermography of CT are being carried out on yearly basis apart from other tests. Inspite of doing all the best possible maintenance, failure of CTs are observed at all voltage levels (400 / 220 / 132 kV). Failure status of CT is indicated at table- 1.

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S.No Make Population No.of failure No.of failure

(3-15 yrs) Newly

commissioned 1 Manufacturer-1 695 74 5 2 Manufacture -2 1046 5 17 3 Manufacture -3 168 0 4 4 Manufacture -4 174 0 3 5 Manufacture -5 872 4 3 6 Manufacture -6 42 3 0 7 Manufacture -7 132 0 0

TOTAL 3129 86 32 Table-1

The cases of failure of CTs can be divided into two categories viz. failure of CT within a year of commissioning and failure of CTs after three years and more in service. Power Grid have established two nos. oil Lab for conducting various test as per IEEE-C57.104 and DGA of oil is being used as one of important tool, for identifying the fault of transformer and reactors since last 15 years. With successful interpretation of DGA results, failure of number of transformers and reactors were avoided in the past. As CT is also oil filled equipment, DGA of CT was also started since last 4 – 5 years on case to case basis. Initially, DGA of CT was opposed by the manufacturer with the pretext that CT has limited oil capacity and functioning of CT may deteriorate due to breakage of hermetically sealing of the CT. But with our experience, it was found that DGA is a very important tool, for identifying the manufacturing process defects or aging affect of CT at the very initial stage. The maximum admissible values for sealed instrument transformer without any action to be taken on the transformer are as below (as per IEC 60599)

H2 CO CO2 CH4 C2H6 C2H4 C2H2 300 300 900 30 50 10 2

The paper discusses about two Case Studies, which correlates evolution of gases with inconsistency in manufacturing in CTs.

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CASE STUDY – I: In one of the newly commissioned projects, 48 Nos. of 400 / 220 kV CTs were supplied in one manufacturing lot in the year 2004 – 05. The CTs were commissioned in July – Aug. 2005. One no. CT of 400 kV has failed after 51 days of charging of the CT. As all the test results both at factory and site were perfectly in order for all the CTs including that of failed CT, the manufacturer contended that it may be an isolated case and CT has failed due to lightening affect as the CT failed in rainy season under thunder storm. But it was decided that DGA of oil of all the CTs shall be carried out and accordingly DGA was carried out and results are tabulated as under: Sl. No. Sample

Date TGC (%)

N2 (%)

O2 (%) H2 CH4 C2H4 C2H6 C2H2 CO CO2

Total Furans

1 30-Oct-05 3.35 2.24 0.84 83 1 0 0 0 63 184 0 2 30-Oct-05 3.37 2.2 0.71 156 2 0 0 0 54 216 0 3 30-Oct-05 3.65 2.37 0.8 223 1 0 0 0 53 243 0 4 30-Oct-05 2.91 1.98 0.7 65 1 0 0 0 53 153 0 5 30-Oct-05 4.37 2.91 1.04 88 1 0 0 0 86 197 0 6 30-Oct-05 2.91 2.02 0.66 69 1 0 0 0 84 169 0 7 30-Oct-05 2.77 1.84 0.74 216 2 0 0 0 26 198 0 8 30-Oct-05 3.35 2.29 0.85 79 1 0 0 0 53 181 0 9 9-Dec-05 7.48 5.35 1.69 741 35 6 2 46.8 49 1380 0

10 9-Dec-05 9.06 6.67 1.97 128 2 0 0 0 95 276 0 11 9-Dec-05 4.1 2.85 1.03 199 3 0 0 0 87 585 0 12 9-Dec-05 5.43 3.65 1.55 138 1 0 0 0 35 475 0 13 9-Dec-05 5.42 3.75 1.43 210 1 0 0 0 45 221 0 14 9-Dec-05 3.66 2.46 0.98 134 2 0 0 0 59 195 0 15 9-Dec-05 3.81 2.52 1.09 162 2 0 0 0 78 324 0 16 9-Dec-05 2.2 1.46 0.6 123 2 0 0 0 40 292 0 17 9-Dec-05 3.37 2.31 0.9 96 2 1 0 1.3 104 434 0 18 9-Dec-05 4.83 3.49 1.19 94 2 0 0 0 104 281 0 19 9-Dec-05 4.39 3.01 1.21 118 2 0 0 0 84 226 0 20 9-Dec-05 3.67 2.54 1.01 158 2 0 0 0 65 208 0 21 8-Dec-05 5.84 4.23 1.45 133 2 0 0 0 89 183 0 22 8-Dec-05 6.88 5.02 1.61 128 2 0 0 0 58 253 0 23 8-Dec-05 4.88 3.68 1.06 123 2 0 0 0 74 487 0 24 8-Dec-05 3.37 2.36 0.89 182 3 1 0 2.8 72 245 0 25 8-Dec-05 9.11 6.65 2.94 94 2 0 0 0 60 420 0 26 8-Dec-05 3.08 2.12 0.8 127 3 0 1 0.3 59 229 0 27 8-Dec-05 2.64 1.77 0.76 207 2 0 0 0 33 232 0

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28 8-Dec-05 2.94 2.07 0.75 421 1 0 0 0 52 254 0 29 8-Dec-05 2.5 1.72 0.67 262 2 0 0 0 29 178 0 30 17-Dec-05 9.37 7.51 1.36 922 2 0 0 0 118 787 0

31 17-Dec-05 8.54 6.73 1.4 361 5 0 2 0 102 234 0

32 17-Dec-05 8.51 6.45 1.51 308 2 0 0 0 70 1113 0 33 17-Dec-05 8.89 6.62 1.74 343 1 0 0 0 105 1049 0 34

17-Dec-05 8.07 6.41 1.16 606 2 0 0 0 101 266 0 35 17-Dec-05 8.02 6.5 1.06 749 3 0 0 0 61 242 0 36 3-Jan-06 7.72 6.43 1.06 562 2 0 0 0 91 268 0 37 3-Jan-06 8.8 6.87 1.61 649 1 0 0 0 139 512 0 38 3-Jan-06 9.16 7.47 1.32 1143 2 0 0 0 149 785 0 39 3-Jan-06 7.55 6.25 0.98 749 2 0 0 0 110 174 0 40 3-Jan-06 8.68 7.34 1.15 533 2 0 0 0 132 601 0 41 3-Jan-06 10.5 8.26 1.96 187 2 0 0 0 96 381 0 42 3-Jan-06 9.95 8.43 1.16 518 2 0 0 0 125 534 0 43 3-Jan-06 9.62 8.12 1.14 482 2 0 0 0 121 545 0 44 3-Jan-06 9.37 7.97 1.17 466 1 0 0 0 113 241 0 45 3-Jan-06 9.06 6.98 1.58 427 2 0 0 0 100 1170 0 46 3-Jan-06 9.83 7.9 1.22 3053 80 0 8 0 224 642 0 47 3-Jan-06 9.08 7.12 1.63 578 2 0 0 0 151 357 0 48 3-Jan-06 9.62 7.83 1.23 3077 70 0 7 0 216 346 0

As can be seen from the results, in majority of the CTs, content of Hydrogen is comparatively high, even after only few days of charging of CT. To investigate the matter of high generation of dissolved gas in majority of CTs, it was decided to select some CTs for detailed high voltage testing at manufacturing works. Total 4 Nos CTs at Sr.No.9, 30, 46 & 48 were selected. These CTs were sent to the manufacturer works for detailed testing.

a) 400 KV CT at Sr.No. 9 Following tests were carried out in CT

1. Capacitance and Tan Delta test up to 1.1 Um/Square root3. 2. HV one minute power frequency test at 630 KV 3. PD at 291 KV and 420 KV 4. Long duration Power Frequency test (6 hour) with simultaneous

monitoring of PD. 5. Capacitance and Tan Delta test upto 1.1 Um / Square root 3 6. Lightning Impulse test as per IEC – 60044-1

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7. Switching Impulse test as per IEC – 60044-1 8. PD Test up to 170 KV 9. Capacitance and Tan Delta test up to 170 KV. The test results were normal up to Sr.No.7 i,e switching Impulse test, while carrying out PD test after completion of switching Impulse test, PD was found very high i.e. exceeding meter range at 170 kV. The CT was again tested for C & Tan Delta value at different voltage, but bridge did not balance at 1.1 Um/ square root 3 and test could not be completed.

As can be seen that CT has passed majority of the tests except PD test after the impulse testing. All the wave form during impulse test were matching, but it is very clear that CT has failed during the impulse test as PD was very high immediately after impulse test. Subsequently, CT was taken for internal inspection and the observation were as follows. 1. Tan delta test connection was normal. 2. Secondary windings were normal. 3. The physical distance between two foils was found varying between

15 mm to 40 mm, where as internal standard of manufacture says 30 mm +-5.

4. Puncture marks were observed on the straight limb between first main grading and fifth partial foil at the distance of 920 mm and 1300 mm from top.

5. At 1300 mm, the flashover marks were dominant. (Figure-1) 6. The first grading foil in the puncture / flashover zone was not intact

whereas the same was perfectly alright in balance portion. 7. Sharp edged were found in primary conductor at about 1300mm from

top.(Figure-2).

(Fig – 1) (Fig-2)

Puncture Mark Sharp edges Photographs with puncture mark and sharp edges are shown in Fig. 1 & 2.

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b) 220 KV CT at Sl. No. 46: As per past experience of 400 KV CT, since CT has passed majority of the high voltage power frequency tests except the impulse test. It was decided to carry out the Lightening Impulse Test as per IEC-60044-1, immediately after Capacitance & Tan delta test of CT. Accordingly following Tests were carried out.

1. Capacitance and Tan Delta up to 1.1 Um / Square root 3 2. Lighting Impulse test as per IEC -60044-1 in which CT failed during

positive impulse test. It was observed that Capacitance & Tan Delta was normal, it had also passed negative impulse test, however, it failed on first positive Impulse shot. (Graph-1)

(Test waveform) (First full voltage waveform)

(Second full voltage waveform) (Graph-1)

(Positive Impulse waveform)

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Subsequently, CT was taken for internal inspection and the observation were as follows. 1. Tan delta connection was normal. 2. Secondary windings were normal. 3. Puncture mark was observed on the straight limb just below the

secondary core. (figure -3) 4. Placement of insulating grading foil was checked and found OK. 5. In insulation, wrinkles and deep depression near the failed point was

observed. The wrinkles were observed in both paper and semi conductor layer and aluminums layer.(figure-4)

6. The puncture mark was on the outward side in the final 8 grading layer and then on the opposite side for balance 5 grading layers.(figure-5)

7. The built of insulation on the curved portion at the bottom was largely uneven and was abnormal.(figure-6)

It is very clear that CT was failed due to poor workmanship during manufacturing of CT core.

(Fig-3) (Fig-4)

Puncture mark Wrinkles on paper

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(Fig.5 Puncture mark) (Fig -6 Insulation on curved portion uneven)

Photographs with puncture mark & inconsistency in manufacturing is given in Fig. 3, 4, 5 & 6.

c) 220 kV CT at Sl. No. 48: CT was put to 1 minute Power frequency test and at 368 KV ( 80 % of 1 minute power frequency test). CT failed in the above test and build up of volatage was tried again at 20 KV, PD level was found to be beyond range of meter. Further test was discontinued.

d) 220 KV CT at Sl. No. 30:

1. Partial discharge test at Um was carried out and same was found to be 7 pC as against pre-dispatch value of 1-2 pC. The CT was pre-stressed at 368 KV ( 80 % of 1 min HV) CT withstood the voltage.

2. Lightning Impulse test at 1050 KV was conducted and CT failed during 2nd positive shot.

CTs of this lot were supplied after detailed type testing to prove its design capability. Generation of gases immediately after commissioning indicates the discrepancy in manufacturing quality.

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CASE STUDY – II: In one of the projects, 16 Nos. 400 kV CTs were supplied. The CTs were commissioned in the year 2002 after carrying out all the pre-commissioning tests. In August 2005, one of 400 kV CT had failed. As we had never witnessed the failure of this design CT in our network, it was decided to investigate in details about the healthiness of other CTs. Detailed testing of all the balance CTs were carried out and results are tabulated as Table-2.

Measured on 04.09.05

Sr.No Phase N2

Pressure N2 pressure Measured value at 10

KV Pre-commissioning

value at 10 KV found filled Capa. Tandelta Capa Tandelta

1 R 0.8 1.05 1.124 0.0029 1.117 0.0028 2 Y 0.2 1.05 1.133 0.0047 1.124 0.0029 3 B 0.4 1.05 1.143 0.0026 1.133 0.003 4 R 0.8 1.05 1.133 0.0011 1.133 0.002 5 Y 0.6 1.05 1.119 0.0021 1.118 0.0018 6 B 0 1.05 1.145 0.0019 1.144 0.0025 7 R 0.7 1.05 1.129 0.0021 1.117 0.0026 8 Y 0.6 1.05 1.119 0.0022 1.117 0.0022 9 B 0.4 1.05 1.125 0.0023 1.129 0.0023

10 R 0.6 1.05 1.142 0.0029 1.121 0.0028 11 Y 0.6 1.05 1.123 0.0021 1.107 0.0019 12 B 0.4 1.05 1.123 0.0029 1.107 0.0029

13 R 0.4 1.05 1.135 0.003 1.122 0.0034 14 Y 0.3 1.05 1.124 0.0028 1.105 0.0031 15 B 0.2 1.05 1.146 0.0028 1.177 0.0036

(Table-2)

Tan Delta values of CTs. In one number CT Nitrogen pressure was found ‘zero’, oil sample of the CT was tested for BDV and PPM and no abnormality was found while in one no. CT (at Sl. No.2) the appreciable rise in tan delta value with respect to previous values was observed, although, absolute value of tan delta was well

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within the limit, as per IEC 60044-1. To further investigate the reason of increase in Tan delta value, oil sample of the CT along with some other CTs (at Sr.No.2,6,7 & 15) were taken for DGA and the results for the same are tabulated as Table-3.

CTSl No

Sample Date

TGC (%)

N2 (%)

O2 (%) H2 CH4 C2H4 C2H6 C2H2 CO CO2

Total Furan

15 6-Sep-05 8.08 6.65 1.31 69 2 0 1 0 93 1115 0

6 5-Sep-05 8.73 7.06 1.55 87 2 0 0 0 112 1210 0

7 5-Sep-05 8.72 6.64 0.95 78 1 0 0 0 82 791 0 2 5-Sep-05 7.79 5.51 0.55 10633 1174 1 182 1.3 99 998 0

Table – 3 ( DGA of CTs )

The CT with appreciable increase in tan delta value was found also with high content of Dissolved Gas in the oil. CT was withdrawn from service and sent to the manufacturing works, for detailed high voltage testing. The modalities of the test were pre-decided and carried out as per the following details: 1. Capacitance and Tan delta at different voltages and up to 267 KV - Results were same as per site values. 2. HV test at 80% of 1 minute Power Frequency Voltage (504 kV) - It

withstood. 3. Voltage was reduced to PD measuring voltage i.e 1.2Um/square root 3

(291kV) and kept for 6.5hrs and PD was measured continuously - PD was found less than 5 PC.

4. Voltage was again raised to 420 KV and kept for one hour. PD was measured and found that less than 5 PC.

5. Again Capacitance and Tan Delta was measured and there was no appreciable changes.

6. CT was heated in a oven up to 95 Deg.C, Capacitance and Tan Delta was measured and found with marginal increase in Tan Delta value.

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7. CT was again tested for PD at 291 KV for one hour and found that PD was less than 5 PC, where as while measuring PD at 420 KV kept for 30 minutes, PD was 7-8 PC. 8. Tan Delta was measured at surface temperature of 57 Deg and no

appreciable change was observed. Inspite of extensive high voltage / thermal stress testing, we did not find any appreciable change in Partial Discharge and Tan delta values at the factory. To rule out the possibilities of deterioration of oil, it was decided to change the oil of the CT and reprocess the CT. DGA of CT oil was done after reprocessing . Subsequently CT was again tested for partial discharge for six hours at 291 KV, although PD was less than 5pC but DGA of oil after test indicates generation of gases again. DGA results are indicated in Table -4.

Sl No

Sample Date

TGC (%)

N2 (%)

O2 (%) H2 CH4 C2H4 C2H6 C2H2 CO CO2

Total Furans

1 29-Jan-06 8.81 5.8 2.7 21 48 1 14 0 12 382 0 2 29-Jan-06 8.76 5.66 2.69 0 1 1 1 0 8 1038 0

Table – 4

(DGA of CTs) From the table, it is very clear that inspite of change of oil, generation of gas from the CT has not stopped. It is also clear from the detailed testing that none of the high voltage tests mentioned above were able to identify the reason of the generation of gas. As per our experience in Case Study-1, CT shall be tested for impulse test as per IEC 60044-1. CT shall be subsequently internally inspected for identifying the reason for generation of gases.

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OUTCOME OF INVESTIGATION: From the case studies, following can be concluded:

i) DGA of CT oil is reliable tool for ascertaining the healthiness of CT.

ii) DGA test is very sensitive test for ascertaining healthiness of CT,

which cannot be diagnosed in high voltage test other than Lightening Impulse Test. As can be seen in Case Study – 2, the CT has passed all tests despite the generation of high content of Gas and also in Case Study – I (a), CT had passed all the tests except the Impulse Test.

iii) DGA of CT oil after commissioning is an essential tool to ascertain

the manufacturing quality of lot. iv) In the service span of CT, if any appreciable changes are observed

in the routine test , oil sample of CT shall be taken for DGA, for ascertaining the healthiness of CT, as indicated in Case Study – 2.

CONCLUSION: It can be concluded that correlation of DGA findings with the factory testing can be established and DGA of CT oil can be used as an effective tool, for ascertaining the healthiness of CT in service and accordingly, the service utilities can introduce DGA test as commissioning test for CTs. POWERGRID had already made DGA test of CT oil as mandatory after commissioning and included the same in technical specification for ensuring manufacturing quality of CT.

Reference:

1. IEC – 60044-1 2. IEC – 60599 3. IS-9434:1992 4. POWERGRID norms.