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Dissolved Gas Analysis (DGA) of Current Transformer (CT) Oil – a Reliable Tool to Identify Manufacturing Defects

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Dissolved Gas Analysis (DGA) of Current Transformer (CT) Oil – a Reliable Tool to Identify Manufacturing Defects 1 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 2 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. 3 S.No Make Population No.of failure No.of failure Newly (3-15 yrs) 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. 4 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 TGC N2 O2 Total Date (%) (%) (%) H2 CH4 C2H4 C2H6 C2H2 CO CO2 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 5 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.
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