ct failure

8/3/2019 Ct Failure

http://slidepdf.com/reader/full/ct-failure 1/13

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 ourpast 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 AnalysisCT - Current Transformer

CVT - Capacitive Voltage Transformer

PD - Partial Discharge

8/3/2019 Ct Failure


2

Nomenclature:

Name Symbol

Nitrogen N2

Oxygen 02

Hydrogen H2

Carbon monoxide CO

Carbon dioxide CO2Methane 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 standardtest 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.

8/3/2019 Ct Failure


3

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


4 Manufacture -4 174 0 35 Manufacture -5 872 4 3



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.

8/3/2019 Ct Failure


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. SampleDate

TGC(%)

N2(%)

O2(%) H2 CH4 C2H4 C2H6 C2H2 CO CO2

TotalFurans

1 30-Oct-05 3.35 2.24 0.84 83 1 0 0 0 63 184 02 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 87 585 012 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 89 183 022 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

8/3/2019 Ct Failure


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

3117-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 033 17-Dec-05 8.89 6.62 1.74 343 1 0 0 0 105 1049 0

3417-Dec-05 8.07 6.41 1.16 606 2 0 0 0 101 266 0

3517-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 96 381 042 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

8/3/2019 Ct Failure


6

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.

8/3/2019 Ct Failure


7

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)

8/3/2019 Ct Failure


8

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 largelyuneven 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

8/3/2019 Ct Failure


9

(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 indicatesthe discrepancy in manufacturing quality.

8/3/2019 Ct Failure


10

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 PhaseN2

Pressure N2 pressureMeasured value at 10

KVPre-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

8/3/2019 Ct Failure


11

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.

CT

Sl

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

11

2 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 noappreciable 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.

8/3/2019 Ct Failure


12

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

NoSampleDate

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.

8/3/2019 Ct Failure


13

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.

ct failure

Documents