2012 IEEE 10th International Conference on the Properties and Applications of Dielectric Materials

July 24-28,2012, Bangalore, India

An Attempt to Investigate the Transformer Failure by using DGA and SFRA Analysis

Shubhangi S Patil and Sushil E Chaudhari

High Voltage Product Technology Centre,

Global R&D centre, Crompton Greaves Limited, Mumbai, India

Email: shubhangi.patil@cgglobal.com

Ahstract- A sudden increasing power demand leads to manufacture of large number of oil immersed power transformers and other electrical power equipments.

Power transformers are the most vital equipment in power system. Any failure in transformer leads to malfunction of whole power system. Unfortunately, the

failure rate of these transformers is very high in India,

25 % per annum, which is not favorable as compared to international units of 1-2 %. Failures happen due to internal reasons or operational hazardless. Transformer

insulation deteriorates as the function of temperature, moisture and time. The core and winding losses, stray losses in tank and metal support structures are the

principle sources of heat which cause oil and winding

temperature rise. There are multiple reasons for overheating such as improper cooling, excessive eddy currents, bad joints, blocked radiators, overloading,

improper earthing and harmonic contents in power supply. This leads to accelerated aging of oil and

cellulosic solid insulation, which generate the gases

within transformer and further leads to permanent failure. To prevent such failures, effective analysis and diagnosis needs to be investigated. The type of gases generated and amount of gas concentrations in oil

efficiently evaluated using Dissolved Gas analysis (DGA). Various other electrical diagnostic tests like winding resistance test, short circuit impedance, oil

analysis and sweep frequency response analysis (SFRA)

are also helpful for identification of abnormalities and probable fault area. SFRA technique is widely accepted and used for transformer mechanical condition

assessment. Based on the type and concentration of gases generated in oil along with application of SFRA

test on transformer can help to identify the abnormal

areas prior to catastrophic failure. An attempt has made for the investigation on relation of DGA with SFRA

response. Case studies are presented here for the transformers which have higher fault gas concentrations

(DGA). Additional diagnostic tests and analysis, inspection and history data has found supportive in investigation.

Keywords- Power

Deterioration, Overheating, shift and Failure Analysis

INTRODUCTION

transformer, Insulation

DGA, SFRA, Resonance

To assure the efficient and reliable operation of

transformer in power system, various failure causes and

978-1-4673-2851-7/12/$31.00 ©2012 IEEE

effects are essential to study and consider. Power transformer can fail due to any combination of

mechanical, electrical or thermal stresses. Such failures are sometimes catastrophic and almost always include irreversible internal damage. The regular monitoring of dissolved gases DGA can provide useful information

about the condition of the transformer and prior

information of the faults [1, 4, and 8]. Monitoring can best be done by observing the trend of the generation of

the various gases. The relative distribution of the gases is used to evaluate the origin of these gases generation, and the rate at which the gases are formed can be used to assess the intensity of the fault. Together, these kinds of

information provide the necessary basis for the evaluation of any fault and the necessary remedial actions. DGA can give the indication of internal

abnormalities inside the transformer but may not exactly pinpoint towards the exact fault area [5]. Similarly, the mechanical condition assessment is efficiently done

using SFRA [2, 3]. SFRA is a well known diagnostic

test for detecting internal abnormalities and identifying the affected areas. After intimation of transformer

abnormal behavior, SFRA testing is one of the most preferred and always been used after the failures of

transformer which involves internal inspection and dismantling of transformer [2]. As compared to other diagnostic methods, SFRA technique provides the wide

range of reliable information about the mechanical status of a transformer's active part. But at the same time, this technique has less sensitivity for minor faults identification in relative sub frequency range. In actual

practice SFRA is used by utilities after permanent failure of transformer. During operation, minor faults developed inside the transformer progresses to major

fault and further leads to failure. To avoid permanent failure and to have proper maintenance, faults prediction at early stage becomes essential. The use of SFRA technique as regular check can be the better approach

for assessment of mechanical condition of transformer.

Study of effect of various faults on SFRA response having minor deviation but consistently growing nature

along with the generation of gas concentrations can be helpful to achieve the objective [5, 6].

Thus the combination of oil test, DGA and electrical

test, SFRA has tried to use for detecting transformer

internal faults prior to failure. To attempt this concept, the cases of transformers which have higher total

combustible gas concentrations are considered under study.

Case Study 1:

167MVA, 400/220 kV 3 phase auto transfonner was claimed for gassing problem. So the transfonner was

continuously monitored by DGA to avoid catastrophic failure and or prolonged outage. The DGA results revealed that the transformer had severe internal

overheating. The concentrations of CH4, C2H6 and C2H4 had substantially found in oil. The transfonner was suspected to undergo the arcing inside. DGA test results

only could give the warning of arcing inside the

transformer but did not indicate the area of fault.

In further study, due to abnormal behavior of the

transfonner, electrical tests like voltage ratio, excitation

current, impedance test, winding resistance test, insulation resistance test and magnetic balance test were carried out. All the test parameters were well within

acceptable limit which showed the healthiness of the

transformer. Still there was abnormality in DGA, the transformer was continuously monitored. The gas

trending was observed as below,

%TVGC 14

i!l 12 D%TDGcl - -� 10 E - -'S OJ 8 - - - -!! 8 6 - - - -�

o 0 n=

'" 4 1- - - - -5

D 0-0-� 2

-0-- - - -

0

,fy� ,fy,,'b ,fy� ,fy� ,fy� ,fy� ,fy� ,fy� ,fy� ,fy� ,fy<SJ ,fy<SJ ,fy<SJ cr· cr· <:9. �. �. �. <SJ. 0· ,'" ",. ,,'" <.Y. "",. c0. ",,,. ,"'. ". ,'b. ,,'b. ,,'b. ",,,,. ,'" " . ,,,. ,,0" " .

llme duration

Fig.O I. Rate of percentage total dissolved gas consumption (TDGC)

1600

1400 -1-12

1200 __ CH4

�ooo __ C2HE

] 800 --C2H4 -<: ,--g 600 ___ co

400 .,,- ------ CO2

I 200

�-/ ..... ---:: 0

110 1/5 1/10 1/15 sampling time

Fig.02. Trend of dissolved gas analysis

DGA trend indicated that the total dissolved gas content (TDGC) rate was increasing continuously. The

hydrocarbon gases were increased rapidly except CO

and CO2 gases. They did not show any abnonnal rise which indicates the cellulose was not involved in the fault. The liquid insulation deterioration lead to generation of hydrocarbon gases like CH4• C2H6 and

C2H4 gas concentrations indicated the severe thennal

fault inside the transfonner. It may lead to generate the carbon in oil due to pyrolysis of mineral oil. There are

2

many causes of severe overheating like bad contacts or

connections, small hot areas in the core, short circuit in the core, overheating of copper due to eddy currents and circulating currents in the core and the tank. This

infonnation was not sufficient to locate the fault. Additionally, SFRA was considered for further investigation.

Geometrical changes within and between the elements of network reflects as deviation in its frequency response. Differences between a deviated and original SFRA

signature are an indication of positional or electrical

variations of the internal components. Different failure modes affect different parts of the frequency range 20 Hz - 2 MHz. Due to diversity of structures of core

and windings, distribution of pole points of transfer function is different and thus it helps to reflect as deviation in SFRA response to state the condition of

core and winding. In this case, suspected transformer was analyzed for SFRA response taken at different time

intervals. The responses showed that one phase of transformer which has minor magnitude deviation of

1.44% - 3% and resonance shift of 8.05% to 10.5% in low frequency region «1 kHz) and in 20 - 30 kHz

frequency range which considered as insignificant in

earlier stage. But after the detailed analysis and evaluation with the change in gas concentrations was revealed that the fault is progressing in nature and affected areas were broken core bolt and bottom

insulation plate was damaged.

Fig. 03 SFRA response for HV winding at low frequency range «1 kHz)

:�� -50-

"'­

-EO-

065-

:!O 100 l K 1� lOOK 1M

Fig.04. SFRA response for HV winding··at medium frequency range (20 - 30 kHz)

Transformer core is a nonlinear component. If any part

of core insulation becomes defective or laminated structure of core is bridged by conducting material which permits the sufficient level of eddy current flow

which further causes severe overheating particularly at that area. The insulated core bolts are usually used for tightening of core. The insulation of the core bolts failed near the top yoke joint provided the easy path for

circulating currents to flow which led to overheating [5, 7]. With the desire of detecting such overheating sources before major fault gets created, regular basis

SFRA responses are analyzed for different time interval

to study the behavior of fault. The deviated SFRA responses are shown in fig. 03 and actual photograph of broken core bolt is shown in fig. 05.

Fig.05. Damaged core bolt near to top yoke joint

Observations in DGA and SFRA analysis:

Observations were drawn from the change of gas concentrations during the time period considered for

assessment and from the resonance shift with magnitude variation in SFRA response. The observations are listed below:

Gas Interval I Interval 2 Interval 3 interval 4

concentrations

%TGC 1.47 4.45 8.65 12.25

DGA CH4 9 27 83 88

C2H6 5 13 26 27

C2H4 19 51 180 186

Magnitude -78.629 -80.101 -81.73 -82.93

change normal 1.44% 2% 2.88%

SFRA Resonance shift 579.443 626.09 691.63 764.02

towards higher normal 8.05% 10.46% 10.46%

frequency

Fig.06. Black marks observed on 5mm bottom insulation plate

3

Fig.04 shows the SFRA response with deviation

observed in 20 - 30 kHz frequency range and deviation found progressing in nature at every interval. The capacitance value has changed due to damage in

bottommost 5 mm insulation plate and that is reflected

in SFRA response in terms of resonance shift towards the high frequency. The magnitude variation of 15 %, 20 % and 26 % with the factory results at regular check

is also observed. Black marks were observed on insulation plate after dismantling but such small

carbonization of cellulose was not supported by DGA.

Small deviation with consistent increase and resonance shift was closely observed here in case of core bolt fault,

monitored and correlated to respective gas

concentrations and DGA trends to identify the location of internal fault of transformer. The abnormality indication given by DGA and resonance shift in SFRA

for typical type of fault helped to strengthen the decision

making for transformer and to overcome the outages.

With similar methodology, investigation study is

performed on transformer which was having high gas generation issue. The detail of analysis is described below:

Case Study 2:

315MV A, 400/220 k V three phase transformer abnormality was reflected by high fault gas concentrations found in DGA. The CH4, C2H6 and C2H4 gases confirmed severe overheating. The trend of

individual gases was plotted. But DGA could not able to locate the fault. DGA trends for various intervals are shown in following fig. 07.

%TDGC

� 14 D%TDGcl

� 12+�----------------------------------�-Q. �10+----j ;: :3 8

j � fU-rju,-"-r"'-ru,-um-,'-Y=[m"-ru,-u-[J���L .c,u-01],-LL-c"'-r�'"-r'-�J�- ""-,u,-u,

Fig.07. Rate of percentage total dissolved gas consumption (TDGC)

2500

5'2000 +-------------------------�---� � 1500 +_---------------cr-"""'------------� �1000+---------/��-----------------� c5 500 t--f--��------==><=><"""�-----

�H2

_CH4

---*-C2H4

-C2H6

�C2H2

-+--co

-C02

1/5 1/10 1/15 1/20 1/25 Time

Fig.08. Trend of dissolved gas analysis

To check the status of internal abnormalities of

transformer, SFRA test was performed. All the three phases of HV winding were showing close correlation to each other except minor magnitude deviation of 3.15%

and resonance shift of 6.35% with one of the either phase, B phase. Minor deviation showed healthiness of

transformer but after revisiting data, that deviation was also considered in analysis and monitored with changes

in DGA results. Due to overheating problem indicated by DGA results, SFRA response of B phase at different interval has investigated to check the effect of behavior

of overheated transformer active part on SFRA response. The response shown in fig. 10 has shown clearly the deviation of B phase with magnitude variation and slight resonance shift.

Fig.09 SFRA response of HV winding in healthy condition

Fig. 1 0 SFRA response check at different interval for snspected B

phase of HV winding

After the disassembly of transformer, damage to insulation layer over lead and small black marks on lead were observed. Thus the combination of oil test, DGA and electrical test, SFRA could help to pinpoint the

affected area and identify the cause of overheating before transformer breakdown.

CONCLUSIONS In the paper, two case studies for transformers having

overheating issue are represented, discussed and

analyzed with DGA and SFRA diagnostic techniques.

4

Monitoring of fault gas concentrations with minor

resonance shift observed in SFRA can help to diagnose the fault at prior level to prevent failures. Strengthening the decision making of fault prediction in transformers

can be enhanced through maintaining proper history data and regular monitoring of DGA and SFRA. It is crucial to study the effect of various faults on gas concentrations and gradual escalation in resonance shift.

REFERENCES [1] IEEE standard C57.104-2008 Guide for interpretation of gases

Generated in oil hnmersed Transformers

[2] G Mathew Cannedy, Charles Sweetser, "Field experiences with

FRA" Doble, 2008

[3] Mechanical condition assessment using FRA, CIGRE 342, 2008

[4] December 2010, DGA for mineral oils and load tap changers and

improved DGA diagnosis", CIGRE 443, 2010 [5] T. Kobayashi, Y. Marnetani, T. sono, Y. Yogama, K. Miyagi, Y.

shirasaka, "New technology of diagnose for area of abnormal

overheating on actual transformers", CIGRE 2011

[6] T. Kobayashi, Y. Mametani, T. sono, Y. Yogama, K. Miyagi, Y.

shirasaka, "Using DGA and FRA to Develop New Technology for the

Diagnosis of Areas Subject to Abnormal Overheating on Actual

Transformers", CIGRE 2006

[7] Nuchit Tipcharoen, Kreingsak Srisuk, "Study of transformer

overheating investigation using frequency response analysis", International conference for MEA, 2008 [8] IEC 60599, Edition 2.1, 2007-05, Mineral oil-impregnated

electrical equipment in service - Guide to the interpretation of

dissolved and free gases analysis.