Procedia Engineering 68 ( 2013 ) 477 – 483

Available online at www.sciencedirect.com

1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysiadoi: 10.1016/j.proeng.2013.12.209

ScienceDirect

The Malaysian International Tribology Conference 2013, MITC2013

Diagnosis of OLTC via Duval Triangle Method and Dynamic Current Measurement

Mohd Shahril Ahmad Khiara*, Yasmin Hanum Md Thayoobb, Young Zaidey Yang Ghazalic, Sharin Ab Ghania, Imran Sutan Chairula

aUniversiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia bUniversiti Tenaga Nasional,43000 Kajang, Selangor, Malaysia

cTenaga Nasional Berhad (Distribution), 46000 Petaling Jaya, Selangor, Malaysia

Abstract

Power transformers studies have shown that the main cause of power transformers failures is the On Load Tap Changer (OLTC). OLTC failure occurs due to the insulation degradation and contacts failure inside it. Other than that, contacts wear influences the OLTC performance and this phenomenon has not been deeply investigated. The worn contacts will cause hotspots in the OLTC tank and cause the insulating oil to degrade faster. Consequently, this will lead to the damage of complete transformer unit. Thus, the research work that has been carried out applied three different stages related to the diagnosis of OLTC. Firstly, fault detection of power transformer’s OLTC is done mainly based on the results of the DGA interpretation by using the latest Duval Triangle method. Then, the results that obtained from latest Duval Triangle method are compared with the results carried out from Dynamic Current Measurement (DCM). Finally, research validation through visual inspection is applied to validate the results obtained from both DGA and DCM.

© 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia. Keywords: Contact resistance; dynamic current measurement; fixed contact; OLTC; transition contact

Nomenclature

ppm parts per million H2 hydrogen CH4 methane C2H6 ethane C2H4 ethylene C2H2 acetylene

* Corresponding author. Tel.: +6019-318-8185; fax: +606-555-2261. E-mail address: mohd.shahril@utem.edu.my

© 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia

478 Mohd Shahril Ahmad Khiar et al. / Procedia Engineering 68 ( 2013 ) 477 – 483

1. Introduction

Power transformers play a crucial role in power systems networks [1]. In addition, power transformers are among the most expensive equipment [2]. Therefore, the condition monitoring of power transformers using various method such as Dissolved Gas Analysis (DGA) and Frequency Response Analysis (FRA) are required consecutively to prolong the lifespan of power transformer [3].

Generally, the critical part of power transformers is the On Load Tap Changer (OLTC). Hence, the existence of a tap changer is being considered as the only moving element inside a power transformer. The mechanism of transforming the output voltage by selecting number of turns of the winding is done via tap changer [4].

Most power transformers studies indicated that the main cause of power transformers failures is the OLTC [1]. The OLTC results in higher failure rates compared to other electrical components since their mechanical parts are usually in operation as compared to others. Furthermore, their failures have effected on the damage of completetransformer unit. Thus, condition based maintenance should be a better option to observe the condition of the OLTC [1].

In this paper, the diagnosis of the OLTC is done by 3 different stages. Firstly, fault detection of power transformer’s OLTC is done mainly based on the results of the DGA interpretation by using the latest Duval Triangle method. Then, the results that obtained from latest Duval Triangle method will be compared with the results carried out from Dynamic Current Measurement (DCM). Finally, research validation through visual inspection is applied to validate the results that were obtained from both DGA and DCM.

1.1. Principle of OLTC’s Operation

While in operation, the sequence of OLTC’s switch comprised several taps. Fig 1(a)-(e) shows the sequence of OLTC in which [5]:

A main transition contact labeled as “M” Two others transition contacts labeled as “T1” and “T2” First fixed contact labeled as “A” Second fixed contacts labeled as “B”

(a) (b) (c)

(d) (e)

Fig. 1. A switching sequence of an OLTC with a main contact and two transition contacts [5].

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Fig. 2. DCM graph which illustrates the OLTC’s switching movement related to Fig 1 [5].

Fig 2 shows the DCM graph which illustrates the OLTC’s switching movement related to Fig 1(a)-(e) [5]. Furthermore, Fig 1(a) shows the initial position of the OLTC where the main transition contact labelled as “M” is on the first fixed contact (A). A switching sequence begins only when the OLTC contact starts to move.

Furthermore, Fig 1(b) illustrates the starting movement of OLTC where the main contact (M) leaved the fixed contact (A) and the transition contact (T1) is currently on the fixed contact (A). Through this process, the load current (IL) is transferred to the transition contact (T1) through the transition resistor (RT) of T1. Thus, the current will starts to decreased sharply.

Moreover, Fig 1(c) shows the T2 contact move on the fixed contact B. In this position, T1 and T2 contacts are in parallel in the current measuring circuit. As a result, circulating current (IC) will be generated between both transition contacts through the tap windings while these both transition contacts share the load current. Due to this process, total transition resistance becomes 1/2R, causing a slower decrease in the current.

Additionally, Fig 1(d) illustrates the current has been transferred to the next tap (fixed contact B). It flows through T2, with the same situation as in Fig 1(b).

Finally, Fig 1(e) shows the complete process of OLTC’s switching operation. This happened when the load current has been transferred to the next tap (fixed contact B) in which the main transition contact (M) is finally on the fixed contact B.

2. Research Works

2.1. Results of DGA

Through oil sampling process, the oil and the gases from OLTC of Transformer 1 is separated into nine different gases measured in ppm unit. This gases is then transferred into Duval Triangle to obtained type of faults occurred inside this transformer.

Hence, Fig 3 shows the fault coordination of OLTC for Transformer 1 after the faults plotting process via Duval Triangle has been done. As a result, the diagnosis shows that the condition of this OLTC is at “X3”. Table 1 shows the summary of the OLTC’s fault indication for Transformer 1. Hence, this OLTC is considered to have a light coking or increased resistance of contacts or severe arcing.

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Fig. 3. Fault coordination of OLTC for Transformer 1 by using latest Duval Triangle.

Table 1. Summary of the OLTC’s faults indication for Transformer 1 using Duval Triangle Method

Zone Identification Recommended Actions

X3 Fault T3 or T2 in progress (mostly) with light coking or increased resistance of contacts or severe arcing

Test or inspect the OLTC for signs of light coking or resistance of contact or severe arcing

2.2. Results of Dynamic Current Measurement

Fig 4(a) illustrates the pattern of current recorded by using DCM before the maintenance has been done. Based from the theory described in Section 1.1, a distortion can be clearly seen at fixed contact for all taps positions as in Fig 4(a). In addition, this OLTC was considered to be faulty due to the shape which was not in a “V-Shape” as described in [5].

On the other hand, Fig 4(b) expressed the pattern of current recorded by using DCM after maintenance ended which can be clearly observed that the distortion was still existed. However, the shape of the graph in Fig 4(b) acquainted that the OLTC was found to be healthier as compared to the graph in Fig 4(a). The reason was due to the graph obtained in Fig 4(b) was in a “V-Shape” as expected [5].

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(a)

(b)

Fig. 4. A pattern of current recorded via DCM for red phase (a) before maintenance (b) after maintenance.

2.3. Research Findings Validation through Visual Inspection

Finally, the OLTC was taken out for visual inspection. These last parts of observations were done only to the red phase to validate the findings by DGA and DCM. When the OLTC was taken out, high arcing was observed at the fixed contacts at tap 6 and tap 7 (Fig 5). This revealed exactly as the analysis carried out by the DCM that the red phase experienced a high switching arcing.

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Fig. 5. A high arcing was observed at fixed contacts at tap 6 and tap 7 for red phase.

Moreover, arcing mark was found at the three transition contacts of red phase as shown in Fig 6. In addition, these transition contacts were eroded and worn out due to the switching arcs [6].

Fig. 6. A closed up image of erosion marks at the OLTC’s transition contacts for red phase.

3. Conclusion

The research work that has been carried out discovered that condition based maintenance should be a better option to observe the condition of the OLTC. Hence, the findings revealed that it was significance to apply DGA via latest Duval Triangle method at the first stage to provide early indication of OLTC’s breakdown. At the second stage, the advanced technique on OLTC diagnosis using DCM can be applied if faults were detected by DGA via latest Duval Triangle method. The main reason is that it could directly figure out the exact types and location of the

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OLTC critical parts which comprised of the fixed and transition contacts.

Acknowledgements

The authors like to acknowledge the funding support provided by Universiti Teknikal Malaysia Melaka (UTeM) under grant no. PJP/2012/FKE/(44C)/S01049. Greatest gratitude goes to Dr. Yasmin Hanum Md Thayoob and Mr. Young Zaidey Yang Ghazali for their continuous support and magnificent assistance throughout this research project.

References

[1] Khiar, M.S.A., Thayoob, Y.H.M., Ghazali, Y.Z.Y., Ab Ghani, S.; Chairul, I.S., 2012. “Condition assessment of OLTC using duval triangle and static winding resistance test,” IEEE International Conference on Power Engineering and Optimization (PEOCO) Melaka, Malaysia, pp.432-435. [2] Chairul, I.S., Ghani, S.A., Khiar, M.S.A., Thayoob, Y.H.M., Ghazali, Y.Z.Y., 2012. “Kraft paper insulation's life assessment and effects of oxygen and moisture to paper insulation's deterioration rate,” IEEE International Conference on Power and Energy (PECon), pp.728-731. [3] Ab Ghani, S., Md Thayoob, Y.H., Ghazali, Y.Z.Y., Khiar, M.S.A., Chairul, I.S., 2012. “Evaluation of transformer core and winding conditions from SFRA measurement results using statistical techniques for distribution transformers,” IEEE International Conference on Power Engineering and Optimization (PEOCO) Melaka, Malaysia, pp.448-453. [4] J.J. Erbrink, E. Gulski, J.J. Smit, L.A. Chmura, R. Leich, P.P. Seitz and B. Quak, 2010. “Condition assessment of on load tap changers using dynamic resistance measurements,” International Conference on High Voltage Engineering and Application (ICHVE), pp. 433-436. [5] J.J. Erbrink, E. Gulski, J.J. Smit, P.P. Seitz, B. Quak, R. Leich, R. Malewski, 2010. “Diagnosis of Onload Tap Changer Contact Degradation by Dynamic Resistance Measurement,” IEEE Transactions on Power Delivery, Vol.25, No.4, October 2010, pp. 2121-2131. [6] J. J. Erbrink, E. Gulski, P. P. Seitz and R. Leich, 2008. “Advanced on-site diagnosis of transformer on-load tap changer,” IEEE International Symposium on Electrical Insulation, pp. 252-256.