Ranjana SinghJabalpur Engineering College

Jabalpur , M.P. , Indiaranjana_n_singh@yahoo.co.in

Amarjit SinghGyan Ganga Institute of Technology and Science

Jabalpur , M.P. , Indiaamarjit_singh_india@yahoo.co.uk 1

Failure of distribution transformer affects the economy of nation in the form of loss of revenue, materials, repairing charges etc. , besides the inconvenience caused by interruption of power supply to consumers. Along with other causes, the main cause of failure of distribution transformers is prolonged overloading. In this paper three distribution transformers are analyzed and results are verified by dissolved gas analysis.

Keywords- causes of failure; cold load pickup current; distribution transformer; harmonics; overloading

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Unfortunately the failure rate of these transformers is very high in India, around 25% per annum, which is not favorably comparable to international norms of 1-2%. Transformer insulation deteriorates as a function of time and temperature. The winding I²R losses, the core losses, and the stray losses in the tank and metal support structures are the principle sources of heat that cause the oil and winding temperature rise .This leads to ageing and decomposition of both oil and cellulose which liberate gases within the transformer. The amount and type of gases formed and dissolved into the oil can be determined with the help of Dissolved Gas Analysis (DGA).

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All three transformers under consideration were supplied by different manufacturers and have the same specifications as follows - 200kVA, 11/0.433 kV, Dy 11, 50 Hz, 3-phase, high voltage winding current - 10.5 A, low voltage winding current - 266.67 A,Aluminium wound. Figures are of Case B in which all three phases of HV and LV windings were severely burnt.

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Transformer installed in rural area which failed after 23 months i. e. within warranty period of 3 years (Manufacturer – X) :

Diagnosis Results: Failure of one phase of HV windings is caused by overloading. Overloading/unbalanced loading due to power theft by hooking the mains is one of the major causes of transformer failure [5]. It may be assumed that stealing of power by some of the consumers would have been done by hooking the approachable bottom most conductor i.e. B phase of the system.

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It was found by inspection that one phase of HV windings was damaged severely as shown in Fig. 1. Fig. 2 gives the view of charred paper insulation. Fig. 3 shows the burnt condition of LV winding of same phase.

Fig. 1. Burnt HV windingsFig. 2. Charred paper

insulationFig.3. B phase of

LV winding

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Transformer installed in urban area which failed after only 11 months i.e. within warranty period of 3 years (Manufacturer – Y) :

Diagnosis results : This transformer was supplying residential and some commercial buildings. The large number of personal computers and adjustable speed drives/air conditioners in commercial buildings generate very high levels of harmonics and can result in effective overloading of the distribution transformer supplying the buildings. Cold load pickup (CLP) current may also be considered as one of the reasons of its failure. In India load shedding of 1 to 6 hours is a routine of daily life. CLP current is many times greater than pre outage current which causes heavy overloading of the distribution transformer.

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ig. 4 shows one phase of burnt LV windings. As the conductor is made of Aluminium, it comes out in the melted form, which can be clearly seen on the top of the winding.

ig. 5 gives the condition of fresh oil, oil of less severely damaged transformer i.e. case A and oil of most severely damaged transformer i.e. case B respectively. The color of oil clearly shows the effect of overheating on the liquid insulation. In A, charred paper insulation present in the oil can be seen at the bottom of the jar.

Fig. 4. Fig. 5. Oil Samples

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Transformer installed in rural area which failed after 4 years and 4 months i.e. beyond warranty period (Manufacturer –Z) : Diagnosis results : It is again the case of excessive overloading on B phase. The reason of powdered form (as shown in Fig. 7 on next slide) is an impurity of Iron in Aluminium i.e. pure Aluminium is not used for conductor. The excessive overloading on B phase would have been caused by power theft by hooking. With this unauthorized connection they run their agricultural pump sets.

Results of DGA of all cases (given in Table I) also agree with the diagnosis. As, ethylene is the principal gas with some amount of ethane and methane, the cause of which is overheating of the oil. Presence of carbon dioxide and carbon mono oxide indicate the paper degradation.

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It was found by inspection that one phase of both HV and LV windings were severely burnt/damaged as shown in Fig. 6.

The overloading has increased the temperature of B phase of LV winding to such an extent that Aluminium conductor of winding became brittle and came out in the powdered form as shown in Fig. 7.

Fig 6. Burnt B phase of HV windings Fig 7. B phase of LV

windings in powdered form

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Amount in ppmComponent

Normal Case A Case B Case C

Hydrogen H2 0.6646 36.451 57.464 41.97

Methane CH4 0.3326 50.508 310.667 114.76

Ethane C2H6 2.31E - 03 5.957 112.778 67.12

Ethylene C2H4 6.07E - 02 122.088 2512.33 571.14

Acetylene C2H2 0.1126 1.13E - 03 0.0197 0.121

Carbon di-oxide

CO2

104.53 156.56 3539.12 246.44

Carbon mono-

oxide CO1.5542 3.451 101.745 21.64

TABLE I. Results of dissolved gas analysis

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It was found that overloading, due to various reasons such as power theft by hooking the mains, harmonics and CLP current etc., is the prime cause of failure. Two main reasons for low maintenance are - (i) Most of them are located in remote areas and special attention cannot be given to the operating conditions of the transformers. (ii) A shortage of maintenance personnel working in State Electricity Boards (SEBs). Some states in India have given one point legal connections per family free of charge to all families who are economically weak. This highly appreciable gesture of the government is misused by almost 90 percent public, by drawing more power than legally permitted, to run their appliances without paying for them.Solution to the problem of power theft is to use “armored cable”. The armored cable network will cut short the problem of illegal electricity connection and reduce distribution losses [6].

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The authors would like to express their sincere gratitude to Mr. R. Chansoria, Proprietor of Transformer Manufacturing and Repairing Company for providing photographs and details of transformers. 1.IEEE Guide for loading Mineral – Oil – Immersed Transformers, IEEE C57.91 –19952.D.P.Gupta, “Loading and life of transformers” TRANSFORMERS, Bharat HeavyElectricals Limited Bhopal (M P), Tata McGraw-Hill Publishing Company Limited.3.P. K. Sen, Pansuwan Sarunpong, “Overloading and loss-of-life assessment guidelines of oil-cooled transformers”, Rural Electric Power Conference, 2001 Little Rock, AR, USA, pp B4/1-B4/8, April-May 2001.4.N. A. Muhamad, B. T. Phung, T. R. Blackburn, K. X. Lai, “Comparative Study and Analysis of DGA Methods for Transformer Mineral Oil”, IEEE Power Tech,Lausanne, pp 45-50, July 20075.“Transformer precaution”, [Online]. Available http://www.mahadiscom.in/emagazine/jan07. , [Accessed Jan 9, 2009]6.Staff Reporter, “MPERPDCL succeeds in reducing distribution losses to 1.5 per cent”, The Hitvada newspaper, Jabalpur Edition, dated Nov.15, 2009

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- Ranjana Singh & Amarjit SinghIndia

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