Condition Assessment of High Voltage Power Transformer Using Dissolved Gas Analysis

Transcription

1 ISSN 348-7X September 5 Condition Assessment of High Voltage Power Transformer Using Dissolved Gas Analysis Jugasri Joy Sarma M.E. Scholar, EE Department, Assam Engineering College, Guwahati, India Dr. RunumiSarma Professor, EE Department, AssamEngineering College, Guwahati, India ABSTRACT Economically transformers constitute one of the largest investments in a Power system. For this reason, transformer condition assessment and management is a high priority task. If a transformer fails, it would have a significant negative impact on revenue and service reliability. This project deals with Dissolved Gas Analysis (DGA) which is a widely used technique to estimate the condition of oil-immersed power transformers. The objective of this project is mainly to analyze available data from DGA, and investigate the types of fault using different types of diagnostic methods like key gas method, IEC gas ratio method, Rogers method, cellulose condition test method, total dissolved combustible gas method and Duval triangle method. Both manual calculation and specific MATLAB programming have been performed for proper detection of faults from available data from DGA. Transformer data have been collected from Regional Testing Institute Guwahati, Assam. Keywords- transformer, dissolved gas analysis (DGA), parts per million (ppm), fault types, diagnostic methods, Duval Triangle. I. INTRODUCTION Transformer is one of the most important and critical component of electrical transmission and distribution system. Much attention is needed on maintenance of transformers in order to have fault free electric supply and to maximize the lifetime and efficiency of a transformer. Dissolved gas analysis is a powerful diagnostic technique for detecting incipient faults in oil filled transformers long before they develop into major faults. The transformer in operation is subjected to various stresses like thermal and electrical, resulting in liberation of gases from the hydrocarbon mineral oil, which is used for insulation and cooling. The components of solid insulation also take part in the formation of gases, which are dissolved in the oil. An assessment of these gases would help in diagnosing the internal faults. The health of the oil is reflective of the health of the transformer itself. II.FORMATION OF GASES IN TRANSFORMER OIL IN SERVICE Mineral insulating oils are complex mixtures of hydrocarbon molecules, in linear (paraffinic) or cyclic (cycloaliphatic or aromatic) form, containing CH 3, CH and CHchemical groups bonded together. Scission of some of the C-H and C-C bonds as a result of thermal or electrical discharges will produce radical or ionic fragment such as H*, CH 3 *, CH *, CH* or C*, which will recombine to form gas molecules such as hydrogen ( H-H ), methane ( CH 3 -H ), ethane (CH 3 -CH 3 ), ethylene ( CH =CH ) or acetylene ( CH CH ). More and more energy is required to form the above chemical bonds. Hydrogen (H),methane (CH 4 ) and ethane (C H 6 ) are thus formed at low energy level, such as in corona partial discharges or at relatively low temperatures ( <5 C ), ethylene (C H 4 ) at intermediate temperatures, and acetylene (C H ) at very high temperatures ( > C ) such as in arcs. The gases dissolve in oil or accumulate above it and are analyzed by DGA. III. FAULTS DETECTABLE BY DGA Partial discharge: A fault of low level energy which usually occurs in gas-filled voids surrounded by oil impregnated material. The main cause of decomposition in partial discharges is ionic bombardment of the oil molecules. The major gas produced is Hydrogen. The minor gas produced is Methane. 74

2 ISSN 348-7X September 5 Thermal faults: A small amount of decomposition occurs at normal operating temperatures. As the fault temperature rises, the formation of the degradation gases change from Methane (CH4) to Ethane (CH6) to Ethylene (CH4). A thermal fault at low temperature (<3 C) produces mainly Methane and Ethane and some Ethylene. A thermal fault at higher temperatures (>3 C) produces Ethylene. The higher the temperature the greater the production of Ethylene. Arcing: Fault caused by high energy discharge. The major gas produced during arcing is Acetylene. Power arcing can cause temperatures of over 3 C to be developed. Fault involving insulation: If the cellulose material (insulating paper etc.) is involved carbon monoxide (CO) and carbon dioxide (CO) are generated. A normally aging conservator type transformer having a CO/CO ratio above 7 should be considered normal and below 3 should be regarded as perhaps indicating a fault involving cellulose, provided the other gas analysis results also indicate excessive oil degradation. IV. DIAGNOSTIC METHODS BASED ON DISSOLVED GASES There are many DGA methods to interpret fault type. The methods used in this paper are Key gas method, IEC gas ratio method, Cellulose condition test method, Total combustible gas method, Duval triangle method and extension to Duval triangle. IV(A):Key gas method The following table indicates whether a gas has crossed its permissible limit that would possibly indicate it to be involved in the fault. Table : IV(B):Totalcombustible gas method The Westinghouse guidelinefor the limits for total combustible gasesand recommended actions are given in the table below. Table : Total Combustible Recommended Action Gases - 5 ppm Normal ageing, analyse again in 6- months 5 ppm Decomposition of oil in excess of normal ageing, analyse after 3 months (monitoring) 5 ppm More than normal decomposition, analyse frequently to establish trend (in every month) 5 ppm and above Substantial decomposition, possibly fault, to be confirmed 75

3 ISSN 348-7X September 5 IV(C) :IEC gas ratio method The Basic Gas ratios recognized in the International Electro technical Commission (IEC) Standards is equivalent to Doernenberg ratios and Rogers ratios in the ANSI/IEEE C57.4. Three gas ratios are used in this method. They are methane/hydrogen, acetylene/ethylene, and ethylene/ethane. Table 3: IEC 599 C H /C H 4 CH 4 /H C H 4 /C H 6 Ratios of characteristic gases < >3 Case Characteristic faults no. No fault Partial discharge of low energy density 3 Discharge of low energy Discharge of high energy 5 Thermal fault of low temperature <5 C 6 Thermal fault of low temperature range 5 C-3 C 7 Thermal fault of medium temperature range 3 C-7 C 8 Thermal fault of low temperature range >7 C IV(D) : Rogers ratio Central Electric Generating Board (CEGB) of Great Britain has employed a method developed by Rogers, IEEE method (Duval, 6), in which four ratios of fault gases are calculated to generate a four digit code presenting in Table 4 and 4.. Table 4 illustrates circumstance of developing the digits and Table 4. describes the fault diagnosis assigning to each of the digits Table 4: Ratio Range Code CH 4 /H..< R< R<3 3 C H 6 /CH 4 < C H 4 /C H 6 < R<3 3 C H /C H 4 <.5.5 R<

4 ISSN 348-7X September 5 CH 4 /H CODES C H 6 /C H 4 C H 4 / C H 6 Table 4.:Rogers ratio fault diagnosis C H /C H 4 Normal DIAGNOSIS 5 Partial discharge, Slight overheating < 5 C, Slight overheating 5 - C Slight overheating - 3 C General conductor overheating Winding circulating currents Core and tank circulating currents,overheated joints Flashover, no power follow through,, Arc, with power follow through Continuous sparking to floating potential 5, Partial discharge with tracking IV(E):The Duval triangle Michel Duval of Hydro Quebec developed Duval Trianglemethod in the 97s using a database of thousands of DGAs and transformer problem diagnosis. This method has proven to be accurate and dependable over many years and is now gaining popularity. In this method concentration (ppm) of methane (CH 4 ), ethylene (C H 4 ), and acetylene (C H ) are expressed as percentages of the total (CH 4 + C H 4 + C H ) and plotted as a point (%CH 4, %C H 4, %C H ) in a triangular coordinate system on a triangular chart which has been subdivided into fault zones. The fault zone in which the point is located designates the likely fault type which produced that combination of gas concentrations. The Duval Triangle method, like any other DGA diagnostic method, should be applied only when there is some suspicion of a fault, based on an increase in combustible gas or some other suspicious symptom. Fig :Duval triangle 77

5 ISSN 348-7X September 5 Table 5: Faults detectable by Duval triangle Symbol Fault Examples PD Partial discharges Discharges of the cold plasma(corona) type in gas bubbles D D T T T3 DT Discharges of low energy Discharges of high energy Thermal fault, T<3 C Thermal fault 3 C<T<7 C Thermal fault T>7 C Thermal electrical fault and or voids, with the possible formation of X-wax in paper Partial discharges of the sparking type, inducing pinholes, carbonized punctures in paper. Low energy arcing inducing carbonized perforation or surface tracking of paper, or the formation of carbon particles in oil. Discharges in paper or oil, with power follow-through, resulting in extensive damage to paper or large formation of carbon particles in oil, metal fusion, tripping of the equipment and gas alarms. Evidence by paper turning brownish (> C) or carbonized (>3 C). Carbonization of paper, formation of carbon particles in oil. Extensive formation of carbon particles in oil, metal coloration (8 C) or metal fusion (> C). Sometimes both thermal and electrical fault occurs inside the transformer. These faults accelerate the decomposition of electric fluid and solid insulation. IV(F): Extension to the Duval triangle a) Duval triangle for low temperature faults & faults in paper and mineral oils: There are five zones in this Duval triangle such as: PD- Corona partial discharge, S- Stray gassing of mineral oil (T< C), C- Hot-spots with carbonization of paper, O- Over-heating, N/D- Not determined. In zone C, the probability of having carbonization of paper is 8 %. b) Duval triangle for thermal faults & faults in paper and mineral oils: This Duval Triangle method is used for high temperature faults. It uses three gases for the analysis such as methane, ethylene and ethane. It is used to get more information about the faults identified as Thermal faults i.e. T and T3 in classical Duval triangle. It should not also beused for faults like D, D.There are seven zones in this Duval triangle such as: PD- partial discharge, S- Stray gassing of mineral oils (T< C), C- Hot spot with carbonization of paper, O- Over heating (T<5 C), T-Thermal faults of high temperature (3 C<T<7 C), T3-Thermal faults of very high temperature (T>7 C), N/D-Not determined. In zone C, the probability of having carbonization of paper is 9 %. Fig : Duval triangle for low temperature faults & faults in paper and mineral oils 78

6 ISSN 348-7X September 5 Fig 3: Duval triangle for thermal faults & faults in paper and mineral oils IV(G): Cellulose condition test Condition of cellulose can be determined by calculating CO /CO ratio. Case no. Table 6: Cellulose condition test Ratio= C /CO Characteristics >7 Normal cellulose 3 4 5<ratio<= 7 3<ratio<= 5 Problem with cellulose Rapidly deteriorating cellulose insulation <3 Strong indication of decomposition of cellulose V.CASE STUDY Transformer data Power and voltage class: 6MVA, 3/33 kv Age of transformer: year Table 7: DGA data of transformer Gases Ppm level of gases Methane (CH 4 ) 6 Ethane (C H 6 ) 4 Ethylene (C H 4 ) 88 Acetylene (C H ) 37 Hydrogen (H ) ND Carbon monoxide (CO) 56 Carbon dioxide (CO ) 4875 TGC, µl/litre 3654 ND-not diagnosed 79

7 ISSN 348-7X September 5 V (A): Key gas method Fig 4: Bar diagram ppm level of different gases present in transformer Ppm level of methane suggests partial discharge, ppm level of ethane suggests overheating, ppm level of ethylene suggests severe overheating,ppm level of acetylene suggests arcing, Ppm level of carbon dioxide suggests severe overheating and involvement of cellulose; to be confirmed after diagnosis by following methods. V(B):Total combustible gas method Fig 5: Bar diagram of ppm level of total combustible gases present in transformer In this case ppm concentration of total combustible gases is 493, which indicates substantial decomposition of oil, possibly fault, to be confirmed. V(C): IEC gas ratio This test indicates thermal fault of medium temperature range 3 C-7 C This test does not give any result. V (D): Rogers ratio method 8

8 ISSN 348-7X September 5 V(E) : Duval triangle Fig 6: Duval triangle for transformer The red asterisk mark is the fault point indicated by the gases methane, ethylene and acetylene and it is located in the T3 i.e. Thermal fault >7 C zone. To get more accurate result the extension of Duval triangle that is Duval triangle for high temperature faults & faults in paper and mineral oils is checked. V (F):Duval triangle for thermal faults & faults in paper and mineral oils Fig 7:Duval triangle for thermal faults & faults in paper and mineral oils The red asterisk mark is in the fault zone indicated by the gases methane, ethylene and ethane and it is located in C-hot spot with carbonization of paper, zone. V(G): Cellulose condition ratio test In this case CO /CO = 3.5 i.e. (CO/CO)>7 which means cellulose condition is normal. V(H) :Conclusion of the case Here all the gases except carbon monoxide and hydrogen has crossed permissible limit. Thus it can be guessed that there might be heating due to methane but presence of ethane, ethylene, carbon dioxide indicates severe overheating. Though methane is present since hydrogen is the major gas in partial discharge and its value is zero so it can be considered that level of partial discharge is negligible. Thus depending on the various test results it can be almost confirmed as a case of thermal fault of high temperature with occurrence of hotspots and carbonization of paper. 8

9 ISSN 348-7X September 5 VI: CONCLUSION DGA should be performed at regular interval of time to study the trend of gas. An annual DGA is the most important test for liquid insulation Thus to save a transformer from catastrophic damage concern should be shown. Prevention is better than cure. VII: ACKNOWLEDGMENT The authors would like to thank the faculty members of Electrical Engineering Department of Assam Engineering College, Guwahati for their support and appreciation to this work. VIII: BIBLIOGRAPHY [] EnaNarang, Er. ShivaniSehgal ander.dimpy Singh, Fault Detection Techniques for Transformer Maintenance Using Dissolved Gas Analysis, International Journal of Engineering Research & Technology (IJERT), Vol. Issue 6, pp. -7 August. [] Sukhbir Singh, Dheeraj Joshi and M.N. Bandyopadhyay, Software Implementation of Duval Triangle Technique for DGA in Power Transformers, International Journal of Electrical Engineering, Vol. 4, No. 5, pp ,. [3] Sukhbir Singh and M.N. Bandyopadhyay, Duval Triangle: A Noble Technique for DGA in Power Transformers, InternationalJournal of Electrical and Power Engineering, Vol. 4, Issue-3, pp.93-97,. [4] Debashis RanjanPatra, SonalSagarBoda, Department of Electrical EngineeringNational Institute of Technology, Rourkela, Condition assessment of high voltage power transformer using dissolved gas analysis. [5] Michel Duval, Dissolved Gas Analysis and the Duval Triangle. [6] M. Duval, A Review of Faults Detectable by Gas-in-Oil Analysis in Transformers, IEEEElectrical Insulation Magazine, Vol.8, No.3, pp.8,. [9] Joseph B. DiGiorgio,Northern technology and testing, Dissolved gas analysis of mineral oil insulating fluids. [] Mohammad Golkhah,SaharSaffarShamshirgar and Mohammad Ali Vahidi, Artificial neural networks applied to DGA for faultdiagnosis in oil-filled power transformers,journal of Electrical and Electronics Engineering Research, Vol. 3(), pp. -, January. [] N.K.Dhote, Dr. J.B.Helonde, Diagnosis of Power Transformer Faults based on Five Fuzzy Ratio Method, WSEAS transactions on Power systems, Issue 3, Volume 7, July. [3] N.A. Muhamad, B.T. Phung, T.R. Blackburn, K.X Lai The University of New South Wales,School of Electrical Engineering and Telecommunications, Sydney 5, Australia, Comparative Study and Analysis of DGA Methods for Transformer Mineral Oil. [4] P.Werle, C. Radigk, W. Sorgatz, M. Hahn, V.Wasserberg Comparison of Different DGA (Dissolved Gas Analysis) Methods for the Condition Assessment of Power Transformers. 8