Simulation of Fault Diagnosis in the Oil Immersed Transformer using Dissolved Gas Analysis. R.Arumugama, Mrs.Indra Getzy Davidb and Dr.M.Rajaramc Electrical and Electronics Engineering Department, Government College of Engineering, Tirunelveli, a
[email protected] ,
[email protected], crajaramgct@ rediffmail.com
Abstract: The IEEE standard classifies transformer conditions into 4 discrete deteriorating states, with the criteria of combustible gases as by product of insulation deterioration .Dissolved Gas Analysis is a diagnostic and maintenance tool used in machinery. Duval has used three hydrocarbon gases to diagnose the faults. In this paper the fault diagnosis is achieved by means of DGA using modified Duval’s Triangle simulation method. Index terms- dissolved gas analysis (DGA), fault diagnosis, power transformer. I. INTRODUCTION Most of our electrical facilities were commissioned in the late 1950’s .In South India the electrical switch gear and systems are nearly 50 years old in certain substations. Depending upon the environment and application of the equipment they may be at or nearer the end of their useful lives. Transformers are some of the most efficient Electrical machines with some large units able to transfer 99.75%of their input power to their output. Small transformers do not generate significant heat and cooled by air circulation and radiation of heat .Power transformers that are rated up to several hundred kVA can be adequately cooled by the natural convection air cooling, sometimes assisted by air circulator. In larger transformer part of the problem is removal of the heat. Some power transformers immersed in the oil that both cooled and insulate the windings. Temperatures above rated will damage the winding insulation. Mineral oils are used in the transformer tank for insulation and also as a media for heat transfer .The oils are the mixture of many different hydrocarbon molecules which decompose under high thermal and electrical stress within the transformer during the period of service .The critical changes are the breaking of carbon-hydrogen and carbon-carbon bonds as a result of which different gases are formed due to the presence of individual hydrocarbon and the distribution of energy and temperature in the neighborhood of the fault. The IEEE has provided the interpretation of the gases generated in the transformer oil and the corresponding standards for evaluating the condition of transformer oil insulation based on Dissolved Gas Analysis results.[1]Dissolved gas Analysis data is used to estimate the failure rate of deterioration of insulation oil in the transformer. Dissolved gas analysis, or DGA, is a diagnostic and maintenance tool used in machinery. The study of gases from transformers can be used to give an early indication of abnormal behavior of transformer and may indicate appropriate action that may be taken on the equipment before it suffers great damage. TABLE 1. MAIN GASES ANALYZED BY DGA Hydrogen
H2
Methane
CH4
Ethane
C2H6
Ethylene
C2H4
Acetylene
C2H2
Carbon monoxide
CO CO2
Carbon dioxide Oxygen
O2
Nitrogen
N2
The conventional Buchholz relay and gas collector relay are universally used. However, they have limitation that enough gas must be generated, first to saturate the oil full and then to come out of solution and collect in the relay, typically 10-15% of gas by volume of oil, has to be generated for it to come out of the solution, often by the time the bucholz relay was never meant to be a diagnostic device. The DGA is a very sensitive technique and detect gas in ppm (microlitre/litre) of oil. The main gases formed by decomposition of oil and paper are summarized in Table 1. These gases dissolve in oil or accumulate above it and are analyzed by DGA. Some laboratories also report the contents of C 3 and C4 hydrocarbon gases formed.
II.FAULTS There are four basic types of faults, which can occur in the transformer:
Arcing or high current break down. Low energy sparking or partial discharges. Localized overheating or hot spots General over heating due to inadequate cooling or sustained overloading.
Each of the fault results in thermal degradation of the oil either alone or in combination with paper insulation. This gives rise to the evolution of various hydrocarbon gases, hydrogen and oxides of carbon, in quantities depending on the type of fault. Heavy current arcing is characterized by the evolution of significant quantities of Hydrogen (H2) and acetylene (C2H2). If the arcing also involves paper insulation, the oxide of carbon will also be present. Partial discharge usually results in evolution of hydrogen and lower order hydrocarbons. Localized heating or hot spot gives rise to Methane and Ethane in appreciable amount. Prolonged overloading or impaired heat transfer can cause CO and CO2 to be generated due to overheating paper insulation. To ensure uninterrupted and economical supply the trouble free performance of vital electrical equipments like power transformers during service is a matter of great importance. They are often subjected to complex environmental condition and variable thermal and electrical stresses. Efforts have been made to assess the health of the transformer during service through a series of diagnostic tests. In a new transformer, typical hydrocarbon gases concentration for good new oil after vacuum filtration would be within 5 ppm DGA shall be repeated once a month after commissioning and then at intervals as found necessary. Mineral insulating oils are complex mixtures of hydrocarbon molecules, in linear (paraffinic) or cyclic (cycloaliphatic or aromatic) form, containing CH3, CH2 and CH chemical 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*, CH3*, CH2*, CH* or C*, which will recombine to form gas molecules such as hydrogen ( H-H ), methane ( CH3-H ), Ethane ( CH3-CH3 ), ethylene ( CH2=CH2 ) or acetylene ( CH ≡CH ). More and more energy is required to form the above chemical bonds. Hydrogen (H2), methane (CH4) and ethane (C2H6) are thus favored at low energy level, such as in corona partial discharges or at relatively low temperatures ( < 500 °C ), ethylene (C2H4) at intermediate temperatures, and acetylene (C2H2) at very high temperatures ( > 1000 °C ) such as in arcs.
III.FAULT DIAGNOSIS The main diagnostic methods used are: The IEEE methods ( Dornenburg, Rogers and key gases methods ) The IEC ratio codes The Duval Triangle The Dornenburg, Rogers and IEC codes compare gas ratios such as CH4/H2 , C2H2/C2H4 and C2H4/C2H6. The key gas method is based on the 2 or 3 main gases formed. And the Duval Triangle on the relative proportions of 3 gases (CH4, C2H4 and C2H2). One drawback of the gas ratio methods (Dornenburg, Rogers, IEC) is that some DGA results may fall outside the ratio codes and no diagnosis can be given (unresolved diagnoses). This does not occur with the Triangle method because it is a closed system rather than an open one.
Fault Codes: PD Partial Discharge D1 Low energy electrical discharge D2 High energy electrical discharge DT Indeterminate thermal fault or Electrical discharge T1 Low range thermal fault (below 300C) T2 Medium range thermal fault (300-700C) T3 high range thermal fault (above 700C) The most severe faults, in terms of type and location, are generally considered as : high-energy arcing D2 in paper (and in oil). medium-to-high temperature faults T2-T3 in paper (> 250 °C) low energy arcing D1 in paper (tracking, arcing) high temperature faults T3 in oil (> 700 °C) The less severe faults, which can often be tolerated for relatively long periods of time as long as they don’t evolve into a more severe one are: low-energy discharges PD/D1 in oil (corona, sparking) low temperature faults T1 in paper (< 150 °C) medium temperature faults in oil (< 500 °C). these faults are difficult to find by visual inspection
IV. TESTING DGA is one such powerful diagnostic tool which helps to detect faults at an early stage by detecting abnormal changes in the composition of gasses dissolved in the transformer oil, before the other protective gadgets like buchholz relay and the other respond. DGA has proved to be reliable means of establishing the healthiness of a transformer which have tripped by suspected maloperation of differential protection (due to charging inrush or C.T Circuit problem) or Buchholz relay ( due to air suction or control suction problem) can be returned to service with more confidence on the basis of D.G.A results. According to the IEC/IEEE standards, eight fault types [5] are identified as shown in the Table2 .In this paper, the range of gases from the table is taken as reference for a lookup table and the input is tested for diagnosing the faults. TABLE 2.DGA DATA OF REFERENCE SEQUENCES No
Fault type
H2
CH4
C2H6
C2H4
C2H2
1 2
No fault T <150 thermal fault 150
700… thermal fault Low energy Partial High energy Partial Low energy Discharges High energy Discharges
0-15 20-160
0-4 10-130
0-11 15-33
0-3 5-96
0-0.2 0-0.4
27-160
20-245
33-39
20-50
0-0.4
27-181
90-262
41-42
28-63
0-0.2
56-173
260-340
42-172
480-928
7-38
43-55
24-58
12-66
300-560
1-3
340-675
34-66
29-66
10-21
1-3
565-980
53-93
34-58
12-47
3-6
32-200
6-107
1-11
13-154
13-224
3 4 5 6 7 8 9
The internal inspection of hundreds of faulty equipment has led to the broad classes of faults indicated in Table 3, detectable by visual inspection and by DGA:
TABLE 3.EXAMPLES OF FAULTS DETECTABLE BY DGA Symbol
Fault
Examples
PD
Partial discharges
Discharges of the cold plasma (corona) type in gas bubbles or voids, with the possible formation of X-wax in paper.
D1
Discharges of low energy
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.
D2
Discharges of high energy
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.
T1
Thermal fault, T <300 °C
Evidenced by paper turning brownish (> 200 °C) or carbonized (> 300 °C).
T2
Thermal fault, 300
Carbonization of paper, formation of carbon particles in oil.
T3
Thermal fault, T >700 °C
Extensive formation of carbon particles in oil, metal coloration (800
THE DUVAL TRIANGLE
The Duval Triangle was first developed in 1974 [2]. It uses three hydrocarbon gases only (CH 4, C2H4 and C2H2). These three gases correspond to the increasing levels of energy necessary to generate gases in transformers in service. The Triangle method is indicated in Figure 1. In addition to the 6 zones of individual faults mentioned in Table 2 (PD, D1, D2, T1, T2 or T3), an intermediate zone DT has been attributed to mixtures of electrical and thermal faults in the transformer. C2H2 and C2H4 are used in all interpretation methods to represent high energy faults (such as arcs) and high temperature faults. H2 is preferred in several of these methods to represent very low energy faults such as PDs, where it is produced in large quantities. CH4, however, is also preventative of such faults and always formed in addition to H2 in these faults, in smaller but still large enough amounts to be quantified. CH4 has been chosen for the Triangle because it not only allows identifying these faults, but provides better overall diagnosis results for all the other types of faults than when using H 2. This good performance of the Triangle with CH4 might be related to the fact that H2 diffuses much more rapidly than the hydrocarbon gases from the oil through gaskets and even metal welds. Therefore, gas ratios using H 2 are probably more affected by the loss of this gas than those using hydrocarbons gases only, which have much lower and comparable diffusion rates. The three sides of the Triangle are expressed in triangular coordinates (X, Y, Z) representing the relative proportions of CH4, C2H4 and C2H2, from 0% to 100% for each gas. In order to display a DGA result in the Triangle, one must start with the concentrations of the three gases, (CH 4) = A, (C2H4) = B and (C2H2) = C, in ppm.
First calculate the sum of these three values: (CH4 + C2H4 + C2H2) = S, in ppm, then calculates the relative proportion of the three gases, in %: X = % CH4 = 100 (A/S), Y = % C2H4 = 100 (B/S), Z = % C2H2 = 100 (C/S). X, Y and Z are necessarily between 0 and 100%, and (X + Y + Z) should always = 100 %. Plotting X, Y and Z in the Triangle provide only one point in the Triangle.
Fig.1.Coordinates and Fault zones of the Triangle
Transformer I (10 MVA) Date of DGA
H2
CH4
C2H6
C2H4
C2H2
CO2
27.11.’04 08.07.’05
2 1
2 2
Tr Tr
3 4
0 2.01
991 2536
13.07.’06
1
1
6
1.87
1104
12.07.’07
5
7
2
7
0.55
1145
04.10.’08
1
10
4
17
0.23
842
Date of DGA 27.11.’04 08.07.’05
H2 1 1
CH4 2 3
C2H6 2 5
C2H4 15 45
C2H2 0 0
CO2 442 2514
13.07.’06
2
11
85
0
1006
12.07.’07
4
40
19
52
0.24
965
04.10.’08
1
32
28
32
0
1420
5
Transformer II (10 MVA)
16
Transformer III (16 MVA) Date of DGA
H2
CH4
C2H6
C2H4
C2H2
CO2
27.11.’ 04 08.07.’05
2 7
1 2
Tr 1
Tr 5
0 4.76
968 5138
13.07.’06
4
5
2
14
2.15
1196
12.07.’07
10
6
2
14
3.61
940
04.10.’08
5
3
2
9
1.83
2225
Transformer IV (16 MVA) Date of DGA
H2
CH4
C2H6
C2H4
C2H2
CO2
27.11.’04 11.07.’05
3 1
2 2
1 1
4 4
0 1.68
1504 2813
13.07.’06
1
9
3
13
11.21
1356
12.07.’07
3
9
3
12
9.89
1233
04.10.’08
11
22
4
32
53.02
1031
The above tabulated Data’s from. TEDC/METRO, TRICHY for the four Transformers at Palayamkottai.
VI.CONCLUSION The data collected from the transformers (DGA test & data) which are located in Palayamkottai and Coimbatore of south India , were given as inputs to the fault diagnosis coding in JAVA and also to the modified polygon and results are obtained, i.e. faults are diagnosed.
REFERENCES [1].IEEE standards C57.104 TM-2008 “IEEE Guide for the Interpretation of Gases generated in Oil Immersed Transformers”,IEEE power and Energy Society Revision of Std C57,104-1991. [2]Delta-X Research, “Duval Triangle”,HTML file from Google Search. [3].Michel Duval IREQ Canada “A Review of faults detectable by Gas – in- Oil Analysis in Transformers”,2002 IEEE Electrical Insulation Magazine May/June 2002 – vol.18,No.3 Page 8-17. [4]. Michel Duval, “Dissolved Gas Analysis and the Duval Triangle”,AVO Technical Papers 2006 Conference,New Zealand. [5]PengZheng-hong,SongBin, “Application of datamining techniques based on Grey Relational Analysis in Oli Immersed Power Apparatus Fault Diagnosis”,2006,International Conference on Power System Technology.
