Condition Assessment of Current Transformers Chemical and Electrical Analysis of Transformer Oil
7 seems to be weaker in field-aged samples.
However, they get worst with the increasing of acidity and decreasing of IFT.
5.2.2 Chemical Testing
5.2.2.1 Dissolved Gas Analysis (DGA)
Table 4 shows the contents of disslove gasses of field-aged oil samples and the evaluated faults under Basic gas ratio and Duval triangle methods. UKU/01, 02, 05, 06, 09 indicate excess of H2, CH4, C2H6 and C2H2. UKU/13 shows excess of CO2, C2H4 and C2H2. For UKU/14 oil sample, CO2, CH4, C2H6, C2H4 and C2H2 are in excess amounts. UDW/01 indicates excess of CO2 and CH4. IEEE key gas analysis gives interpretation of paper overheating for UKU/11 and oil overheating with arcing for UKU/13, 14 while fails to interpret about other samples. It could be noted that for UKU/13, 14 samples, all Basic Gas Ratio, Duval Triangle and key gas analysis methods give same diagnosis but for other cases they are different to each other.
5.2.2.2 Moisture Content (MC)
Figure 7 shows the MCs at 20oC for field aged samples. The limits of MC for CTs are ≤ 20 ppm for >170 kV and ≤ 30 ppm for <170 kV [13]. According to the results UKU/07, 08, KOT/01 and UDW/01 exceed the limits.
Figure 7 – MC at 20 oC of Field Aged Samples 5.2.2.3 Acidity
IEC 60422 indicates 0.5 mg KOH/g oil as the critical limit and requirement of frequently testing when it exceeds 0.3 mg KOH/g oil [13]. According to the Table 5, all the field aged samples exceed the 0.3 limits and UKU/01, 07 violate critical limit of 0.5.
5.2.2.4 Inter Facial Tension (IFT)
It is considered that 15 mN/m as the minimum IFT limit of CTs [13]. Some IFT results couldn’t obtain due to the breaking of the bubble when measuring. UKU/02, 04, 06 represent very low values while KOT/01, UKU/01, 03, 09 with better IFT values [Table 5].
5.2.3 Electrical Testing
5.2.3.1 Breakdown Voltage (BDV)
For CTs greater than 170 kV voltages recommended minimum BDV value is 50 kV and for other voltages minimum of 40 kV [13]. So in our case, the minimum breakdown voltage is 40 kV.
Figure 8–BDV Values of Field Aged Samples According to the Figure 8 UKU/05, 06, 12, 13, KOT/01 and UDW/01 showed very poor breakdown voltage values.
5.2.3.2 Frequency Dielectric Spectroscopy Maximum conductivity value for CTs voltages higher than 170 kV is 1000 pS/m and 1428.5 pS/m for voltages less than 170 kV at 90 oC [13]. Table 5 shows that only UKU/08 violates the acceptable limit of conductivity.
Recommended action limits of Tan δ at 90 oC and 40 Hz to 60 Hz for CTs are given as maximum of 0.2%(>170 kV) and maximum of 0.3%(<170 kV). According to the Table 5, the
Tan δ values for all field aged samples are
within the acceptable limits.
It was checked whether there was any correlation between the chemical and electrical parameters. Since DGA is main tested parameter in the chemical test, it was used for comparison. In electrical tests, since conductivity is calculated from -1 gradient, it gives the correct information of the properties of the liquid insulation than the loss tangent value taken at 50 Hz. So, conductivity was taken as the parameter characterizing the electrical tests. Figure 9 shows the variation of
Tan δ at 50 Hz at 70oC with conductivity at 70oC. It shows clearly that Tan δ increases with increasing of conductivity. The relationship is nearly linear except the lower conductivity levels. Therefore, conductivity calculated at -1 gradient can be used rather than the loss tangent measured at 50 Hz. Figure 10 shows the variation of hydrocarbon gases in log scale with conductivity. According to that graph CH4 and C2H6 gases shows somewhat similar variation
while C2H4 and C2H2 gases in similar variation.
It can be noted that when the conductivity level was above 200, the generated gases were in the
Figure 9 – Variation of Tan δ with
Conductivity at 70oC
same order. However, when the conductivity is below 200, a good corelation between the gas content and the conductivity cannot be established.
Figure 10 – Variation of Hydrocarbon Gases
with Conductivity
Table 5 – Test Results of Visual Inspection, FDS, IFT, Acidity, Moisture Content and BDV
Sample Name Colour Condu ctivity at 900C [pS/m] Tan δ at 50 Hz @ 700C [%] Per mitti vity at 700C IFT [mN/ m] Acidity [mg KOH/g of oil]) MC @ 20oC [ppm] Activa tion Energ y [eV] Break Down Volta ge [kV] NT Pale Yellow(1) 39.69 0.079 2.27 42.1 0.22 - 0.17 18.6 UWT1 Amber(4) 39.6 0.205 1.85 15.4 0.42 - 0.11 13.8 UWT2 Brown(5) 117.96 0.024 1.86 27 0.42 - 0.22 29.4
UWT3 Dark Brown(6) 22.88 0.003 1.78 15.8 0.44 - 0.24 16.8
UDT1 Amber(4) 86.97 0.045 2.39 19.2 0.58 - 0.23 20.4
UDT2 Brown(5) 39.95 0.004 1.85 14.6 0.61 - 0.34 24.5
UDT3 Dark Brown(6) 31.26 0.151 1.89 13.8 0.61 - 0.1 14.6
SWT1 Pale Yellow(1) 18.21 0.008 1.82 29.9 0.5 - 0.15 13.5 SWT2 Yellow(2) 69.1 0.008 1.79 24.2 0.49 - 0.55 30 SWT3 Amber(4) 26.18 0.004 1.79 20.7 0.5 - 0.24 31.7 SDT1 Pale Yellow(1) 22.31 0.008 1.88 16.1 0.37 - 0.11 14.5 SDT2 Pale Yellow(1) 18.81 0.004 1.78 23.7 0.35 - 0.14 18.3 SDT3 Amber(4) 51.12 0.005 1.76 20.8 0.37 - 0.31 31.3 UKU/01 Yellow(2) 758.99 0.074 1.97 34.1 0.52 13.7 0.57 46.1 UKU/02 Yellow(2) 220.67 0.045 1.86 9.8 0.37 13.48 0.11 62.4 UKU/03 Yellow(2) 601.78 0.054 1.84 33.5 0.41 13.11 0.49 57.9 UKU/04 Yellow(2) 96.23 0.023 1.78 15.6 0.39 29.32 0.06 41.1 UKU/05 Yellow(2) 448.76 0.038 1.77 - 0.41 12.6 0.5 30.3
UKU/06 Bright Yellow(3) 226.75 0.044 1.81 10.1 0.48 13.06 0.09 33.2 UKU/07 Bright Yellow(3) 465.11 0.082 1.92 - 0.53 31.64 0.13 40.5
UKU/08 Amber(4) 1825.2 0.135 1.83 25.7 0.49 31.77 0.53 74.1
UKU/09 Bright Yellow(3) 364.23 0.038 1.81 33.2 0.43 19.27 0.4 65.7
UKU/10 Yellow(2) 192.25 0.027 1.83 16.4 0.34 14.92 0.3 56.3
UKU/11 Yellow(2) 61.65 0.015 1.86 20.3 0.33 13.94 0.19 44
UKU/12 Yellow(2) 39.23 0.013 1.87 21.1 0.39 13.67 0.13 17.7
UKU/13 Bright Yellow(3) 27.43 0.125 0.60 16.7 0.48 26.56 0.07 25.6 UKU/14 Bright Yellow(3) 119.85 0.011 1.75 18.4 0.37 23.55 0.43 42.9
KOT/01 Water White(1) 29.41 0.111 1.81 42.5 0.35 31.86 0.12 22.2
he following conclusions were found in our 5. Ayman H. El-Hag, Yasser Adel Saker and
tu y Ibrahim Yehia Shurrab, “Online Oil Condition
Unsealed transformer insulations
Monitoring Using a Partial-Discharge Signal”, IEEE Trans on PD, Vol. 26. No. 2, pp. 1288-1289, When comparing sealed and unsealed ageing
aspects, ageing under sealed condition usually gives worse results than those under unsealed condition [4]. This is mainly due to produced by-products or catalysts during aging process and evaporation is easier in unsealed condition so that existence of catalyse in the sealed samples may help to accelerate the ageing process [4]. However in our study, unsealed oil samples showed faster ageing than sealed oil samples. It was found that unsealed wet condition resulted the worst insulation damage than the others (see Table 5).
6. Conclusions
T
2. Hinduja D., Gayathri, Kalpage C.S. and M.A.R.M. Fernando, “Laboratory Investigation of Treated Coconut Oil as Transformer Liquid Insulation”, Sixth IEEE International Conference on Industrial and Information Systems (ICIIS2011), Kandy, Sri Lanka, pp. 108-113, August 2011.
3. Ekanayake C., “Diagnosis of Moisture in Transformer Insulation - Application of frequency domain spectroscopy”, Lic Thesis, Chalmers Sweden, 2006.
4. Ten C.F., Fernando M.A.R.M. and Wang Z.D., “Dielectric Properties Measurements of Transformer Oil, Paper and Pressboard with the Effect of Moisture and Ageing”, IEEE Conference on Electrical Insulation and Dielectric Phenomena, Vancouver Canada, pp. 727-730, October 2007.
s d
deteriorate more quickly than the sealed transformers.
The conductivity, loss tangent and permittivity provide useful information about the condition of the tested oil samples and have good correlation with dissolved gas analysis.
Some conventional evaluation methods interpret wrong diagnosis for few cases except UKU/13 & UKU/14.
More studies needs to be done to clearly identify the potential of using tests such as frequency dielectric spectroscopy measurements to estimate other chemical testing parameters.
Acknowledgement
The authors would like express their deep gratitude to CEB Assets management branch for conducting the tests, Departments of Electrical and Electronic Engineering, and Chemical and Processing Engineering, University of Peradeniya for conducting the electrical and chemical tests, Lanka Transformers PLC for providing material samples.
References
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