Condition Assessment of Current Transformers Chemical and Electrical Analysis of Transformer Oil
2 earth at 10 kV and 2.5 kV voltages, respectively.
Measurement of capacitances and internal pressure are also included in those tests. However, oil analysis together with the above tests would give easy and detailed information about the condition of the CT. Unlike larger ventilated type power transformers, CTs have limited oil capacity and removal of 1-2 l of oil for testing is a considerable amount and may affect to the operation of the CT. Since the procedure required for oil sampling and refilling is complicated, utilities are reluctant to perform oil testing very frequently. It is therefore, important of consider tests which require very small amounts of oil as well as provides useful information about the condition of the oil. Frequency Dielectric Spectroscopy (FDS) measurement covering frequency variation of the loss tangent, the permittivity and the conductivity is good in this respect. It also gives good correlation with other tests such as Dissolve Gas Analysis (DGA), acidity, moisture content etc.
This paper presents condition assessment of CTs in Sri Lanka. Electrical and chemical tests were conducted on field-aged and laboratory- aged oil samples to show the potential of monitoring the condition of the oil-filled CTs.
2. Current Transformers and
Faults
The CTs belonging to the group of instrument transformers has a basic function of measuring high currents which can’t be measured with normal measuring equipments. Insulation of CTs consists of solid (paper, pressboard) and liquid (mineral oil) insulations. Mainly solid insulation is used to provide the insulation for the primary and secondary windings whereas the liquid insulation provides transformer insulation as well as cooling the windings. The CT failures can be severe since they are often followed by explosions. Figure 1 illustrates this effect. The consequence of such a failure is not only net disturbances, but also a high risk of personnel injuries and damage to surrounding equipment due to the possible launching of porcelain splinters [1].
The main factors for oil-paper insulation ageing can be listed as; 1) Degradation due to the development of ionization processes, 2) Thermal Ageing, 3) Degradation caused by oxidation processes in oil, 4) Moistening of the insulation [1]. Mainly there are five types of faults identified in CTs [10].
Figure 1 – Failure of Current Transformer When condition assessment of CTs is considered, basically DGA and moisture content (MC) are the major analysing tools used by most utilities.
3. Dissolved Gas Analysis (DGA)
DGA has been used for many years as an effective and reliable tool to detect incipient faults in mineral oil filled transformers. This method is based on the analysis of the concentration and rate of gases generated and dissolved in transformer oil, and associates the kind of failures with the presence of those gases [10]. There are mainly seven types of dissolved fault gases namely Hydrogen (H2), Carbon monoxide (CO), Carbon dioxide (CO2), Methane (CH4), Ethane (C2H6), Ethylene (C2H4), Acetylene (C2H2).
3.1 DGA Evaluation Techniques
The IEC 60599 gives typical values of fault gases used for CTs. Two evaluation methods are commonly used to evaluate DGA in transformers namely Basic Gas Ratio and Duval triangle Method [10]. The IEEE analysis method uses the concept of key gases. For example, low intensity PD or corona produces mainly H2. Same way, the key gas C2H2 is for arcing; C2H4 for overheating oil and CO is for overheating of cellulose [11].
4. Sample Preparation and Test
Procedure
4.1 Laboratory Aged Samples
Twelve virgin transformer oil samples were prepared according to the details given in Table 1 and Table 2. The sample sets were divided into four categories: dry sealed, wet sealed, dry unsealed and wet unsealed, to represent the sealed and unsealed transformers as well as ageing with and without moisture. In addition, Cu, Al, Fe and Zn metal substances were added to each sample to see the catalytic effect of the
3 Sample No. Aging Time [hours] Temperature Volume [ml]
NT New Transformer Oil (NT)
Unsealed Wet Transformer Oil (UWT)
UWT1 200 1200C for upto 200 hours then between 1100C-1300C 1000 UWT2 438 1000 UWT3 640 1100
Unsealed Dry Transformer Oil (UDT)
UDT1 200 1200C for upto 200 hours then between 1100C-1300C 1000 UDT2 438 1000 UDT3 640 1100
Sealed Wet Transformer Oil (SWT)
SWT1 336
1200C
1000
SWT2 840 1000
SWT3 1176 1000
Sealed Dry Transformer Oil (SDT)
SDT1 336
1200C
1000
SDT2 840 1000
SDT3 1176 1000
materials used in real transformers [12]. The total ageing period was about seven weeks which gives satisfactory ageing [4].
Table 1 – Details of the Oil Samples
years, 15 years). Sampling were done under very dry condition in mid day time (around 300C and humidity less than 50%). The details of the oil samples are shown in Table 3.
Table 3 – Details of Field Aged Oil Samples Sample No. Power Station Voltage [kV] Ageing Time [Years] KOT/01 Kotmale 220 26 UKU/01 Ukuwela 132 35 UKU/02 Ukuwela 132 35 UKU/03 Ukuwela 132 35 UKU/04 Ukuwela 132 34 UKU/05 Ukuwela 132 34 UKU/06 Ukuwela 132 34 UKU/07 Ukuwela 132 33 UKU/08 Ukuwela 132 33 UKU/09 Ukuwela 132 33 UKU/10 Ukuwela 132 35 UKU/11 Ukuwela 132 35 UKU/12 Ukuwela 132 35 UKU/13 Ukuwela 33 26 UKU/14 Ukuwela 33 26 UDW/01 Udawalawe 33 49
Table 2 – Materials Added to Each Sample
Transformer Material Quantity [g/l]
Aluminium (Al) 0.5
Zinc (Zn) 0.5
Copper (Cu) 2.5
Iron (Fe) 2.5
Press board 100
For unsealed samples six stainless steel containers of 1500 ml and for sealed samples six auto clave glass bottles of 1 000 ml were taken. First, they were rinsed and washed with hot water (1000C) and then, washed by methanol and dried. Afterwards, containers and bottles were washed by transformer oil.
4.2 Field Aged Samples
Sixteen field-aged oil samples were taken from CTs selected in three power stations covering 25-50 service years. Samples were taken from the CTs immediately after they have been removed from the service. Sampling has been done according to the specification given by IEC. There are test lugs provided in CTs to take oil samples for evaluation after long service (10
4.3 Ageing Procedure
Sealed oil samples were aged inside a drying oven for a temperature of 120 0C up to 1176 hours continuously. Pressboards were added to wet samples and kept for whole ageing period. Unsealed oil samples were aged inside the drying oven initially for 200 hours at 1200C, and then for 1200 hours as a cyclic manner (nearly half a day ageing and a half day resting) at between 1100C and 1300C. Pressboards were added to wet samples and kept initially for 40 hours.
The three samples in each category were aged with different ageing times of 2, 4 and 7 weeks. The empirical 10 degree rule which states the ageing rate is doubled in every increment of 10°C was used to recalculate the equivalent ageing time for laboratory aged samples [4]. 4.4 Test Procedure
Both field-aged and laboratory-aged oil samples were analyzed by visual inspection (colour), electrical and chemical tests. The electrical study included the measurement of breakdown voltage and frequency dielectric spectroscopy covering frequency variation of the loss tangent, the permittivity and the conductivity. The chemical analysis represents
4
0
dissolved fault gases, acidity, inter-facial tension and moisture content measurements. 4.4.1 Visual Inspection
The removed laboratory aged samples were carefully checked for any reduction of oil level. Significant reduction of oil level was observed in unsealed samples confirming the evaporation. All laboratory and field aged sample colours were recorded.
4.4.2 Chemical Testing 4.4.2.1 Interfacial Tension (IFT)
IFT measures the surface tension at the interface between two liquids (in our case oil and water) which do not mix. The interfacial tension between oil and water provides a means of detecting soluble polar contaminants and products of deterioration [13].
The IFT of the samples were measured from Easy Dyne measuring device which used the du Noüy ring method to find the IFT. The IFT was measured five times and the average value was taken as the IFT value.
4.4.2.2 Acidity (Neutralization number) Acids are usually formed from oil oxidation or from atmospheric contamination. In the presence of water, acids may cause corrosion in the transformer. So that acidity of oil is important in the condition assessment. As the process of oxidation progresses in, the amount of sludge increases reducing the heat transfer capacity of the mineral oil [13].
oil increases with increasing temperature and neutralization value [13]. Moisture content was determined only on field aged oil samples due to the amount of laboratory aged oil was limited.
4.4.2.4 Dissolved Gas Analysis (DGA)
Dissolved gases are analyzed using the MYRKOS Transformer Fault Gas Analyzer. Selected field and laboratory aged samples were subjected to this analysis.
4.4.3 Electrical Testing 4.4.3.1 Breakdown Voltage
Breakdown voltage is an indicator of dielectric strength i.e. the ability to withstand electrical stresses without failure. Breakdown voltage was tested using MEGGER OTS 80AF/2 test equipment across a sphere gap of 2.5 mm. BS148:1984 standard is used for this test.
4.4.3.2 FDS Measurements
FDS measurements were conducted by Insulation Dielectric Analyzer (IDA 200) and a three terminal oil test cell (geometric capacitance of 70 pF). FDS measurements were done from 1 kHz to 1 mHz for a voltage of 50 V. 75 ml samples were taken and FDS tests were conducted in two different temperature levels for each sample. i.e. at room temperature about 270C and high temperature about 700C.
The frequency variation of the capacitance of the oil samples can be written as;
In the process of acidity measurements, 50 mg of analytical grade NaOH flakes were dissolved
C
f
C ' jC" C0
' j"
C0The permittivity is given b;
(1)
in 500 ml of de-ionized water in a volumetric flask to make 0.1%w/v lye solution. One ml of oil was dissolved in 10 ml of pure commercial
' "
'
j
"
(2) grade isopropyl alcohol (IPA) in a titrationflask. The flask was gently warmed and stirred until all the oil was completely dissolved in the alcohol and turned clear. Two drops of phenolphthalein was added to the solution in the titration flask. Then, the sample was titrated with 0.1% w/v lye solution until the colour
where, is the conductivity and is the susceptibility. Usually the variation of susceptibility with respect to frequency is negligible oil and the relative permittivity ’ can directly be obtained from ’ at 1 kHz i.e. at our case. The loss tangent can be obtained as, turned to pink (magenta). The acid value (AV)
tan C"
f
"
f
of the oil was estimated as in [2] and aciditywas expressed in KOH mg/Oil g by using KOH
C '
f
'
f
0 (3)equivalents and density values obtained in IFT. 4.4.2.3 Moisture Content (MC)
Water may originate from the atmosphere or be produced by the deterioration of insulating materials. The solubility of water in transformer
The loss tangents at 50 Hz were used for the analysis. The conductivity was obtained at -1 gradient of the log-log plot of tan Vs frequency. The conductivity is temperature dependent and follows Arrhenius law as;
e
0
E
KT (4) and wax in transformer oil. It can be noted that decreasing IFT with the unsealed condition is From the conductivity values obtained for room
temperature (200 C-300 C) and high temperature (700 C), the activation energy E was obtained. From this activation energy, conductivity at 90°C was estimated.
5. Results and Discussion
5.1 Laboratory Aged Samples 5.1.1 Visual Inspection (Colour)
Table 5 shows the colour variations of oil samples. Colour changes in unsealed transformer oil were high compared to those of sealed samples. The reason could be when the unsealed transformer oil was exposed to air the oxidization with sufficient oxygen from surrounding may produce substances with colour changes.
5.1.2 Chemical Testing
5.1.2.1 Dissolved Gas Analysis (DGA)
Table 4 shows the DGA of some of the selected oil samples. SWT3 shows excess C2H6 indicating oil overheating. Basic Gas Ratio method gives correct diagnosis for this case while duval triangle method gives wrong interpretation for SWT3 sample. CO represents basically paper overheating and wet samples shows more CO due to adding of pressboards. It can be assumed that produced CO in unsealed samples not appears in the results due to unsealed condition.
5.1.2.2 Inter Facial Tension (IFT)
Figure 2 – Variation of IFT with Aging Time Figure 2 shows the variation of IFT with aging time for laboratory aged oil samples. IFT values usually reduce due to the presence of moisture or any other substances. Fast deterioration of IFT normally indicates the formation of sludge
higher than the sealed condition. 5.1.2.3 Acidity
Acidity of laboratory aged oil samples is shown in the Figure 3. Acidity level of all transformer oil samples increased in first few days and then came to a stable level. Lowest acidity values show in sealed dry samples compared to other samples confirming lower contact with oxygen and moisture.
Figure 3 –Variation of Acidity with Aging Time
5.1.3 Electrical Testing
5.1.3.1 Breakdown Voltage (BDV)
Figure 4 – Variation of BDV with Aging Time
Figure 4 show the variation of breakdown voltage with respect to ageing time of the laboratory aged samples. According to the figure, the breakdown voltage for unsealed samples initially increased proving that the oil had been gone through a purification process rather than the ageing. However, the values reduced at 600 hours of ageing time confirming a deterioration of the oil samples. The sealed samples showed a different behaviour by first by reducing and then increasing of the breakdown voltage. The reason could not be clear.
Table 4 – Test Results of Dissolved Gas Analysis, Basic Gas Ratios and Duval Triangle Method Sample
Name Dissolved Gases in ppm Basic Gas Ratios Diagnosis
H2 CH4 C2H6 C2H4 C2H2 CO CO2 C2H2 C2H4 CH4 H2 C2H4 C2H6 Basic Gas Ratio Duval Triang le UWT3 15 0 0 0 0 13 452 - 0 - PD - UDT3 21 0 0 0 0 14 483 - 0 - PD - SWT3 26 79 204 0 0 573 2784 - 3.038 0 T1 PD SDT3 28 0 0 0 0 68 919 - 0 - PD - KOT/01 0 4 6 0 0 0 1357 - - 0 T1 PD UKU/01 439 22961 7757 31 32 134 760 1.032 52.303 0.004 T1 PD UKU/02 1232 35011 9172 15 12 84 541 0.8 28.418 0.002 T1 PD UKU/05 1084 25741 6938 12 10 88 718 0.833 23.746 0.002 T1 PD UKU/06 727 38619 10652 9 6 114 924 0.667 53.121 0.001 T1 PD UKU/09 1009 29141 9794 11 11 81 900 1 28.881 0.001 T1 PD UKU/11 140 0 73 0 0 143 1205 - 0 0 PD - UKU/13 0 1 130 1750 1090 0 10180 0.623 - 13.46 D2 D2 UKU/14 0 4065 146 900 907 0 5426 1.008 - 6.164 D1 D1 UDW/01 0 4410 28 0 0 0 8020 - - 0 T1 PD
5.1.3.2 Frequency Dielectric Spectroscopy Figures 5, 6 show variation of conductivity, loss tangent at 50 Hz for laboratory aged samples. Recommended conductivity level for new transformer oil at 900C is 16.5 pS/m [13]. Unsealed conditions have higher conductivity than sealed conditions. However, the conductivity levels for unsealed samples reduced after 600 hr.
Figure 5 – Variation of Conductivity at 90 oC with Aging Time
Also an unsealed wet condition shows the highest value of conductivity in transformer oil. Decomposition of Pressboard/paper in presence of air and metal substances such as Fe and Cu would be the reason for it.
Figure 6 shows that in first few days loss tangent reduced in transformer oil. This implies that all samples had undergone purification in
first few days [2]. Unsealed transformer oil has started to age before sealed transformer oil. The variation of permittivity was comparatively lower as expected for insulating materials than other two parameters (loss tangent and conductivity). However, slight decrease could be noted [see Table 5]. Decreasing in permittivity in first few days proved that the oil had been gone through a purification process.
Figure 6 – Variation of Tan δ at 50 Hz at 70 oC with Aging Time
5.2 Field Aged Samples 5.2.1 Visual Inspection (Colour)
Table 5 shows that the colour of field aged samples. In general, field-aged samples shows limited changes in colour compared to those of laboratory aged ones. Oxidization process
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