The Effect of Moisture on the Breakdown Voltage of Transformer Oil
Power transformers utilize oil as a heat transfer medium and a dielectric material together with cellulose. Breakdown voltage (dielectric strength) is one of the most important parameters of transformer oil. It is measured when the transformer is taken into use and typically monitored by sampling during its operational lifetime.
It has been reported that breakdown voltage is affected by several factors, such as moisture, particles, acidity, and pressure [3,4,5,6]. The objective of this study is to investigate the dependency of the breakdown voltage on moisture using a representative set of oil samples and to point out the advantages of online moisture measurement.
Materials and Methods
The transformer oil samples used in this study (Table 1) were three mineral oils, one synthetic ester-based oil, and two mineral oil samples taken from transformers in operation. In addition, aging of the transformer oil was simulated for two of the mineral oils by using an oxidation treatment. This was done by feeding air into the oil vessel at 150 °C, catalyzed with a piece of copper. After a few days the oil colour had changed to dark brown and the Total Acid Number (TAN) had increased to about 1.6.
The breakdown voltage of the oil samples was measured at room temperature according to the IEC 60156 standard using a BA100 Breakdown Analyzer (b2 electronic GmbH). The measurement sequence (6 breakdowns) was typically repeated a few times at each moisture level. The moisture level of the oil relative to saturation (water activity in oil) was determined using a Vaisala MMT330 transmitter with the probe installed in the test cell of the breakdown analyzer. The transmitter has a capacitive sensor in which the capacitance is affected by the amount of water molecules that penetrate the sensor polymer, which in turn depends on the water activity in the surrounding oil.
The methods for determining the breakdown voltage of oil are described in the IEC 60156 [1] standard, and guidelines for interpreting the results in terms of transformer maintenance are presented in the IEC 60422 [2] standard. For example, for new trans formers above 170 kV the recommended lower limit is 60 kV, whereas breakdown voltages below 50 kV indicate a need for oil reconditioning [2].
In order to vary the moisture level of the oil, saturated nitrogen flow was fed into the oil under constant stirring. A saturator cell, immersed in a temperature-controlled bath, was used to control the humidity of the nitrogen flow and thereby the moisture in the oil sample. The flow was stopped before the breakdown voltage measurement sequence was started. The test cell, equipped with the probe and gas line, is shown in Figure 1.
Figure 1. The breakdown voltage analyzer test cell equipped with a moisture measurement probe and a gas line for feeding humid nitrogen.
Results and Discussion
The results of the breakdown voltage as a function of the relative saturation of moisture in oil are presented in Figure 2. Each point represents the average of six breakdowns. The breakdown voltages for each oil sample were normalized with the corresponding dry oil value to account for the variation between different oil samples (60…88 kV).
It is observed that the breakdown voltage remains almost unchanged when the relative moisture saturation is below 20 %. Above that it decreases rapidly. This behaviour seems to be similar for all the oil samples tested. One explanation for the unexpected similarity between the new and aged samples could be that according to [4] it is the low molecular acids, rather than TAN, that contribute to the breakdown voltage.
Figure 2 also includes a model function fitted to the measured data. Although approximate, such a model can be used to interpret the changes in the oil moisture in terms of the breakdown voltage.
It should be noted that the moisture level of the oil in the above breakdown voltage model is expressed as relative saturation rather than moisture content in ppm. This is important because the water solubility, i.e. the ppm value corresponding to a given relative saturation, can vary significantly between different oils and is also affected by the oil temperature and age [7].
Figure 3 illustrates the difference between relative saturation and ppm for two oil samples with very different water solubility. For both samples the two solubility coefficients required for the ppm calculation were determined by coulometric titration and temperature variation, as described in [8]. The comparison in Figure 3 shows that measuring moisture relative to saturation rather than in ppm is more informative regarding the breakdown voltage. This is also reported in [3,4].
In a typical transformer maintenance routine the breakdown voltage is monitored by taking an oil sample for laboratory analysis, for example every two years. It has been reported, however, that in addition to slow moisture change processes such as cellulose degradation, there may also be considerable short-term variation in the moisture level of the transformer oil due to changes in the transformer loading and ambient temperature [9].
Table 1. Oil samples used in the study.
Figure 2. Measured dependency of the breakdown voltage on the relative moisture saturation with an approximative model fitted to the data. Each point corresponds to the average of six breakdown voltage measurements performed according to the IEC 60156 standard.
Figure 3. Comparison between the dependency of the breakdown voltage on relative moisture saturation (a) and moisture content in ppm (b). Oil sample 1 is a typical mineral oil whereas oil sample 6 is a synthetic ester-based oil with very high water solubility. The oilspecific solubility coefficients are shown in the inset.
The change in relative moisture saturation can be caused either by migration of moisture between the oil and cellulose insulator, or the temperature-dependency of water solubility. Taking into account the above breakdown voltage results, this indicates that breakdown voltage may fluctuate during transformer operation; this is also pointed out in [10]. Knowing the moistureinduced momentary change in breakdown voltage could be useful for transformer operation and maintenance. This can be achieved with online measurement of relative moisture saturation.
Conclusions
The effect of moisture on the breakdown voltage of transformer oil was investigated using a set of eight oil samples comprising both new and aged oils. The results were very similar for all the oils tested when moisture is expressed relative to saturation: the breakdown voltage remains high when moisture saturation is below 20 % but then decreases significantly as relative moisture saturation increases. In order to provide a convenient interpretation of moisture changes in terms of breakdown voltage, a model function was fitted to the data. This is considered to be particularly useful for online moisture measurement, which allows the detection of short-term breakdown voltage changes and reliable monitoring of trends, as well as elimination of the measurement errors that can result from improper oil sampling.
»»References ›› [1] IEC 60156 International Standard, Insulating liquids – Determination of the breakdown voltage at power frequency – Test method, Second edition, International Electrotechnical Commission, 1995 ›› [2] IEC 60422 International Standard, Mineral insulating oils in electrical equipment – Supervision and maintenance guidance, Third edition,
International Electrotechnical Commission, 2005 ›› [3] CIG RE 349, Moisture equilibrium and moisture migration within transformer insulation systems, CIGRE working group A2.30, 2008
›› [4] M. Koch, M. Fischer, S. Tenbohlen, The breakdown voltage of insulation oil under the influences of humidity, acidity, particles and pressure, International Conference APTADM, 2007, Wroclaw, Poland ›› [5] P. J. Griffin, Water in transformers – so what!, National Grid Conference on Condition Monitoring in High Voltage Substations, Dorling, 1995 ›› [6] E. Gockenbach, H. Borsi, Performance and new application of ester liquids, Proceedings of 2002 IEEE 14th International Conference on Dielectric Liquids, 2002, Graz, Austria ›› [7] T. Gradnik, M. Koncan-Gradnik, N. Petric, N. Muc, Experimental evaluation of water content determination in transformer oil by moisture sensor, IEEE International Conference on Dielectric Liquids, 2011, Trondheim, Norway ›› [8] Vaisala MMT330 User Guide, retrieved from
www.vaisala.com
›› [9] C. Feely, Transformer moisture monitoring and dehydration – Powercor experience, TechCon Asia-Pacific, 2006, Sydney, Australia ›› [10] B. Buerschaper, O. Kleboth- Lugova and T. Leibfried, The electrical strength of transformer oil in a transformerboard-oil system during moisture non-equilibrium.
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