Impulse Breakdown Voltages of Ester-based Transformer Oils Determined by Using Different Test Methods

Q. Liu (1), Z.D. Wang (1) and F. Perrot (2)

(1)The University of Manchester, Manchester M60 1QD, UK (2)AREVA T&D, Stafford, ST17 4LN, UK

Abstract- Increasing interests on ester-based oils as potential alternatives to mineral oil, stimulate researches of oil properties and facilitate cross comparison among various studies. Lightning impulse breakdown voltage, one of the most important oil property parameters, is regarded as the criterion for large power transformer insulation design. In this paper, three different testing methods including rising-voltage method (refer to ASTM and IEC Standards), up-and-down method and multiple-level method, were employed to determine the lightning impulse breakdown voltage of two ester-based oils, Midel 7131 as synthetic ester and FR3 as natural ester. Meanwhile, mineral oil Gemini X was also tested as the baseline for benchmarking. The results clearly indicated that test methods have significant influence on breakdown voltage measurement values. Nevertheless, mineral oil showed higher lightning impulse breakdown voltage than ester-based oils irrespective of the testing methods being employed.

I. INTRODUCTION Ester-based oils which are non-toxic, less flammable and environmental friendly, are being considered in recent years as a potential replacement of traditional mineral oils. At the moment, these oils have been applied in low-voltage or medium-voltage transformers (up to 132kV) at certain countries and regions [1, 2]. However, for large power transformers, the engineering practice of replacing mineral oil by ester-based oils requires a significant amount of research and development activities. Transformer oils should withstand not only power frequency AC voltage but also transient impulse voltage (such as lightning impulse and switching impulse). Oil breakdown under transient voltage is normally regarded to be predominated by the oil intrinsic properties. Furthermore, impulse breakdown voltage as BIL is used as criterion for large power transformer insulation design. Therefore the increasing interests on ester-based oils press for the comprehensive investigations on impulse breakdown performance of these oils. Different methods including rising-voltage method [3, 4], up-and-down method [5, 6] and multiple-level method [7] have been used for oil impulse breakdown tests in both academic and industry fields over the past decades. Each method has its own pros and cons, and also the validity range: up-and-down requires breakdown voltage follows normal distribution; multiple-level method needs to consider the independence of breakdown events and rising-voltage method suffers small sample size limited by testing time consideration, etc (e.g. IEC 60897 and ASTM D3300). Furthermore, that the methodology

influence on the test results is suspected, especially when we compare various oils. This may lead to a confusing situation for evaluating potential alternatives to mineral oil, e.g. ester-based oils. Consequently, the above mentioned three methods were first introduced and compared in the paper, and they were then employed to determine the lightning breakdown voltage of transformer oils including two ester-based oils, Midel 7131 as synthetic ester and FR3 as natural ester, and one commonly used mineral oil Gemini X.

Some basic properties of these three oils, selected from Product Data Sheets, are given in TABLE I.

TABLE I

BASIC PROPERTY OF OILS: GEMINI X, MIDEL 7131 AND FR3 Property Unit Gemini X Midel 7131 FR3 Density@20 kg/dm3 0.882 0.97 0.92(25 ) Viscosity@40 mm2/s 8.7 28 34 Pour point -60 -60 -21 Flash point, PM 144 275 316

Acidity mg KOH/g <0.01 <0.03 0.04

Water content mg/kg <20 50 30 Breakdown voltage (AC) - before treatment - after treatment

kV

IEC 2.5mm 40-60 >70

IEC 2.5mm >75

ASTM 2.0mm 56

Dielectric dispassion factor@90 <0.001 <0.03 3.0(100 )

II. EXPERIMENTAL DESCRIPTION

A. Pre-processing of Oil Samples Particles were reported [8, 9] to have less influence on lightning breakdown voltage than AC breakdown voltage, since they do not have sufficient time to be polarized or migrated under lightning impulse voltage. However, a well controlled pre-processing procedure including filtering, dehydrating and degassing was still performed on all the oil samples, in order to compare the exact methodology influence and oil property difference. First, the “as-received” oil (new oil barrels delivered directly from the oil manufacturers; once delivered, they are stored in a cool, dry, isolated and well ventilated storage area.) was filtered by Nalgene® MF 75 nylon membrane Filter whose pore size is 0.2μm. A HIAC/ROYCO 8000A 8-Channel Particle Counter with a HIAC/ROYCO ABS2 Automatic Bottle Sampler was used to measure the particle numbers in oil samples. It was found that the particle number of oil after one filtering “pass” decreases dramatically, only one tenth of

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2009 Annual Report Conference on Electrical Insulation and Dielectric Phenomena

the original particles is left in the filtered oil. Based on this, all the samples were only filtered once in practice and we defined these oils as filtered oils. Particle numbers of unfiltered and filtered oils are shown in Fig. 1. The cumulative particle number larger than 5μm of filtered oils including the background influence (such as oil container) could reduce to 500 per 100ml oil or even lower compared to unfiltered oils.

Fig. 1. Particle numbers of unfiltered and filtered oils

Second, the filtered oils were degassed and dehydrated in vacuum oven under 10 mbar at 85 ˚C for 48 hours, then were given a further 24 hours to cool down to ambient temperature also under vacuum condition. Water content of oil samples was measured by using Karl Fisher titration analysis. The results of water content of oils after processing are shown in TABLE II.

TABLE II WATER CONTENT AND RELATIVE HUMIDITY OF OILS AFTER PROCESSING

Oil Water content (ppm) Relative humidity at 20 (%) Gemini X 3 5.5

Midel 7131 73 2.7 FR3 25 2.3

B. Test Setup A Haefely 10 stages impulse generator with maximum charging voltage of 2 MV was used to provide 1.2μs/50μs standard lightning impulse. The same brand high voltage divider and DIAS 733 analyzing system were used to record and calculate the impulse waveform. A test cell with volume of about 250ml, was made of high mechanical stress Perspex glass, taking IEC 60897 recommendation into account, as shown in Fig. 2. Sphere-to-sphere electrodes whose diameter is 12.5mm were fixed at a 3.8mm gap distance. A current limit resistor was connected between the lower electrode and the earthing point.

III. METHODOLOGY AND PROCEDURE A. Rising-voltage method Rising-voltage method can be performed using all types of voltage [10]. For a single test, the applied voltage should be increased from an initial voltage level at a constant rate till the

breakdown occurs. Repeat the single test procedure after a certain rest interval, until a significant number of breakdown voltages are obtained. It is widely and successfully used in AC breakdown tests due to easy control of voltage increasing process. However for impulse tests, the voltage increasing is discrete, which can only be increased shot by shot or step by step. Both IEC 60897 and ASTM D3300, two popular standards for liquid lightning breakdown tests, adopt rising-voltage method. The main difference between them is that one shot per step for IEC standard while three shots per step for ASTM standard, as shown in Fig. 3 (a) and (b). T1 is the time interval between successive shots; T2 is the time interval between continuous tests and U is step voltage.

Fig. 2. Lightning breakdown test cell with sphere electrodes

The detailed test procedure used in this paper is given as follows: after filling the oil sample, degas the test cell with oil sample together in vacuum oven for 20 minutes. Keep the test cell standing still for 5 minutes before starting the test. The initial voltage level is 140kV; increase the voltage step by step with step voltage U of 10kV and time interval T1 of 60 seconds till a breakdown occurs.

Fig. 3. Sketch of methodology for impulse test

(a)Rising-voltage method (1shot/step); (b) Rising-voltage method (3shots/step) (c) Up-and-down method; (d) Multiple-level method

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B. Up-and-down method Up-and-down method, proposed by Dixon and Mood, permits an estimation of 50% probability breakdown voltage, when the breakdown voltage is normally distributed [10]. It is widely used in impulse breakdown tests to save testing time and also has the advantage that getting the breakdown voltage at a certain breakdown probability of 50%. Procedure of general up-and-down method is shown in Fig. 3 (c). The voltage is initially raised from an initial value at which with certainty no breakdown would occur, in steps of a fixed amplitude U, until the first breakdown occurs. Next, the voltage is reduced by the same step U until no breakdown occurs, and then raised again until breakdown occurrence, and so on so forth. After a significant number of useful shots, the 50% breakdown voltage can be calculated by simple averaging the applied voltages. In the present tests, the initial voltage level is 150kV, the step voltage U is 10kV and the time interval T1 is 60 seconds. “25 useful shots” is the end criterion for tests while the lowest voltage level taken into account has at least two shots. C. Multiple-level method Multiple-level method, also named as constant-voltage method, can be performed using all types of voltage. For impulse tests, they represent the ‘classic’ method of determining breakdown probability [10], as shown in Fig. 3(d). Apply a certain number of shots at different voltage levels and record the corresponding number of breakdowns at each voltage level. Based on the results, draw the cumulative frequency function. In IEC 60060-1, Normal distribution is suggested to model the test results and then obtain the 50% probability breakdown voltage. In this paper, 20 shots with time interval T1 of 60 seconds were applied for each voltage level. Step voltage U between consecutive voltage levels is 20 kV. According to previous test experience, the initial level of Gemini X is set at 240 kV while that of Midel 7131 and FR3 is set at 200 kV.

IV. RESULTS AND DISCUSSIONS A. Rising-voltage method For rising-voltage method, different increasing rates, 1shot/step and 3shots/step referring to IEC 60897 and ASTM D3300 respectively, were investigated to see the influence on the test results.

Thanks to the good energy control of current limit resistor, 15 breakdowns for each type of oils at 1shot/step increasing rate were obtained without changing the electrodes and oil samples. Then, weibull distribution was used to fit the test results and further to find out the breakdown voltage at 50% breakdown probability, as shown in Fig. 4. Mineral oil Gemini X shows the highest 50% breakdown voltage of 276.4 kV, followed by Midel 7131of 258.0 kV and then FR3 of 239.3 kV.

Following the 1shot/step study, 14 breakdowns for each type of oils at 3shots/step increasing rate were carried out, 5 of which were obtained without using the current limit resistor, but changing the electrodes and oil samples after each breakdown. Weibull fitting of the results are shown in Fig. 5.

Again, mineral oil has higher 50% breakdown voltage than ester-based oils. In this case, the breakdown voltage of Midel 7131, 205.0 kV, is slightly higher than that of FR3, 200.4 kV.

Fig. 4. Results of rising-voltage method at 1 shot/step increasing rate

Fig. 5. Results of rising-voltage method at 3 shots/step increasing rate

In summary, the parameters of weibull fitting and thereby

the 50% breakdown voltage, U50%, are listed in TABLE III. Comparing the results, it can be found that tests by using 3shots/step increasing rate always result in lower breakdown voltages than tests by using 1shot/step. It can be explained as follows: it is reasonable to assume that more shots at a certain voltage level (e.g.3shots/step) could leave more chances for a breakdown to occur at that voltage level, so 50% probability breakdown voltage value under 3shots/step increasing rate would be lower compared to that under 1shot/step.

TABLE III BREAKDOWN VOLTAGES OBTAINED AT 1SHOT/STEP AND 3SHOTS/STEP (KV)

Oils Gemini X Midel 7131 FR3

1 shot/step a(Scale) 287.0 267.5 250.4 b(shape) 9.7 10.1 8.0 U50% 276.4 258.0 239.3

3 shots/step a(Scale) 261.4 214.7 210.1 b(shape) 9.9 7.9 7.7 U50% 251.9 205.0 200.4

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B. Up-and-down method Lightning breakdown voltages of various oils by using up-and-down method are shown in Fig. 6. Since up-and-down method is a coherent test procedure before reaching the end criterion, we did not change the oil sample and electrodes after each breakdown during one sample’s testing process. In this case, successive shots around the breakdown voltage level could damage the oil sample or introduce ‘space charge’ issue. The big ‘triangles’ in Fig. 6, which are not the same as the ideal up-and-down trend, might indicate the above mentioned influences. However the general trends of different oils still level-off. Overlapping of the results among various oils indicates that there is only small difference among the oils by using up-and-down method. After calculation, the oil sequence from high to low for impulse breakdown voltage is given as: Gemini X of 232.8 kV, followed by Midel 7131 of 223.2 kV, and then FR3 of 210.0 kV.

Fig. 6. Results of up-and-down method

C. Multiple-level method It is paradoxical to use multiple-level method in liquid impulse breakdown tests: apply a small number of impulses per voltage level, the scatter of cumulative frequency would be large; on the other hand, apply large number of impulses per voltage level, the change of oil condition (discharge by-products, space charge accumulation) especially at high voltage level, should be carefully considered and also the time consuming issue pops up. Consequently in this paper, a small scale of tests has been done by using this method; further studies are still on-going.

Lightning breakdown voltages of various oils by using multiple-level method are shown in Fig. 7. For Gemini X, the results jump from 0% at 260kV to 100% at 280kV, so the 50% breakdown voltage is regarded as 270 kV. For Midel 7131, large scattering existed for its probability distribution, so further verifying tests on some voltage levels were done to confirm the results. At last, the 50% breakdown voltage is calculated as 248.9 kV. For FR3, the results look surprisingly good, and the 50% breakdown voltage is 230.8 kV.

Fig. 7. Results of multiple-level method

D. Summary of methodology influence and oil property All the results are summarized in terms of methodology influence and oil property, as shown in Fig. 8 and Fig. 9 respectively. As seen in Fig. 8, different methods do influence the breakdown voltage values. For rising-voltage method, results by using 3 shots/step (refer to ASTM D3300) are lower than results by using 1 shot/step (refer to IEC 60897), due to more chances under 3 shots/step to make breakdown occurs at a certain voltage level. Multiple-level method and 1 shot/step rising-voltage method almost give the same values, which are higher than those obtained using the other two methods.

Fig. 8. Comparison in terms of methodology

It is clear in Fig. 9 that mineral oil Gemini X shows better

lightning impulse withstanding ability than ester-based oils no matter which methods are used. Considering mineral oil Gemini X as the baseline, the percentage reductions of breakdown voltage for ester-based oils are indicated. The ranking sequence from the highest to lowest breakdown voltage is shown as: Gemini X > Midel 7131 > FR3. The worst case for ester-based oils happens by using 3shots/step rising-voltage method, where lightning breakdown voltages of

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ester-based oils are about 20% lower than that of mineral oil Gemini X.

It can also be found from Fig. 9 that different methods have big influence on the absolute breakdown voltage values rather than the relative ranking when comparing the oil property.

Fig. 9. Comparison in terms of oil property

V. CONCLUSIONS

Rising-voltage method including 1shot/step and 3shots/step,

up-and-down method and multiple-level method were studied comparatively to determine the lightning breakdown voltages of ester-based oils, with mineral oil as the baseline for benchmarking, under 3.8 mm gap distance of sphere-to-sphere electrodes configuration. The results indicated that different methods have big influence on the absolute breakdown voltage values. Results of 1shot/step rising-voltage method conformed to that of multiple-level method, both of which were obviously higher than results by using 3shots/step rising-voltage method and up-and-down method. No matter whichever test methods were used, ester-based oils showed lower lightning breakdown voltages than mineral

oil. The percentage reduction compared to mineral oil in the worst case was about 20%.

ACKNOWLEDGMENT

The authors would like to express their gratitude to AREVA T&D, EDF Energy, Electricity North West, M&I Materials, National Grid, Scottish Power, TJ H2b Analytical Services and United Utilities for their financial and technical contributions to form the research consortium on ‘Application of Alternative Oils in Large Power Transformers’ at The University of Manchester.

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