Characteristics of Dielectric Fluid

Characteristics of Dielectric Fluid describe how insulating liquids withstand voltage, control heat, and age in transformers. Important properties include dielectric strength, permittivity, moisture resistance, oxidation stability, and thermal conductivity.

In practical transformer operation, the Characteristics of Dielectric Fluid determine whether insulating fluids can maintain electrical integrity, remove heat efficiently, and resist chemical degradation over long service periods. Variations in these properties directly influence insulation life, fault risk, and maintenance planning.

Dielectric liquids are selected not only because they are electrically insulating, but because they behave predictably as a dielectric material across a wide temperature range under high voltage stress.

When an electric breakdown occurs, it is usually preceded by localized electric discharges that reveal weaknesses in fluid purity, geometry, or thermal balance. This same physics is exploited in electrical discharge machining EDM, where controlled discharges shape metal using dielectric liquids as both insulation and cooling media.

In transformer service, the goal is the opposite: to prevent discharge entirely by maintaining high dielectric and high thermal stability within the fluid. These demands explain why different types of dielectric fluids are engineered to balance electrical strength, heat transfer, and chemical resilience rather than excelling in only one property.

Why the Characteristics of Dielectric Fluid Matter

1. Introduction

For many years, synthetic ester fluids were seen as specialist materials, used only in unusual transformers, such as those in rolling stock, offshore installations, and steel plants, where fire safety was a prime consideration. However, in recent years, users have realized that ester-based liquids can offer a more mainstream alternative to mineral oil. Although these fluids are more expensive, overall project costs can be lower when factors such as reduced fire protection are considered.

In service environments where mineral oil remains dominant, understanding the aging behavior of transformer oil provides an essential reference for comparing ester performance.

In some space-constrained urban environments, ester-based liquids may even become the preferred option, with the flammability and potential environmental impact of mineral oil making the design of modern installations extremely challenging. Designers evaluating alternative liquids often begin with guidance on oil for transformers to balance thermal, electrical, and environmental constraints.

2. Standard AC Breakdown Testing

Standard test methods for assessing the breakdown voltage of liquids typically employ small electrode gaps of ≤2.54mm. The electrode configuration can vary from spherical to VDE-type “mushroom” to disc. This type of testing is primarily used to evaluate the cleanliness of a liquid, since it provides very limited information about the actual dielectric performance. Our research shows that all the different types of liquids discussed in this paper give very similar results for a given electrode arrangement in this type of testing. 

This could lead to the conclusion that all these liquids are equal in their dielectric performance, or that given a good result in the AC breakdown test, one liquid is in some way superior to another. However, the true picture is more complex, as the electrical stress distribution is influenced by many factors, such as electrode geometry, spacing, and material types. Another key factor in the dielectric behavior is the shape of the applied voltage waveform. AC voltage in the form of a clean sine wave is usually expected at frequencies of 50-60Hz, depending on the geographical location. However, this is rarely the case with harmonics and other distortions of the pure waveform.

In addition, the prevalence of surges on the network must be accounted for; in testing, this is usually characterized by two different types of event, either lightning surge or switching surge, and there are standard waveforms established to test these.

Ongoing Condition Insight

Ongoing condition insight is strengthened when laboratory findings are paired with routine transformer oil analysis, which tracks moisture, particles, and oxidation products over time.

So any dielectric system in a transformer must withstand AC conditions, switching impulse, and lightning impulse, as well as chopped lightning impulse if this is specified. There may also be a requirement to withstand DC fields in some cases, which adds an extra level of complexity.

When considering a new dielectric medium, therefore, all these aspects need to be tested, and initially, researchers will look to comparisons with existing materials of known behavior to assess likely changes. As stated previously, in terms of short-gap AC behavior, ester-based liquids are very similar to mineral oil, which gives some confidence that they can be used. For distribution class equipment up to 33kV, the change to ester has required little in the way of detailed electrical design evaluation, since the electrical margins are large due to the need for excess solid insulation to provide mechanical strength. However, as the voltage level rises, there is less electrical margin and a greater need for routine impulse testing, both of which require larger steps to evaluate the design. So, to begin using ester-based liquids in power-class transformers, we need to check impulse behavior over similarly short gaps to those in the AC tests, and this is where we start to see some differences emerge.

3. Impulse Strength of Short Electrode Gaps

There are standard methods for measuring impulse breakdown, including ASTM D3300, which is a popular one. The electrode arrangement for this test can be either needle-to-sphere or sphere-to-sphere. In the first instance, researchers began work with small electrode gaps using a sphere-sphere setup, such as the example shown in Fig 1, used by the University of Manchester.

Fig.1. Arrangement for short gap impulse tests

During testing, a number of methods were used to increase the test voltage, following recommendations in various standards. This showed a lower impulse breakdown strength for the ester liquids, and Fig 2 summarizes the results, with the maximum difference in breakdown voltage of approximately 20%. In this case, the tested mineral oil was Nynas Nytro Gemini X, the synthetic ester M&I Materials MIDEL 7131, and the natural ester Cargill Envirotemp FR3. These observations are consistent with broader properties of transformer insulating oil related to ionization, space charge, and pre-breakdown dynamics. Because liquid behavior directly affects electrical margins, dielectric performance must always be considered alongside the transformer insulation system's structure.

Fig. 2. Results of impulse breakdown testing to various methods

4. Partial Discharge Inception

To further understand the mechanism behind the observed behavior, researchers began examining highly divergent arrangements, such as a sharp needle with a tip radius of 6.5µm and a sphere with a radius of 12.5mm, as this allows observation of phenomena in a liquid at manageable voltage levels.[3] This allowed the study of partial discharge inception, when the liquid begins to yield to the electrical field. When the researchers subjected this arrangement to AC, they discovered that the PDIV of ester-based liquids with a 50mm gap is very close to that of mineral oil.

In fact in the case of natural ester a higher PDIV was found than in mineral oil. This suggested that the reason for the difference in impulse breakdown behavior does not lie in discharge inception, although some differences were observed in this study, especially in polarity, between mineral oil and esters. Mineral oil exhibits a strong tendency to PD only in the positive half-cycle of the AC waveform, i.e., when the needle is at positive polarity. In the negative half-cycle, the required voltage to form PD is much higher than the PDIV. In ester-based liquids, the situation is somewhat different; PD was observed in the negative half-cycle at voltages much closer to the positive half-cycle PDIV, as shown in Fig. 3.

Thermal viscosity and gas solubility also link dielectric behavior to overall transformer cooling performance under high stress.

Fig. 3. PDIV in positive (left-hand chart) and negative (right-hand chart) half cycles

This indicates that the electrical behavior differs between the liquid types. It also raises questions about how mineral oil-filled transformers are tested, e.g., is testing only with negative impulses a valid practice?

5. Streamer Propagation Behavior

Streamer propagation differences between esters and mineral oil are linked to acceleration voltage thresholds. These differences explain why impulse breakdown behavior diverges more strongly in highly non-uniform electric fields.

These relationships are ultimately governed by the fundamental characteristics of dielectric fluid that control ionization, heat transfer, and aging dynamics.

Streamers can be characterized by four different modes, as shown in Fig. 4.

Fig. 4. Streamer velocities and modes

The principle behind the connection between streamer mode and breakdown can be demonstrated with a simplified example. Taking a gap size of 50mm, if it is assumed that the liquid is only subjected to the voltage necessary to sustain propagation for 5 µs, then the streamer will need to attain a velocity of 10 km/s, or, in other words, be of Mode 3-4, to bridge the gap and cause a breakdown. Otherwise, the streamer will only be characterised as a partial discharge. The transition from Mode 1/2 to Mode 3/4 can be characterised by the acceleration voltage.

A variety of researchers have examined the acceleration voltage principle with esters, and all agree that this is one area in which these liquids differ from mineral oil. The charts in Fig. 5 show the behavior when the electrode system is extremely divergent, with esters having a substantially lower acceleration voltage than mineral oil, especially under positive polarity.

Fig. 5. Acceleration voltage under Positive polarity and Negative polarity at 50mm spacing

6. Testing with More Realistic Electrode Arrangements

Although this difference in acceleration voltage would appear to prevent the use of esters at higher voltages, as the electrode arrangement becomes less divergent, inception becomes more important for the withstand level. This supports the findings of researchers who have studied behavior across varying levels of electrode divergence, from homogeneous to highly divergent.

When considering the design of real-world equipment and transformers for transmission levels, the more homogeneous case actually represents the majority of configurations. Needle-to-plate type situations are avoided in good design and manufacturing, as they are electrically weak and prone to discharges.

Research examining impulse behavior under more realistic conditions has focused on tap changer contacts, since these represent a more divergent part of power transformer designs. In this case, the arrangement shown in Fig. 6 was used, and the results obtained under impulse conditions showed very little difference between ester and mineral oil.

Fig. 6. Tap changer contacts used for natural ester evaluation

Fig. 7 shows the Weibull distribution for results obtained in this arrangement. This gives some confidence that even though the situation with a needle and plate looks unfavourable, as soon as the configuration starts to reflect the real-world situation, the difference between esters and mineral oil becomes much smaller.

Fig. 7. Weibull distribution of lightning impulse breakdown under positive polarity

7. Laboratory Testing of Creepage Discharge and Flashover

Another area where divergence becomes important is along long creepage paths, where an effectively concentrated electric field is present at one end, with a very long distance to the lower potential. A popular arrangement for testing creepage behavior is the so-called Weidmann setup, with a paper-wrapped or bare conductor in contact with a pressboard barrier, as shown in Fig 8.

Fig. 8. Weidmann electrode arrangement

When this type of arrangement has been tested over gap sizes up to 35mm, it has been found that esters give flashover results similar to those of mineral oil, as shown in Table 3. The difference between the liquids in this arrangement is small, not even as large as that found in small oil gaps. This suggests that, even though design modification may be necessary, there is not the very large difference that might be assumed if the acceleration voltage were used in an extremely divergent setup.

8. Testing in Prototype Transformers

Another area where more focus may be required with an ester-based liquid is over very long creepage paths far beyond the distances tested with the Weidmann arrangement, since the fundamental investigations indicate that propagation is key. Experience with real transformer prototypes has shown that failure modes along extremely long paths support the faster-propagation model. Researchers from Brazil found that when testing a single-phase 245kV prototype transformer in natural ester, designed to mineral oil rules, the natural ester failed at 100% of the Basic Insulation Level (BIL) rating when tested with a lightning impulse along a long-gap discharge path, as shown in Fig.
3. This unit had an HV winding with a center connection coil.

Fig. 9. Model of winding showing discharge path

The designers of this transformer noted that although they experienced this failure, it does not prevent the use of esters at higher voltage. However, there may need to be greater design margins and closer attention to peak-stress areas and long creepage paths. 

This is a theme that is often mentioned in the industry when discussing ester-based liquids and the necessary design changes. It is important to note that a growing number of manufacturers have conducted their own research in addition to published works; to date, several transformers have successfully operated at 400 kV+ with esters. There are also many other projects in development, and the expectation is that in the coming years, esters will move from being a product used in niche applications to one deployed in mainstream installations.

9. Conclusion

Over the last 15 years, extensive research has been conducted to understand the electrical behavior of ester-based liquids under a range of conditions. This has been driven by a desire for safer, more environmentally friendly transformers.

The laboratory-based test arrangements across highly divergent fields indicate differences in streamer propagation behavior between esters and mineral oil, suggesting that designers may need to focus on specific portions of the dielectric structure. Evaluations with more realistic electrode arrangements indicate that although there is a difference in behavior, this will not prevent the use of esters at higher voltages. The experience in real-world applications, where esters are now used in 400kV+ power transformers, also supports this assertion.

The key aspects for designers when considering ester-based liquids are to design a discharge-free transformer; extra margin may be needed over long creepage paths and in divergent arrangements to compensate for the higher probability of propagation. This could be summarized by saying that with mineral oil, discharges may occur without flashover, but with ester, there is a higher probability that a discharge will become a breakdown.

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