2016/06/28

Service: interfacial tension

Interfacial tension is a much-discussed topic in conjunction with routine oil checks, primarily of older transformers. This part of the routine checks which is performed at Smit Transformer Services in its own oil laboratories, is frequently confused with breakdown voltage, due to the similarity of the terms (at least in the Dutch language). Breakdown voltage is the decisive factor for the insulating properties of oil and is expressed in kV/cm or kV/2.5 mm. Interfacial tension has almost nothing to do with breakdown voltage.

Interfacial tension
Interfacial tension describes the tension of the interface between two liquids, in this case between water and oil. Mineral transformer oil must feature a certain interfacial tension to prove that it has the conforming quality degree, is clean and free of impurities. This interfacial tension is determined by allowing oil to float on a bowl filled with pure water. As water and oil do not mix, an interface forms between the two liquids. Subsequently, a precision balance is used to determine the force required to break up this interface by means of a special ring.

 

MilliNewton per meter?
The significance of routine checks of the interfacial tension of oil from transformers in operation is easy to explain.
Clean and high-quality transformer oil has a high interfacial tension. It is expressed in mN/m (milliNewton per meter). The interfacial tension of new oil is above 50mN/m; if this oil is poured into the transformer, this value will decrease globally by 5 mN/m to 45mN/m, due to the initial absorption of impurities, depending on the quality and cleanliness of the transformer. This value is a good starting point for the initial situation of a new oil-filled transformer.

 

Ageing of oil
Subject to the effect of oxygen and humidity, but also due to the thermal load, transformer oil ages at the cost of quality, a phenomenon known as the oxidation process. This ageing process can be monitored by routine checks of the interfacial tension during transformer operation. The value decreases gradually over the years, based on the initial situation, e.g. starting from 45 mN/m.

In case of considerable ageing of the oil, the oil's structure finally turns into sludge (German “Schlamm”, Dutch “drab”). This substance, which is heavier than the oil as such, is deposited on surfaces and may disturb the natural circulation of the oil for cooling purposes. This may result in inadmissible temperature increase in the windings and eventually to damage to the insulation.

 

Drab, sludge, Schlamm
Thus, intervention is called for before this process of sludge formation commences.
Many examples from practical application have proved that sludge may form once the interfacial tension falls below 22 nN/m.
Based on this, Smit Transformer Services recommends subjecting the oil to a precise inspection for sludge once the limit of 22 mN/m is reached. This is easily done by taking an additional oil sample. As the decreasing tendency of the interfacial tension is generally very slow, this aspect can be inspected in case of the next routine check.
If no sludge formation is detected, the interfacial tension may be allowed to decrease to 15 mN/m before action has to be taken, without giving rise to concern. However, during this decreasing process, it is recommended to consider the acidity index and the tangent delta within the routine check once the interfacial tension has decreased to almost 15 nN/m.

In case of sludge formation, action should be taken immediately.
Smit Transformer Services recommends action to be taken in any case as soon as possible, if the declining trend is clearly foreseeable in case of a transformer with a long service life ahead. In this case, the test for sludge may even be foregone.

 

Oil change or oil regeneration
The measures to be taken concern oil change or oil regeneration. We will deal with this topic in more detail in the next issue of ePOWER.
If you want to obtain information right now, please contact servicesmit-trafo.nl. Our specialists will provide you with expert advice.