Introduction

Dielectric strength in power transformers is one of the key properties of insulating liquids in withstanding electrical stresses within a power transformer without breaking down. Failure of the dielectric strength can result in flashovers and major damage to the transformer. It is thus very important to maintain the dielectric strength above the minimum operating limits during the life of the transformer.

Ensuring optimal dielectric strength is imperative for the proper functioning of power transformers. A primary rationale for upholding robust dielectric strength lies in its role in preserving insulation integrity. This involves creating a barrier between conductive elements, thereby averting inadvertent electrical contact, and mitigating the potential for short circuits or faults. Such faults can have severe consequences, leading to catastrophic failures, fires, explosions, and environmental contamination, incurring substantial remediation costs.

Another critical aspect of maintaining effective dielectric strength, through regular monitoring, is addressing the constant stress endured by insulation due to voltage surges and overloading. This proactive approach enables the early detection of degradation, facilitating the implementation of timely and effective measures. By doing so, the long-term sustainability of the transformer is safeguarded, contributing to its reliability, and minimizing the risk of unplanned disruptions in the power supply.

The other advantage of using a liquid insulating fluid is that the combination with paper insulation results in a higher dielectric strength capability.

How to test for dielectric strength

The dielectric strength of a liquid insulating medium like mineral oil, is the measure of its ability to withstand electrical stress without breaking down and is usually expressed in terms of voltage per unit thickness where electrical breakdown occurs.

The testing method generally prescribed is that of ASTM D877, ASTM D1816, or IEC 60156 test method. Contamination of the oil sample may affect the results and all precautions must be taken to abide by the sampling and testing methodology as prescribed in these standards.

Dielectric strength is usually measured in 2.5 mm/kV suggesting that the liquid insulation can withstand 2.5 millimeters thickness for every kilovolt of electrical stress. For example, if there is a 10 kV electric potential difference 2.5mm/kV × 10 kV = 25mm of thickness is required without breaking down. The following table provides a high-level comparison of the most common international standards for dielectric strength in insulating fluids.

	ASTM D1816	ASTM D877	IEC 60156
Electrode Shape
Gap Size	2 mm or 1 mm	2.54mm	2.5mm
Time Between Breakdown	1-1.5 min	1 min	2min
Number in sequence	5	5	6

The following is the summary of limits as prescribed by IEC60156

	Voltage Class	IEC 60156 (2.5 mm gap)
		Good	Fair	Poor
New Mineral Oil, prior to energization	≤72.5 kV	>40 kV	30 to 40 kV	<30 kV
	72.5 to 170 kV	>50 kV	40 to 50 kV	<30 kV
	≥ 170 kV	>60 kV	50 to 60 kV	<50 kV

What affects the dielectric strength of mineral oil?

The dielectric strength of mineral oil can be influenced by factors such as temperature, impurities, mechanical loading, fabrication details, voltage type, and aging.

• Temperature: has a significant influence on the dielectric strength of insulating materials with an inversely proportional relationship. The dielectric strength tends to decrease as the temperature rises. This is due to the increased thermal motion of molecules within the material, making it more susceptible to breakdown under electrical stress. This makes the temperature at which dielectric strength is measured very significant thus the dielectric strength is often measured at room temperature at around 20-25 degrees Celsius and then adjusted for temperature variations.
• Mechanical loading – which may be caused by transportation, installation, short circuits, thermal expansion and contraction, dynamic loading, overloading, and vibration can introduce imperfections that act as leakage current paths of least resistance, reducing its dielectric strength.
• Fabrication details – such as flow lines or weld lines can also act as channels of least resistance, lowering the dielectric strength.
• The type of voltage applied can also affect the dielectric strength over time since AC voltage has a greater stress on insulating material than DC voltage of the same value.
• Aging and environmental factors can also cause a decrease in dielectric strength over time. Oxygen in the air causes oxidation of the oil with aging and the production of moisture, acids, and sludges which then further contributes to the ageing process. High moisture levels with elevated temperatures can also lead to creating bubbles in the oil reducing the dielectric strength and causing failure.

What to do when the dielectric strength is low?

A low dielectric strength of the insulating medium is indicative of a potential risk of electrical breakdown or failure. The following steps can be followed to improve or remedy the situation.

1. The first step is to Identify the root cause of the low dielectric strength – common causes are usually a bad oil sample. A resample is always recommended.
2. Further testing of the oil may be required. Usually, tests for moisture, acidity, sludge, interfacial tension, particle count, and other impurities may be conducted. If these identify bad quality oil the remedial actions must follow.
3. Once established that there is a true low dielectric condition other reasons could be contamination, aging, thermal stress, or the presence of impurities.
4. If contamination is identified one of the considerations can be oil filtration.
5. If the quantity of oil in the transformer is small, then replacement of the oil can be considered. This can be effective for small transformers with very little oil.  

Proactive approach to maintaining a good dielectric strength

Other types of insulating medium

Some alternative fluids have benefits such as high fire (360°C) and flash points (330°C) for indoor use or environmentally friendly attributes. Common examples may include FR3, Midel, R-Temp, Envirotemp, and Silicon oil.

Natural esters are also environmentally friendly and biodegradable allowing for fewer clean-up measures when experiencing leaks, spillage, or failures of the units. Table 3 provides the minimum dielectric strength limits as defined by ASTM D877.

Type of Insulating Liquid	ASTM D877
Mineral oil	45 kV
Silicon oil	40 kV
Synthetic ester	43 kV
Natural ester	56 kV

Conclusion

Maintaining the dielectric strength within the prescribed limits is very important for maintaining the life of the insulation and preventing major failure on power transformers. For further application of how dielectric strength can be used to monitor the health of a power transformer read the Transformer Age Index Model (TAIM).

References

1. ASTM D877 – Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes.  
2. ASTM D1816 – Standard Test Method for Dielectric Breakdown Voltage of Insulating Oils of Petroleum Origin Using VDE Electrodes.  
3. IEC 60156 Insulating Liquids – Determination of the breakdown voltage at power frequencies – Test method.