OTS PB and OTS AF series of insulating oil test sets

Put simply, a dielectric breakdown voltage test is a measure of the electrical stress that an insulating oil can withstand without breakdown. The test is performed using a test vessel that has two electrodes mounted in it, with a gap between them. A sample of the oil to be tested is put into the vessel and an AC voltage is applied to the electrodes. This voltage is increased until the oil breaks down – that is, until a spark passes between the electrodes. The test voltage is then immediately turned off. The voltage at which breakdown occurred is the test result, and is typically evaluated by comparing it with guidelines set out in various standards, or in the oil manufacturer’s specifications.The exact method of performing the test is determined by the standard that is being used. The standard typically defines parameters such as the size and shape of the electrodes, the gap between them, the rate at which the test voltage is increased, how many times the test is repeated, and whether or not the oil is stirred during the test

There are many types of organisation that benefit from carrying out tests on insulating oil. These include:

• Utility contractors (principally in substations)
• Utility companies (principally in power stations and substations)
• Rail companies (locomotive high voltage step-down transformers and switchgear)
• Oil test laboratories (providing testing services)
• Transformer and switchgear manufacturers (quality control of oil)
• Oil companies (testing new oil during manufacture)
• Heavy industry and manufacturing (asset maintenance programs)

While the generic term ‘oil’ is almost universally used to describe insulating fluids, there are currently five different types of insulating fluid in common use. These are:

• Mineral oil
• High molecular weight hydrocarbon (HMWH) fluids
• Silicone fluids
• Synthetic ester fluids
• Natural ester (vegetable oil) fluids

 All of these oil types can be tested for dielectric breakdown voltage and tested with Megger OTS range test sets. Mineral oil is the most common insulating fluid and has been in use since the late 19th century. There are many mineral oil filled transformers that have been in continuous use for more than 50 years. Mineral oils are refined from either naphthenic crude or, more recently, from paraffinic crude. HWMH, silicon, synthetic ester and natural ester fluids are more recent developments and are often preferred because they are much less flammable than mineral oil. ASTM D5222 specifies that for insulating fluids to qualify as ‘less flammable’ they must have a fire point of at least 300 ºC. The five fluids differ significantly in the way they behave in the presence of moisture. Mineral oil is the least satisfactory, and even small amounts of water significantly reduce its breakdown voltage. Silicone fluid is also quickly affected by small amounts of moisture, whereas ester fluids behave very well in the presence of moisture and can typically maintain a breakdown voltage of greater than 30 kV with more than 400 ppm water content. This is one of the reasons that esters last much longer in service.

IEC 60156 is an international standard that appears in many forms as IEC member national committees from various countries have adopted it. Examples are British Standard BS EN 60156 and German VDE 0370 part 5. IEC 60156 specifies the use of either spherical or mushroom-shaped electrodes, the same as those used in the ASTM D1816 standard. The IEC standard differs in several ways from D1816, but the main difference is that the IEC standard allows the optional use of a stirring impeller, the use of a magnetic bead stirrer, or even no stirring at all. The standard states that differences between tests with or without stirring have not been found to be statistically significant. A magnetic stirrer is only permitted when there is no risk of removing magnetic particles from the oil sample under test. When oil is used as a coolant, in which case it circulates, it would be stirred during testing. For example, typically, transformer oil circulates when it is used as a coolant. Therefore, an oil sample from a transformer should be stirred to ensure the best chance of detecting particle contamination. Oil from a circuit breaker is ordinarily static, so particles would naturally fall to the bottom of the tank, where they are unlikely to cause a problem. So in static use applications, an oil sample is usually not stirred. The dielectric breakdown values from the IEC 60156 method are usually higher than those from the ASTM methods. The higher dielectric breakdown values are partly because of the differences in voltage ramp-up speed and electrode gap compared with D1816, and electrode shape compared with D877 (the IEC electrode shape provides a more uniform electric field). The result is that for well-maintained transformers, the breakdown voltages may be higher than a 60 kV test instrument can reach. The inability to quantify a breakdown voltage higher than 60 kV may not be a problem when evaluating new oil from a supplier or even for in-service oil. However, frequently an actual breakdown voltage value is required. Therefore, when testing according to IEC 60156, an instrument capable of applying a higher voltage is advisable. As with D1816, dissolved gas in the oil sample may reduce breakdown values, but the effect is much less pronounced with the IEC 60156 standard.

Dielectric breakdown voltage testing is an important element in the maintenance programme of any item of oil-insulated electrical equipment. However, to get the maximum benefit from this type of testing, Megger strongly recommends that the oil is tested at least once a year and preferably twice a year. The results should be recorded, as trending the data will make it easier to identify sudden or unexpected changes. If a sudden change in the results is found, the transformer can be inspected for leaks, the oil level can be checked, and the water content of the oil evaluated. If contamination is confirmed, it will often be possible to dry and filter the oil, thereby reconditioning it rather than having to replace it with expensive new oil.

ASTM D877 is an older standard and is generally not very sensitive to the presence of moisture. For that reason, it is not widely used for in-service applications. In 2002, IEEE revised C51.106, Guide for the Acceptance and Maintenance of Insulating Oil in Equipment. IEEE removed the values for D877 from their criteria for evaluating in-service oil in transformers. Generally, ASTM D877 is recommended only for acceptance testing of new oil received from a supplier in bulk loads or containers to ensure the oil was correctly stored and transported. Typically, a minimum breakdown value of 30 kV is specified. The ASTM D877 standard specifies the use of disc-shaped electrodes that are 25.4 mm (1 inch) in diameter and at least 3.18 mm (0.125 inch) thick. These electrodes are made of polished brass and are mounted to have their faces parallel and horizontally in line in the test vessel. The edges are specified to be sharp with no more than a 0.254 mm (0.010 inch) radius. It is good practice to inspect the sharp edges regularly to ensure they have not become too rounded. Excessively rounded edges will have the effect of falsely raising the breakdown voltage, possibly passing oil that should have failed the test. It is also important that the electrodes are kept very clean, with no pitting or signs of corrosion; otherwise, breakdown values can be falsely low.

ASTM D1816 has become widely used in North America, even outside the standard’s stated application of petroleum-origin insulating oils and viscosity limits. D1816 is more sensitive than D877 to moisture, oil ageing, and oxidisation, and is more affected by the presence of particles in the oil. The IEEE revision of C51.106 in 2002 added breakdown voltage limits for new and in-service oil using D1816. ASTM D1816 specifies the use of mushroom-shaped electrodes 36 mm in diameter. As with D877, the electrodes are made of brass and must be polished to be free of any etching, scratching, pitting, or carbon accumulation. The oil is stirred throughout the test sequence, and a two-bladed motor-driven impeller is specified. The standard prescribes the impeller dimensions and pitch as well as the operating speed, which must be between 200 rpm and 300 rpm. The test vessel must have a cover or baffle to prevent air from coming into contact with the circulating oil. The D1816 standard, although generally accepted as more valuable than D877, has one significant limitation: when testing in-service oil, this test method is very sensitive to dissolved gases. Excessive amounts of gas in the oil can lower the test results to the point that a perfectly good sample of oil, with low moisture and particle content, will fail the test. It is important to bear this in mind when testing oil from small gas-blanketed transformers and, in some cases, free-breathing transformers.

The dielectric breakdown voltage test is a relatively quick and easy way of determining the amount of contamination in insulating oil. Usually the contaminant is water, but it can also be conductive particles, dirt, debris, insulating particles and the by-products of oxidation, and ageing of the oil.For in-service equipment, the dielectric breakdown voltage test offers a useful and convenient way to detect moisture and other contamination in the oil before it leads to a catastrophic failure. The information gained from the test can also be used as an aid to:

• Predicting the remaining life of a transformer
• Enhancing operational safety
• Preventing equipment fires
• Maintaining reliability

Dielectric breakdown voltage testing is also carried out on new oil before it is used to fill equipment, and as part of the acceptance testing for deliveries of new and reprocessed oil.