OTS PB and OTS AF series of insulating oil test sets

Two things are particularly important when taking oil samples. The first is to ensure that the proper sampling procedure is followed, and the second is to ensure that all of the essential information is properly recorded. If the sample is to be sent to a test house for testing, the test house should be able to advise on the information needed, but it’s important to bear in mind that the condition diagnosis will only be as good as the information supplied. The test house should also advise on the volume of the sample and the type of container to use. For oil samples from transformers, the information that oil test laboratories generally require is:

• Description of the sample
• List of tests to be performed
• Transformer nameplate information
• Type of transformer
• Type of insulating fluid
• Any leaks noted
• Insulating fluid service history (has it been dried, etc)
• Transformer service history (has it been rewound, etc)
• Type of breather
• Type of insulation, including temperature rise rating
• Details of cooling equipment (fans, radiators, etc)
• Temperature of top of fluid, read from gauge
• Actual fluid temperature measured
• Fluid level
• Vacuum and pressure gauge readings

 For load tap changers, it is also advisable to record the counter reading, the selector range, and the sweep range. Sampling should be performed in accordance with the appropriate standard. Hints and tips for taking oil samples:

• For a sample to be truly useful, it must be representative of the oil in the equipment. This means that cleanliness is extremely important.
• Samples are normally drawn from a drain valve or sampling cock. This must be cleaned both inside and out before the sample is taken to ensure that dirt does not fall into the sampling container.
• The drain valve is at the bottom of the equipment, where all of the sludge, water and contaminant particles collect. It is important therefore, to flush the system thoroughly to ensure that the sample is drawn from the main bulk of the oil. This may involve removing two litres of oil, and even more if the equipment has been out of service for some time.
• Do not be tempted to use old engine oil bottles. Even a few p.p.m of engine oil will cause the sample to fail a breakdown test.
• Do let the oil flow down the side of the sample bottle, or use a clean tube run to the bottom of the bottle; it will prevent air being mixed with the oil.
• Do store the oil samples in glass or clear plastic bottles in the dark, mineral oil will deteriorate if exposed to UV light.

 Safety

• Before taking samples, ensure that you have all of the required permissions and permits.
• Have everything you need to lock out/tag out to hand.
• Make sure that the PCB (polychlorinated biphenyl) content of the oil, if any, is known and that the equipment is labelled. PCB is very hazardous and requires special handling.
• Use all of the correct personal protective equipment (PPE) and correctly rated tools.
• Check the area for electrical and tripping hazards.
• Check for wildlife – snakes, bees, etc like transformers!
• Check that the transformer is under positive pressure – are the pressure gauges reliable? Could they be blocked or broken? NEVER try to take a sample from a transformer under negative pressure. Air could be drawn into the transformer and cause it to fail.

 Sampling equipment

• Take extra sample bottles and syringes – they’re often needed
• Ensure that the sample bottle seals are airtight
• Use only ground glass syringes
• If rubber hose is used, discard after each sample is taken

 Flushing the systemWhen flushing the system, a spare sample bottle is usually repeatedly filled and emptied into the waste. It is good practice to measure the oil temperature using the last bottle that will be discarded, as this avoids having to put the thermometer into the actual sample. Taking the sampleWherever possible, try to take samples during times of relatively steady loads and temperature – in other words, when the equipment is at equilibrium. This is particularly important with transformers as, if the sample happens to be taken after the transformer has cooled following a long period of running at full load, the breakdown voltage of the oil will be much lower than normal. This is because moisture in the paper insulation will have migrated to the oil during the period of full load, and will not yet have had time to migrate back. This is usually considered to be a normal phenomenon, but it is possible that it may also be a factor in so-called ‘sudden death’ transformer incidents where, for no apparent reason, a seemingly healthy transformer suddenly fails. This is another good reason for recording as much information about the transformer as possible and for trending results to look for unexplained changes. Do not take samples when it is raining or snowing, or when the relative humidity is above 50 %, as there is a high probability that samples taken in these conditions will be contaminated. Do not take samples when it is windy, as dust blown by the wind may contaminate the sample. Try not to take samples when the ambient temperature is high, as perspiration is a common source of contamination problems. 

Successful dielectric breakdown voltage testing depends not only on obtaining a good sample, as discussed in the previous section, but also on ensuring that the test vessel is properly prepared. The preparation of the test vessel can be divided into two key elements: the first is storing, cleaning, and filling, and the second is setting the electrode gap. Storing, filling, and cleaning test vesselsIEC 60156 recommends that a separate test vessel assembly is used for each type of insulating fluid that it is required to test. This standard requires that the test vessels are filled with dry insulating fluid of the type that they will be used to test, then covered and stored in a dry place. ASTM offers an alternative option of storing the vessels empty in dust-free cabinets. Immediately prior to testing, vessels stored full must be drained and then all internal surfaces, including the electrodes, rinsed with fluid taken from the sample to be tested. The vessel should then be drained again, and carefully filled with the test sample, taking particular care to avoid the formation of bubbles. If the vessel was stored empty, or if it is to be used for a different type of fluid from that with which it was filled during storage, it should be cleaned with an appropriate solvent before the rinsing and filling procedures described above are followed. ASTM D1816 specifies the use of a dry hydrocarbon solvent such as kerosene, which meets the requirements of D235. Solvents with a low boiling point should not be used as these evaporate rapidly, cooling the vessel and giving rise to the risk of condensation. Solvents commonly used include acetone and, in the USA, toluene. Toluene is banned in Europe. Use lint-free, clean room wipes to clean the vessel. Do not use paper towels as they may introduce particles that hold moisture, causing breakdown values to be dramatically reduced. Touching the electrodes or the inside of the vessel should be avoided and during cleaning, the electrodes should be checked for pitting or scratches that may cause breakdown voltage values to be decreased. Setting the electrode gapSetting the electrode gap accurately is very important, as the results obtained are only valid if the gap is correct. A big problem is movement of the electrodes after the gap has been set and for this reason, many users of oil test sets check the electrode gap frequently – sometimes before every test. A better solution is to use test sets where the electrodes can be locked in position, such as the instruments in Megger’s latest OTS range. Megger recommends the use of flat, smooth gap gauges. The latest Megger gauges have a black anodised coating, which not only provides a smooth surface but also shows when the gauge is worn, as the shiny aluminium starts to show through the coating. Hints and tips for vessel preparation:

• If rinsing the test vessel with the sample oil before testing, it is most important to immediately fill the test vessel with the oil sample to be tested. Any significant delay will result in the oil film on the vessel’s walls absorbing water from the air, and since the walls have a large surface area, this will contaminate the oil sample and reduce the breakdown voltage once it has been mixed with the sample.
• Pour the oil into the vessel swiftly with minimum turbulence so as not to entrap air.
• Allow the sample to stand for a few minutes before the testing to allow air bubbles to clear.
• Do not leave the sample in the vessel to stand for too long before testing as it will absorb water from the air in the headspace above it. This will reduce the breakdown voltage.
• If you are using an impeller stirrer that utilises a baffle plate to exclude air from the oil sample ensure that:
  • Oil does not pass over the upper surface of the baffle plate
  • Oil is in full contact with the underside of the baffle plate
  • The use of a magnetic bead for IEC60156 will circulate oil in the lower portion of the test vessel, whereas the impeller will circulate all of the oil in the test vessel. The magnetic bead therefore has the advantage that moisture absorbed by oil in contact with air is not stirred into the sample, avoiding unwanted contamination.
• Remember that the rules of cleaning and preparing the vessel also apply to the magnetic bead, impeller, baffle plate and electrodes, not just the vessel walls.
• When performing continuous testing of many oil samples, such as in laboratory environments it is important to clean or rinse the test vessel between every sample tested.
• Always refer to the appropriate test standard to ensure the preparation is performed as specified.

You should:

• Store electrodes in a suitable container
• Immerse electrodes in clean mineral insulating oil

You can keep electrodes in a test vessel, left to stand overnight, with the last oil sample tested.

The 400 ml vessel supplied meets the requirements for most testing standards. A 100 ml vessel is also available that complies with ASTM D877.

The breakdown voltage of an oil sample increases significantly with temperature. For example, a natural ester sample with a breakdown voltage of around 35 kV at 30 ºC could easily have a breakdown voltage of nearly 60 kV at 70 ºC. For this reason, all oil test standards specify that the temperature of the sample must be recorded in the test report.  The trending of test results to identify changes in breakdown voltage is only valid if the sample and ambient temperatures for all results have been taken into account. Some breakdown testers measure oil temperature automatically. This helps to ensure that the sample temperature has been measured and avoids the possibility of introducing contamination by placing a thermometer into the oil sample.

The simple answer is yes; new oil can fail a breakdown test. Sometimes, users suspect their test set is faulty because it is failing new oil. When the test set is checked, however, almost invariably no fault is found.

IEC 60156 recommends that a separate test vessel assembly is used for each type of insulating fluid required to test. This standard requires that the test vessels are filled with dry insulating fluid of the type you will use them to test, then covered and stored in a dry place. ASTM offers an alternative option of storing the vessels empty in dust-free cabinets.

Cleaning the outer surfacesYou should:

• Disconnect the instrument
• Wipe the instrument using a clean damp cloth with isopropyl alcohol

Cleaning the test chamberMake sure the test chamber is always kept clean, particularly before a test.You should:

• Wipe away any spilled oil
• In the chamber
• Outside of the test vessel using a lint-free cloth
• Use the drain facility at the rear when lots of spilled oil is in the test chamber
• Unclip the clear tube and drain the oil into a beaker or other suitable container

Cleaning the inside of the test vesselYou should:

• Follow the instructions given in the relevant test specification
• Use a small volume of the next sample of oil you are measuring in the case of no instructions

You should:

• Use isopropyl alcohol
• Immerse the electrodes in clean insulating oil for a couple of hours before use

You should:

• Use a clean soft cloth and brass cleaner
• Use minimal pressure to avoid removal of excessive electrode material
• Use a clean cloth with isopropyl alcohol after removing the dirt
• Immerse the electrodes in clean insulating oil for a couple of hours before use
• Discard pitted or scratched electrodes and fit new electrodes