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Check the gap between the electrodes and make sure the vessel is cleaned according to the standards.
Megger offers a voltage check metre that can be fitted to the instrument in place of the measuring vessel. Doing so allows you to compare the voltage shown on the check metre with that shown on the instrument display. Check meters are not sufficiently accurate to use as a calibration standard. Still, they provide an excellent way of detecting changes in instrument calibration. You should record the check meter readings each time you carry out a voltage check to identify changes quickly. If any significant change is detected, you should not use the instrument until you have returned it to Megger or an accredited service centre for servicing and recalibration.
Indicators that you need to send your OTS into Megger or to an accredited service centre for repair include your OTS not booting up or not building voltage.
Interpreting test results
There are several key factors to consider to carry out effective and reliable insulating oil dielectric breakdown testing. You’ll need to know that your results are valid, considering standards and their specific conditions that must be met. You also need to know if your insulating fluid meets manufacturing standards.
This extract of a chart comparing standards shows that each standard specifies different conditions that must be met if the test results are to be accepted as valid. You can find the full chart in our ‘Guide to insulating oil dielectric breakdown testing’.
|Standards||ASTM D1816||ASTM D877||IEC 60156|
|Procedure A||Procedure B|
|Valid test conditions||
If breakdown does not
Tests must be repeated if range of BD voltages recorded
|Tests must be repeated if the range of BD voltages recorded are more than 92 % of mean. If the range of 10 BD voltages is more than 151 % investigate why.||Expected range of standard deviation/mean ratio as a function of the mean provided as a chart.|
Mean is the average of the breakdown values recorded in the test sequence. For example, if the breakdown values are 33 kV, 37 kV, 32 kV, 35 kV, 38 kV, and 34 kV, the mean value would be the total of these results – 209 – divided by the number of results – 6 – which gives a mean value of 209/6 = 34.83 kV. (Note that in this example, there are six results as required by the IEC standard. The ASTM standards require either five or ten results.)
Range of breakdown voltage is referred to in the ASTM standards. For example, D877 specifies that the test sequence must be repeated if the range of breakdown voltages recorded is more than 92 % of their mean value. Two examples will make this easier to understand.
In the first example, the breakdown voltages recorded are 43, 45, 52, 40, and 38 kV. The lowest value is 40 kV and the highest is 52 kV, so the range is 12 kV. The mean of the recorded values is 43.6 kV, so the range is only 12/43.6 x 100 % = 27.5 % of the mean value. These test results are, therefore, valid.
In the second example, the breakdown voltages recorded are 33, 45, 52, 18, and 20 kV. The lowest value is 18 kV and the highest is 52 kV, so the range is 34 kV. The mean of the recorded values is 33.6 kV, so the range is 34/33.6 x 100 % = 101 %. This is above the 92 % limit, which means that the test must be repeated.
Standard deviation: IEC 60156, there is a graphical representation of standard deviation – otherwise known as the coefficient of variation – versus the mean breakdown voltage. Calculation of the mean has already been covered, but what about the standard deviation? IEC 60156 does not explain how to calculate this. The procedure, however, is to calculate the difference between each of the six test results and the mean value of those test results, then square each of the differences and add them together. Divide the figure obtained by 2, and then take the square root. The final answer is the standard deviation for the set of test results.
IEC 60156 states that, for the test results to be considered valid, the following procedure must be followed:
- Perform six tests
- Calculate the mean of the results
- Calculate the standard deviation (see above)
- Divide the standard deviation by the average value, noting that scatter is expected and acceptable (see the chart at the end of IEC 60156)
- If the value is acceptable, conclude testing
- If not, perform six more tests
- Repeat the calculations using all 12 results
An insulating fluid manufacturer normally quotes typical new and in-service fluid breakdown values in its data sheets. In addition, the test standards refer to oil condition standards that provide guidance about the acceptability of results.
D877 is usually only recommended to accept new oil from a supplier. However, some oil testing laboratories still recommend its use for specific in-service applications. In these cases, a breakdown voltage of 30 kV or more is usually considered acceptable, with values below 25 kV unacceptable. Values between 25 and 30 kV are considered questionable. For new oil, a minimum value of 30 kV is normally specified.
|Oil type||New oil|
|Mineral oil||45 kV|
|Silicone oil||40 kV|
|Synthetic ester||43 kV|
|Natural ester||56 kV|
D1816 is more widely used and is accepted by the IEEE as the test method to be used for dielectric breakdown testing for the acceptance and maintenance of insulating oil. The IEEE C57.106 standard incorporates the D1816 limits – which are shown below – for new and in-service oil. Note that the values provided in this table are for mineral oil.
IEEE Guide for acceptance and maintenance of insulating oil in equipment
|Applications||Voltages class/group||D1816 (1 mm gap)||D1816 (2 mm gap)|
|New mineral insulating oil as received from supplier||Not specified||>20 kV||>35 kV|
|New mineral insulating oil received in new equipment,
prior to energisation
|≤69 kV||>25 kV||>45 kV|
|69 to 230 kV||>30 kV||>52 kV|
New mineral insulating oil - processed from equipment, prior to energisation
|230 to 345 kV||>32 kV||>55 kV|
|≥345 kV||>35 kV||>60 kV|
|Service-aged insulating oil - for continued use (Group 1)||≥69 kV||>23 kV||>40 kV|
|69 to 230 kV||>28 kV||>47 kV|
|≥230 kV||>30 kV||>50 kV|
|Shipments if new mineral insulating oils, oil circuit breaker (OCB)||OCB||>20 kV||>30 kV|
|New OCB insulating oil - after processing, prior to energisation||OCB||>30 kV||>60 kV|
|Service-aged OCB insulating oil - for continued use||OCB||>20 kV||>27 kV|
|New mineral oil for load tap changer (LTC), prior to energisation||LTC||>35 kV||>55 kV|
|Service-aged LTC insulating oil - for continued use||LTC - Neutral||>20 kV||>27 kV|
|LTC - ≤69 kV||>25 kV||>35 kV|
|LTC - >69 kV||>28 kV||>45 kV|
IEC 60156 uses acceptance values that are contained in two further standards: IEC 60296 and IEC 60422.
IEC 60296, fluids for electrotechnical applications: Unused mineral insulating oils for transformers and switchgear. As its title indicates, this standard applies only to new, unused oil as received from the manufacturer, which must have a dielectric breakdown voltage of 30 kV or more, determined using the IEC 60156 test method. Oil that has been vacuum filtered in a laboratory must have a minimum dielectric breakdown voltage of 70 kV.
IEC 60422, mineral insulating oils in electrical equipment: Supervision and maintenance guide. This standard prescribes acceptable dielectric breakdown values for new oil (after filling but before energising) and for in-service oil. The values are:
|Equipment voltage||Dielectric BD voltage|
|≥72.5 kV||>55 kV|
|>72.5 kV ≤170 kV||>60 kV|
|>270 kV||>60 kV|
|Equipment voltage||Dielectric BD voltage|
|≥72.5 kV||>40 kV||30 - 40 kV||>30 kV|
|>72.5 kV ≤170 kV||>50 kV||40 - 50 kV||>30 kV|
|>270 kV||>60 kV||50 - 60 kV||>50 kV|
The IEC recommends that if values are in the ‘fair’ range, testing should be performed more frequently, and that the test results should be cross checked with other testing methods. If the test results are in the ‘poor’ range, the oil must be brought back into a good state by reconditioning. This might, for example, involve filtering and drying the oil.
User guides and documents
Software and firmware updates
OTS Test Standards
The attached file will update all the test standards of your OTS to the latest versions. Do not change the file name or it will not work. Please follow the instructions below:-
1. Extract the attached file (stdSeqs.db) to a USB memory stick
2. Insert the memory stick into the Type A USB port on the front panel of the OTS (or the Type A USB port on the rear of the OTS)
3. On the OTS, navigate to the Tools menu with the Hammer & Wrench symbol
4. Scroll down and select Manage test standards
5. On the next screen select Update Standards (USB) and the instrument will upload the new file from the USB stick.
6. The instrument will now have the latest standards installed ready to use.
For Older OTS (Firmware version 1.15) use "OTS-Test-Standards-V0-10.zip"
For updated OTS (Firmware version 3.xxx) use "OTS-Test-Standards-V0-30.zip"
OTS-Test-Standards-V0-30.zip is not compatible with OTS Firmware version 1.15
OTS-Test-Standards-V0-10.zip is not compatible with OTS Firmware version 3.xxx
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.
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.
- Use isopropyl alcohol
- Immerse the electrodes in clean insulating oil for a couple of hours before use
- 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.
- 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
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.
- 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.
- 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.
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