Megger OTS60SX-2 User Manual
The Megger guide to
insulating oil dielectric
breakdown testing
The word ‘Megger’ is a registered trademark
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Contents
1. Introduction ......................................................................................................................... 3
2. What are the methods for testing oil? .................................................................................. 3
3. What is oil dielectric breakdown voltage testing? ................................................................. 4
4. Who needs to perform dielectric breakdown voltage tests? .................................................. 4
5. Which types of insulating oil can be tested? ......................................................................... 4
6. Why, when and how often is oil testing performed? ............................................................ 6
7. What are the test standards and their differences? ...............................................................6
7.1 ASTM standards (USA) ............................................................................................... 8
7.2 IEC standards (international) ...................................................................................... 9
8. How should an oil sample be taken? .................................................................................. 10
9. Preparing the test vessel ..................................................................................................... 12
10. Why measure oil sample temperature? ...............................................................................13
11. How do I know whether my test results are valid? ..............................................................14
12. How do I know if my fluid has passed the test? ..................................................................14
12.1 USA – ASTM and IEEE standards .............................................................................. 15
12.2 International IEC standards ...................................................................................... 15
13. Can new oil fail a dielectric breakdown test? ......................................................................16
14. Can I verify my test instrument performance? ....................................................................17
15. How do oil test sets detect dielectric breakdown? ..............................................................17
15.1 Testing silicone oil .................................................................................................... 18
16. Choosing an oil dielectric breakdown voltage test set .........................................................18
16.1 On-site versus laboratory testing .............................................................................. 18
16.2 General instrument selection considerations ............................................................18
16.3 Selection considerations specific to laboratory instruments .......................................24
16.4 Selection considerations specific to portable instruments .........................................25
17. Megger OTS range summary .............................................................................................. 27
Acknowledgments
Megger gratefully acknowledges the support of John Noakhes of TJ/H2b Analytical Services for his help in compiling this publication.
2 www.megger.com The Megger guide to insulating oil dielectric breakdown testing
1. Introduction
Oils that combine a high flashpoint with high dielectric strength have long been used as an insulating medium in
transformers, switchgear and other electrical apparatus. To ensure that the dielectric strength of the oil does not
deteriorate however, proper maintenance is essential, and the basis of proper maintenance is testing.
For over 100 years, Megger has been a world leader in the development and manufacture of test equipment for
electrical power applications. The famous Megger trademark was first registered in 1903, and jealously guarded
by the company. Megger’s experience in the design and production of oil test sets also reaches back to the early
20th century, when pioneering equipment was produced by Foster Transformers, a company that became part of
Megger Group in 1968.
Today, all of Megger’s oil test set design, development and manufacture is carried out at the company’s
manufacturing facility in Dover, England.
2. What are the methods for testing oil?
For in-service equipment in particular, there are many test techniques for evaluating the condition of the insulating
oil. If the technique of dissolved gas analysis is excluded, oil tests can be divided into two basic groups.
The first group includes tests that are concerned with the immediate condition and acceptability of the insulation
in an item of electrical equipment. This group includes dielectric breakdown voltage testing as well as moisture
measurement by the Karl Fischer (KF) method, and determination of insulation condition by measuring the dielectric
dissipation factor.
The Megger OTS range of dielectric breakdown voltage test sets
The Megger KF range of moisture content test sets
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The second group includes tests that look at the degree of degradation and aging of the equipment’s insulation
system. These tests include interfacial tension, acidity (neutralisation value), resistivity and visual determination of
colour and appearance of the insulating oil.
Typical oil colour specimens
The darker the colour the more contaminants present
Good Fair Bad
While all of these tests are useful in particular circumstances, the most convenient and most universally applicable is
dielectric breakdown voltage testing, which is why it has been selected as the subject of this technical guide.
3. What is oil dielectric breakdown voltage testing?
Put simply, a dielectric breakdown voltage test is a measure of the electrical stress that an insulating oil can
The exact method of performing the test is determined by the standard that is being used, as will be explained later.
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.
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.
4. Who needs to perform dielectric breakdown voltage tests?
There are many types of organisation that benefit from carrying out tests on transformer oil. These include:
Utility contractors (principally in substations)
Utility companies (principally in power stations)
Rail companies (locomotive HV 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)
5. Which types of insulating oil can be tested?
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
4 www.megger.com The Megger guide to insulating oil dielectric breakdown testing
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.
This table is not exhaustive, but does give a good indication of the types of insulating fluid that are used in various
applications. It also shows which types of fluid can be tested with the Megger OTS range of dielectric breakdown
voltage test sets.
Equipment Fluid Type Example/Sub-type
Capacitors
Medium and high voltage
cables
Bushings Mineral oil Various types Yes
Oil filled circuit breakers Mineral oil Various types Yes
Transformers
LTC (Load Tap Changers) Mineral oil Various types Yes
Synthetic aromatic hydrocarbons PXE Yes
Aromatic esters Various types Yes
New Synthetic hydrocarbons Polybutenes Yes
Old Mineral oil Various types Yes
Mineral oil Shell Diala AX Yes
Perflurocarbon (PFC) 3M PF-5060 Ye s
High molecular weight
(HMW) oil
Silicone Dow Corning 561 Yes
Low flammability
fluids
Old fluids
Gases Sulphur Hexafluoride SF6 No
Old gases Freon R-113 Vapotrans No
Synthetic hydrocarbons Polyalohaolefins (PAOs) Yes
Synthetic polyol esters Envirotemp® 200 Yes
Vegetable oils - natural ester Envirotemp® FR3 Ye s
Hydroflurocarbon Vertrel
PCBs - Polychlorinated
biphenyls
Tetrachloroethylene/
perchloroethylene (PCE)
Various types Yes
®
VX Yes
®
Askarel
®
Pyranol
Phenochlor
Askarel® (contained 50%)
Wecosol
Can be tested with Megger OTS
®
®
No - Harzardous - requires special
No - Harzardous - requires special
range?
handling
handling
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6. Why, when and how often is oil testing performed?
BDV (kV)
Period between tests (e.g. years)
6.1 Why and when?
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 aging 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.
6.2 How often?
Dielectric breakdown voltage testing is an important element in the maintenance programme of any item of oilinsulated 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.
50
45
40
35
30
25
20
1 2 3 4 5 6 7 8
7. What are the test standards and their differences?
There are many test standards for insulating liquids, but they are derivatives of three main standards. Two of these
are from ASTM International (USA) and the other is from the IEC (Europe). These main standards are:
ASTM D877 – Standard Test Method for Dielectric Breakdown Voltage of Insulating
Liquids Using Disk Electrodes.
ASTM D1816 – Standard Test Method for Dielectric Breakdown Voltage of Insulating
Oils of Petroleum Origin Using VDE Electrodes.
IEC 60156 Insulating Liquids – Determination of the breakdown voltage at power
frequencies – Test method.
There is also a Japanese standard not based on these ASTM or IEC standards.
The following table is not exhaustive, but it shows the main differences between the ASTM and IEC standards.
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Standards ASTM D1816
ASTM D 877
Procedure A Procedure B
Origin USA USA USA Europe
IEC 60156
Electrodes
Oil sample stirring
Laboratory test
temperature
Outside test
temperature
Test voltage
Breakdowns
Test voltage
switch off
time following
breakdown
Time between filling and start
of test
Equivalent standards (adopted
into)
Notes on testing silicon oil
Special conditions
Comments
Shape
Gap size 2 mm or 1 mm* 2.54 mm 2.54 mm 2.5 mm
Impeller yes
Magnetic
bead
Liquid
Ambient 20 - 30 ºC Must record Must record Within 5 ºC of oil sample
Liquid
Ambient Referee tests 20 - 30 ºC Must record Must record Within 5 ºC of oil sample
Rise rate 0.5 kV/s 3 kV/s 3 kV/s 2 kV/s
Frequency 45 - 65 45 - 65 45 - 65 45 - 62
Definition <100 V <100 V <100 V 4 mA for 5 ms
Number in
sequence
Time
between
breakdown
Normal (e.g.
mineral oil)
Silicon oil Not specified Not specified Not specified <1 ms
no option optional
At ambient - must
record
At ambient - must
record
5** 5*
1 to 1.5 min 1 min n/a 2 min
Not specified Not specified Not specified <10 ms
3 - 5 min 2 - 3 min 2 - 3 min
None None None
Can be used provided discharge energy in
sample <20 mj
* If breakdown does
not occur at 2 mm,
reduce gap to 1 mm
** Tests must be
repeated if range of
BD voltages recorded
are more than 120%
of mean with 1 mm
electrode gap and 92%
of mean with 2 mm
electrode gap
Test vessel requires
cover or baffle to
prevent air from
contacting circulating oil
not stirred not stirred
20 - 30 ºC must
record temperature
as collected and
when tested
Must record Must record 15 - 25 ºC
*Tests must be repeated if range of BD
voltages recorded are more than 92% of
mean. If range of 10 BD voltages is more
than 151% investigate why
Used if any insoluble
breakdown products
in oil completely
settle between
breakdown tests
20 - 30 ºC must
record temperature
as collected and
when tested
1 - 5 different
samples
Can be used if
modified in accordance
with D2225 if
procedure A cannot
be used
Used if any insoluble
breakdown products
do not settle
between breakdown
tests
BS EN 60156
CEI EN 60156
IRAM 2341
UNE EN 60156
FN EN 6056
OK if test instrument can comply with
voltage switch off time requirements
Expected range of standard deviation/
mean ratio as a function of the mean
provided as a chart
*With some stand/stir timing differences.
Test cell/vessel must be transparent.
Reconditioned/reclaimed oil to BS148
is tested to IEC60156 following update
in 2009.
optional
15 - 25 ºC
for referee tests
6
2 min
SABE EN 60156
VDE0370 part 5
PA SEV EN 60156
AS1767.2.1
NRS 079-1*
IS6729*
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There are many standards based on IEC 60156 and in addition Japanese standard JIS C2101 also includes
requirements for dielectric breakdown voltage testing. JIS C2101 calls for spherical electrodes similar to those
specified by IEC 60156, but requires a different sequence of five breakdowns. JIS C2101-99 (M) for mineral oils calls
for two oil samples to be tested each with a sequence of five breakdown tests. JIS C2101-99 (S) for silicon oils calls
for a sequence five tests but each test is performed on a different sample of oil.
7.1 ASTM standards (USA)
There are two standard test methods from ASTM International. The first is ASTM D877-02 (latest edition 2002) –
Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes. The second is
ASTM D1816-04 (latest edition 2004) – Standard Test Method for Dielectric Breakdown Voltage of Insulating Oils of
Petroleum Origin Using VDE Electrodes. Although this is essentially an American standard, it borrows from VDE, a
German standards organisation.
ASTM D877
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 the IEEE revised C51.106, Guide for the Acceptance and
Maintenance of Insulating Oil in Equipment the values from D877 were removed from their criteria for evaluating
in-service oil in transformers. Generally ASTM 877 is recommended only for acceptance testing of new oil received
from a supplier, either 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. The sharp edges should be regularly inspected to ensure that 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. Instrument users should regularly
inspect electrodes, cleaning and polishing as required.
The Megger OTS instrument test set up screens for D877:
ASTM D1816
ASTM D1816 has become widely used over the years, even being used outside the standard’s stated application of
petroleum origin insulating oils and viscosity limits. D1816 is more sensitive than D877 to moisture, oil aging and
oxidisation, and is more affected by the presence of particles in the oil. When the IEEE revised C51.106 in 2002,
breakdown voltage limits for new and in-service oil using D1816 were added.
ASTM D1816 specifies the use of mushroom-shaped electrodes 36 mm in diameter. As with D877, the electrodes
are made of brass 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 impellor is specified. The standard prescribes
the impellor dimensions and pitch as well as the operating speed, which must be between 200 rpm and 300 rpm.
To prevent air coming into contact with the circulating oil, the test vessel must have a cover or baffle.
8 www.megger.com The Megger guide to insulating oil dielectric breakdown testing
The D1816 standard, although generally accepted as more useful than D877, has got 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
even, in some cases, from free-breathing transformers.
The Megger OTS instrument test set up screen for D1816:
The insulating fluid manufacturer will usually quote typical breakdown values for both new and in-service fluid in
the insulating fluid data sheet. In addition the test standards will refer to another oil condition standard that will
provide guidance as to what is acceptable. Test houses will also provide guidance as to what is acceptable.
7.2 IEC standards (International)
The International Electrotechnical Commission (IEC) defines only one method for oil breakdown dielectric
voltage testing. This is IEC 60156 Insulating Liquids – Determination of the Breakdown Voltage at Power
Frequencies – Test Method.
IEC 60156
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 those used in the ASTM D1816
standard. The IEC standard differs in a number of ways from D1816, but the main difference is 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. The
use of 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 and therefore circulating it would be stirred during testing. For example oil
from a transformer normally circulates if it is used as coolant, so an oil sample taken here would normally be stirred
to ensure the best chance of detecting particle contamination. Oil from a circuit breaker is normally static in use so
particles would naturally fall to the bottom where they are unlikely to cause a problem. So in static use applications,
an oil sample would usually not be stirred.
The dielectric breakdown values from the IEC 60156 method are usually higher than those from the ASTM methods.
Possibly this is in part 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.
This may not be a problem when evaluating new oil from a supplier or even for in-service oil, but often an actual
breakdown voltage value is required. When testing to IEC 60156, therefore, the use of 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 than with the IEC 60156 standard.
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Some Megger OTS instrument test set up screen examples for IEC60156:
8. How should an oil sample be taken?
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, this information that oil test laboratories generally require is:
Description of the sample
List of tests to be performed
Transformer name plate 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, and is not, therefore, discussed in
detail in this technical guide.
In the USA, there are two standards for sampling:
D923 – Standard Practices for Sampling Electrical Insulating Liquids
D3613 – Standard Practice for Sampling Electrical Insulating Oils for Gas Analysis and Determination
of Water Content
Internationally, there are two further sampling standards:
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