Transformer oil Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled wet transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and so ...

, a type of insulating and cooling oil used in

transformer In electrical engineering, a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces ...

s and other electrical equipment, needs to be

test Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film) ...

ed periodically to ensure that it is still fit for purpose. This is because it tends to deteriorate over time. Testing sequences and procedures are defined by various international standards, many of them set by

ASTM ASTM International, formerly known as American Society for Testing and Materials, is a standards organization that develops and publishes voluntary consensus technical international standards for a wide range of materials, products, systems and s ...

. Transformer oil testing consists of measuring

breakdown voltage The breakdown voltage of an insulator (electrical), insulator is the minimum voltage that causes a portion of an insulator to experience electrical breakdown and become electrically Conductor (material), conductive. For diodes, the breakdown vo ...

and other physical and

chemical A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combin ...

properties of samples of the oil, either in a laboratory or using portable test equipment on-site.

Motivation for testing

The transformer oil (insulation oil) of voltage transformers and current transformers fulfills the purpose of insulating as well as cooling. Thus, the dielectric quality of transformer oil is essential to secure operation of a transformer. As transformer oil deteriorates through aging and moisture ingress, transformer oil should, depending on economics, transformer duty and other factors, be tested periodically.

Electric utility An electric utility, or a power company, is a company in the electric power industry (often a public utility) that engages in electricity generation and distribution of electricity for sale generally in a regulated market. Electric utilities are ...

companies have a vested interest in periodic oil testing because transformers represent a large proportion of their total assets. Through such testing, transformers' life can be substantially increased, thus delaying new investment of replacement transformer assets. BA75-b2hv-Breakdown-during-transformer-oil-testing

BA75-b2hv-Breakdown-during-transformer-oil-testing

On-site testing

Recently time-consuming testing procedures in test labs have been replaced by on-site oil testing procedures. There are various manufacturers of portable oil testers. With low weight devices in the range of 20 to 40 kg, tests up to 100 kV rms can be performed and reported on-site automatically. Some of them are even battery-powered and come with accessories.

Breakdown voltage testing procedure

To assess the insulating property of dielectric transformer oil, a sample of the transformer oil is taken and its

is measured. The lower the resulting breakdown voltage, the poorer the quality of the transformer oil. * The transformer oil is filled in the vessel of the testing device. Two standard-compliant test electrodes with a typical clearance of 2.5 mm are surrounded by the dielectric oil. * A test voltage is applied to the electrodes and is continuously increased up to the breakdown voltage with a constant, standard-compliant

slew rate In electronics and electromagnetics, slew rate is defined as the change of voltage or current, or any other electrical or electromagnetic quantity, per unit of time. Expressed in SI units, the unit of measurement is given as the change per seco ...

of e.g. 2 kV/s. * At a certain voltage level breakdown occurs in an

electric arc An electric arc (or arc discharge) is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma ( ...

, leading to a collapse of the test voltage. * An instant after ignition of the arc, the test voltage is switched off automatically by the testing device. Ultra fast switch off is highly desirable, as the carbonisation due to the electric arc must be limited to keep the additional pollution as low as possible. * The transformer oil testing device measures and reports the

root mean square In mathematics, the root mean square (abbrev. RMS, or rms) of a set of values is the square root of the set's mean square. Given a set x_i, its RMS is denoted as either x_\mathrm or \mathrm_x. The RMS is also known as the quadratic mean (denote ...

value of the breakdown voltage. * After the transformer oil test is completed, the insulation oil is stirred automatically and the test sequence is performed repeatedly: typically 5 repetitions, depending on the standard. * As a result the breakdown voltage is calculated as mean value of the individual measurements.

Types of test

*Color; e.g., ASTM D1500. *Dielectric breakdown voltage; e.g., D 877, ASTM D1816 * Dissolved gas analysis; e.g., ASTM D3612 *Dissolved metals; e.g., ASTM D7151 *

Flash point The flash point of a material is the "lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture". The flash point is somet ...

fire point The fire point, or combustion point, of a fuel is the lowest temperature at which the liquid fuel will continue to burn for at least five seconds after ignition by an open flame of standard dimension. At the flash point, a lower temperature, a su ...

; e.g., ASTM D92 *Interfacial tension; e.g. D 971 *

Furan Furan is a Heterocyclic compound, heterocyclic organic compound, consisting of a five-membered aromatic Ring (chemistry), ring with four carbon Atom, atoms and one oxygen atom. Chemical compounds containing such rings are also referred to as f ...

ic compounds; e.g., ASTM D5837 * Karl Fischer moisture; e.g., ASTM D1533 *Liquid power factor; e.g., ASTM D924 * Neutralization number; e.g., ASTM D974 * Oxidation inhibitor content; e.g., ASTM D2668 *

Polychlorinated biphenyl Polychlorinated biphenyls (PCBs) are organochlorine compounds with the formula Carbon, C12Hydrogen, H10−''x''Chloride, Cl''x''; they were once widely used in the manufacture of carbonless copy paper, as heat transfer fluids, and as dielectri ...

s content; e.g., ASTM D4059 *

Relative density Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

(

specific gravity Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

); e.g., D 1298, ASTM D1524 *

Resistivity Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity i ...

; e.g., ASTM D1169 *Visual examination; e.g., D1524

International transformer oil testing standards

* VDE370-5/96 * OVE EN60156 * IEC 60156/97, * ASTM1816-04-1 * ASTM1816-04-2 * ASTM877-02 * ASTM877-02B * AS1767.2.1 * BS EN60156 * NEN 10 156 * NF EN60156 * PA SEV EN60156 * SABS EN60156 * UNE EN60156 * IS:6792 * IS 335

References