Test Laboratory for Transformer Materials
Top class laboratory diagnostics
Do you know the chemical age of your transformer? There are numerous complementary methods for diagnosing the transformer ageing process early and reliably. The Siemens Energy Test Laboratory for Transformer Materials is completely focused on the laboratory diagnostics of oil-filled electrical equipment. The highly qualified personnel are constantly developing new methods for ageing diagnostics and are using this knowledge for the benefit of our customers.
Demonstrable competence
Accredited as an independent test laboratory in accordance with ISO 17025 (Certificate)
- DKE 182 – liquids and gases for electrotechnical applications
- DKE 181 – Solid electrical insulating materials
- DKE 171 – magnetic alloys and steels
- Cigré D1 – materials and emerging test techniques
- Various Cigré working groups on the subjects of Dissolved Gas Analysis, corrosive sulphur, insulating liquids and new ageing markers
- IEC TC 10 working groups – fluids for electrotechnical applications
Our services include insulating oil analyses and material testing. The diagnostic test procedures vary between ageing analysis or condition detection of oils /insulating materials and the new state assessments of oils /insulating materials.
The lifespan of a transformer largely depends on the ageing of the solid and liquid organic insulating materials. In normal operating conditions, below the maximum permissible load, and with regular maintenance and care, a transformer's lifespan can be extended to well over 30 years.
Nevertheless, during the working life of a transformer abnormal events could occur, for example transient over voltages, overheating during emergency operation, dynamic stress and faults in the cooling. These processes result in an accelerated ageing of the transformer materials.
Whether it is a case of new construction, operation, retrofit or repair, it is always a matter of reliably recognizing material degradation and rectifying it. For this purpose an extensive knowledge of the materials as well as a specific, precise analysis is fundamental. In order to avoid costly breakdowns, minimize downtimes and extend the lifespan of a transformer, the performance of a regular trend analysis and targeted maintenance are indispensable.
The combination of moisture in the oil and in the cellulose is an indicator of ageing. The most important method for the diagnosis of the ageing process as a result of dielectric, thermal, dynamic and chemical ageing in oil and oil-paper insulation is the Dissolved Gas Analysis (DGA). The Siemens Energy Material Testing Laboratory for Transformers offers a comprehensive service, from the collection of samples to analysis and assessment.
Oil sampling
The sampling device, developed in collaboration with the customers, offers the following advantages:
- always ready for use and contains all the necessary equipment – from thread adapters to multi-use pliers
- compact and stackable, all connections have self-closing couplings
- guarantees an easy, secure and reproducible sample collection
During the mostly decades-long service life of a transformer, materials too are subjected to considerable ageing processes. Through our tests we are able to determine the condition of the materials, and offer our clients the possibility to plan any necessary maintenance measures early.
Analyses of solid insulating materials, conductive metallic materials (grain-oriented electrical steel and copper materials) as well as coating materials and sealing materials, are carried out.
During the natural aging process of the oil and insulating parts, especially in the case of thermal or electrical failures, cracked gases form, and are dissolved in the oil. The rate of decomposition and the type of gases change during defective operation, which could be a result of thermal overloading and/or electrical faults. Based on the quantity or type of fault gases, the gas increase rates and the proportions between the gases, the type of failure can be deduced.
Partial discharges with lower energy mainly lead to the formation of hydrogen and methane, as well as small quantities of ethane. Thermal overheating results in the pyrolysis of hydrocarbons. In temperatures between 300-700°C ethylene and propylene accompanied by larger quantities of CO and CO2 prevail. Over 700°C mainly ethylene, propylene and hydrogen form, and above 1000°C acetylene are also formed. Electrical discharges (arcs and spark discharges) cause separation of hydrogen and acetylene, as well as methane and ethylene. By thermal-oxidative cellulose degradation, larger quantities of CO and CO2 are formed.
Through DGA mainly slowly developing faults can be detected. The gradual development of gas concentration allows for a trend analysis, which in turn makes it possible to make a prognosis on the lifespan of the transformer. The following failure sources can also be understood by means of DGA:
- Identification of Hot spots
- partial discharges and discharges
- formation of pollution layers on contacts
- abnormal cellulose degradation
- localized metal overheating
- saturation of the transformer oil with air, which can cause the BHR to trip, without a transformer failure
- Catalytic effect of coatings and other transformer materials
- The analysis of Buchholz gases gives additional information about the damage – acute or subtle
- Leaking diverter switch
The following cannot be detected by means of DGA:
- the fault location
- acute faults, that develop within seconds or minutes
- temperatures, that are below 150°C degrees for a long time, e.g. caused by faulty cooling operation and leads to the degradation of the paper and oil.
The maintenance of insulating liquids is based on IEC norm, VDE 0370 part 2 (= IEC 60422). This norm is valid for oils that are already contained in transformers. For new oil the norm VDE 0370 part 1 (= IEC 60296) is valid.
The following oil values are decisive for the regulation of measures:
- Color and appearance
- Breakdown voltage
- Water content
- Neutralization value (acidity)
- Loss factor
- Interfacial tensions
- Inhibitor content
The color and appearance of transformer oil are useful for comparative evaluation: a quick color darkening or dark oil is a sign of oil ageing. The analyzed oil color is assigned a number ranging from 1 to 8, whereby the level of discoloration is indicated by means of the rising color number.
By analyzing the appearance, undesirable by-products can be detected. Cloudiness or sediment indicate free water, insoluble sludge or dirt particles. If these are present, then also the breakdown voltage and/or the loss factor and eventually also other oil values are out of the norm and measures must be taken.
The breakdown voltage (Ud) indicates how well insulating oil can withstand an electrical load and is therefore decisive for the operational efficiency of a transformer. The breakdown voltage is measured according to VDE 0370 part 5 (=IEC 60156). The initial measures to be taken in case of a drop below the insulation voltage limit values of the relative transformer type, depend on the values of the other characteristic oil values.
Water formation is caused by the ageing of cellulose- insulating material (paper, pressboard, laminated wood), ageing of oil and infiltration of humidity from the environment by means of badly maintained air dryers and/or defective sealing systems.
There is an equilibrium between the water content in oil and the moisture content in the solid insulation, it reduces the breakdown voltage and accelerates the ageing process. This equilibrium though, is temperature and time-dependent.
As there is never an even temperature in an operating transformer, -oil temperature in the tank is much higher at the top than at the bottom, and the same counts for the temperature distribution in the windings – the moisture distribution curves are only a rough estimate. In the moisture distribution curves the water content in the oil can theoretically be distinguished from the moisture content in the cellulose, based on the temperature. This is however valid only for insulation paper and not for pressboard and laminated wood. A precondition is constant temperature and a set equilibrium.
Through progressive ageing (Oxidation) of the oil, polar decomposition products develop. These deteriorate the dielectric properties of the oil. The result of far advanced oil ageing is sludge formation. Sludge greatly compromises the windings because it leads to the formation of sediments, which prevent heat removal. This heat accumulation in turn causes the winding paper to age intensely. A timely diagnosis of the incipient acid formation is therefore important, so that timely counteractive measures can be initiated.
Literature: Cigre Brochure D1.30 – „Oxidation Stability of Insulating Fluids“
The dielectric dissipation factor of an insulating material is the tangent of the dielectric loss angle.
The dielectric loss angle, is the angle in which the phase difference between the applied voltage and consequential current diverges from π/2 rad, when the capacitor's dielectric comprises only of insulating material.
A rising dissipation factor is an indication of oil ageing or oil contamination. The dissipation factor is strongly influenced by polar components and is therefore a very sensitive parameter.
In addition to the neutralization value and the dissipation factor, interfacial tension is another indicator of sludge formation in the transformer. This test measures the concentration of polar molecules in oil, which form during the ageing process. The higher the concentration, the lower the interfacial tension and the likelier it is for the insulating oil to form sludge.
Inhibitors are age protecting agents which delay the decomposition of the insulating oil. The IEC 60296 accredited inhibitor DBPC (Di-tertiary-butyl-para-cresol), with a weight percentage of 0.25 ± 0.040 , is used. Inhibited oils are preferred for transformers > 200 MVA, for heavily loaded transformers such as traction transformers, for oven transformers or at special customer request. When the accelerated oil ageing process is caused by a transformer malfunction, the original inhibitor content (0,3), can be restored by adding a calculated amount of DBPC in powder form, once the failure has been fixed. Even in the case of normal DBPC- deterioration, the oil can be re-inhibited, although an oil change would be less expensive. This decision can be made only based on the results of the other oil value tests: if these are normal then a re-inhibition can be successful. The inhibitor content is determined using infrared spectrometry or gas chromatography with MS-detector.
The decomposition of paper is caused by the processes of hydrolysis, pyrolysis and oxidation. The degree of polymerization (DP-value) of paper was defined by IEC 60450 and it counts the number of polymerized glucose rings. During the paper decomposition, the DP-value is reduced and the tensile strength decreases. New cellulose has a DP-value of 1000-1100, aged cellulose on the other hand has a value of only 150-200, which means the end of the transformer's lifespan.
As it is not possible to take paper samples during running operation, the condition of the solid insulation is estimated based on the cellulose decomposition products (2- Furfural). For this purpose a regular trend analysis is necessary.
The furan content in oil depends on:
- Oil temperature
- Neutralization value
- Sludge content
- Oil/paper ratio
- Type of oil (inhibited, non-inhibited)
- Type of paper (thermostabilized, non thermostabilized)
An exact link between the furan content and DP-value cannot be deduced at present. However it is possible to make a trend analysis: based on the change of the furan content, information about thermal conduct of the solid insulation over the years can be obtained.
The testing and approval of new insulating liquids, or mineral oils, synthetic or natural ester or silicone liquids is also part of the laboratory's work.
Using our computer controlled and monitored oxidation equipment we are able to set and monitor the test conditions much more precisely than is required by the standard specifications. The result is a much better reproducibility and safe assessment.
The solid insulating materials in the windings include purified cellulose solid materials like pressboard and insulating paper as well as bonded laminates (e.g. laminated board, various fiber- reinforced synthetic materials).
In the material testing laboratory the physical properties of these materials are tested, hence verifying their applicability for transformer construction.
The following tests are carried out:
- Determination of thickness and density
- Chemical analysis to ensure that there is no metal contamination
- Shrinkage behavior
- Moisture content
- Bending strength, compressive strength and tensile strength
- Compatibility with insulating liquids
- Impregnation behavior
- Conductivity and ph value of the aqueous extract
- Ash content
Transformer cores are made up of grain-oriented electrical steel. There are three different qualities: conventional grain-oriented steel, highly permeable grain-oriented steel and domain refined electrical steel.
In the material testing laboratory the following tests are carried out on the grain-oriented electrical steel:
- Surface resistance using the Franklin Tester
- Magnetic loss measurement, using a Single Sheet Tester (SST)
- Magnetostriction
Copper materials are mainly used in the windings and in the leads as single flat conductors, multiple flat conductors and continuously transposed conductors. The condition of the enamel paint and of the paper wrapping is decisive for safe transformer operation.
The following tests are carried out on copper materials:
- Resistance of the enamel insulation after accelerated ageing in insulating liquids
- Thickness and hardness of the enamel insulation
- Adhesion and elasticity during bending and elongation
- Proof stress and elongation at break of the copper
- Breakdown voltage varnished copper conductors
- Determination of the solidification factor of continuously transposed conductors with epoxy bonding
The tank, tank cover, manhole cover and valves of a transformer are sealed using sealing materials that are resistant to insulating liquid, for example:
- Nitrile Butadiene Rubber (NBR)
- Fluoric rubber (FPM)
- Fluorosilicone rubber (FMQ)
- Asbestos-free fibrous materials (FA 200)
Outside coating materials are used as corrosion protection for tanks, radiators, coolers and other construction parts. Interior coatings on the other hand are used to protect the transformer from possible metal particle contamination.
The main coating materials used are environmentally friendly. These can be for example, water-based, “High Solid” or cathodic dip coating.
In the material testing laboratory we inspect the condition of the coating using the following procedures:
- Layer thickness measurement
- Cross-cut test
- Effect of the internal coating materials on the insulating liquid (DGA/oil values)
Adhesives, cast resins, desiccants, glide waves and detergents are only some of the numerous auxiliary materials used in transformer production or repair. The behavior of each of these auxiliary materials must be understood to the smallest detail, particularly in their interaction with other materials. We therefore test the adhesion of glues, the adsorption behavior of desiccants and the compatibility of different auxiliary materials with insulating liquids.
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Siemens Energy Transformers
Chemical-physical material testing laboratory
Katzwanger Str. 150
90461 Nürnberg, Germany
E-Mail: testlab.energy@siemens-energy.com
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