Source of Colour Used In Oil

Chemical contamination is typically colour absorbed from the type of varnish used on the windings or resins used in the cellulose pressboard packers and spacers. It can also be a result of using too high temperature or inadequate vacuum during oil filling or subsequent treatment causing damage to the oil cellular structure. Rusting inside the transformer will also cause colour contamination. Colour introduced by these mechanisms cannot be removed by Fullers Earth Treatment.

Aging is from the normal oxidation process and most of the colour introduced by oxidation can be removed through Fullers Earth. Particulate Contamination can be drawn in from the atmosphere via silica gel breathers, carbon from switching or arcing faults, residual contamination from manufacture and assembly. Colour from particulate contamination can generally be removed quite quickly by Fullers Earth and even Purification.

FIELD EXPERIENCE:

Over more than 25 years of experience in Oil Reclamation we find that Transformers and Oil from India, China and Korea tend to have very much darker in-service oil compared to the same oil condition of other sources. We believe this is a result of the materials used in construction as well as the base stock from which the transformer oil is refined.

STANDARDS for REGENERATED OIL CONTRACTS:

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1. TNB Transmission, Malaysia: Contract Acceptance Specification for On-line Reclamation of 132 kV, 230 kV and 400kV Transformers – Color max 2.5

2. PLN Transmission, Indonesia: Contract Acceptance Specification for On-line Reclamation of 150 kV and 500kV Transformers – Color max 3.0

3. NGCP, Philippines: Contract Acceptance Specification for On-line Reclamation of 240kV Tranformers – Color max 2.0

4. Transpower, New Zealand: Contract Acceptance for Online Reclamation of 230kV Transformers – Not Specified

5. TransGrid, SPI Power Net, ETSA, Powerlink, Australia: Contract Accetance for On-line Reclamation of HV Transmission Transformers – Not Specified

The post Colour in Oil appeared first on ESI.

Transformers are the single most expensive item of high voltage plant in a substation. It is a recognised fact that deterioration of transformer oil, through either oxidation and/or contamination, can cause premature and at times catastrophic failure. Similarly oil will deteriorate at a greater rate as a transformer is operated at progressively higher loads.

Traditionally, transformers have been designed with reasonable overload capacity and conservatively loaded. Recent moves towards corporatisation/privatisation of electrical utilities, and the inevitable economic constraints that go with that, demand tighter design criteria and enhanced performance from network assets. Loading of transformers has increased with an attendant need to endure maximum integrity of performance.

It makes good economic sense, and gives confidence when loadings are increased, to regularly test and maintain transformer oil in good condition.

Sludge Information

The normal aging process of a transformer results in sludge formation. It will appear faster in a heavily loaded, hot running transformer. The formation of sludge can be further hastened by the presence of moisture and free access to oxygen. Sludge builds up in progressive layers with varying degrees of hardness depending on how the unit has been operated.

Acids are first formed in the oil by the oxidation of its components, and the rate of build up of acid formation is an exponential one. After a significantly high concentration of acids has occurred they then attack the iron, copper, varnishes, paints and cellulose insulation. These materials coming into solution combine to form sludge.

Sludge then precipitates out of solution and forms a heavy tarry substance, which adheres to the insulation, the sidewalls of the tank, the core, and lodges in ventilating ducts and cooling fins. Sludges are also formed in the cellulose fibres of the insulating system itself.

Sludges, once deposited on the insulation and formed in the insulation system, immediately go to work to increasingly leech out varnishes and cellulose materials from the insulation, resulting in severe shrinkage of the insulation.

Since formation of sludge is one of the prime causes for premature failure, it was imperative that a solution be found to remedy the problem.

On-Site Reclamation and Transformer Conditioning

Reclamation of transformer oil is energy efficient and less expensive today than refilling an older transformer with new oil. Further, it must be appreciated that simply replacing poor quality oil with newer, or reclaimed oil is not the answer, as the oil will in turn, rapidly deteriorate due to re-contamination by internal components of the transformer. The complete solution is not just simple reclamation of transformer oil – the sludges must be dissolved and removed from the transformer itself.

Regeneration of oil and desludging of transformers has been carried out successfully in overseas countries for many years using a well proven technique of hot oil purging and filtering via Fuller’s Earth adsorbent bed. (Adsorption is the tendency of a liquid, gas or small particle to cling to the surface of another substance by physical rather than chemical means.).

Most of the contaminants in oil, including water, are polar in nature and are therefore easily adsorbed. There are several types of adsorbent material, the most widely used and accepted is Fuller’s Earth, with alternatives such as activated alumina and molecular sieves being less attractive and far more expensive.

Transformer oil itself can be reclaimed in one pass through a Fuller’s Earth bed of proper size and utilisation. Additional passes of hot oil redissolve decay products in and on the transformer insulation, core and coils, thus effectively scrubbing out the vital parts of the transformer.

The number of recirculations through the filtering system to accomplish internal cleaning of a transformer depends on a number of factors, including the:

• Equipment used
• Quality and quantity of Fuller’s Earth
• Condition of the oil
• Age of transformer

It is recommended that hot oil cleaning should occur before an acidity level (Neutalisation Number (NN)) of 0.15mg KOH/gm and an Interfacial Tension (ITF) of 24dynes/cm is reached. These figures represent the maximum oil degradation beyond which the oil is unable to keep the sludges in solution.

When oil test indicate an acidity level greater than 0.3mg KOH/gm and an IFT less than 17.9dynes/cm, complete desludging procedures are required.

It is recommended that the desludging process takes place with the transformer energised – the mechanical vibration helps the hot oil dissolve sludges in the innermost recesses of the insulating system while the heat from the copper and iron losses aids in the general heating of the oil.

Mains Feature of the ESI System

Fuller’s Earth type LVM 60/90 which is commonly used in earlier technology oil reclamation systems, is not suitable for reactivation. Hardened grades such as RMV, Porocel and Tonsil can be reactivated up to 500 times. A number of grades of Fuller’s Earth are mixed in the ESI system to provide a balanced and optimum treatment of oil with respect to acidity correction, discolouration, improvement of dissipation factor and rehabilitation of many other properties.

The Reclamation plants used by ESI are known as mobile generation plants, they have multiple regeneration columns and flow rates of 1 – 6,000 litres/hr (MRP6) and 1 – 2,000 litre/hr (MRP2).

Reclamation, degassing and reactivation operations are all controlled by a programmable Logic Controller and monitored by a computer, which will be located within the plant. This high degree of automation ensures a maximum safety, reliability and simplicity of operation. All vital operating parameters are read, monitored and adjusted according to program, and stored on hard disk for retrieval as and when required.

Oil Quality and Transformer Life Expectation

ESI guarantee of oil quality on completion of reclamation would include :

ACIDITY max 0.03 mgKOH/g

INTERFACIAL TENSION min 40 dynes/cm

DIELECTRIC STRENGTH min 60 KV

MOISTURE max 10ppm

INHIBITOR (DBPC) min 0.25%

New oil contains natural inhibitors, which are consumed during the normal aging process, and as a result reclaimed oil will oxidise, or age, at a faster rate than new oil. For this reason it is extremely important to add an inhibitor (the most common inhibitor being DBPC) to minimise the rate of oxidation of the reclaimed oil.

Independent studies by SD Myers and Hydro-Quebec have shown that optimum time to reclaim and re-inhibit oil is not at the time when the inhibitor has been fully depleted, but prior to the oil showing signs of aging as measured by acidity, IFT and other classic tests.

In summary: it is vital to keep transformers in good condition by maintaining the oil in good condition. This is achieved by carrying out regular condition monitoring tests and where indicated, by regenerating and inhibiting the oil.

Ecology and the Economy

There are a number of very distinct advantages in reclaiming oil with a reactivatable Fuller’s Earth system:

• The residue created in the process of oil reclamation is negligible (less then 0.1%) but is still reuseable. (In the process of reactivation, the contaminants are removed from the Fuller’s Earth and concentrated within a small quantity of transformer oil. This oily residue can be used as feedstock in engine oil re-refining plants).
• Oil loss is minimal compared to previous technologies, less than 0.2% of reclaimed volume.
• Transformer oil is becoming a scarce resource and its’ recycling as opposed to replacement makes sound economic and environmental sense now and in the future.
• Fuller’s Earth is also a valuable resource and reactivation within the columns extends its’ life 500 times
• The new reclamation system is economical to run in comparison to existing types of reclamation.

Protection Against Oil Spills

All ESI plants offer a high degree of reliability with respect to spillage protection during operation. Any oil leakage from various interface devices such as relief valves, de-aerators, vacuum pumps etc, is monitored by level sensing devices and any spillage is contained within the fully bunded plants. If there is an indication of an oil spillage, the plants will shutdown immediately and automatically isolate themselves from the oil supply.

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Once that same oil has been in service for 20 or 30 years then passed through a passive microscopic filtration process to remove all contaminants and many of the same engineers refuse to consider its use. Why? Most common answer is, “we don’t want second hand oil in our equipment”. Surely that is an emotive response more suited to the ‘arts sector’ than practical, fact based engineering types?

What is Oil Refurbishment?

Refurbishment of oil has been carried out using adsorption by Fullers Earth for decades and is often still used as the final stage in oil refining or re-refining. Adsorption is the tendency of a liquid, gas or small particle to cling to the surface of another substance by physical rather than chemical means.

Fullers Earth is a hydrated magnesium and aluminum silicate with a unique crystalline structure. Once activated through high temperature, this clay possesses up to 13 hectares of surface area per kilo. Most of the contaminants found in serviced oil are polar in nature and are therefore easily adsorbed by the Fullers Earth.

When coupled with fine particulate filters (0.3 micron) plus a high vacuum degasser/dryer system, virtually all oxidation by-products can be removed and the oil returned to original, new oil specifications. The Refurbishment process also removes corrosive sulfur and metals from the oil.

Once the natural inhibitors consumed in the oxidation process have been replaced by a synthetic anti-oxidant, usually 2,6 Di-tert-butyl-4-Methylphenol (also called DBPC and BHT), the refurbished oil is often more stable than new oil.

Two major factors involved in the poor performance of New Oil following retro-filling

1. Retro-filling a transformer will not remove sludge deposits from the core, windings, radiators, and floor of the transformer. A thorough flushing can only remove 10 to 15% of the sludge deposits within a transformer. As soon as the transformer is re-filled and energized, the sludge deposits begin to contaminate the new oil and degradation occurs very quickly.

2. Even in sludge-free transformers, the cellulose insulation and spacers retain approximately 10% of the total oil volume within the unit. This cannot be drained out and will begin to contaminate the new oil as soon as the unit is re-filled.

In-situ Oil Refurbishment gives a vastly superior performance mainly because during refurbishment, hot clean oil is circulated through the transformer, between 6 and 16 passes, depending on the severity of the sludging. The clean oil is returned to the top of the transformer above the aniline point for transformer oil, which is the point at which the oil will dissolve its own oxidation by-products. The dissolved sludge is then drawn into the refurbishment plant, via the main tank drain valve, and removed by the activated Fullers Earth filter media. This not only removes surface contamination but begins to clean deposits embedded in the cellulose, particularly the outer insulation layers.

Typically a Fullers Earth Refurbishment plant will produce twice the daily volume of refurbished oil in a ‘tank to tank’ situation (where input and finished product oil specifications are similar) compared to in-situ transformer work. This is due to the extra time and adsorption required to dissolve and remove contaminates from within the transformer.

A decrease in Transformer Top Oil Temperatures of 8 degreeC has been observed in transformers following in-situ oil refurbishment due to the removal of sludge deposits. I suggest this is mainly due to the removal of sludge in the radiators. The actual decrease in ‘Hot Spot Winding Temperature’ is likely to be closer to twice this value as similar sludge deposits also blanket the core and block cooling ducts. A reduction in operating temperature of 8 degreeC will double the life expectancy of a transformer.

Is ‘Super Refining’ really so Super?

Over the last 20 years almost all the transformer oil brought into Australia and New Zealand has come from the same Venezuela crude oil base. Notable exceptions were the High voltage DC link Transformers New Zealand inter-island and Australia Bass link.

When the Venezuela crude was first introduced the most notable difference was the higher aromatic content compared with previous oils. Whether this is totally because of the base oil or the newer refining techniques (Hydrogenation) is unclear, but Transpower (NZ) had to adjust their new oil acceptance criteria, which specified a maximum of 10% aromatics, to cater for the new oil which had a typical aromatic content of 14%. From a service provider’s point of view, the biggest difference we noticed was that, even with very low moisture content, this oil produced a lot more foam in the high vacuum degassing chamber during processing. Our conclusion was that the oil contained a lot more light ends which boiled off under vacuum.

Hydrogenation, as used in modern refining and re-refining plants, uses hydrogen on a catalytic surface to chemically convert unwanted molecules into desired ones. The severity, temperature, pressure and velocity can all be controlled to produce the desired output (5). These parameters must be set by skilled chemists who have a full understanding of both the input oil and the service requirements of the product. Once the parameters are correctly established, in large scale plants with constant base oil as the input, the product from there-on should only vary slightly over time. In smaller plants, with variable input oil specifications, I suspect consistency in output very much depends on the skill and knowledge of the operators and chemists involved.

Refurbishment plants using Fullers Earth, do not have this problem, the refurbishment process simply extracts unwanted compounds, rather than attempting to convert them into desired ones. The worst that can happen is that a small percentage of the un-wanted compounds remain in the oil after processing. This process is therefore much more suited to field use or where the input stock is variable.

What about chemical reactions within the transformer?

Some have described an energized transformer as a huge chemical reactor. Not only is Iron, Copper, Paper, Solvents, Moisture, Oxygen and other compounds linked by fluid (Oil), electrical stress, leakage currents and magnetic fields are there to mix it all up.

In recent times a number of failures have occurred in transformers due to corrosive sulphur reacting with copper conductors, forming metal sulfides in the paper insulation. Since the metal sulfides are conductive, the dielectric breakdown strength of the paper is reduced leading to breakdown between conductor strands on a disk or between disks. This has ultimately caused the failure of some major assets including a 500 kV shunt reactor and a 450 MVA auto transformer (6). The following graph illustrates the effect of adding paper covered and bare copper to an oil oxidation test.

The amount of Sulphur present in transformer oil depends on the original crude oil used and on the degree and method of refinement. This Sulphur is normally stable and actually improves the oxidation stability of the oil. It appears however that under high levels of stress, high temperature plus electrical stress, the sulphur can become corrosive and lead to chemical reaction with copper described.

What are Passivators, do they help?

Metal Passivators are a compound which can chemically react with the surface of a metal forming a microscopic

protective coating against catalytic reaction. They are not new and in fact passivated transformer oil was specified in transformer oil from 1955 by Shortland County Council, NSW, following successful trials on a number of small transformers with exposed copper conductors (8).

With the recent problem of corrosive sulphur and metal sulfides, some transformer manufacturers and oil companies

are recommending their use in ‘at risk’ transformers. The affected (or at risk) units reported so far are Reactors, HVDC and Step up transformers working near to rated load or overloaded and/or at high ambient temperatures. Most of these units have been fitted with rubber bag type conservators and were filled with non inhibited or partly inhibited oils

The passivator recommended by most authorities today (Siemens and Nynas included) is Irgamet 39, added at aconcentration of 100ppm. This passivator can be added to the oil during hot oil filtration or the final stages of the oil refurbishment process. As shown in Table 1. Refurbishment by Fullers Earth can remove corrosive sulphur from the transformer oil but it can be expected some will leach out of the oil impregnated cellulose, reaching a point of equilibrium, after 6 – 9 months.

Conclusion

Refurbished transformer oil is generally oil that is tens of thousands of years old, plus 20 or 30 years of service. In Australia and New Zealand, this oil is almost always from excellent base stock and which had oxidation compounds plus particulates, gas and moisture removed in the refurbishment process.

The suitability and stability of this oil has been well proven in field use for well over 50 years and has been purchased by a number of Utilities and Transformer manufacturers as an alternative to purchasing ‘new’ oil. Most Transformer Manufacturers will honour new transformer warranty obligations where refurbished oil is requested by Clients, as long as the oil meets agreed specifications.

In existing transformers, the performance of this oil, following in-situ refurbishment, is clearly superior to simply refilling with new or re-refined oil. Refurbishment is normally carried out with the transformer on-line and therefore there is no down-time, switching or load shedding required.

An added advantage of in-situ refurbishment is the removal of contaminants such as sludge and moisture from within the transformer, thus extending the life and reliability of the serviced units. All this at a lower cost than the price of new oil.

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