Definitions

 

Degradation (of performance) (from IEC Electropedia)

An undesired departure in the operational performance of any device, equipment or system from its intended performance

Note – The term “degradation” can apply to temporary or permanent failure.IEV ref.161-01-19 [source]

 

 

Mineral insulating oil - mineral oil, natural esters - natural ester, synthetic organic ester - synthetic ester

Mineral insulating oil

insulating liquid derived from petroleum crudes
Note – Petroleum crude is a complex mixture of hydrocarbons with small amounts of other natural chemical substances.
IEV ref.212-17-02 [source]

Natural esters (from IEC 62770)
vegetable oils obtained from seeds and oils obtained from other suitable biological materials and comprised of triglycerides
IEC 62770, ed.1.0 (2013-11)

Synthetic organic ester (from IEC Electropedia)
insulating liquid produced from acids and alcohols by chemical reaction
Note – These esters include mono-, di- and polyol-esters.
IEV ref.212-17-08 [source]

 

 

Reclaiming - regeneration, reconditioning - physical treatment

Reclaiming (from IEC Glossary)
Elimination of soluble and insoluble contaminants from an insulating liquid or gas by chemical adsorption means, in addition to mechanical means, in order to restore properties as close as possible to the original values ​​or to the levels proposed in this standard
Published in:IEC 60480, ed.2.0 (2004-10) – Reference number:3.3.5 – Source:IEV 212-09-05 (modified) [source]

Reconditioning (from IEC Glossary)
Process that eliminates or reduces gases, water and solid particles and contaminants by physical processing only
Published in:IEC 60422, ed.4.0 (2013-01) – Reference number:3.5 [source]

Depolarisation (from IEC Glossary)
Process of removing electrical polarisation from an electrical insulating material until the depolarisation current is negligible
NOTE Depolarisation is generally recommended before measuring the resistive properties of an electrical insulating material.
Published in:IEC 62631-1, ed.1.0 (2011-04) – Reference number:3.12 [source]

 

 

Introduction

 

Properties and deterioration/degradation of oil

(Sea Marconi translation of Chap. 4 of IEC 60422 Ed.4-2013

The performance of a mineral oil in an insulation system depends on some basic oil characteristics that can affect the overall performance of the electrical equipment.In order to satisfy its multiple roles of dielectric, cooling liquid and arc extinction capabilities, oil must possess certain properties, in particular:

• high dielectric strength to withstand the electrical stresses imposed by operatingn
• Viscosity sufficiently low so that its ability to circulate and transfer heat is not compromised
• Appropriate low temperature properties up to the lowest temperature provided for at the installation site
• Oxidation resistance to maximise useful life

Mineral oil degrades in service due to conditions of use.In many applications, the insulating oil is in contact with air and is subject to oxidation.High temperatures accelerate degradation.The presence of metals, organo-metallic compounds or both can act as oxidation catalysts.Oil colour changes, formation of acidic compounds and, at an advanced oxidation stage, the occurrence of sludge may occur.Dielectric properties and, in extreme cases, thermal properties can be compromised.

During service, in addition to oxidation products, many other undesirable contaminants such as water, solid particles and polar compounds soluble in oil can accumulate in the oil and affect its electrical properties.The presence of such contaminants and any other oil degradation product is indicated by a change of one or more properties as described in Table 1.

 

 

Mineral insulating oil is a mixture of predominantly paraffinic, naphthenic or aromatic hydrocarbons, mainly resulting from the distillation and refining of crude oil.The obtained oil can then be added and/or blended (mixed) for specific applications.
In several countries, it is a common operating practice for oil recovery and recycling.In order to meet this regulatory requirement, IEC formalised the IEC 62701 standard, cancelled in February 2015 and subsequently reconsidered (January 2016) in terms of merging with standard 60296 ed.4 by decision of the IEC Standardisation Management Board (SMB).According to these guidelines, new mineral-based insulating oils will not only be those classified as "virgin" but also those classified as "recycled".

In this scenario, the management of the life cycle of oils with different characteristics creates much more complex situations (e.g. in the phases of formalisation of purchase and acceptance requirements of supplies).

Degradation processes occur during the various phases of the life cycle of the transformer and are distinguished by "degraded electrical insulation of oil" and "chemical degradation of oil" (see differences below):

 

| Degraded electrical insulation of oil | Chemical degradation of oil

Causes | Physical contaminants such as water, particles, dissolved gases resulting from external sources (atmospheric) or internal sources (degradation of materials through thermal or electrical stress) | Oxidation polar compounds (sludge), and consumption of additives (if present) resulting from degradation of internal materials through thermal or electrical stress

Effects | Reduction of the discharge voltage | Reduction of chemical-physical properties such as total acidity, dielectric dissipation factor, interfacial tension, resistivity, concentration of additives and oxidation stability

Countermeasures | Physical treatment (microfiltration, dehumidification and vacuum degassing) or selective adsorption in the case of water in oil | Selective reclamation or depolarisation treatments by Sea Marconi (also effective against corrosive and acidic compounds)

 

The mechanisms of chemical degradation of oil are the result of complex reactions (e.g. catalytic oxidation) between the organic substances of the oil and the materials contained within the transformer.The magnitude of the phenomenon depends on some peculiar characteristics such as type of insulating liquid, type of equipment (power, voltage, etc.), its load profile (percentage of nominal load, duration in hours), conditions of environmental severity in which it operates and supervision and maintenance policies.
Degradation is mainly characterised by variation in time (pejoratively) of some specific properties (symptomatic indicators) compared with the values ​​of the same indicators for a new (unused) insulating liquid.The evolution of oil degradation processes acts to accelerate the ageing process of solid insulators (papers).

 

 

List of major Sea Marconi publications on the subject:

• J. Diana, V. Tumiatti, G. Camino – “Diagnostic testing of oil samples and interpretation of results” – Proceedings of the Conference – Power Transformer Maintenance – Faculty of Engineering – University of Pretoria – R.S.Africa, 26-27 May 1998.
• V. Tumiatti - "Analysis of technical fluids as a diagnostic instrument of degradation for effective prevention of failures" - Seminar of the International Research Institute on Production Maintenance for Hydraulic Systems and Lubrication Systems, Milan, 25-26 November 1998.
• V. Tumiatti, R. Actis, A. Armandi, G. Di Iorio, G. Camino – “Diagnostic testing of oil samples on electrical transformers” – (to be presented for SMI'99 – 3rd International Conference on Industrial Plant Maintenance , Bologna, 17-20 February 1999).
• S. Kapila, P. Nam, V. Tumiatti, A. Armandi “Evaluation of Analysis Techniques for Fingerprinting Mineral Transformer Oil” CIGREWG15.01.TF06 – Leatherhead (UK) 13.01.1999.
• M. Pompili, F. Scatiggio, V. Tumiatti.(2009).Insulating liquids: new perspectives and regulatory evolution.U & C. Unification and Certification, Vol. LIV.; p. 41-44, ISSN:0394-9605

 

 

Regulatory framework

– IEC 60296:2012, Fluids for electrotechnical applications – Unused mineral insulating oils for transformers and switchgear
– IEC 60422:2013, Mineral insulating oils in electrical equipment – Supervision and maintenance guidance
– IEC 60944:1988, Guide for the maintenance of silicone transformer liquids
– IEC 60666:2010, Detection and determination of specified additives in mineral insulating oils
– IEC 61203:1992, Synthetic organic esters for electrical purposes – Guide for maintenance of transformer esters in equipment
– CIGRE Brochure 413:2010, Insulating Oil Regeneration and Dehalogenation

Given the predominant spread on the market of mineral insulating oils, the reference standards are IEC 60296 and IEC 60422; the former for new mineral insulating oils, the latter for mineral insulating oils in operation.An oil is considered new (unused) as long as it is outside the transformer (or other electrical equipment), i.e. in drums or tanks; in this case, IEC 60296 applies.When the oil is inside the transformer, the liquid is defined as used, in operation, and IEC 60422 applies.

 

 

Causes

The "Chemical degradation of oil" criticality is caused mainly by the mechanisms of normal ageing and thermal stress of oil, and secondly by problems of cross-contamination and the use of improper practices in the handling of insulating liquids and the transformer.The aforementioned improper practices have an impact on the life cycle of the electrical equipment with insulating fluids:

 

Causes in relation to life cycle phases

Causes of the "PCBs in oil and transformer" criticality | When it may occur (life cycle phases)

Deficiency of requirements for purchase of new insulating liquid (seeIEC 60296) | Requirements and purchase

Deficiencies in quality control for individual lots or individual supplies of insulating oil | Acceptance of insulating oils

Deficiency in analytical procedures for checking chemical degradation of oil | Oil acceptance, factory test, installation and pre-energisation, operation, old age, post-mortem

Cross-contamination through the use of oil, plants, tanks or containers contaminated by oxidised, polar and/or incompatible compounds (for topping up, impregnating or filling) | Transformer construction, factory test, installation and pre-energisation, exercise, old age

Recycling of oil and other materials contaminated by oxidation products or polar compounds | Post-mortem

 

 

 

Signs (visual inspection) – Symptoms (analysis)

Signs (visual inspection)

[ALT img:Chemical degradation of oil]

Oil affected by "chemical degradation of oil" appears dark in color and sometimes with a pungent odour.The appearance may show signs of sediment or particulate matter.
In the case of internal inspection of the transformer, there are deposits of sludge at the bottom of the transformer casing, on insulating papers and in oil circulation ducts used for cooling the windings and the papers themselves.

 

Representative sampling

During external inspection of the transformer it is necessary to take representative samples of insulating oil in accordance with the reference standard and the operating instructions attached to the sampling kits (read more).

 

Symptoms (analysis)

The specific symptom of the "chemical degradation of oil" criticality is related to the presence of the following diagnostic indicators with typical values that do not conform with those recommended by IEC 60422:

• Water in oil (IEC 60814)
• Appearance (ISO 2049)
• Colour (ISO 2049)
• Total acidity – TAN (IEC 62021-1 or IEC 62021-2)
• Dielectric dissipation factor (IEC 60247)
• Particles (IEC 60970)

 

There are other co-factors useful for completing the diagnostic picture:

 

• Interfacial tension (ASTM D971, EN 14210)
• Additives:Passivators (BTA, Irgamet 39, Irgament 30); oxidation inhibitors (DBPC, DBP)
• DBDS (IEC 62697-1)
• Dissolved metals (ASTM D 7151)
• Oxidation stability (IEC 61125)
• Sediments and sludge (Annex C of IEC 60422 Ed.4-2013
• Oil fingerprint
• Copper deposition tendency test

 

Sea Marconi test reports are compliant (EN ISO/IEC 17025) concerning the indication of measurement uncertainty (except for the aspect that is not a numerical test, and for the ISO particle code).

 

In natural ester oils, additives can be up to 5% (0.3% in mineral oils); therefore their degradation by-products are decisive indicators.

 

 

Through oil analysis

It is possible to assess the state of chemical degradation
Contact us

 

 

Acidity analysis in the field using the SM-TAN Kit (Total Acidity Number)

[ALT img:Chemical degradation of oil | SM TAN kit case]

The kit makes it possible to obtain the result in very few minutes, with high repeatability and reproducibility; it can also be used on particularly dark specimens and is suitable for the analysis of liquid matrices.

The test can be performed by non-specialised personnel thanks to the illustrated support manual, tutorial videos and Sea Marconi's support service (read more).

 

 

Diagnosis

For diagnosis of the "Chemical degradation of oil" criticality, Sea Marconi uses its own diagnostic metrics, namely:

• visual signs on the transformer (and those from any internal inspection) are interpreted;
• through analysis of oil the symptoms are identified, i.e. the symptomatic indicators (aspect, colour, TAN, dielectric dissipation factor, particulate matter) and their characteristic values.

 

The limits given in IEC 60422 are intended as "recommended"; as occurs for dissolved gas limits, it is advisable to determine typical values of warning and alarm on a statistical basis divided by families (of equipment and oil) regarding one's machine pool.

 

• the database is used to study family or subjective case histories (in the search, for example, for failures in twin machines);
• factors of uncertainty, speed and evolution over time (trends) of symptomatic indicators are taken into consideration and monitored during the life cycle phases;
• on the basis of assessment of these key factors, the specific criticality is classified according to type and priority, and type and priority of corrective actions are identified at the same time.

 

 

Real example

Cat A transformer (see Table 2 IEC 60422), GSU ​​elevator type generation (breathing with conservator and silica gel)
Voltage:400 kV, Power:250 MVA
50,000 Kg of non-inhibited paraffin-based mineral oil
Total acidity of 0.25 mg KOH/g ("poor" value compared with Table 5 IEC 60422),
Dielectric dissipation factor = 0.27 ("poor" value compared with Table 5 IEC 60422)
Interfacial tension = 20 mN/m ("poor" value compared with Table 5 IEC 60422)
Dissolved copper = 0.97 mg/kg ("poor" value compared with Table 5 IEC 60422)
Colour = 6 dark ("poor" value compared with Table 5 IEC 60422)
Paper weight = 2,500 kg

Through the test of oxidation stability (IEC 61125:1992) it is possible to measure an amount of sludge equal to 0.2% in mass (of oil) which, on 50.000 kg of oil, means having about 100 kg of sludge resulting from degradation of the oil itself.

Impregnating oil cannot be drained completely; typically, 10-15% remains inside the transformer, absorbed from the papers, and in the interstices and dead spots of the machine.This means that in case of an oil change, the new filling oil would be contaminated by old undrained oil.

 

 

Prevention

1.It is recommended that strategic information be updated through a "dynamic inventory" of oils and transformers, indicating the values ​​of symptomatic markers.

2.It is recommended that maintenance practices be changed:

buying new mineral oils compliant with IEC 60296, or natural ester oils compliant with IEC 62770, or synthetic ester oils compliant with IEC 61099 or silicone oils compliant with IEC 60836.It is advisable to select the oils by comparing products for the intended applications (contact us for support)
checking the oils in the acceptance phase of supplies according to the prescribed methods
requiring that the factory tests are conducted using oils free of degradation compounds and monitoring, also in subsequent phases of the life cycle, that the treatments on the oil and transformer are not a source of cross-contamination

3.From an asset management point of view it is recommended acting preventively with a depolarisation treatment against acidity and not wait for the oil to reach the thresholds specified in IEC 60422 (critical acidity if > 0.15, > 0.20, > 0.30 mgKOH/g depending on the different categories of transformers); in fact, an acidity between 0.07 and 0.10 mgKOH/g has already shown phenomena of corrosion by dissolved metals (C4), dangerous sludge formations with negative effects on insulating papers.

 

What are the prevention actions to be taken on electrical equipment with insulating liquids other than mineral ones?
Concerning natural ester oils and synthetic esters, the prevention actions are the same, but it is advisable to choose countermeasures after careful assessment of cost-benefit, cost-effectiveness and environmental impact (biodegradability and fire safety).For silicone oils in operation, the treatments recommended by the standard (IEC 60944:1988) are "vacuum treatment and filtration" and "molecular sieves and filtration".

 

 

Treatments

 

Open

– type, size, and total mass of electrical equipment;

– installation of electrical equipment;
– financial value of the electrical equipment and cost of decontamination/disposal;
– type and amount of insulating liquid;
– concentration of PCBs in electrical equipment;
– state of degradation and effects on the functionality of the electric equipment;
– possible coincidence between decontamination activities and other maintenance activities;
– environmental impact associated with possible failures and consequent losses of contaminated oil.

The following are countermeasures for the "Chemical degradation of oil" criticality, a result of the recommendations of IEC 60422 (Table 5, page 31) improved according to state of the art and the use of BAT and BEP:

Monitoring symptomatic indicators (see symptoms above).If the first symptoms of criticality (such as a high rate of paper ageing on a transformer with less than 10 years of life) occur, it can be scientifically predicted that the machine will have a cycle of life that is much lower than that expected, and therefore it is appropriate to plan a profound revision of the transformer or, more likely, its replacement in the next 3/5 years.In this condition it is recommended that the frequency of symptomatic analysis be increased in order to monitor trends.

Apply appropriate oil treatments in order to reduce critical factors and in particular to keep the moisture in solid insulators (as well as acidity, oxygen and sludge) low and reduce any catalysing effects such as metals in the oil.

Suggested actions include:

 

 

Depolarisation by Sea Marconi

[ALT img:Chemical degradation of oil | MDU 3 modules]

It is a process performed on site, keeping the transformer in service (and under load) without having to empty it.The operation is carried out using a Modular Decontamination Unit (MDU) specifically created by Sea Marconi.The transformer is connected to the DMU by flexible hoses; the oil contaminated with DBDS is sucked from the lower part of the transformer and transferred into the DMU, which heats it, filters it, degasses it, dehumidifies and depolarises it before pumping it back into the upper part of the transformer.This creates a closed loop and every time the oil is circulated the degradation compounds are removed and at the same time the oil returns to optimal conditions.(read more)
For example, IEC 60422 considers the acidity parameter critical if > 0.15, > 0.20, > 0.30 mgKOH/g depending on the different categories of transformers.However, acidity ranging from 0.07 to 0.10 mg KOH/g has already shown phenomena of corrosion by dissolved metals (C4) and dangerous sludge formations.It would thus be advisable to intervene with a depolarisation treatment before the oil reaches the indicated acidity thresholds and which contributes to the reduction in the thermal life of the insulating papers.

 

 

Regeneration through percolation

This countermeasure is described in IEC 60422 para.11.3.2.This is a physical-chemical process that eliminates or reduces soluble and insoluble polar contaminants from oil.
The process involves three phases:
1) the oil extracted from the bottom of the transformer is heated and circulated through a filter to remove particles.
2) the oil is then circulated through one or more cartridges containing fuller's earth or other material suitable for removal of soluble polar contaminants.
3) the oil is finally circulated through an oil treatment system (vacuum or centrifugal dehydration) to remove water and gases.

 

This treatment is not effective for certain species of organic compounds, nor for PCBs, nor for the corrosive sulfur compounds, which require specific chemical reactions (e.g. hydrogenation) to be removed.Also, when treatment involves the reactivation of fuller's earth, the "corrosive sulfur from sulfur combustion by-products (C3)" criticality may occur.

 

 

Oil change

It is worth bearing in mind that, despite changing the oil, 10-15% of the old contaminated oil remains impregnated, i.e. absorbed, in the transformer papers, which release it over time (the time it takes to reach equilibrium is about 90 days).The old oil thus contaminates the new oil, and consequently it is impossible to completely remove the contaminants with a single oil change.In addition, from a technical-operational point of view, this activity is delicate and complex.On the basis of the class of transformer, oil change requires numerous steps (emptying, vacuum application, filling, treatment of new oil), each of which requires appropriate technologies and staff with specific skills.If, for example, the vacuum applied to the transformer is not properly created, filling may cause air bubbles that cause partial discharges.In addition to this, it is worth bearing in mind that the oil change produces enormous amounts of hazardous waste to be disposed of (the replaced oil) (read more).

Assess any criticalities linked to compatibility/miscibility resulting from the use of liquids other than those of the original impregnation

 

Assess treatments in terms of mass balance, energy balance, emissions balance, cost-benefit, cost-effectiveness in the given time.

 

What are the prevention actions to be taken on electrical equipment with insulating liquids other than mineral ones?
Concerning natural ester oils and synthetic esters, the prevention actions are the same, but it is advisable to choose countermeasures after careful assessment of cost-benefit, cost-effectiveness and environmental impact (biodegradability and fire safety).For silicone oils in operation, the treatments recommended by the standard (IEC 60944:1988) are "vacuum treatment and filtration" and "molecular sieves and filtration".

 

 

Warnings

• Oil sampling must be carried out by qualified professionals according to procedures
• laboratory analyses must be carried out using methods set by reference standards, as guaranteed by accredited laboratories
• the countermeasures to the "Chemical degradation of oil" criticality, in this case oil changes and depolarisation of the transformer must be carried out
  - using technologies that are safe and fit to meet the requirements of BAT and BEP
  - using staff with specific skills and trainin
  - relying on operators able to demonstrate an extensive application case history and able to certify interventions carried out with quality assurance (ISO 9001)