Definitions

 

Sludge (from IEC Electropedia)

Mixture of insoluble degradation products which are formed in an insulating liquid as a result of ageing
IEV ref. 212-18-17 [source]

 

Total sludge

Sum of the sludge resulting from the degradation of the oil (insulating liquid) and the papers (solid insulators)

 

Paper, kraft paper, insulating paper, cardboard/precompressed cardboard

Paper (from IEC Electropedia)

cellulosic paper of certain types, frequently characterised by their relatively high rigidity
Note – In general the term paper is used for cellulosic papers if not otherwise specified.[source]

Kraft paper (from Wikipedia)

Electrical insulation paper (from Wikipedia)

(Paper) board - cardboard, precompressed cardboard (from IEC Electropedia)
Generic term applied to certain types of cellulosic paper, often characterised by their relatively high rigidity
Note – For some purposes, materials of grammage (mass in grammes per square metre surface area) less than 225 g/m2 are considered to be paper, and materials of grammage of 225 g/m2 or above are considered to be board.[source]

 

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 the 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]

 

Desludging

Process of removing insoluble sediments and deposits from the transformer

 

 

Introduction

In electrical transformers, insulation is mainly ensured by the joint use of solid material (kraft paper) and insulating liquids (especially mineral oils) in intimate contact with each other.

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.
Sea Marconi translation of Chap. 4 (Properties and deterioration/degradation of oil) of IEC 60422 Ed.4-2013.

Like oil, paper is also subject to degradation; its degradation products are solid, liquid and volatile (gas) compounds, some soluble in oil (furans, methanol, ethanol), others insoluble in oil (particles, sludge).Sludge derived from oil and that derived from paper add up to form the total sludge.

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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 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
• IEC TR 62874:2015, “Guidance on the interpretation of carbon dioxide and 2-furfuraldehyde as markers of paper thermal degradation in insulating mineral oil”
• CIGRE Technical Brochure 227, 2003 “Life Management Techniques for Power Transformer”
• CIGRE Brochure 323, 2007 "Ageing of Cellulose in Mineral Oil Insulated Transformers"

 

 

Causes

“Insoluble deposits (sludge)” criticality is mainly caused by mechanisms of normal ageing and thermal stress of both the oil and the papers.Concauses also include problems of cross-contamination due to improper handling of solvents and transformers.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 "Insoluble deposits (sludge)" criticality | When it may occur (life cycle phases)

Lack of purchase requirements for new insulating liquid (check oxidation stability according to IEC 60296) | Requirements and purchase

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

Deficiency in analytical procedures for checking chemical degradation of oil and degradation of paper | 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, operation, 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)

The direct visual signs of this criticality are only highlighted by internal inspection of the transformer.In case of failure (or end-of-life) of twin machines, for example, it is good practice to perform a total sludge diagnostic (through sampling, analysis and interpretation) in order to declare the criticality and classify comparative references.In the presence of "insoluble deposits (sludge)", the following are observed:
- Insoluble deposits(e.g. sludge or copper sulfide) on insulating papers and at the bottom of the casing
- Obstructions of oil circulation ducts used for cooling windings and papers.

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 morei ).

Symptoms (analysis)

Due to intimate contact with the papers, insulating oil becomes a vector of symptomatic indicators of the criticality.Through oil analysis it is therefore possible to identify and quantify the total sludge.
The specific symptom of the "Insoluble deposits (sludge)" criticality is related to the presence in oil of the following diagnostic indicators with non-conforming typical values:

 

Sludge indicators derived from degradation of oil

Sediments
and sludge (Annex C of IEC 60422 Ed.4-2013)
Water in oil (IEC 60814)
TAN Acidity (IEC 62021-1)
Dissipation factor (IEC 60247)
Interfacial tension (ASTM D971, EN 14210)
Particles (IEC 60970)

Sludge indicators derived from degradation of paper

Oxygen
CO2 – carbon dioxide
CO – carbon oxide
2FAL – 2furaldehyde and other furan compounds
Methanol
Ethanol
Symptomatic hot spot gases (methane, ethane, ethylene)

There are also co-factors useful to complete the diagnostic framework (resulting from the oil analysis):

Additives:Passivators (BTA, Irgamet 39, Irgamet 30); Oxidation inhibitors (DBPC, DBP)
DBDS (IEC 62697-1)
Dissolved metals (ASTM D 7151)
Oxidation stability (IEC 61125)
Oil fingerprint

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

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).

 

Using oil analysis
it is possible to estimate the amount of total sludge in the transformer
Contact us

 

 

Diagnosis

For the diagnosis of the "Insoluble deposits (sludge)" criticality, Sea Marconi employs its own diagnostic metrics, namely:

• visual signs on the transformer (and those from any internal inspection) are interpreted;
• through analysis of oil, symptomatic indicators and their characteristic values are identified;

 

The limits of the "sediments and sludge" properties indicated in IEC 60422, i.e. 0.02%, are to be understood as "recommended"

 

• 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.

 

Changing the insulating fluid changes the diagnostic assessments of degradation processes.In natural esters, for example, additives can be up to 5% in mass(0.3% in mineral oils); therefore their degradation by-products are decisive indicators.

 

 

Real example

Cat A transformer (see Table 2 IEC 60422), GSU ​​elevator type generation (breathing with conservator and silica gel)
Year of construction:1978
Voltage:400 kV, Power:250 MVA
Total transformer mass:220,000 kg
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)

The indicators that identify degradation of the papers are added to those of the aforementioned degradation of oil

CO2 => 16,500 μl/l (higher than the typical value, "high ageing rate" 98 percentile)
2FAL = > 6.5 mg/kg (higher than the typical value, "high ageing rate" 98 percentile)
Methanol = 1200 μg/kg (higher than the typical value of the transformer family)
Ethanol = 300 μg/kg (higher than the typical value of the transformer family)

The DP of this transformer has decreased in 35 years from 1000 to 200, understood as a mean value, which conventionally corresponds to the end of thermal life.At the same time, a loss of paper mass of 25% is estimated; in fact, its weight decreases from the initial 2,500 kg to 1,875 kg.
The insulating papers are impregnated with non-inhibited paraffinic oil.After the impregnation cycle (typically under vacuum, 60-80 °C, and at least 72 hours), up to 150-180% of the initial mass of kraft paper becomes impregnated with oil, with a weight range between 2,812 kg and 3,375 kg (compared with 1,875 kg dry).

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.

625 kg (paper weight loss) – 103,125 kg – estimated volatile part equivalent to that of water = approximately 418 kg, which is the sludge resulting from paper.

Through the "sediment and sludge" test on oil it is possible to measure a quantity of sludge equal to 0.2% by mass (of oil) which means that 50,000 kg of oil would give about 100 kg of sludge resulting from degradation of oil.

The total sludge is therefore given by the sum of sludge resulting from paper (418 kg) and that resulting from oil (100 kg), that is to say approximately 518 kg, which settles unevenly on the bottom of the casing, on solid insulating parts and on oil circulation ducts.It is experimentally demonstrated that sludge deposits reduce the cooling capacity of the windings, increasing local temperature by up to 6-8° C and thus doubling the speed of deterioration of the papers according to the well-known Arrhenius equation.This criticality has a significant impact on reduction of the residual thermal life of the transformer and requires timely and effective prevention and risk mitigation actions.

By analogy, this is what happens to people: the accumulation of fat in certain parts in the circulatory system leads to a reduced quality of life of the subject and a reduction in life expectation.

 

 

Prevention

"Insoluble deposits (sludge)" are the result of an extremely complex and prolonged phenomenon that develops during the life cycle of the oil and of the transformer.Deposits and sludge resulting from oil and from papers cannot be removed by the normal means of oil treatment (physical treatment, regeneration, etc.) listed in Table 5 of IEC 60422.

It is like deposits of fat in veins that cannot be removed by simple dialysis.

Nevertheless, the formation of deposits (sludge) can be prevented or mitigated through appropriate operational practices, (e.g. analytical control of the oil, treatment of oil and papers, load profile management, cooling of the machine).
If the transformer belongs to a family of equipment affected by the same criticality, ad hoc maintenance practices can be defined for optimising the various critical factors (metaphorically, it is like suggesting a personalised diet and increased physical activity to a person suffering from diabetes).

 

Prevention actions during the life cycle of the transformer

• Monitoring symptomatic indicators (see symptoms above).If the first symptoms of criticality appear, it is recommended increasing the frequency of symptomatic analysis of indicators 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.

 

The suggested treatments include:

 

Depolarisation

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.

 

Physical treatment

This 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 and dehumidifies it before pumping it back into the upper part of the transformer.This creates a closed loop which, every time the oil is circulated, is able to restore the values ​​of the main physical parameters of the oil (water, gas, particles).(read more)

 

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 the elimination of soluble polar contaminants. 3) The oil is then circulated through an oil treatment system (dehydration under vacuum or centrifuge) to remove water and gases.
This treatment is not effective for certain species of organic compounds that require specific chemical reactions (e.g. hydrogenation) in order 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

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 products of oxidation or polar compounds with a single oil change..(read more)

Impregnating oil cannot be fully drained (typically 10-15% remains inside the transformer, about 6-7% absorbed by the paper, and in the interstices and dead points of the machine); consequently, in the case of an oil change, the new filling oil is contaminated by the old undrained oil

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

 

 

• It is also recommended to change maintenance practices as regards:
  -the drafting of purchase requirements for both oils and electrical equipment for specific applications with a focus on design/sizing criteria
  - the acceptance of oils and equipment employing the best practices of supervision and control, in accordance with prescribed methods.Ask the supplier for a certificate of compliance of the properties of oil and insulating papers
• It is recommended updating strategic information through a "dynamic inventory" of oils and transformers, with indication of symptomatic markers of the "insoluble deposits (sludge)" criticality.

 

In case of failure of a twin transformer, an internal inspection of the transformer being analysed is recommended.In fact, as a result of paper sampling, subsequent laboratory analysis and interpretation of results, it is possible to identify the causes of the failure and prevent the same event on machines of the same family.On the latter, it is also advisable to undertake an in-depth investigation that also includes electrical and thermal tests in order to detect any defects in design or construction of the transformer.

 

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

For defining the priorities of action and choice of countermeasures, it is above all necessary to take the following indicators into account:

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 "Insoluble deposits (sludge)" 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:

 

Transformer desludging

[ALT img:Insoluble deposits (sludge) | MDU 3 modules]

This is the solution proposed and employed by Sea Marconi.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, in combination with a specific setting of transformer oil temperature (> of 76 °C aniline point) it is possible to realize a continuous solubilisation of sludge in the oil and the gradual removal of sludge.
It is recommend performing this activity with the transformer in operation and under full load service by adopting specific transformer operational protocols.(read more)

 

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

 

What are the treatments for electrical equipment with insulating liquids other than mineral ones?
With regard to natural ester oils and synthetic esters the therapies are the same; however, it is recommended choosing countermeasures after careful evaluation in terms 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

• Sampling of oil, and even more so of papers, must be carried out by qualified operators according to protocols and procedures
• laboratory analyses must be carried out using methods set by reference standards, as guaranteed by accredited laboratories
• actions to prevent the "Insoluble deposits (sludge)" criticality, namely oil depolarisation treatments and internal transformer inspections 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 training
  - relying on operators able to demonstrate an extensive application case history and able to certify interventions carried out with quality assurance (ISO 9001)