Definitions

 

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]

Nomex, TUP (thermal upgraded paper)

Nomex (from IEC Electropedia)

TUP (from IEC 62874:2015)
Thermally upgraded kraft paper

 

Degree of polymerisation (of a polymer) (from IEC Electropedia)

the average value of the number of monomeric units in the molecules of a polymer
Note – Different average values (number, mass, or viscometric average) can be determined for the same material.[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]

 

Power transformer (from IEC Electropedia)

A static piece of apparatus with two or more windings which, by electromagnetic induction, transforms a system of alternating voltage and current into another system of voltage and current, usually of different values ​​and at the same frequency for the purpose of transmitting electrical power – IEV ref. 421-01-01 [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

 

Electrical insulation

[ALT img:Degradation of insulating papers]

In electrical transformers, insulation is mainly ensured by the sum of solid materials, such as kraft paper, and insulating fluids, mainly mineral oils.This important innovation was invented and claimed by well-known scientist Nikola Tesla from patent No. 655,838 "Method of Insulating Electric Conductors" of 14 August 1900, "my invention of any kind of fluid capable of meeting the requirements (75 ...) as oil, may be used (130 ...)", and many other patents.

The solid insulator, also called insulating papers or precompressed cardboards, is mainly derived from kraft paper production processes (see definitions for further details).The result is a product that offers surprising properties, both mechanically and electrically.In this sector, kraft paper has found one of its most important applications, especially in the isolation of electrical equipment up to very high voltages.Over time, through the use of specific additives, kraft paper has been improved, especially in its behaviour in relation to the temperature giving rise to Thermal Upgraded Paper (TUP).Products based on synthetic polymers are also available on the market, for example Dupont's Nomex material, a compound with a meta-aramid base.

As for the liquid insulator, in addition to mineral oils, there are also natural esters, synthetic esters, silicone fluids and, in the past, also PCB-based Askarel.

 

 

Kraft paper

The insulating papers are impregnated with oil or other insulating liquids.After the impregnation cycle (typically under vacuum, 60-80 °C, and at least 72 hours), the kraft paper manages to become impregnated with oil at up to 150-180% of its initial mass.
Kraft paper covers copper or aluminium conductors in order to insulate them electrically, and is thus exposed to thermal, electrical and mechanical stress.
The main property of the paper is the DP (IEC 450:1974) degree of polymerisation.This parameter characterises the material properties that are primarily: tensile strength, elongation, bending strength, tensile modulus, loss factor, specific resistivity.A typical new Kraft paper has a DP between 1000 and 1500.
During the real life cycle of the transformer, the DP progressively decreases until reaching the value of about 200 (reduction of 80% over the new) which conventionally matches the end of"thermal" life.In this condition, the paper loses its mechanical properties but not its electrical properties, which instead remain suitable for ensuring the required insulation.

[ALT img:Degradation of insulating papers | heart]

Electrical insulation can be considered the heart of the transformer; if it fails, the direct consequence is electrical failure (metaphorically like a heart attack).In the presence of strong electrical power arcs, the failure can trigger the insulating oil, which is combustible, causing transformer explosions and fires, and possible major accidents.

 

 

Thermal life of papers

Simplifying greatly, we could say that the thermal life of solid insulators (based on Kraft paper without specific anti-ageing additives) is estimated at about 160,000 hours at the nominal load of the transformer.
Specifically, for an ​​elevator type generation transformer (GSU) of a thermal power plant with an operational availability of 7500 hours/year and an average load profile of 80%, in the absence of specific criticalities, a conventional thermal life is estimated at about 25 years.For the same transformer, but installed in a hydroelectric plant, and thus with an average 40% load profile (seasonality of water), in the absence of specific critical issues, a conventional thermal life is estimated at about 50 years.On the other hand, shunt reactors are sized to operate intensively at values close to the nominal load and thus with an expected lower thermal life.

The operational life of the transformer not depends only on the thermal life of the papers but also on other co-factors such as electrical defects which, evolving into electrical failures, interrupt the operational availability of the machine.In the presence of this criticality, it is necessary to consider the option of replacement of the transformer; in this case, it is advisable to choose a machine that meets the requirements of eco-design, in particular in terms of reduction of load losses and reduction of emissions in terms of CO2 equivalents.(link to the directive)

 

 

Degradation of paper

Thermal degradation processes of paper are the result of the interaction of 3 mechanisms: hydrolysis, oxidation, pyrolysis.
Paper degradation processes are extremely complex, if added together then the effects of the degradation of oil (given the oil-paper interaction) give rise to mechanisms affected by a range of critical factors that are difficult to formalise quantitatively.The critical factors that determine the ageing of insulating papers are temperature, water, oxygen, whether the system is closed or opened, thermal cycles and the relationship with the load profile of the transformer.

Sea Marconi has conducted a series of experiments to determine the relationship between the degradation of paper and that of oil.One of these tests, in accordance with IEC 62535 (in a 20 ml vial with 10 ml of oil, a copper specimen weighing about 3 g of weight, wrapped with approximately 23 g of paper at 150 °C is inserted for 72 hours), showed that was a progressive loss of paper weight (up to 25%) and a DP reduction (up to 80% compared with the new value and 60% less compared with an initially non-acidic oil) which increased the acidity of the oil analysed (TAN).(see graph below).

[ALT img:Degradation of insulating papers | DP paper acidity graph]

Loss of paper weight and DP decrease as acidity increases

The main products of degradation of insulating papers are: water, acids, CO2, CO, furan compounds, methanol, ethanol, particles.These compounds amalgamate with the sludge resulting from the ageing of the oil, forming the total sludge.

[ALT img:Degradation of insulating papers | isolation of enemies]

 

 

Click here to access Sea Marconi's major publications on the topic:

Regulatory framework

Main regulatory references

• 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"
• CIGRE Brochure 343, 2008 "Recommendations for Condition Monitoring and Condition Assessment Facilities for Transformers"
• CIGRE Technical Brochure 349, 2008 "Moisture Equilibrium and Moisture Migration within Transformer Insulation Systems"
• CIGRE Technical Brochure 445, 2011 "Guide for Transformer Maintenance"
• CIGRE Technical Brochure 494, 2012 "Furanic compounds for diagnosis"
• IEC 60076-7: 2005 Ed.1, “Power Transformers – Part 7 Loading guide for oil immersed power transformer”
• CIGRE WG A2-30, 2007 "Moisture equilibrium and moisture migration within transformer insulation systems"

 

 

Causes

The "Degradation of insulating papers" criticality is caused mainly by normal ageing mechanisms and by particular conditions of thermal, electrical and mechanical stress on insulating papers.

 

Causes in relation to life cycle phases

 

Causes of the "Degradation of insulating papers" criticality | When it may occur (life cycle phases)

Lack of purchase requirements for insulating papers | Requirements and purchase

Deficiency in quality control for individual batches or single insulating paper supplies (example initial DP before impregnation) | Acceptance of insulating papers

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

Loss of protection gas and moisture accumulation on solid insulators | Transport and installation of the transformer

Deficiencies in paper dehumidification treatments (e.g. vapour processes phase).A good insulating paper has a water impregnated value of between 0.5 and 1% by mass | Construction, transport, installation and pre-energisation, operation, old age

Accumulation of air and moisture (for example, during oil change or other electromechanical maintenance) | Factory test, installation and pre-energisation, operation, old age

 

 

The critical factors that characterise the ageing of insulating papers are temperature, water, oxygen, whether the system is closed or opened, thermal cycles and the relationship with the load profile of the transformer.Oil degradation processes are extremely complex, depend on many factors and call for mechanisms that are difficult to formalise quantitatively.Paper thermal degradation processes are the result of the interaction of the following 3 mechanisms:

1.HydrolysisThrough desorption of paper and the breakdown of cellulose chains, this phenomenon generates water as the main degradation product (in the presence of hot spots typically at a temperature of < 150 °C).

2.OxidationThis phenomenon typically occurs in the presence of hot spots, at temperatures between 150 °C and 300 °C, and is characterised by an increase in oxygen demand.The result is a decrease in the concentration of oxygen in the oil (for a silica gel respirator transformer it changes from 20000 μl/l to values if 5000 0 10,000 μl/l) which combines with the carbon originating from the degradation of cellulose forming CO2 and CO.This process simultaneously produces other by-products of cellulose, including furan compounds (in particular 2FAL), methanol and ethanol.

3.PyrolysisThis is a thermochemical decomposition mechanism typically occurring in hot spots at temperatures above 300 °C, such as to break the paper molecules with the formation of particles, water, methanol, ethanol and other characteristic gases.

The products of paper degradation are solid, liquid and volatile compounds (gas).These include those soluble in oil (furans, methanol, ethanol) and those insoluble in oil (particles, sludge) which amalgamate with the sludge resulting from the ageing of the oil.Oil and paper derived sludge add up to form the total sludge.

 

 

[ALT img:Degradation of insulating papers | Impact of Temperature]
Schematic diagram showing the ageing rate k, depending on the various degradation mechanisms (Fig. 1 taken from IEC TR 62874:2015)

[ALT img:Degradation of insulating papers | Impact pf Humidity and Oxygen]
Relationship between the properties of the insulating papers and the degree of polymerisation of the papers (Fig. 2 taken from IEC TR 62874:2015)

 

 

Signs (visual inspection) – Symptoms (analysis)

Signs (visual inspection)

The direct visual signs of this criticality are only highlighted by internal transformer inspection.In the case of failure (or end of life) of twin machines, for example, it is good practice to perform a paper diagnostics (through sampling, analysis and interpretation) in order to determine an experimental reference for the transformer concerned.In the presence of "degradation of insulating papers" the following can be observed:

- signs of paper embrittlement (loss of mechanical properties),
- insoluble deposits (e.g. sludge or copper sulfide) on the insulating papers
- obstructions of the oil circulation ducts used to cool the windings and the papers themselves.

Through the effect of intimate contact with insulating papers, insulating oil becomes a valuable vector for indirectly diagnosing the state of the papers themselves.Through oil analysis, it is thus possible to identify and quantify paper degradation products to evaluate the thermal life consumed.

 

Paper degradation results in a loss of mass, and clamps can loosen with a progressive increase in vibrations.Over time, the resulting mechanical stress is added to electrical stress and thermal stress, significantly increasing the probability of failure (video of transformer noise).

 

 

Representative sampling

Should it be decided to carry out an internal inspection of the transformer, following a failure or in order to carry out a thorough inspection, it is strongly recommended to take samples of the insulating papers in accordance with relevant protocols and procedures.In particular, it is advisable to select the paper at the top, bottom, and middle of the both primary and secondary windings for each phase, taking multiple paper samples in areas with greatest darkening or embrittlement of the papers themselves.

 

It is important that appropriate sampling protocols be used and that sampling kit be used to ensure preservation of the sample until the phase of preliminary analysis.Likewise, it is important that the kit offers suitable data collection tools (transformer plate data, progressive numbering of each sample, sampling point with three-dimensional mapping of conductors, etc.).

 

During the normal life cycle of equipment it is necessary to take representative samples of the 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 "Degradation of insulating papers" criticality is related to the presence of non-conforming oil values ​​for the following diagnostic indicators:

 

Open

• Water in oil (IEC 60814)
• Oxygen
• CO2 – carbon dioxide
• CO – carbon monoxide
• 2FAL - 2furaldehyde and other furan compounds
• Methanol
• Ethanol
• Particles (IEC 60970)
• Gases symptomatic of hot spots (methane, ethane, ethylene)

To these are added diagnostic indicators derived from insulating paper analysis (following any internal inspection and sampling):

• DP
• Water in papers
• Deposited metals (e.g. copper)

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

• Acidity TAN (IEC 62021-1)
• Dissipation factor (IEC 60247)
• 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

 

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).

 

Through oil analysis,
it is thus possible to identify and quantify paper degradation products to evaluate the thermal life consumed

Contact us

 

 

Diagnosis

For diagnosis of the "Degradation of insulating papers" 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 the papers (if available as a result of internal inspection) and oil, the symptoms and their values are identified.In particular, for some indicators such as 2FAL and CO2, the standard (IEC 62874:2014) shows typical values based on the age of the equipment, the type of electrical equipment used and type of oil.As regards the other symptomatic indicators not "covered" by the standard, it is recommended that typical values be obtained of warning and alarm by means of statistical analysis on 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.

 

Changing the insulating fluid changes the diagnostic assessments of degradation processes.For example, natural ester fluids have a much higher natural ability to dissolve water than a mineral oil

 

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)

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.

 

 

Prevention

"Degradation of insulating papers" is an irreversible process that can, however, be prevented or mitigated through specific actions.
By arranging appropriate operational practices (e.g. analytical oil control, oil and paper treatment, load profile management, cooling of the machine), it is possible to reduce the probability of failure and prolong the operating life of the transformer under examination.If the transformer is a member of a family of equipment affected by failure due to the same criticality, ad hoc operational practices can be defined, optimising the various critical factors (metaphorically, it is like suggesting a personalised diet combined with increased physical activity to a person suffering from diabetes).

 

The replacement cost of a 250 MVA, 400 kV, elevator transformer (GSU) can be realistically of the order of € 2,000,000, including dead time, oil emptying, logistics for decommissioning and waste disposal, purchase of a new transformer (and new oil), transport, installation and testing.

 

Prevention actions during the life cycle of the transformer

• 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:

 

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)

 

Depolarisation

[ALT img:Degradation of insulating papers | MDU]

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 mg KOH/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.

 

Application of cartridges for dehumidifying the transformer

This activity is accomplished by means of a unit which is placed on the transformer and operates continuously as a loop circuit under load and has columns with molecular sieves for selective adsorption of moisture and other polar compounds.(read more)

 

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 contaminants 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 that maintenance practices be modified as regards:

- the drafting of purchase requirements for both oils and electrical equipment for specific applications with particular regard to design/sizing criteria
- the acceptance of oils and equipment using the best supervisory and control practices, in accordance with the prescribed methods.Ask the supplier for a certificate of compliance of the properties of oil and insulating papers

 

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

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

There is no therapy able to "rectify" the "degradation of insulating papers" criticality.Degradation is an irreversible process that can only be managed through appropriate prevention and/or mitigation actions.
Paper replacement is not a viable option because the cost would be close to the cost of fully replacing the transformer.
When the paper DP reaches a value of 200, which conventionally represents the end of thermal life, it is advisable to plan replacement of the transformer and modify the maintenance practices that may have contributed to ageing of the insulating papers.

 

The priority is to avoid degradation by intervening preventively on the symptomatic indicators.

 

 

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
• action to prevent the "Degradation of insulating papers" criticality, in this case the treatments of oil depolarisation and internal inspections of the transformer must be carried out
  - using technologies that are safe and suitable for the purpose, that 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)