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 Electropedia IEC)
Insulating liquid produced by acids and alcohols by chemical reaction
These esters include mono-, di- and polyol esters.
IEV ref.212-17-08 [source]

Power transformer

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]

 

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:Water in transformer (paper and oil) | water temperature migration]

If the temperature drops, the water passes from the oil to the papers, and vice versa with an increase in temperature

In electrical transformers, insulation is mainly ensured by the oil and paper system.
The construction of transformers provides for operational practices such as to guarantee a concentration of water in paper typically less than 0.5% -1% in mass, in relation to the contractual requirements defined between the manufacturer and the purchaser (ref. 5.4.3 IEC 60422).The limit on water in oil depends on the category of transformer; for new transformers (> 170 KV) prior to energisation, the maximum permissible concentration of water in oil is 10 mg/kg (ref. Tab 3 IEC 60422).

After the initial drying of the new transformer, the water concentration increases in relation to the transformer type, environmental conditions, operating conditions, and oil and paper degradation processes.The increase in water in the transformer has external causes (e.g. entry from the atmosphere) and internal causes, due to complex mechanisms of degradation of the oil-paper system.The latter include processes of chemical-physical degradation of the paper known as "hydrolysis processes".During the life cycle of the transformer, there is a condition of continuous modification of the balance between the water absorbed into the paper and that dissolved in oil.

The migration of water between paper and oil depends mainly on the temperature and the "operating profile" of the transformer.

 

Relationship with temperature

The migration of water between paper and oil depends mainly on the temperature and the "operating profile" of the transformer.The higher the temperature rises, the more we observe the migration of water from papers to oil; the lower the temperature drops, the more water passes from oil to the papers.

[ALT img:Water in transformer (paper/oil)]
Taken from the presentation "World of moisture & Moisture Management" by V. Sokolov, presented at the My Transfo 2004 conference, Turin 20/10/2004

The thermal profile of the transformer is not homogeneous in all its internal parts; in particular it changes according to the single winding, in the high, medium and low parts, the circulation and cooling ducts and the geometry of the refrigeration system, and the deposits of sludge that can change over time.A major study of these conditions is contained in the CIGRE WG 230 document of 2007 titled "Moisture equilibrium and moisture migration within transformer insulation systems", whose convenor was Victor Sokolov (deceased in 2008), one of the top experts in the field.

Water in the transformer causes a progressive reduction of the paper-oil insulating properties (discharge voltage, dissipation factor), generating electrical defects (e.g. partial discharges) that can evolve into electrical discharges and power arcs with electrical faults in the transformer.

[ALT img: dangerous effects of free water]
[ALT img: vapour bubble]

 

99% of the water contained in power transformers is found in cellulose materials.

 

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”
• IEC 60422:2013, “Mineral insulating oils in electrical equipment – Supervision and maintenance guidance”
• 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

[ALT img:Water in the transformer (paper/oil)]

Taken is from the presentation "World of moisture & Moisture Management" by V. Sokolov presented at the My Transfo 2004 conference, Turin 20/10/2004

The "Water in the transformer (paper and oil)" criticality is caused mainly by mechanisms resulting from external influences, with moisture entering from the atmosphere, and internal influences, resulting from the degradation mechanisms of solid and liquid insulators that generate water (hydrolysis of cellulose).

 

Causes in relation to life cycle phases

 

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

Lack of transformer drying requirements in the construction phase (< 0.5% -1%) | Requirements and purchase

Lack of quality control for individual batches or individual supplies of paper and oil (example:Initial paper DP before impregnation, water content in oil) | Acceptance of insulating papers and oil

Lack of controls and analytical procedures (and sampling) for checking the water content of oil | Oil acceptance, factory test, installation and pre-energisation, operation, old age

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

 

 

Main sources of water contamination

1. Residual moisture in "thick structures"
   The residual moisture in a new transformer after its construction should be less than 1% with a target of 0.5%.An excessive residual moisture of - 4% may persist in some thick insulation components, particularly in laminated and plastic cardboards
2. Entry from the atmosphere
   1. 1.Absorption of water from the atmosphere due to direct exposure of air insulation (especially during installation or maintenance operations)
      1. 2.Entry of moisture in the form of molecular flux (Knudsen) due to the difference in the concentration of water in the atmosphere and that in the oil in the casing (negligible)
         1. 3.Viscous flow of moist air into the transformer due to the difference between atmospheric pressure and pressure in the casing (main contamination)
3. Decomposition of cellulose due to ageing
4. Decomposition of oil due to ageing
5. Release of "bound" water from oil and particles

 

 

Signs (visual inspection) – Symptoms (analysis)

Signs (visual inspection)

Oil* affected by the "Water in the transformer (paper and oil)" criticality can have different characteristic aspects.

* here we mean the sample of oil representative of the total mass of fluid present in the transformer casing.To do this, it is necessary to take the sample at the bottom of the transformer casing.In the presence of accessories (isolators, switches, etc.), the sampling is repeated at the bottom of each accessory.During the sampling phase, it is absolutely necessary to indicate the sampling point and the oil temperature at the time of the activity

 

[ALT img:Water in the transformer (paper and oil)]
Taken from the presentation "World of moisture & Moisture Management" by V. Sokolov, presented at the My Transfo 2004 conference, Turin 20/10/2004

A. oil with clear water-oil separation (free water).
In this case the aqueous fraction is deposited at the bottom of the sample, while the overlying oily fraction appears in the form of whitish emulsion.Referred at room temperature (e.g. 20 °C)**.This is the worst condition

B. oil with an appearance of milky emulsion.Reported at room temperature (e.g. 20 °C)**.

C. oil with an opaque, whitish appearance.Reported at room temperature (e.g. 20 °C)**.

D. oil which has no characteristic signs because the water contained is completely solubilised.Reported at room temperature (e.g. 20 °C)**.

** If the temperature decreases, the separation step is accentuated and the water on the bottom increases (the water has specific weight 1 and mineral oil 0.875 mg/cm3).
If the temperature rises, the phenomenon of phase separation is reduced by the natural solubilisation of the water in oil.

 

In order to establish the amount of water in oil, the aliquot to be analysed is not the water but the most homogeneous oily fraction

 

In cases of internal inspection of the transformer, signs of free water or drops of water can be observed mainly at the bottom of the casing and at the bottom of the transformer casing head.These are very dangerous because they are close to high voltage areas.Signs of rust or localised paint stripping can be observed in the same area.In the case of loss of casing seal, windings can sometimes also also show signs of water.

[ALT img: water in suspension]
[ALT img: water in suspension]

 

 

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).

 

It is important that suitable sampling protocols are used and that sampling kits be used that can ensure sample retention until the pre-analysis phase – IEC 60475 “Method of sampling insulating liquids”.Likewise, it is important that the kit offers suitable data collection tools (transformer plate data, progressive numbering for each sample, etc.).

 

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 take papers from the top, bottom and middle of the both individual primary and secondary windings for each phase, taking multiple paper samples in areas with greater signs of criticality (see below).

 

 

Symptoms (analysis)

The specific symptom of the "Water in the transformer (paper and oil)" criticality is related to the presence of the following diagnostic indicators with typical values ​​not compliant with those recommended by the IEC 60422 standard:

• Water in oil (IEC 60814)

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

• DP, degree of polymerisation (IEC 450:1974)
• Water in papers (IEC 60814)

 

[ALT img:Water in the transformer (paper and oil)]

To avoid interference in the matric and avoid overestimates,
Sea Marconi has developed and uses use the sample preparation system
in a controlled atmosphere with vial on the
revolving table (designed and marketed by Sea Marconi),
analysed automatically using the Karl Fisher method (read more).

 

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

• Acidity TAN (IEC 62021-1)
• Particles (IEC 60970)

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).

Depending on their type, size and concentration, particles distributed in oil can take on water (bond water), reducing insulation properties locally.This condition can trigger partial discharges until evolving into electrical discharges and electrical failure.

 

 

Diagnosis

For diagnosis of the the "Water in the transformer (paper or 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 the oil and papers (if available following internal inspection) the symptoms are identified and co-factors evaluated.One of these is undoubtedly the temperature, both sampling and ambient temperature.At the same time it is necessary to evaluate the degradation of the oil, the type of oil (between paraffins and naphthenics there are different degrees of solubility, which is even more evident for natural esters, which can reach solubilisation up to 10 times more than mineral oils ) and concentration and type of additives.
  
  The limits recommended limits by IEC 60422 regarding oil in a just-filled transformer and before energisation are:

[ALT img:Water in the transformer (paper and oil)]

Taken from Table 3 – Recommended limits for mineral insulating oils after filling
in new electrical equipment prior to energisation – IEC 60422 ed.4-2013

 

The same standard (on page 28) indicates the recommended limits of water in oil for a transformer in operation:

[ALT img:Water in the transformer (paper and oil)]

Taken from Table 5 – Application and interpretation of tests – IEC 60422 ed.4-2013

 

Starting from the analytical results of the "water in oil" test, there are several methods (with different levels of precision) for evaluating the state of cellulose hydration, that is, the water absorbed by the papers.First of all, it is necessary to distinguish between direct methods, which involve the analysis of paper samples taken from internal inspection, and indirect methods, through mathematical formulas and diagrams.

Technical standard IEC 60422, for example, enables (see table below) estimation of the moisture conditions of the solid insulator (papers).To obtain this estimate, it is necessary to calculate (see formulas below) the saturation of the water in oil, that is, the maximum concentration of oil that can solubilise in oil at a certain temperature.

[ALT img:Water in the transformer (paper and oil)]

After calculating the water saturation in oil in percentage terms, using the table below (Table A. 1 – IEC 60422 ed.4-2013) it is possible to obtain an estimate of the moisture of the papers: dry, slightly moist, moist, extremely moist.

[ALT img:Water in the transformer (paper and oil)]

Table A.1 – Guidelines for interpreting data expressed in per cent saturation – IEC 60422 ed.4-2013

[ALT img:Water in the transformer (paper and oil)]

T. V. Oommen, “Moisture Equilibrium Charts for Transformer Insulation Drying Practice”, IEEE Transaction on Power Apparatus and Systems, Vol, PAS-103, No.10, October 1984

 

For integration, different equilibrium curves can be used (Fabre-Pichon curves, 1960, Oommen curves, 1983, Griffin curves, 1988, Koch curves, 2005).These are diagrams elaborated on the basis of different application case histories that permit scientific prediction of the average amount of water absorbed into the papers in relation to the average operating temperature.Subsequently, knowing the total weight of paper and oil, it is possible to make a mass balance of the total water present in the transformer, both in paper and in oil.

 

As an alternative to the indirect determination described above, there is also the possibility of employing a direct methodology.
In the presence of an oil-impregnated paper sample, it is possible to determine the amount of water in paper by means of a specific protocol for extracting water through a carrier gas (for example, hot nitrogen).The water extracted is determined using the Karl Fisher quantitative method (method IEC 60814).This concentration (mg/kg) makes it possible to calculate the percentage of water relative to the mass of the paper sample being analysed.

Sea Marconi has recently used this method for an important constructor of international transformers.Paper moisture measurement has made it possible to better understand the process of drying the transformer and optimise the entire production process

• the database is used to study family or subjective case histories (in the search, for example, for failures in twin machines);
  • the factors of uncertainty, speed and evolution over time (trends) of symptomatic indicators are taken into consideration and monitored during the life cycle phases; for this purpose, it is important to disengage from the sampling temperature (which changes in relation to the operating profile), renormalising the analytical result of the water in oil at a temperature of 20 °C (seeIEC 60814].This calculation is performed using the formula [put formula] pag. 13 (IEC 60422) with correction factor (see Fig. 1, page 41, IEC 60422).
  • 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.

 

Temperature is a key factor in the degradation processes of paper and oil insulating materials.Some temperature sensors (internal optical fibres or external sensors located at representative points) can monitor the transformer during its life cycle.These data, modelled using a specific "operating profile algorithm", make it possible to diagnose the current situation more effectively and predict future evolution (prognosis) in order to prevent and mitigate specific criticalities.

 

 

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
Oil mass:50,000 kg of non-inhibited paraffin-based mineral oil
Paper mass:2,500 kg
Paper type: non-TU kraft paper

Cooling:ONAF
Environmental severity: normal, temperate climate height (asl) 250 m, ground level
total acidity of 0.25 mg KOH/g ("poor" value compared with Table 5, IEC 60422),
colour = 6 dark ("poor" value compared with Table 5, IEC 60422)

 

 

Water in paper

Water in paper of new era transformer < 0.5%.The amount of water in oil for a new transformer must be < 10 mg/kg at 40 °C.

Renormalised at 20 °C there are 4.5 mg/kg of water in the oil, thus 0.225 kg of water in the oil of the transformer in the example (4.5 mg/kg x 50,000 kg = 225,000 mg = 0.225 kg).

12.5 kg of water in the initial paper

This gives a ratio of 55.5, which means that for every kg of water in oil there are 55 kg of water in paper

 

 

Water in oil

Water in oil = 40 mg/kg sampled at 40 °C. That value corrected at 20 °C becomes 18 mg/kg, which means that on an oil mass of 50,000 kg, there are 0.9 kg of dissolved water in the oil (18 mg/kg x 50,000 kg = 900,000 mg = 0.9 kg)

Thus, moving from 0.225 kg to 0.9 kg, the value of water in oil is increased by 400%!

The insulating paper certainly has a degradation process under way: initially it had a mass of 2,500 kg and fell to 1,875 kg.

Using T.V. Oommen equilibrium curves, a relative oil saturation rate of about 5% is estimated, defined by the IEC as "extremely wet".

These last two data allow us to obtain the water content in the papers:93 kg (1.875 kg x 5/100)

Summing up, there would be 0.9 kg of water in the oil and 93 kg of water in the paper

It is thus shown that, in the real example, the ratio of water in oil/water in papers for a new transformer is 1 to 55; it is now almost doubled, for every kg of water in oil there are about 100 kg of water in papers

 

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, there has been an estimated 25% loss in paper mass; in fact, its weight has dropped from the initial 2,500 kg to 1,875 kg.

 

 

Prevention

The presence of water in the paper and oil system inside the transformer can be an extremely critical condition, especially when the criticality reaches an "extremely wet insulation" level (Table A1, IEC 60422, page 42), causing unreasonable risks of electrical failure.

This condition cannot be resolved with physical treatment of the oil alone, which would reduce water in the oil but not in the papers.The phases of energisation of the transformer at low temperature with sudden load are particularly critical.In these cases, water bubbles are very likely to form.

The life cycle management strategy is to avoid the formation of water in the oil-paper system as much as possible.Although it is impossible to eliminate this criticality, it can be prevented or mitigated through appropriate operational practices (e.g. analytical control of oil and indirectly of papers, oil treatment, load profile management, cooling of the machine).If the transformer belongs to a family of equipment affected by failure for the same criticality, ad hoc operating practices can be defined that optimise the various critical factors (e.g. by installing dehydrating cartridges).

 

 

Prevention actions during the life cycle of the transformer

• Monitoring symptomatic indicators (see symptoms above).If the first symptoms of criticality appear, such as a high content of water in the oil, it is recommended that the frequency of analyses be increased in order to monitor trends.
• Perform appropriate oil treatments in order to keep moisture low in the papers and in any case in order to avoid reaching the "wet insulation" condition (Table A.1 – IEC 60422 ed.4-2013).
  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)
• Application of cartridges for dehumidifying the transformer
  This activity is accomplished by means of an apparatus 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.
• For breathing machines with conservator, it is advisable to periodically check the drying salts (read more)

 

 

In the case of moist papers, the oil change is not a definitive option because the water absorbed from the papers would not be removed even by changing the oil.The oil change operation could also create air bubbles (and consequent partial discharges) that are trapped in the dead zones of the transformer: under the head or in the radiators.

 

It is recommended performing (and keeping updated) the dynamic inventory of transformers with indication of the markers (indicator) symptomatic of the "Water in the transformer (paper and oil)" criticality throughout all phases of the life cycle.It is also advisable to map the equipment in "wet" and "extremely wet" conditions.

 

Other than those using minerals, what are the preventive measures for electrical machines with insulating liquids?

 

With regard to natural ester oils and synthetic esters, the preventive actions are the same; however, it is advisable to choose countermeasures after careful assessment 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".

 

 

Treatments

For the definition of action priorities and choice of countermeasures, it is necessary to diagnose whether the criticality originates internally (paper degradation) or externally (atmospheric influences).

If the origin of the moisture is internal (degradation of paper and oil) the countermeasures are:

• Physical treatment, in the first instance to be performed off-load and subsequently on-load (transformer under load).This activity can be aided by applying dehumidifying cartridges
• Direct dehydration (cycle B).Direct dehydration of the transformer (papers) consists of heating the windings through circulating the oil.

Then there are other techniques, some of which require handling of the transformer.These techniques require that the transformer be opened and treated, in some cases through drying of the nucleus in autoclave, in other cases using appropriate solvents (derivatives of aviation kerosene).These modes, which are far more expensive, are taken into consideration when, in addition to the water problem, there are other electromechanical criticalities which require replacement of parts or in any case extraordinary maintenance of the transformer.

If the origin is external, it is necessary to identify air inlet points and restore the seals.If necessary, it is advisable to change the drying salts, and consider increasing the amount or geometry of the container to increase the drying power.

 

 

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 "Water in the transformer (paper and oil)" criticality, namely the dehumidification 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)