Definitions

 

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 ]

 

SF ₆ (sulfur hexafluoride)(from Wikipedia)
Sulfur hexafluoride is an inorganic compound with SF6 formula.It is a transparent, odourless, non-toxic and non-flammable gas (under standard conditions); at 1 bar it has a density of 6.164 g/L (about 5.1 times denser than air).

SF6 has octahedral geometry, consisting of six fluorine atoms linked to a central sulfur atom.It is a hypervalent molecule.It is slightly soluble in water but soluble in non-polar organic solvents.It is generally transported in liquefied form.[ source ]

 

Introduction

 

Insulators in oil and SF6

The "SF6 in oil - insulator seal defects" criticality does not concern transformers directly, but rather bushings, accessories that allow conductors to pass from inside the transformer to the outside without coming into contact with the casing.

 

In elevator transformers (GSU), 14% of the failures are due to bushings (>100KV).

CIGRE WG A2.37 - Transformer reliability survey, Dec.2015

 

Bushings are normally placed at the upper part of the transformer and are of different types depending on the voltages/currents in play and the type of external conductor to which they are to be connected.The bushings typically consist of two compartments; the lower one is inserted into the transformer, the upper one is anchored on the transformer casing.The upper compartment can be free or inserted inside metal enclosures filled with, for example, SF6 (sulfur hexafluoride).

For high currents (up to 30 KA), oil-air bushings with a high diameter central conductor are used; for high voltages oil-oil, oil-SF6 or condenser bushings are used, in which the central conductor is wound with layers of insulating paper alternating with layers of conductive material.In high and very high voltage, SF6 gas is commonly used as an insulating medium because it is not subject to ageing, is not toxic or flammable, and has good dielectric properties (nearly 3 times greater than air or nitrogen), and arc-interrupter and thermal properties.

 

Features of SF6

Sulphur hexafluoride (SF6) is a colourless, odourless, non-toxic, non-flammable, chemically inert gas with high dielectric properties, almost 3 times greater than air or nitrogen.

For over 50 years, SF6 has been used successfully in various industrial applications, especially on switches and disconnectors on electricity transmission and distribution lines.In other fields, it is used, for example, in particle accelerators, radar and the electronics industry or in the medical field, and particularly in magnetic resonance diagnostic machines and in eye surgery.

In the past it was also used in tyres, tennis balls, some types of gym shoes and as insulation in double glazing but its use in these applications has been banned since 2007.

In medium and high voltage electrical equipment, SF6 is used for its high electric arc extinguishing properties due to its high dielectric strength and its capacity for recombination.Thanks to these properties, which are superior to other fluids such as air or nitrogen, electrical substations can be constructed with much more compact dimensions.

 

SF6 and the environment

Of the gases that cause the well-known "greenhouse effect", SF6 is the one that requires the highest priority for action in terms of preventing/mitigating global climate change.Along with five other gases - [carbon dioxide (CO ₂ ), methane (CH ₄ ), nitrogen oxide (N ₂ O), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs)] - SF6 was listed in the Kyoto Protocol (1997).Its climate impact is 22,800 times greater than carbon dioxide (CO2) and its residence time in the atmosphere is around 3,200 years (source:Federal Statistical Office:Survey of particular climate-active materials “sulphur hexafluoride” (SF6), Wiesbaden, 2015)

To prevent its dangerous effects on climate, use of the gas has been banned since July 4, 2007 for "civil/domestic" applications, however, in most industrial applications, SF6 gas continues to be used because alternatives with equivalent performance have not yet been identified.

Severe rules exist around the world to reduce SF6 gas emissions into the atmosphere.In the European Union, the F-Gas regulation, (CE) No. 517/2014 , on the limitation of greenhouse gas emissions, came into force in 2014.The regulation sets out the general requirements for the inventory, management and treatment of SF6 gas and other fluorinated gases.

 

 

The "SF6 in oil - insulator seal defects" criticality is mainly caused by sealing defects between the insulator (insulated with oil) and the metallic casing (containing SF6) that encloses the top of the insulator itself.

Sealing defects can result from physical chemical degradation of materials, electrical, thermal and/or mechanical stress.Taken together, these factors lead to a migration of SF6 in insulator oil.If, on the one hand, this leads to the presence of SF6 in the oil (oil compartment of the insulator), on the other hand, it causes contamination of the SF6 in the metal casing enclosing the upper part of the isolator by part of the oil.This latter condition is far more serious than the opposite condition; in particular the oil mist in the part in SF6 makes the latter lose its dielectric properties, endangering the accessory insulation.

 

The seriousness of this criticality lies in the fact that, without particular signs/symptoms, it can give rise to electrical failures with power arcs up to explosion and fire in the insulator, with catastrophic consequences.In fact, following an explosion of an insulator, pieces of porcelain and metal components are projected up to 200 metres from the point of installation with a high risk to people, and are likely to trigger a domino effect on surrounding equipment.In the case of fire, the severity of the event is as serious given the high probability of propagation to the transformer casing.The consequence of a similar event could also cause explosion (and/or fire) of the transformer, with a very serious impact on people, things and the environment.

 

 

 

 

Signs (visual inspection)

The "SF6 in oil - insulator seal defects" does not show any characteristic signs; however, through visual inspection, it is possible to:

• assess the general condition of the accessory (corrosion) and the presence of leaks,
• check the dilation of the bellows (oil/SF6), an indicator of the amount of oil present in the insulator;
• check in particular the general conditions of the capacitive jack (if any), thanks to which it is possible to perform appropriate electrical tests (capacity, dissipation factor).

Representative sampling

Despite the presence of oil in the casing in SF6 being more critical, is much more convenient to take an aliquot of oil (from the oil compartment of the isolator) and analyse it to assess the presence of SF6.This indicator is sufficient to declare criticality because a sealing defect between the insulator and the outer casing would produce both oil in the SF6 and SF6 in the oil at the same time.

Representative sampling of an oil-SF6 insulator is delicate and complex. It is necessary to employ appropriate operating procedures (in accordance with the specifications of the insulator manufacturer), special sampling devices and specifically-trained staff.
The difficulty lies in the presence of a very low volume of oil in the insulator compartment (oil), so it is necessary to sample while compensating with the same quantity of oil (pretreated).The operation must be carried out with extreme care to avoid the smallest infiltration of air bubbles.

Sea Marconi has accumulated great experience in the field, performing this operation on insulators in the EDF nuclear machine pool.

 

 

Symptoms (analysis)

The specific symptom of the "SF6 in oil - insulator seal defects" criticality is correlated with values that do not conform for the following diagnostic indicator:

• SF6 in oil*

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

• Dissolved water (IEC 60814)
• Dissolved gases (IEC 60567) [oxygen, nitrogen, carbon dioxide, carbon monoxide, hydrogen, methane, ethane, ethylene, acetylene] with indications of C3s [propane, propylene and methylacetylene or propyne (C3H4)]
• Dielectric dissipation factor (IEC 60247)
• Particles (IEC 60970)

* Analysis of SF6 in oil, sampling from the oil compartment in the insulator and diagnostic interpretation of analytical results are all devoid of a valid reference method and bibliography.

 

Sea Marconi has developed and validated its own analytical method in-house which bypasses the technical limits that would occur by using the classic DGA.In fact, the latter cannot return the content of SF6 in oil.The reason is very technical: in fact, SF6 contaminates the catalyst of the methanator; more simply, it pushes the normal analytical system out of calibration.

The same research and development approach taken by Sea Marconi for the development of the analytical method described above has also been used to validate the diagnostic interpretation of results model.This has led to establishing what the typical values, alerts and alarms should be in relation to the concentration and speed of formation of the SF6 in oil and the profile of the other dissolved gases (DGA) for each family of equipment (in relation to the manufacturer).

 

For diagnosis of "SF6 in oil - insulator seal defects", Sea Marconi uses its own diagnostic metrics, namely:

• visual signs on the bushing are interpreted;
• through analysis of the oil, the symptoms- that is, the symptomatic indicators (SF6 in oil, dissolved gases, dissolved water) and their characteristic values - are identified;
• the database is used to study family or subjective case histories(in the search, for example, for defects/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.

 

Since there is no real treatment for this criticality, prevention is the only viable strategy for correct life cycle management of the bushing and the transformer to which it is connected.

Prevention actions during the life cycle of the insulator

• Monitoring symptomatic indicators (see symptoms above).If the first symptoms of criticality, such as a high SF6 content in oil, occur, increasing sampling and analysis frequencies (once a week or even once a day) in order to monitor their trends is recommended.This monitoring aims to establish the right time to justify decommissioning the transformer and replacing the accessory with a suitable spare part.
• In addition to regular visual inspections, consider carrying out thermographic surveys to establish the general conditions of the accessory, identify any defects in the connections or phases, and identify any defects in the cooling system
• Consider the performance of electrical tests to determine any isolation losses, partial discharges, capacities, delta Tg.
• It is recommended that strategic information be updated through a " dynamic inventory" of oils and transformers (with their accessories), without neglecting the values ​​of symptomatic markers.
• It is recommended that maintenance and procurement practices be modifed/integrated in such a way as to ensure that the warehouse always has sufficient spare parts(of insulators) for each family/manufacturer

 

The "SF6 in oil - insulator seal defects" criticality does not envisage a real treatment; the criticality can only be resolved through replacement of the insulator.

The life cycle management of the accessory calls for constant monitoring of the content of SF6 in the insulator oil (see prevention above) in order to minimise the risk of failure as much as possible and plan a replacement in complete safety.

 

 

Oil sampling in the insulator must be carried out according to the manufacturer's specifications:

• using appropriate procedures for the purpose and specific sampling devices
• 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)