Introduction

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.

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.

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.

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 a generation step-up 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)

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).

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.