Abstract

The substitution of insulating mineral oils with fluids or plant origin in power transformers has awakened a high degree of interest in recent years. This is due to diverse factors such as the high flash point, high water solubility, which can allow the insulating paper to kept drier, and non-toxicity and biodegradability, which make it attractive from the environmental point of view, among other characteristics.

For this reason, Cemig Distribuição has included the use of transformers with vegetable oil as one of its strategic initiatives for the improvement of its technical and economic performance indicators.

Given that such equipment perform essential roles in electrical power systems, occasional failures can result in vast losses, not only due to damages in equipment, but also due to losses of revenues, contractual fines and decrease of reliability of the power system.  In this context, on-line monitoring of the equipment has an essential role for the diagnosis of its status, which can in many cases detect failures even in incipient phase, aside from pointing out possible causes. With this major damages to the equipment or even its total loss can be avoided.

This work will present experiences with on-line monitoring of this equipment, including some specific aspects relative to the type of insulating fluid employed.

Vegetable oils are ester based organic compounds, natural agricultural products or chemically synthesized through organic precursors.  The synthetic ester based dielectric fluids, also known as POE, have good dielectric characteristics [1] and are more biodegradable than high molecular weight hydrocarbon (HMWH) based mineral oils. Aside from this, it has excellent thermal stability, good properties at high temperatures, pour point close to that of conventional mineral oil and dielectric rigidity and viscosities similar to the formulation with natural ester. However, they present lower flash / fire point and slowed biodegradation rate that the natural ester. Its high cost, compared to other fire-resistant fluids [2], generally limits its use in mobile substations, traction transformer and other special applications.

Natural esters are seed based oils, including fatty fluids, derived from glycerol, known as triglycerides. They were considered inadequate for use in transformers due to its high susceptibility to oxidation. With the increase of demand for viable options to mineral, researches were intensified and diverse oil mixtures were assessed. Upon encountering the best way of balancing them, the perfecting of the oxidation and fluidity characteristics was started. Thus the selection of additives for improvement of fluid performance was started.

Natural ester has a viscosity slightly higher than that of mineral oil, greater fire resistance and superior dielectric rigidity, both in the new fluid and after multiple loaded switching operations. The negative aspect is its relatively high pour point.

An attractive source of natural esters are seed oils.  Available in broad scale and with reduced production cost, these seeds – derived from renewable natural sources, contrary to mineral oils – are used mainly in foodstuffs. The vegetable oil formulation does not present any toxicity to human being and has a degradation time much lower than mineral oils. Aside from this, the products from its complete combustion are only carbon gas and water. The fluid can be filtered, recycled and easily disposed.

In spite of its higher viscosity, the thermal performance of natural ester can be compared with that of the conventional mineral oil. In the worst case, the average temperature increase of the winding of vegetable oil insulated transformers is about 5º higher [1] than that of oil of mineral origin. However, in figure 1, the high flash and fire point of vegetable oil is confirmed. Thus, it is possible to allow the increase of operating temperature of the insulating oil without affecting the transformer’s safety.

Figure 1 – Comparative of typical flash and fire points of insulating fluids

An issue pertinent to the adoption of vegetable oil in transformers is the durability of the insulating paper, that is, the way how the new insulating fluid affects the useful life of the solid insulation. Published studies [1-3] show that, under the same conditions, the aging of kraft paper in vegetable oil is much slower than in conventional mineral oil. The main degradation factors of kraft paper in transformers are: temperature (thermokinetic degradation) and amount of water (thermo-hydrolytic degradation).

In terms of relative saturations, a point of equilibrium must be obtained between the paper and the insulating oil in normal operation. Natural esters can accommodate a greater amount of water than mineral oils, causing more water to be displaced from the paper to the fluid. This is one of the advantageous characteristics of natural esters used as insulating agents, because there is significant increase of the paper’s useful life.

Figure 2 shows a comparative test of the aging rate of the thermostabilized paper with mineral oil and with natural ester, where this is significantly slower. It is estimated that the thermostabilized paper in a natural ester transformer with average temperature increase of 85º has the same useful life of an identical paper in a mineral oil transformer with average temperature increase of 65ºC. In other words, for the same useful life, the thermostabilized paper impregnated with vegetable oil can operate at a temperature 20ºC higher than the paper with mineral oil. The importance of this result is huge in the design of new transformers or in the repowering of used transformers, since the extension effect of the useful life of the insulation represents lower design costs or increase of useful life of the equipment.

Figure 2: Comparative of the aging of the insulating paper with different insulating fluids.

As the vegetable oil technology is still found in experimental phase and tests against diverse electrical, thermomechanical and chemical stresses, on-line monitoring was considered of fundamental importance for the assessment of its performance in power transformers.

The on-line monitoring system also carries out the collection and storage  of all monitored information, allowing further studies to be developed, encompassing the analysis in a broad range of situations.

The transformer chosen for installation of the monitoring system was a Tusa transformer, reformed in 2007, by ABB, and filled with insulating vegetable oil in the main tank and also in the switching key. In table 1, some data relative to this transformer are presented.

After establishment of all needs and consultation of the literature [4-8], a specification was elaborated that contemplated them and considered the state-of-the-art in on-line monitoring system, guiding the choice of devices and employed architecture.

For the measurement of variables in the transformer a decentralized architecture was chosen, as figure 3 illustrates, where specialized modules for each desired measurement are observed. This architecture presents various interesting characteristics, shown in table 2.

Figure 3: Comparative of the aging of the insulating paper with different insulating fluids

Main characteristics of the employed Decentralized Architecture

Table 3 that follows described in detail the measurements carried out by each of the IED type sensors installed in the transformer.

Measurements carried out by IED type sensors

The use of vegetable oil as insulating and cooling medium in high voltage power transformers is recent if compared with mineral oil. Thus, constant supervision of its performance is necessary, both for preventing possible failures and for knowing better its behavior in the course of time in real working situations.

The use of an on-line high performance and low cost monitoring system comes to satisfy this need. This system must be carefully specified so as to really obtain the desired performance, where this factor is very important, since the insulating fluid to be monitored i vegetable oil.

The experience with the monitoring of this transformer with vegetable oil has shown to be very promising, reaching the expected results and mainly, providing the tools for better understanding of vegetable oil and its particularities.

[1]     C. Patrick McShane, Marcelo Neves Martins: Desenvolvimento e aplicação de fluido dielétrico de base vegetal para transformadores de distribuição e potência. IV Conferência Double, Belo Horizonte, Brasil, agosto 2003.

[2]     D. A. Halleberg: Less-flammable liquids used in transformer. IEEE Ind. Applicat. Mag., pp 459-463, Jan/Feb 1999.

[3]     IEEE Standard test procedure for thermal evaluation of oil-immersed distribution transformers life test. ANSI/IEEE C57.100-1986, 1986.

[4]     R. Farquharson, GE HARRIS Energy Control Systems, “Technology Solutions for Improving the Performance Reliability of Substations and T&D Networks”, 2001 Western Power Delivery Automation Conference, April 10-12, 2001, Spokane, Washington

[5]     Alves, M, “Sistema de monitoração on-line de transformadores de potência”, Revista Eletricidade Moderna, maio, 2004

[6]     Daniel C. P. Araújo, et al, “Sistemas de Monitoramento e Diagnóstico de Transformadores de Subestações”, Décimo Segundo Encontro Regional Ibero-americano do CIGRÉ, Foz do Iguaçu-Pr, Brasil  –  20 a 24 de maio de 2007

[7]     Daniel C. P. Araujo, Alvaro J. A. L. Martins e Neymard A. Silva “As Vantagens da Revitalização de Transformadores de Potência Utilizando Repotenciação e Óleo Vegetal”, Anais do Seminário Brasileiro de Sistemas Elétricos, SBSE 2006, Campina Grande, Paraíba, Brasil, http://www.sbse2006.ufcg.edu.br/anais/132_sbse2006_final.pdf

[8]       Siemens, “Novos Conceitos em Sistemas de Energia de Alta Confiabilidade” Siemens Energia, Encarte Especial, Janeiro, 2001