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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.


CEMIG Distribuição S.A. Marcelo A. Costa
CEMIG Distribuição S.A. Álvaro J. A. L. Martins
Treetech Sistemas Digitais Ltda. Marcos E. G. Alves
Treetech Sistemas Digitais Ltda. Daniel C. P. Araújo


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.

Data of the monitored transformer

TypeThree Phase Regulating Transformer
Voltage138kV - 13,8kV
ManufacturerTusa (Reformed by ABB in 2007)
Voltage Class138kV
Year Manufacture1977 (2007)
Oil Volume15,500 liters
Type of OilBiotemp, insulating vegetable oil, manufactured by ABB


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

The sensors are IEDs (Intelligent Electronic Devices) that send the information directly to the data treatment block of the monitoring system
Modular system, facilitating future expansions and maintenance
IEDs already existing in the control and protection systems can be integrated to the monitoring and data acquisition systems, avoiding costs of additional sensors
There is no centralizing element - additional costs are avoided
There is no centralizing element - possible additional points of failure are avoided
Failure in an IED results in loss only of part of the functions - other IEDs remain in operation
Operating temperature -40 to +85ºC, adequate for installation at the yard, along with the transformer
Installation together with the transformer, at the yard - only serial communication (twisted pair, optical fiber or GPRS cellular modem) for interconnection with the data treatment block of the monitoring system
Typical 2.5 kV insulation level - designed for the high voltage substation environment.

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

IEDsAcquired Data
Temperature Monitor- Oil temperature
- Temperatures of the hottest point of the windings
- Load currents
- Alarms and disconnections due to high temperatures
Gas Monitor in the oil- Dissolved hydrogen in the transformer oil
- Alarms due to high / very high gas
- Oil temperature at the point of measurement
Transformer Humidity Monitor- Relative saturation (%) of water in the transformer oil
- Water content in the transformer oil (ppm)
- Ambient temperature
- Oil temperature at the point of measurement
Switch Humidity Monitor- Relative saturation (%) of water in the loaded switch oil
- Water content in the loaded switch oil (ppm)
- Oil temperature at the point of measurement
Membrane Relay- Rupture of the conserving tank bag
Voltage and Current Transducer- Switch motor voltages
- Switch motor currents
- Active / reactive / apparent powers of the switch motor
Data acquisition Module- Alarm contacts (buchholz relay, relief valve, oil levels, etc.)
- Status of forced ventilation groups
- Switch under operating load
- Time of operation of the switch under load
Bushing Monitor- Capacitance of the bushings
- Delta tangent of the bushings
Voltage Regulator Relay- Phase voltages
- Phase currents
- Active / reactive / apparent powers


Once the sensors are installed in the transformer, it is necessary for the measurements to be transferred to a data treatment system, where the calculations and the algorithms will be carried out in order to obtain useful information, such as diagnoses and prognoses of the transformer’s status. The employed architecture for this function is shown in figure 4.


Figure 4: Architecture for transmission of measurements, data treatment and remote access to the on-line monitoring system

As it is observed in figure 4, the sensors in the power transformer are interconnected with each other and with the GPRS cellular modem in a serial communication network. Future expansions of the monitoring system, for inclusion of new sensors in the transformer or new equipment of the substation (circuit breakers, CTs, PT, lightning rods, etc.) can be performed easily, by simply adding new sensors in the communication network.

Then, the modem transmits the measurements of the sensors to the monitoring software using the mobile telephony network, through the GPRS (General Package Radio Services) data transmission protocol, which is the same one used, for example, for transactions with credit cards using wireless terminals. The information is received in a computer located in an IDC (Internet Data Center), described as follows.


The data provided by the IED sensors located in the transformer are received by a computer where the Treetech Sigma4web monitoring software is executed. In this application it was opted to execute the software in a computer located in an Internet Data Center (IDC) contracted and administered by Treetech. The IDC is an independent company, specialized in data storage and processing services, which has the entire infrastructure needed for reliability and availability of the monitoring system, including:

  • Servers with high availability (24 hours x 7 days / week);
  • Contingency for power failure, with no-breaks and emergency generator groups;
  • Redundant Internet access band, ensuring availability of access to teh system;
  • Automatic daily backups;
  • Firewall;
  • Https protocols (safe site), through SSL (Secure Sockets Layer) standard;
  • Physical safety, with strict access control.

The main gains obtained with this solution, where the monitoring software Sigma4web is hosted in an IDC instead of in Cemig’s facilities, were the following, whereas the monitoring system is a tool for maintenance engineering:

  • Assurance of monitoring software update, once its execution in the IDC is administered directly by its manufacturer;
  • Assurance of update of hardwares (servers, etc.) according to growth of the monitoring software, for example, with the inclusion of other transformers;
  • Assurance of data integrity, due to backups performed automatically;
  • Assurance of continuous software execution, without risk of downtimes due to power shortage. Possibility (optional) of redundant servers;
  • Assurance of access to monitoring information from any locality, whether in the regional areas of Cemig, which cover a large geographical area, or outside the company – for example, during trips or out of office hours, in any part of the world;
  • Overload of the company’s in-house IT staff is avoided with the regular maintenance of the system, which would include the execution of backups, no-break operation supervision, software updates (operating system, antivirus, monitoring software, etc.), firewall administration, etc.
  • High expenses with adquisition, maintenance and periodic update of hardwares and software licenses are avoided.


The main functions of this software can be grouped into two categories, Digitalization and Data functions and Monitoring functions:

Digitalization Functions:

  • On-line data acquisition from the sensors
  • On-line presentation of measurements, alarms and statuses.
  • Storage of measurements, alarms and statuses in historical databases.
  • Query of measurements, alarms and statuses stored in historical bases in form of graphs or tables.
  • Automatic sending of notifications or alarms by e-mil or SMS.

Monitoring Functions:

  • Data treatment through algorithms
  • Data treatment through mathematical models
  • Obtainment of diagnosis of the current transformer status
  • Obtainment of prognosis of the future transformer status
  • Detection of defects still in incipient phase.

As it is observed in the foregoing, the Monitoring functions aims to transform the measurements from the sensors into useful information for maintenance, which are diagnoses and prognoses of equipment status. For this case, Sigma4web has the so-called “Engineering Modules”, where the algorithms and mathematical models for diagnoses and prognoses are.

Just as it occurs with the IEDs used for the acquisition of measurements, the monitoring functions (Engineering Module) of the system are also organized in a modular way, allowing the functions you want to install to be chosen freely, aside from facilitating future expansions by simply adding new software modules and their corresponding IEDs. The Engineering Modules used in the application are described in Table 4 that follows.

Data Treatment Modules for Monitoring, Diagnosis and Prognosis

Engineering ModulesMonitored Functions
Sigma Ageing• Insulation Useful Life Loss
• Useful life loss rate
• Extrapolation of the remaining time of life, in years.
Sigma Forecast• Forecast of Future Temperatures
• Forecast of occurrence of alarms or disconnections
• Calculation of remaining time for alarms and disconnections.
Sigma Efficiency• Efficiency of the natural and forced cooling systems
• Alarms in case of low cooling efficiency
Sigma Fan• Hours of operation of the forced cooling groups since start of operation
• Hours of operation of the forced cooling groups since the last maintenance
• Average hours of daily cooling operation
• Extrapolation of remaining time for inspection or maintenance
• Advance notifications for inspection or maintenance.
Sigma Hydro• Water content in the Oil and in Paper
• Acceleration of insulation life loss due to excess water
• Bubble Formation Temperature
• Free Water Formation Temperature
• Alarms due to risk of bubble formation or free water
Sigma Chroma• On-line measurement of dissolved hydrogen in the oil
• Alarms due to high, very high or trend of increase of H2 content,
• Off-line gas chromatography analysis database
• Calculation of gas increase rates
• Automatic reports for off-line gas chromatography analyses
Sigma Torque• Loaded Switch motor torque
Sigma Simulation• Loading simulation based on current load and temperature conditions
• Loading simulation with user load and ambient temperature curves
Sigma SpecialistAnalysis of notifications and alarms issued by the monitoring system for indication of:
• Probable causes (diagnosis)
• Recommended actions
• Future consequences (prognosis)


The assembly of the gas and humidity monitoring system was done during the installation of the transformer in SS Pará of Minas 1, in December 2007 and January 2008. Some photos of the installation of the devices are in Figure 5.


Figure 5 – Installation of sensors in the transformer

Immediately after entry into operation of the transformer, system performance started to be assessed. Then the need of survey of the real solubility constants of the oil used was established, such that the sensors presented values coinciding with the vegetable oil. Through the measurements stored in the monitoring system and oil chromatography and physical-chemical analysis database it is possible to gather these constants in an accurate way and then calibrate the sensors. Table 5 shows values sampled by the monitoring system after calibration with the right constants. These values were compared with the physical-chemical and chromatography analyses performed by Cemig’s laboratory and presented excellent results, with errors smaller than 2% of the reference value, showing system efficacy and precision.

Values presented during the monitoring of H2 and H2O



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,

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

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