Abstract

In 2001, the first commercial online power transformer monitoring system in Brazil was installed at Alumar, one of the largest aluminum production complexes in the world. The monitoring system was installed on a three phase 343MVA 230-34.5kV transformer with two OLTCs (On-Load Tap Changers), one on the high voltage and another on the medium voltage side. This article describes the system’s architecture, based on a decentralized data acquisition philosophy and using intelligent devices, and also its monitoring and diagnosis features. We have also mention here some of our practical experiences during the implementation and operation of this system in the last five years, including early detection of a mechanical problem in one of the OLTCs.

ALUMAR is considered one of the world’s largest alumina and aluminum industrial complexes, a  consortium  of  the  companies  Alcoa,  Bhpbilliton, Alcan  and Abalco. The complex currently has an approximate annual output capacity of 1.5 million tons of alumina and 380 thousand tons of aluminum. The plant is basically split into two areas: Reduction and Refinery.

The Reduction area has received this name because of the alumina electrolytic reduction process required to produce aluminum, which is carried out there. A significant quantity of electric power is required for the alumina reduction to take place. The average demand of electric power in this area of ALUMAR is of 765MW, and its average monthly power usage is 570GWh. ALUMAR is connected to the basic power grid, through which Eletronorte delivers power to it at a voltage of 230kV.

The process of alumina reduction requires a reliable power supply without interruptions, which must be kept steady for over two hours.  If this happens, it would freeze the entire production line, resulting in losses which could amount to millions of US dollars.

This is why the company’s electrical system and its power transformers are an essential asset in this line of business, a fact that led ALUMAR to purchase a new 343MVA reserve bay that was added to the existing four in 2001.  Each existing bay has a 300MVA 230-34.5kV transformer, with one high-voltage OLTC and a 34.5kV voltage regulator transformer, also equipped with one OLTC. However, for the new bay, the option was to build both transformers in the same tank, resulting in equipment with two OLTCs, one conventional, oil-based arc extinction type, on the 230kV side, and another with vacuum chamber extinction one, on the 24.5kV.

While still being purchased, the equipment was specified in a way to allow installation of an online monitoring system in the future. This system was purchased from, installed on the field and placed in service by Treetech still in 2001, immediately after the transformer had been delivered and commissioned. Therefore, this monitoring system was the first of its kind to be installed and to start operating regularly in Brazil, pioneering the acceptance of online transformer monitoring devices, which has been quickly growing.

The  online  monitoring  system  installed on the transformers at ALUMAR (Sigma,  by  Treetech)  uses  a  modular  and  decentralized architecture [1],[2], based on Intelligent Electronic Devices (IEDs) installed on the control panel of the transformer casing, from where data is sent  via a serial interface to a computer at the plant’s control room, which runs the software to store the data received, making it available and able to be processed, as shown in Figure 1. These three main parts of the monitoring system architecture are described below.

Some of the IEDs perform primary transformer control functions and are therefore used in the transformer regardless of the presence of a monitoring system. The existing equipment is integrated into the monitoring system through one of its serial communication ports, in order to work simultaneously as sensors to supply data to the system, but without increasing costs.

Other  sensors  were  installed  specifically  to be used by the monitoring system, but also based on the philosophy of decentralized IEDs integrated  to  the  system  through  their  serial ports.   In the few   cases where it was impossible to integrate them to the system via serial interface, either because the devices are not intelligent devices or because the manufacturer does not offer open protocols at serial ports (only proprietary protocol), universal data acquisition modules were used, able to receive multiple digital and/or   analog signals and digitizing them, thus making them available through open protocol serial ports.

It was then possible to integrate every sensor, both intelligent and conventional, to the monitoring system through serial communication port. This was also a reason why no centralizing equipment was installed on the transformer casing, and simplified both the monitor’s design and installation, reducing the initial costs and, last but not least, also reduced the TCO (Total Cost of Ownership) of the system as it increased its reliability and availability.

Another beneficial feature of a decentralized architecture, with IEDs, is the system’s modularity, allowing free choice of monitoring variables and also facilitating future expansions, by simply adding new IEDs. Several factors may have to be considered when making these choices, including transformer asset value, its importance in the production chain (or in energy generation, transmission or distribution systems), among others.

Figure 1 – Monitoring System Architecture

The physical medium used for communication in this case is a copper, shielded twisted pair cable. Even though fiber optic solutions were available and possible, at higher costs, there is a conviction, based on the features of the RS-485 communication standard, that this option could be used with satisfactory results.  Among the RS-485 features mentioned above is the fact that the RS-485 operates in differential mode, which associated to the mutual cancellation of interferences in adjacent legs of the twisted pair makes this standard less susceptible to the interferences already expected in substations for this level of voltage.

As expected, the twisted-pair solution has shown to be totally satisfactory in spite of a complication in this kind of layout: the high intensity magnetic fields generated by the high currents used in the aluminum production process.

It is worth highlighting that, as Lavieri et. al. stress: [3], the fact that the IEDs used are developed specifically for the substation environment is essential for this strategy to be successful. Any equipment originally developed for industrial purposes, when used in this type of application, usually shows fragility and lack of reliability and availability of the serial communication ports when they undergo electromagnetic surges and voltage impulses, in addition to extreme outside temperatures.

The data supplied by the IEDs located on the transformer, both direct readings and information obtained from data preprocessing, are received by a computer running the monitoring software, in this application located in the plant control room.

The main functions of this software can be divided into two groups: Data Digitization functions, associated to simple data availability and storage, and Monitoring functions, with the objective of transforming simple data items into useful maintenance information. In the monitoring system at ALUMAR, the following functions were created:

• Data Digitization functions:

–      Readings, alarms and status displayed online

–      Readings, alarms and status stored as history databases.

–      Readings, alarms and status queries, stored in history databases as charts or tables.

–      System can be accessed remotely or locally.

• Monitoring functions:

–      Data processing based on algorithms

–      Data processing based on mathematical model

–      Transformer current status diagnosis

–      Transformer future status prognosis

–       Early failure detection.

In systems in which the purpose is to obtain useful information for transformer maintenance, such as status diagnosis and prognosis, the Monitoring function block is specially important, confirmed in practice in this facility, when a defect in the OLTC was detected early (see item 4.1).

As in the IEDs used in reading data acquisition, the system’s data monitoring functions also have a modular organization, allowing the user to freely choose the functions he desires to install, and also facilitating future expansions, which can be done by simply adding new software modules and their corresponding IEDs. As already explained above, several factors may need to be taken into account in this choice, such as the value of the transformer or its importance in the electrical system, among others.

Following this modular philosophy, the monitoring system in operation at ALUMAR was equipped with the monitoring items described below, considered at that time as the most important for this application,  although it is possible to add any other items among those currently available.

Extreme heat leads to carbonization of the cellulose fibers. On the other hand, moderately intense heat, usually found in transformers, causes rupture of  the  individual  monomers of the cellulose chain,  forming solid residue and releasing carbon monoxide, carbon  dioxide and water. Again, the level of polymerization is reduced, weakening the mechanical resistance features of cellulose.

Since temperature is not distributed uniformly in transformers, the analysis of cellulose deterioration by heat is done taking in consideration the temperature of the hottest spot, since this will be the site where the highest level of degradation will occur.

As shown in the chart in Figure 4, when a given load is applied on a transformer, it is possible to forecast winding final temperature after thermal stabilization.

This allows to calculate whether the rise in winding temperature over the top oil temperature will achieve levels that will cause protection systems to trigger alarm signals or even enable shutdown sequences.

If the forecast for the system temperature exceeds the alarm level value, the monitoring system issues the alarm for this condition, also informing the time remaining before the alarm goes off, a value that is reached based on the thermal time constant of the winding.

Figure 5 shows an example of the evolution expected for the winding temperature, the time to reach the alarm value and the final winding temperature after stabilization. In this example, when an overload  is  applied  to  the  transformer,  the monitoring  system would

initially calculate that 13 minutes remain before   the   alarm temperature (or the shut down temperature) is reached, with calculation being continually readjusted.

Likewise, the extrapolation of temperature rises of the winding temperature over top oil temperature can also be applied to the temperature rise of the oil over the surrounding outside temperature, allowing trend monitoring for future rises in temperature with advance warning of hours.

Figure 5 – Evolution of winding temperature along time

All failure statistics for power transformers show that the On Load Tap Changer is one of the main sources of failure, especially due to the moving parts that conduct and interrupt high currents while subjected to high voltages.

This is why this monitoring function helps in supervising regular commuter wear and tear, which is traditionally done offline, via the preventive factory maintenance schedule. These interventions are usually based on the number of tap changes and equipment operating time, and include visual inspections and contact thickness measurement procedures.

This monitoring function provides useful information to help with on load tap changers maintenance:

• Sum of the total current commuted since the changer was first placed in service, yielding the contact wear rate.
• Total number of operations since the changer was placed back in service after last maintenance
• Calculation of current thickness of the arc interruption contacts, through extrapolation based on previous thickness measurements and number of tap operations
• Total tap changer service time and total service time since last maintenance
• Daily average contact wear and daily average tap changes
• Time in which the contacts will reach minimum thickness or number and time until the number of operations or maximum time period before inspection or maintenance.
• Warnings, with programmable advance, about tap changer inspection or maintenance.

On Load Tap Changers is one of the main sources of power transformer failures. The reason for this, as described previously, is the fact that OLTCs are mechanical, based on moving parts. Thus, mechanical failures of the On Load Tap Changer can cause problems of different magnitudes, starting from equipment unavailability until severe dielectric failures.

In this context, the function that monitors commuter operating times supervises the time required to perform the tap change in each operation of the OLTC, issuing an alarm if this time is different from the times observed during regular operation of the equipment. In item 4.1, there is a description of how this function detected an actual failure in one of the OLTCs.

Because this monitoring system was, at the time, the first online transformer monitoring system to operate commercially, there were many expectations around the system we discussed in this article. In fact, shortly after it went into operation, facts evidenced the gains achieved with the installation of the  system,  when  it  detected  a  defect  in an  On  Load  Tap  Changer  that,  under  other conditions, would remain unnoticed and might cause severe future losses.

This fact shows that the savings that can be obtained from using an online monitoring system to avoid severe failures are substantial, and the price, often used as an excuse to not install monitoring systems is actually low.

To our discussion, we must add the fact that monitoring systems based on decentralized architectures can be assembled modularly, according to each client’s needs and budget, allowing the system to be gradually expanded in the future.

[1] Alves, Marcos, “Sistema de Monitoração On-Line de Transformadores de Potência”, Revista Eletricidade Moderna, Maio/2004.
[2] Amom, Jorge, Alves, Marcos, Vita, André, Kastrup Filho, Oscar, Ribeiro, Adolfo, et. al., “Sistema de Diagnósticos para o Monitoramento de Subestações de Alta Tensão e o Gerenciamento das Atividades de Manutenção: Integração e Aplicações”, X ERLAC – Encontro Regional Latinoamericano do CIGRÉ, Puerto Iguazu, Argentina, 2003.
[3] Lavieri Jr., Arthur, Hering, Ricardo, “Novos Conceitos em Sistemas de Energia de Alta Confiabilidade”, Encarte Especial Siemens Energia, http:// mediaibox.siemens.com.br/upfiles/232.pdf, Janeiro/2001.
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[6] Shroff, D. H., Stannet, A. W., “A Review of Paper Aging in Power Transformers”, IEE Proceedings, vol. 132, Pt. C, No. 6, pp. 312-319, November 1985.
[7] Lampe, W., Spicar, E., Carrander, K., “Continuous Purification and Supervision of Transformer Insulation System in Service”, IEEE Winter Point Meeting, IEEE Paper A 78 111-7, January/February 1978.