Abstract

The step-up transformers at the Euzébio Rocha Thermal Power Plant in Cubatão – SP [São Paulo] are key for the Brazilian national electric power supply. Due to their significance and to their severe operating conditions, Petrobras has always invested on and kept a strict preventive maintenance program for this equipment. Leader in technology and committed to keeping a high power supply quality level, Petrobras installed an online monitoring system on its step up transformers, with the purpose of identifying early possible incipient defects and mitigate catastrophic failure risks even more.

The implementation of this system, with sensors and software, took place in 2014, and four transformers which were already operating at the plant began to be monitored. The implementation of the monitoring system used IEDs which were already in place and took advantage of the modularity and expandability features of the decentralized architecture.

Here we describe the architecture and the solutions used in system implementation, as well as the results obtained, successfully reaching goals such as maintenance focused in reliability and greater personal, equipment and facility safety.

Natural gas-fired Euzébio Rocha Thermal Power Plant, in Cubatão-SP, one of Petrobras’s power plants, contributes with over 200MW to the Brazilian Electric Power System. With the ability to supply a 600-thousand inhabitant city with electric power, it demands great operating efficiency to guarantee seamless electric power supply.

It operates with two step-up transformers: a 225 MVA one and a 70 MVA one, plus two ancillary 16MVA transformers. They entered operation in 2009.

These transformers play a key role in the transmission of the electric power generated by the plant. Due to their significance and to their extreme operating conditions, including high temperature, strongly corrosive atmosphere and the normal dielectric requirements for system operation, Petrobras has always kept a strict preventive maintenance program for them.

However, conventional practices are not enough to ensure plant reliability. Therefore an online monitoring system was implemented to reduce failure risk with the detection of the main types of defects at an incipient phase – the monitoring of the transformer operating status.

Some of the sensors which were needed for the project were already in place in the transformers and were used.

An international research of operating power transformers performance [1], carried out by Cigré with data from failures occurred between 1968 and 1978 involving over 1,000 failures revealed the main causes of downtime, both forced and planned, for several transformer kinds and applications.

Figure 1 shows those data for NLTC transformers in power plants. According to those statistic data, the bushings are the main cause for transformer failure, responsible for one third of the occurrences, with the active part virtually tied with it, in second place, followed by tank and oil. These three subsystems of the transformer together answer for virtually 84% of the equipment downtime.

Based on these statistics and on the previous experience by Petrobras, step-up transformer and ancillary transformer variables to monitor were selected. This was done in addition to the required data processing functions so the measured variables could be turned into useful information for transformer diagnosis and prognosis. That procedure aimed at covering the main transformer subsystems and therefore reducing failure and unavailability risks.

Figure 1 – Statistics of the causes for NLTC transformers downtime [1].

The variables measured in step-up transformers in Mauá and Cubatão plants are shown in table 1, grouped by subsystem.

In order to completely comply with its objectives, the monitoring system must process the data listed above so useful  information is obtained about transformer status, as detailed next

Variables monitored in the step-up transformers grouped by subsystems

To handle the data obtained from the sensors mentioned above, the installed monitoring system has in its software an Engineering Module, which includes the engineering models shown in table 2

Therefore, raw data from sensors allow of useful information to be obtained for transformer status diagnosis and prognosis [2], [3]

Euzébio Rocha Thermal Power Plant ‘s transformer monitoring system architecture is shown in figure 2.

Variables already shown in Table I were measured through intelligent sensors (Intelligent Electronic Devices), designed and tested to be specifically used in the substation yard environment, which allows them to be installed directly in the transformer body.

Those sensors are provided with communication ports that allow them to be directly connected to a communication network, which makes it possible to directly transmit measurements to a transformer monitoring server. The sensors communicate with the server via Modbus-RTU protocol.

The server, in turn, also communicates with the Supervisory Control System of the Thermal Power Plant UTE-EZR through OPC protocol. Therefore the data can be monitored by the operations team, stored in a history server or even be made available for other systems.

In the transformer monitoring server, the Sigma monitoring software, which processes the date shown in Table II, is executed.

Through Petrobras’s Intranet, the users have remote access to the monitoring system through an HTML interface with Web 2.0 technology.

Figure 2 – Transformer monitoring system architecture

1. ELECTRA, “An International Survey on Failures in Large Power Transformers in Service”, Paris, CIGRE, Ref. number 88 1983
2. Ammon, Jorge Alves, Marcos, Vita, Andrew, 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” (“Diagnostic System for High Voltage Substation Monitoring and Maintenance Activity Management: Maintenance Management Activities: Integration and Applications “, X ERLAC – Latin American Regional Meeting of CIGRÉ, Puerto Iguazu, Argentina, 2003.
3. Alves, Marcos, “Sistema de Monitoração On-Line de Transformadores de Potência” (“Online Monitoring System for Power Transformers”), Revista de Eletricidade Moderna (Modern Electric Power Journal), May 2004.
4. Melo, Marcos A.C., Alves, Marcos, “Experiência com Monitoração On-Line de Capacitância e Tangente Delta de Buchas Condensivas” (“Experience with On-Line Monitoring and Tangent Delta Condensive Bushings”), XIX SNPTEE – National Seminar on Production and Transmission of Electric Power. Rio de Janeiro, Brazil, 2007.
5. Alves, Marcos, Silva, Gilson, “Experiência de Campo com Monitoração On-Line de um Transformador 343MVA 230kV, com 2 Comutadores sob Carga” “Experience in the Field with Online Monitoring of a 343 MVA 230kV Transformer with two OLTCs”, IV Workspot – Workshop on Power Transformers, Recife, Brazil, 2005.
6. Alves, Marcos, Vasconcellos, Vagner, “Monitoramento da Umidade no Óleo Isolante de Transformadores de Potência Visando o Aumento da Confiabilidade Operativa” (“Monitoring the Moisture in the Insulating Oil of Power Transformers to Increase Operating Reliability”), V Workspot – Workshop on Power Transformers, Belém, Brazil, 2008.