The case and the customer

The Mesquita Substation is located in the city of Santana do Paraíso – MG, close to the city of Ipatinga, in the eastern region of Minas Gerais. This substation supplies the power of large consumers of electrical transmission system such as USIMINAS, ARCELOR MITTAL and CENIBRA. It went into commercial operation in June 1981. Currently the installed transformation power is 1,200 MVA, which is composed of three 500 / 230 kV autotransformers with 400 MVA each, two 500 kV and five 230 kV transmission lines.

In mid-2005 Cemig, in a pioneering way, installed the Treetech BM condensive bushing monitor in the 550 kV and 245 kV bushings of the Autotransformer T-2 of the Mesquita SS (Substation). On this occasion the system was also installed in a 550 kV free-standing CT (current transformer) which is part of the same bus of this autotransformer. The choice was motivated by suspicions in relation to a specific family of bushings with history of failures in the electrical sector.

Thus, Cemig began to have easy access to all data and diagnoses issued by these sensors. These data are used by Cemig’s maintenance to improve the use of the asset and to foresee possibles failures in-between the maintenances and routine checks.

Efficiency of the Treetech Solution

At the beginning of 2011, the Bushing Monitor System indicated a High Capacitance trend alarm in one of the bushings. Aside from indicating that the High Capacitance alarm would be activated from that moment to nine days, the BM pointed toward a progressive trend in the Tan Delta, which would shift to high within 63 days. Treetech’s technical team was notified and, after thorough analysis, confirmed the recorded data in the BM.

Cemig scheduled the disconnection of the transformer for off-line data analysis, which could prove the situations pointed out by the monitoring system, and prepared itself as well for a possible change of bushings, in case the problem was confirmed.

The increasing trend of the capacitance and tan delta continued as it was foreseen until the day prior to its disconnection, when the monitoring system pointed a sudden progression of the tan delta. From this record, en emergency disconnection was authorized for assessment of the bushing’s condition.

The results obtained through on-line monitoring were confirmed by the tests conducted directly on the bushing, which had to be changed. A later analysis of the bushing oil proved the loss of its insulation capacity and Cemig’s judgment in deciding for the disconnection of the transformer.

Thus, the on-line bushing monitoring offered by Treetech through the Bushing Monitor – BM – proved to be valuable for reduction of maintenance costs through improvements in the equipment conservation planning process and in the intensive use of the condition-based maintenance paradigm, reducing disconnections for routine tests. Additionally, on-line monitoring has provided valuable data on the type / condition and time / duration of progression of the defects, in this component.

A technical article elaborated from this case was presented at the 22^nd National Seminar on Electrical Power Production and Transmission (XXII Seminário Nacional de Produção e Transmissão de Energia Elétrica). To access the article, click here.

The bushing monitoring system granted to Cemig GT for experimental application and installed in an auto transformer of the Mesquita SS reported capacitance variation in a 230 kV bushing. This bushing was replaced and after laboratory tests and dismantling it was established that the bushing had the informed defect.”

Luiz Henrique Silva Duarte, Cemig Transmission Maintenance Planning and Engineering Manager.

The IED monitors continuously the tan delta, capacitance and leakage current in the bushings, CT’s (current transformers) and PT’s (voltage or potential transformers) of the power transformers. Installed in over 800 bushings worldwide, the IED provides complete diagnosis of the insulation condition of these pieces of equipment. Thus, potentially catastrophic failures can be prevented, in that the problems are detected still in the incipient phase.

The figure on the side represents the three parts of the BM (click on the image and learn more about the product):

• the tap adapter ensures the electrical connection between the bushing tap and the IED, weather-proof sealing and protections against overvoltages and overcurrents, aside from preserving the tap against possible accidental opening of the measurement circuit;
• The BM-MM measurement module receives leakage currents from up to three bushings of the same three phase assembly and carries out the measurements of these currents, as well as their mathematical and statistical processing.
• The BM-HMI interface module receives the information from the measurement module and makes it available on its front monitor. Aside from this, it has analog outputs, alarm relays and serial communication ports available to the user.