It is widely known that bushings are one of the main failure points in transformers and power reactors, a concern increased with the occurrence of many failures in the last years, some followed by fires and explosions.
Recent studies performed by Furnas Centrais Elétricas involving many defective bushings showed that the alterations in the capacitance and power factor are safe defects indicators also in initial phase, opening the way to avoid catastrophic failures. The measurement of these parameters is traditionally performed offline, which implies in transformer shut-down in intervals of few months in critical cases.
In order to avoid these shut-offs, in November, 2005, the online monitoring system of capacitance and power factor BM from Treetech started operation at Serra da Mesa Power Plant. 9 HSP and ABB bushings are monitored, 3 of 500kV and 3 of 230kV in a single-phase transformers bank and 3 of 500 kV in a single-phase reactors bank manufactured by Siemens.
The online measurements of capacitance and tan delta are remotely followed by Furnas maintenance team, through the company’s Intranet. This is possible because the serial ports BM have open protocols, allowing the use of the Bushings Monitor in a isolated or integrated way to other monitoring systems.
With installed capacity of 1,275MW, Serra da Mesa plant serves the state of Goiás and the Federal District, besides promoting the connection between South/Southeast/Midwest and North/Northeast interconnected systems, in a complex system which gets more reliable with the online monitoring of Treetech bushings.
Bushing Monitor – BM
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.