SIGMA – Ageing On-line Monitoring Sub-system Insulation Ageing
Under normal operation conditions, a useful life of a transformer is principally determined by the life of its insulation. Whilst an aged insulation would not be directly responsible for ending a transformer’s working life, in the majority of cases, the deterioration of its mechanical properties increases the risk of failure in the presence of forces caused by short-circuit faults, thus making the equipment less reliable.
If the contaminating agents (oxygen, acidity and water) of the oil are controlled, the life expectancy of the insulation will predominantly deteriorate cumulatively due to thermal effects, the rate of ageing being determined by the winding temperature (the hottest spot would be the worst effected).
The SIGMA Ageing Monitoring Sub-system is a software module, which, together with the Treetech Temperature Monitors TM1 and TM2, carries out on- line monitoring of the loss of useful life of the insulation, making important information available for diagnosis and prognosis of the equipment’s state:
- Actual percentage useful life remaining, from 100% (new insulation) to 0% (end of insulation life);
- Average rate of loss of insulation life, in % per day, calculated from a period selected by the user;
- Extrapolation of the insulation life remaining, calculated as a function of the variables above (percentage of life remaining and the average rate of loss of life);
Based on the requirements above, limit values are set by the average daily rate of loss of life together with the remaining life of the insulation, providing alarms, in the event that these limits are overtaken.
Optionally, if the SIGMA Ageing Sub-system is used in conjunction with the SIGMA Hydro Sub-system, the calculation of the water content in the paper performed by the latter will be used to adjust the acceleration of the loss of thermal life of the paper using the value of water content.
The SIGMA Ageing permits remote access to on-line diagnostics and prognosis and to historical data, stored in data banks. Remote access is carried out over the Intranet or Internet network, using any standard Internet browser. The system can also send alert messages by e-mail or text messaging to mobile phones (SMS) when the system diagnoses an abnormal condition, freeing operators from the need for permanent supervision of the system.
SIGMA Monitoring Systems and Sub-systems architecture
The SIGMA monitoring system is based on a Decentralized and Modular architecture, in which Intelligent Electronic Devices (IEDs) are used as sensors (data sources) interconnected to the SIGMA monitoring system over a communication network. Frequently these IEDs are pieces of equipment that are essentially used in transformers in protective, supervisory and control functions, supplying information to the monitoring system through one of their serial ports. This makes centralizing units at the body of the transformer unnecessary, this architecture eliminates the necessity for specific sensors, resulting in a significant reduction in the cost of the system. This is the case with SIGMA Ageing Sub-system, as shown in the diagram.
SIGMA monitoring software, obeying the same philosophy as the Treetech IEDs, is composed of independent and scalable modules called Sub-systems, allowing the user to build their own monitoring system, according to their needs. As such, the Treetech modular systems allow the monitoring of small-scale transformers, eliminating the financial barriers that previously limited the application of monitoring systems to large-scale equipment only.