SIGMA – Hydro On-line Monitoring Sub-system Oil and Solid Insulation Humidity

The presence of an excess water in a power transformer’s insulation system, in either the oil or solid insulation, causes diverse well-known negative effects. These including the lowering of the dielectric rigidity at low temperatures and the migration of water to and from the insulation paper, with the risk of bubble formation and rapid ageing at high temperatures, threatening the useful life and hence the integrity of the equipment.

SIGMA Hydro Monitoring Sub-system  is a software module which, together with the Treetech Moisture  in Oil Monitor  MO and  the  Temperature Monitors TM1 and TM2, carry out on-line  monitoring  of the water in oil and  paper  (solid insulation), making important  information available  for a diagnosis and prognosis of the state of the equipment:

• Actual percentage saturation of water in the oil and oil and winding temperatures;
• Water content in the oil (ppm), calculated based on the percentage saturation of water and oil temperature;
• Water content of the paper, as a percentage of the mass of dry paper, calculated based on the measurement of the percentage saturation of water in the oil and oil and winding temperatures;
• Water content of the paper and percentage saturation of water in the oil, converted to a second measured temperature, enabling an understanding, for instance, what will the water content in the paper and the water saturation  in the oil be if the transformer  were to be de-energized  and cooled down to reach the ambient temperature;
• Water content of the paper and percentage saturation of water in the oil, converted to a Reference Temperature selected by the user, enabling an understanding, for instance, what will the water content in the paper  and the water saturation  in the oil be at the smallest  temperatures  expected during the transformer’s operation;
• Acceleration factor in the thermal loss of life as a function of the water content of the paper;
• Maximum temperature allowed without risk of bubble formation;
• Trend and the time required to reach temperature values and the risk of the bubble formation, calculated with current loading;
• Trend of rise and reduction of water content in the oil (ppm/day) and in the paper (%/day).

Based on the above requirements, limit values for alarms for the various variables, water content saturation, percentages, safe maximum temperatures and the evolution trends are set. This establishes a platform for the Diagnosis and Prognosis of the condition of the transformer, allowing for the detection in the earliest stages conditions as they evolve that will introduce a risk of failure to avoid greater problems later on.

SIGMA Hydro can also be used for on-line monitoring the percentage saturation of water and water content in load tap changer oil, thus increasing the reliability in the operation of this equipment as well.

SIGMA Hydro  facilitates remote access of on-line measurements,  diagnostics and prognostics, as well as to historical data as stored in data banks. Remote access is carried out over the Intranet or Internet network, using a 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 alarm 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.