What actually limits a transformer’s overload capacity is the temperature of its winding. Although windings undergo tests to show that their temperature does not rise above industry standards, such tests give the average temperature across all the winding’s parts. Its hottest area is known as the “hot spot” and is the real limiting factor. If no fibre optic sensor is installed in the winding, the hot spot cannot be accessed for measurement. And even then measurements yield only the current state of overload. “With modelling you can determine future developments,” explains Dolata. New development of thermal model and pilot projects have been started to calculate the hot spot, which in turn enables it to continuously calculate a transformer’s permissible overload.
The implemented thermal model based on principles of IEC standards is ample for calculating the temperatures of the hot spot and top oil in normal cyclic loading conditions where the load factor does not exceed the IEC´s standard 1.3 at a maximum hot spot of 120°C and 105°C for top oil temperature. Furthermore, under steady state condition, the modelling used calculates the time needed to overload the transformer in short-time emergency loading condition. However, in order to cope with sharp fluctuations in load or heavy emergency overloads, where IEC 60354 or 60076-7 requires transformers to withstand overloads of 1.5 times the rated current for up to 30 minutes at a maximum hot spot temperature up to 160°C, the thermal model has to be improved and advanced. The use of such continuous overload diagnostic models enables dynamic load management. The importance of continuous overload monitoring is thrown into sharp relief by the fact that the hot spot temperature of 120 °C, although permissible by international standards, nevertheless increases ageing by a factor of 12 compared to running at a temperature of, for example, 98 °C for no upgraded paper insulation.
Its hottest area is known as the hot spot.
For high-precision monitoring of continuous overload capacity, the insulation’s moisture content should also be factored into the thermal model. As the transformer heats up, moisture migrates from the paper into the oil. When moisture in the oil exceeds 2 %, residual water may become trapped in the paper and then escape in the form of water vapour bubbles. These bubbles would go with the oil flow or get trapped in the winding, causing a possible breakdown of the insulation. Furthermore, water content in the oil of 4 % accelerates ageing by a factor of 20.
Key factors and figures
The lifetime of its paper insulation determines a transformer’s life expectancy. Temperature is one of the main factors of cellulose chain ageing. The figure above shows the sensitivity to temperature of non-thermally upgraded paper in an oxygen-free environment. Its life expectancy is reduced at even faster rates if oxygen or moisture is present. Overload is the main factor causing high temperatures. To determine a transformer’s overload capacity its hot spot has to be calculated. The MS 3000 monitoring and diagnostic system incorporates a thermal model to assess a power transformer’s overload capabilities within the limits set by the IEC.