Increasing the Maximum Load Capacity of Power Transformers Using a Heat Run Test Based Dynamic Thermal Model

Inhalt:
Due to the increased expansion of renewable energies and the increasing energy demand in the next few years according to the grid development plan 2045 in Germany, methods for the flexible management of transmission grids and their operating resources are becoming increasingly important. In particular, the expansion of wind energy generation is leading to a growing need for the expansion of operating resources and increasing investment costs. One way of reducing the costs and increasing the flexibility in the event of an outage is to allow a planned, temporary increase in the maximum permissible load of power transformers. Here, it is essential to take the thermal limits of the operating equipment into account. A heat run is carried out to check the compliance with these limits. The information from that heat run test and the consideration of the cooling of the power transformer can be used for the parameterization of a thermal model according to IEC 60076-7. This allows a simple thermal profile of the transformer to be created and the overload capacity to be determined as a function of the current ambient temperature. This is relevant both for determining expansion scenarios and for the management of operating resources. Within this paper, the parameter results of different heat run tests of different oil-direct-air-forced (ODAF) cooled power transformers with the same rated power (350 MVA) are used to parameterize a thermal model according to IEC 60076-7. By using the same cooling method and the same rated power, the transformers exhibit a similar but slightly varying temperature behavior. This difference in the thermal behavior results in a variance of the resulting overload capability curves from the parameterized models. In this paper, this variance is investigated and an overload potential valid for all investigated transformers is derived from the result. The more accurate the thermal modelling and thus the calculation of the hotspot temperature, the more confident a statement can be made about a possible higher maximum permissible load depending on the current environmental conditions. For an evaluation of a higher maximum permissible load of the transformers, the temperature limits recommended in IEC 60076-7 for normal cycle loading and long-lasting emergency operation are used. As a result, it is possible to estimate both higher permissible loads in normal operation and in the event of a transformer outage without permanently increasing the aging of the equipment.