Minimizing Transformer No-Load Losses at Hydropower Plants : A Study of Effects from Transformer Switch-Off During Stand-by Operation

Sammanfattning: Hydropower is the most important power balancing resource in the Swedish electrical power system, regulating the power supply to match the load. Consequently, several hydropower plants have periods of stand-by operation where the power production is absent but where several devices within a plant are still active. Such a device is the step-up power transformer, which during stand-by operation still generates no-load energy losses. These losses can accumulate to a considerable amount of energy and costs during the long technical lifetime of the apparatus. One option to minimize these no-load energy losses is by turning the transformer off when its generating unit is in stand-by operation. However, when this transformer operational change has been explained to experts in the field, the most common response has been that a more frequent reenergizing of a transformer leads to higher risks for errors or transformer breakdowns. This study aimed to analytically investigate three effects from this operational change. First, the potential of fatigue failure for the windings due to the increased sequences of inrush current. Secondly, the thermal cycling as a consequence of change in present losses. Lastly, the energy and economic saving potentials for hydropower plants where this operational adjustment is applied. The study used both established as well as analytical tools explicitly created for this study. These were then applied on currently active transformers in different plant categories in Fortum’s hydropower fleet. The study primarily showed three things. Firstly, risk of fatigue failure due to the increased presence of inrush currents did not affect the transformer’s technical lifetime. Secondly, the thermal cycling changes were slightly larger with absent no-load losses during stand-by operation. The average temperature for the transformer decreased, which in general is seen as a positive indicator for a longer insulation lifetime and thus the transformer’s technical lifetime. Finally, the created frameworks showed the potential of saving energy and money for all plant categories, where the potential grew with the installed production capacity and the stand-by operation timeshare. Despite the simplifications made to describe the complex reality of a transformer operating in a hydropower plant, this thesis contributes to lay a foundation for future investigation of an easy adjustment to avoid unnecessary energy losses and costs for transformers in hydropower plants