No abstract

Insulation dimensioning requires accurate calculation of dielectric stress levels. These need to be conducted for various critical electrode configurations inside a transformer under different voltage test levels. Advanced computational tools are being used to estimate stress levels, which can then be compared to standard withstand levels. Available commercial tools for electric field calculation mostly use one of the two calculation methods; Finite Element Method (FEM) or Boundary Element Method (BEM). The paper provides a comprehensive analysis of these methods, describing their advantages through real case simulations.

Power transformer in service is exposed to various voltage and current stresses. The ability to withstand short circuit is an essential requirement for power transformers. There are different types of short circuit: single - phase to earth, double - phase with or without simultaneous earth fault and three - phase short circuit. These various short circuit conditions result in different stress conditions for the windings. Mesh analysis and symmetrical components are the two methods most commonly used for determining the magnitude of short circuit currents. In this paper, both methods will be presented with results compared on a real-case transformer. Also, a generic scheme using the symmetrical components approach is designed in order to standardize the short-circuit currents calculation for all power transformer types and to reduce the time required for obtaining results.

During voltage transients, the windings of a transformer are coupled by electric and magnetic fields. To calculate the transients inside the transformer, a network model is typically used. The accuracy of obtained calculation results depends mainly on this model in which the windings are lumped into R, L and C circuit components. The windings are usually represented by discs, or groups of discs, with the corresponding resistances, inductances (self and mutual) and capacitances (series and to the ground) [1]. In case of impulse voltage, wave’s steep front and consequently high frequency oscillations are the main reason why capacitances modelling is of major importance for the calculation of voltage distribution in winding and between windings.

The subject of the analysis is a 300 MVA auto-connected power transformer in different loading conditions with regard to the load losses. During the electrical design time, some operating points of the transformer were analyzed in more detail using 2D electromagnetic field finite element method (FEM) software. The models included 2D magnetic stray field calculation and covered a range of transformer loading cases that covered some that are more difficult to solve with traditional analytic methods based on the static magnetic field calculations. This is due to the presence of a phase shift between the currents through the windings. The results of the static magnetic Rabins’ method field calculation and the FEM method are compared and the best practice method is defined and determined accordingly.