The equivalent T‑network of any two‑winding transformer qualifies as the standard model of three‑phase transformers that is used for the network calculations in electrical three‑phase systems. This equivalent T‑network is derived from the detailed geometrical model of the transformer with some simplification for the network calculations in the low frequency range. This thesis investigates an alternative model of a three‑phase transformer for the calculation of an unbalance network states due to the voltage unbalance conditions. This alternative model reproduces the behaviour of some kind of network failures better than the equivalent T‑network. Furthermore, this model shortly named topology-based model (germ. TK‑Modell) enables the possibilities to involve the geometrical topology of the transformer core.

This thesis investigates the propagation of voltage unbalance in an energy supply system and its impact on the three‑phase transformer. The three‑phase transformer is represented by the three-legs core-type power transformer. Furthermore, the knowledge about the phase-element voltages on the winding of the transformer are obtained during the voltage unbalance. This voltage unbalance on the terminal of the transformer is the reason of the unbalanced network state. The theoretical description of the network unbalance contains the description of behaviour of the three‑phase transformer under consideration of fault conditions. Furthermore, the equivalent circuit of the topology‑base model of the three‑phase transformer is derivate for the calculation in symmetrical components. This equivalent circuit supports the analysis and evaluation of the magnetic coupling between the limbs of the transformer under the voltage unbalance.

Furthermore, the conductor continuity one‑phase fault is investigated in an electrical energy supply system. On the one hand, the network protection equipment has some difficulty with a triggering of this fault type. On the other hand, the behaviour of the electrical network depends on the geometrical dimensions of the transformer's core and on the neutral point treatment of the network during the conductor continuity fault. The main goal of this thesis is the investigation of the phase voltage on the terminal of the power transformer in model network with high integration of renewable energies. This investigation considers different types of vector group and neutral point treatment in the electrical network.

The double-feed asynchronous generator and the full‑size converters represent the wind energy plants modelled in this thesis. As stipulated in the connection conditions at the network connection point, the wind energy plants take the part in the supporting the voltage in the network. Therefore, the wind energy plants are feeding the reactive current in the positive and negative sequence system directly proportional to the under‑voltage conditions in positive and negative sequence system. Thus, supporting the voltage has a positive impact on the reduction of the voltage unbalance; especially in a case of an unbalanced short‑circuit conditions in the electrical network. The part of this thesis is investigation of the participations levels of the wind energy plants with the active supporting of voltage during the one-phase series fault.

Moreover, the unbalanced network states have impact on the ground current and on the neutral point displacement voltage. These depend on the neutral point treatment in three‑phase network and the personal protection measures. Both are important for an assortment of the suitable protection equipment. The unbalanced network states with a special consideration of the one-phase series fault are difficult to show in three‑phase network by standard protection equipment. The method of the detection of the series fault is elaborated in this thesis. The monitoring of the line-to-earth voltage makes the detection of the unbalanced load of limbs possible in a consequence of the conductor continuity one‑phase fault. Due to exceeding, the setting value a trigger or status signal can be sent to a control centre.