## 1 Introduction  
The accelerated growth in recent years of the number of single-phase and three-phase Distributed Energy renewable Resources (DERs), has caused a series of additional technical and economic problems, resulting from the need to ensure a high level of performance of the Electric Power Distribution Systems (EPDS) of low and medium voltage. Among these, the most important are:  
* increasing the power transfer capacity of some lines (sections of lines), transformer substations or transformer stations;
* ensuring the technical conditions that allow for maximizing the energy generated by the distributed sources;
* ensuring the appropriate quality of power delivered to consumers by limiting the level of disturbances produced and propagated in the network (power frequency disturbances, electromagnetic interferences, transients, harmonics and low power factor, overvoltages, voltage fluctuations, voltage unbalances, etc. [1]). The article refers to a particular disturbance present in the three-phase three-wire medium voltage distribution networks containing distributed sources of electrical energy: the active and reactive power flow unbalance on the three phases of lines and transformers, going in two ways through various sections of the network. Unbalances in the active and reactive power flow in a medium voltage distribution network are determined by unbalanced loads fed directly from this network or resulting in propagation from low-voltage networks. Over these the unbalances of active and reactive powers generated by single-phase wind or photovoltaic sources are superimposed, which are unevenly distributed, have an intermittent, variable, and non-simultaneous operation and are present, especially at the single-phase prosumers from the low-voltage networks connected to it. The phenomenon is amplified by the presence of an increasing number of electrical energy storage systems, an important part of which is represented by EVs (electric vehicles) that can be used as controllable loads, energy storage systems, or power supply sources [2]. An important characteristic of these networks is the fact that the operating regimes of DERs can hardly be correlated with those of consumers, thus resulting in numerous time intervals, longer or shorter, in which the generated power is in excess. In these time intervals, the excess power generated is transferred to the higher voltage network, which means changing the direction of power flow through the HV/MV transformer station. As a result of the accentuation of the unbalances, this change of direction is performed in one, two or three phases, at different times and for different durations. In fact, there is an increase in the propagation of unbalance disturbances in the higher voltage network. The increase in power flow unbalances in the network is accompanied by the voltage asymmetry accentuation, the increase in the deviations of the rms voltage values above the admissible limits, the overloading of some sections of the current paths, and the pronounced decrease in the power factor [3]. These disturbances negatively affect the operation of distribution operators' installations by increasing the difficulties of operating the network, mainly in terms of voltage regulation systems, protection, and control systems, especially because of two-way power transfer. The accentuation of voltage asymmetry in the points of common coupling (PCC--Point of Common Coupling) between the network and consumers can be considered as an accentuation of the quality deterioration of the energy delivered to consumers. The three-phase receivers, especially transformers and induction motors, are the most affected by this disturbance. In the case of the three-phase transformer, the asymmetry of the supply voltages causes the increase in losses in the copper and therefore the increase in the temperature in some areas inside. This results in a decrease in efficiency and a reduction in the lifetime of the transformer as a result of accelerating the ageing of the insulation [4; 5].