Mitigation and adaptation to climate change have been major driving forces in modernizing electric power infrastructure. Low power quality has a significant impact on electrical equipment efficiency. Voltage unbalances one of the issues with low power quality. This study used a simulation method for a power supply with a voltage unbalance to evaluate the effect of voltage unbalance on the performance of 3-phase transformers. The research results show that when voltage unbalance occurs, the working point with the maximum efficiency shifts and transformer efficiency is reduced in all load cases. When there is an increase in VUF, the efficiency of the transformer is reduced. With a small VUF, the drop is insignificant, but when the VUF is greater than 3%, the drop starts to increase. In the case of voltage unbalance, when the positive sequence voltage is greater than the phase voltage, both the no-load loss and the load loss increase significantly. Both these losses are reduced when the positive sequence voltage is less than the phase voltage. Due to phase shift, both these losses are almost unchanged. The research results are the starting point for general studies on voltage unbalance and power quality.