Analysis of Losses in Cables and Transformers with Unbalanced Load and Current Harmonics

The power quality problems are more and more serious in the power system. The harmonics and the three-phase imbalance have a greater impact on the line loss of power grid. We focus on the power losses in cables and transformers with unbalanced load in current harmonics basing on the equivalent resistance method. The impact caused by the skin effect is illustrated in order to enhance the accuracy of the theoretical calculation. For this purpose, a mathematical development that takes in account the presence of current harmonic components and unbalanced load in the losses calculation is presented. The reliability of the method is verified in a simulation model. According to the comparative analysis between the simulation and the proposed method, we can conclude that the harmonic resistance calculation method considering skin effect is more accurate than the traditional method. The results show that the additional loss caused by the harmonics has a nonlinear relation with the harmonic currents. At the same time, the additional loss of distribution network under compound disturbance with unbalanced load in current harmonics is studied. Compared with the cases that two phase load are heavy and one phase load is light or one phase load is heavy and other two phase load are light, when one phase load is heavy, one phase load is light and one phase load is the average value of the others, the additional loss of the system is the least one. The additional loss for the case with that two phase load are heavy and one phase load is light is the biggest one.