A Comparative Analysis of the Results from Experimental Investigation of Imbalanced Operation Modes of a Three-phase Transformer and Full-phase Simulation Data

Abstract

Achieving better energy efficiency and quality of electrical power supply in distribution networks is a multipronged problem, which should be solved with reference to unbiased criteria. Data obtained from load flow analysis of the network under study serve as the main criterion in estimating the efficiency of technical solutions and administrative measures aimed at decreasing technical and commercial losses and improving the key electric energy quality indicators. At the same time, achieving better accuracy of a numerical analysis of electrical networks with loads unevenly distributed among the phases, including, in particular, distribution networks, still remains a significant problem in the electric power industry. The single-line equivalent circuits, which are conventionally used in engineering applications, do not allow the steady-state operating conditions of a three-phase electric network containing multiple sources of imbalance to be analyzed with sufficient accuracy. Under such conditions, it is reasonable to make a shift for using a three-phase representation of the network under study. This approach involves the need to construct three-phase equivalent circuits of electrical equipment, determine their parameters and use them for analysis. The article describes a conceptual approach for constructing the three-phase model of a three-phase two-winding transformer with a wye - grounded wye winding connection circuit, which is widely used in distribution networks. The following experiments were carried out on the test bench: open-circuit test, short-circuit test, load operation tests, open-circuit test under zero sequence supply voltage, and open-circuit test and load operation tests with one phase disconnected. Part of the experimental data was used to calculate the parameters of the three-phase equivalent circuit and to model a three-phase transformer in the MATLAB environment based on the described method.A comparison between the simulated and the experimental data testifies that the performed simulation of various transformer operating conditions yields highly reliable and accurate results. In the balanced mode, the maximum and minimum errors of simulating the main parameters were found to be 4.78 and 2.13%, respectively. In the unbalanced mode, the maximum and minimum errors of simulating the main parameters were found to be 6.6 and 0.78%, respectively. The obtained results allow a conclusion to be drawn that the proposed model of a three-phase two-winding transformer with the wye - grounded wye winding connection circuit adequately reflects the properties of a real transformer. The proposed technique allows the key properties of a three-phase transformer to be revealed, namely, the redistribution of magnetic fluxes among the magnetic core legs and the presence of voltage across the secondary winding when one of the primary winding phases is open. It can also be inferred that a three-phase transformer has a balancing effect.