Research on the Power Output Characteristics of a Coupling Transformer in D-FACTS

The series coupling distributed flexible AC transmission system (D-FACTS) device couples the secondary side compensating reactance to the primary side based on the principle of transformer, thus realizing the goal of adjusting the transmission line parameters and controlling the distribution network flow. The coupling transformer is the most important power conversion part in the D-FACTS device but has different working principles of mutual inductor. The magnetizing current of the coupling transformer changes within a large range, which makes the traditional power output model of mutual inductor inapplicable. In this paper, the power output model of coupling transformer was constructed by viewing the coupling transformer as a constant current source. Moreover, the corresponding relationship between the output power of the transformer and related parameters, as well as between variations of reactive output power and active loss power with load changes, was analyzed. Some conclusions were drawn: (1) the output power characteristic curve of the coupling transformer was acquired; (2) given a constant line current, the maximum power was achieved when the system’s capacitive reactance and the internal resistance of the transformer matched, and the maximum power was unrelated to the number of turns of the coupling transformer; (3) the output power changed with the variation of the line current, and there was a proportional relationship between the maximum power and the square of the line current. Next, the relationship between the maximum output power and the air gap thickness of the magnetic core was discussed. Finally, these conclusions were proved accurate via experiments.