A Practical Core Loss Estimation Method for Three-Phase Three-Level Grid-Connected Inverters

In a three-phase three-level grid-connected inverter, the ac output current ripple actually fluctuates four times per switching period with each fluctuation generating some core loss. However, only the two larger fluctuations are considered in most loss estimation methods, resulting in a smaller core loss evaluation. In this letter, an alternative method has, hence, been proposed, beginning with the deduction of fundamental and ripple equivalent circuits from the inverter. These circuits permit current ripple to be calculated, from which its four fluctuations are all considered for forming the dual current-ripple envelopes (DCREs). These DCREs, together with the fundamental current and a selected core loss model, can then be solved to give the estimated fundamental and ripple core loss values. The nonlinear characteristic of the filter inductor has also been considered in the calculation of the DCREs and the core loss. The estimated values have eventually been verified through testing with a 10-kW three-phase T-type three-level grid-connected inverter.