Contents I. Introduction
Conventional line frequency (LF) distribution and traction transformers have low power densities that range from 0.2 to 0.4 kW/kg [1]. Instead of LF transformers, solid-state transformers (SSTs) have been investigated extensively in recent years for medium-voltage (MV) grid-tied applications because of their high power density and controllability [2]–[5]. Fig. 1(a) shows one of the many configurations of SSTs. An isolated dual-active-bridge (DAB) converter that has two bridge converters and one high-frequency (HF) transformer is the key component of the SST [6]–[8]. In particular, a well-designed HF transformer with high efficiency, high power density, low parasitic capacitance, and high voltage isolation is essential for SSTs. However, it is very challenging to develop an HF transformer that simultaneously possesses all the desired characteristics. There is a tradeoff relationship between the required performances of the transformer. For example, the primary and secondary windings of the HF transformer should be located as close as possible to increase the magnetizing inductance and efficiency. However, the proximity of the windings increases the copper losses of the windings and it makes difficult to provide high-voltage insulation. Therefore, many researchers have investigated design methodologies for HF transformers of DABs. Fan and Li developed a planar transformer using printed circuit board windings [9]. The developed planar transformers had uniform circuit parameters as well as low core and winding losses. Du et al. studied the magnetic core materials and winding methods of HF transformers [10]. The study used split-core and separate windings to achieve the required leakage, magnetizing inductance, and high-voltage insulation performance. Bhattacharya et al. studied an oil-free insulation strategy for an HF transformer [11]. They added a thin polypropylene film into an air gap between the C-type cores. Mao and Wei developed a model for determining parasitic capacitances in an HF transformer [12]. The effects of eddy-currents and nonuniform magnetic field distributions are considered in the proposed model. Some studies have demonstrated the use of a DAB converter with inductive power transfer (IPT) coils. Azad et al. proposed a bidirectional IPT system using the DAB topology as the battery charger of an electric wheelchair [13]. However, this article limited its discussion to IPT systems, and a multilevel SST using loosely coupled IPT coils was not explored.
Comparison of conventional and proposed SST topology. (a) Conventional SST. (b) Proposed LCR-SST. (c) Proposed LCR-SST with multiple primary modules and a few secondary modules. (d) Proposed LCR-SST with multiple primary modules and a single secondary module.
