Solid-state transformer (SST) is a recent evolution of power-electronics-based conversion technology that has the potential to revolutionize many fields. It fulfills all the functionalities of traditional low-frequency transformers and enables additional characteristics, such as increased power density, improved output power quality, and flexible power flow regulation. The input-series-output-parallel structure is one of the most common configurations of SST, as its modular structure allows for the easy expansion of the input voltage rating.

When the input-series-output-parallel SST is applied in single-phase medium-voltage AC to low-voltage DC conversion, all SST cells suffer from the second harmonic current from the grid, leading to DC link ripple voltages and increased current stress on components. Large DC link capacitors are employed to smooth the ripple voltage, which reduces the power density of the SST. Although the mitigation of second harmonic ripple voltage in single-phase AC/DC converter has been extensively explored, the unique structure of SST poses additional challenges to this problem, and many existing solutions need to be reevaluated. This thesis provides a comprehensive analysis of the second harmonic components in SST and proposes both software-based and hardware-based solutions. Although the analysis is based on the topology and parameters of an existing SST research platform, the conclusion of this thesis can be generalized to other similar applications.

This thesis first looks into the propagation of the second harmonic current. The SST using open-loop modulated LLC converters as the isolated DC/DC stage is particularly analyzed. An equivalent circuit model is introduced to analyze the coupling effect between the LLC converter and the DC link capacitors. The conclusion indicates that an inappropriate parameter design may cause excessive ripple voltages and reduced efficiency. Based on the analysis, the optimized design for DC link capacitors is provided. A variable frequency control method of the LLC converter is proposed. By adding an additional control loop, the switching frequency and voltage gain of the LLC converter are regulated to prevent the propagation of the second harmonic current, which reduces the ripple voltage and current stress of the SST.

Active power decoupling is a promising solution for the second harmonic ripple mitigation. By adding active power filters to the original circuit, the second harmonic current is diverted from the DC link capacitor and buffered by dedicated components, which reduces the ripple voltage and the required DC link capacitance. Although it has been applied in typical single-phase AC/DC converters, the application of active power decoupling in SST is notably limited. This thesis investigates the feasibility and potential benefit of applying active power decoupling to single-phase SST. Various active power filter topologies are compared, and the buck-type topology is selected. The design of the active power filter, including component sizing, circuit diagram, and control algorithm, is presented in detail. The coupling between different SST cells may lead to instability. The mathematical model of the coupled SST cells is established, and the stability analysis and controller design guideline of multiple active power filters in SST is presented. Results show that a greatly decreased second harmonic ripple voltage is achieved with very small DC link capacitance.