GaN enabled high step-down bidirectional ac-dc converter for grid-tied battery energy storage system(BESS)

With the increasing penetrations of renewable energy resources, the energy storage system (ESS) is becoming necessary to minimize the impact of the variable power generation on the grid operation. Among many different types of storage, the battery energy storage system (BESS), mostly based on the Li-ion battery, is the fastest-growing due to the decreased system cost, and fast real and reactive power dispatch capabilities, which can be used for various applications such as voltage and frequency support, as well as for economic dispatch applications. BESS requires a bidirectional power electronics converter between the dc battery and the ac grid while having the capability to handle a wide range dc battery voltage. For interfacing with a three-phase ac grid, a single-stage configuration is used. The single-stage design can eliminate the ac grid side contactor, as it has a small ac-link capacitor and the inrush current is small. The three-phase single-stage design can also minimize the bulky dc-link capacitor. The proposed design includes three identical single-phase modules. Each module includes an unfolding bridge and a single-stage bidirectional (DAB) or series-resonant dual-activebridge (SR-DAB). This modular topology can also be used for both the single-phase and three-phase grid-connected BESS. The unfolding bridge will rectify the ac voltage to twice the line frequency in ac-dc operation (charging of the battery), or invert the voltage to the ac grid for dc-ac operation (discharging of the battery). In the meantime, DAB can convert ac energy with the absolute value of the sinusoidal voltage to the battery side or converter dc energy to the ac side with a high-frequency transformer, while providing zero voltage switching (ZVS) for the whole ac voltage range. A single-stage DAB topology is proposed in Chapter 2. The power flow for both directions is introduced and the novel combined dual phase shift modulation and variable frequency modulation are explained with advantages over the single phase shift modulation and fixed frequency modulation in terms of the inductor rootmean-square (rms) current. The power factor circuit (PFC) requirement and the ZVS constraints are investigated for the single-stage DAB with dual phase shift and variable frequency modulation. A novel online calculation control algorithm for the single-stage DAB is explained in Chapter 3. The control is proposed to minimize the dc low voltage side maximum turn-off current. A detailed explanation is provided for the control algorithm with the variable frequency and with a fixed frequency range. The extended ZVS ranges are proposed for the control algorithm to guarantee the ZVS over a wide range of the dc battery voltage and loads. A dual loop close loop control is introduced with its capability of dealing with the transit charging/discharging current response. An adaptive deadtime method is utilized to optimize the deadtime loss while working with a varying switching current over line period. A single-stage SR-DAB is proposed to further optimize the turn-off current and (rms) current of the single-stage topology, and is included in Chapter 4. The operation principle of a single-stage SR-DAB is proposed and its numerical expression of the equivalent model is analyzed with its PFC requirement and ZVS constraint investigated. The advantage of the dual phase shift and variable frequency control modulation is explained with its comparison over the single phase shift and fixed frequency modulation. An optimization algorithm is proposed aiming to minimize the system overall loss for the single-stage SR-DAB. A range of comparisons over the switching current and rms current between the single-stage DAB and SR-DAB are made, and the advantage of a single-stage SR-DAB is verified. A comprehensive loss model including a transistor loss such as conduction loss, switching loss, driving loss and deadtime loss, and magnetic loss such as transformer and inductor loss is introduced and well analyzed in Chapter 5. An optimization algorithm aimed to optimize the system loss is introduced based on the comprehensive loss model. With this algorithm, hardware optimizations are conducted and the optimal values of the transformer turns ratio, auxiliary inductor for a single-stage DAB, a resonant inductor and capacitor for a single-stage SR-DAB, the snubber capacitor for the dc low voltage side transistors are determined to ensure optimal performance of the converter. The ac side input capacitance and inductance are also determined to ensure a small switching voltage ripple and guarantee relay-less operation. In Chapter 6, the hardware that is utilized to verify the single-stage DAB and single-stage SR-DAB is explained in detail. Advanced implementation and switching performance of the power stage are presented. The system operation parameters as well as the major components used are included. Experimental results for single-stage DAB and single-stage SR-DAB at 1 kW and 2 kW single phase operation, and three-phase operation are displayed to verify the single-stage concept and present its performance. Thermal image, loss breakdown, and efficiency map/curve are presented. The single-stage DAB and single-stage SR-DAB provide a good solution for three-phase ac to dc battery with bidirectional power flow and requirement of isolation. The operation principle, PFC requirement, and ZVS constraint for both converters is well analyzed in the main content. A loss model is established and a hardware optimization is conducted to ensure the converter is operating at optimal efficiency. Experimental verification is included to verify the capability of the single-stage DAB and single-stage SR-DAB.