Dual active bridge converters in solid state transformers

This dissertation presents a comprehensive study of Dual Active Bridge (DAB) converters for Solid State Transformers (SSTs). The first contribution is to propose an ac-ac DAB converter as a single stage SST. The proposed converter topology consists of two active H-bridges and one high-frequency transformer. Output voltage can be regulated when input voltage changes by phase shift modulation. Power is transferred from the leading bridge to the lagging bridge. It analyzes the steady-state operation and the range of zero-voltage switching. It develops a switch commutation scheme for the ac-ac DAB converters. Simulation and experiment results of a scaled down prototype are provided to verify the theoretical analysis. The second contribution is to develop a full-order continuous-time average model for dc-dc DAB converters. The transformer current in DAB converter is purely ac, making continuous-time modeling difficult. Instead, the proposed approach uses the dc terms and 1st order terms of transformer current and capacitor voltage as state variables. Singular perturbation analysis is performed to find the sufficient conditions to separate the dynamics of transformer current and capacitor voltage. Experimental results confirm that the proposed model predicts the small-signal frequency response more accurately. The third contribution addresses the controller design of a dc-dc DAB converter when driving a single-phase dc-ac inverter. It studies the effect of 120 Hz current generated by the single-phase inverter. The limitation of PI-controller is investigated. Two methods are proposed to reduce the voltage ripple at the output voltage of DAB converter. The first method helps the feedback loop with feedforward from inverter, while the second one adds an additional resonance controller to the feedback loop. Theoretical analysis, simulation and experiment results are provided to verify the effectiveness of the proposed methods --Abstract, page iii

Closed-Loop Control of DC-DC Dual Active Bridge Converters Driving Single-Phase Inverters

The solid state transformer (SST) is a high-frequency power electronic converter as a distribution power transformer. A common three-stage configuration of an SST consists of ac-dc rectifier, isolated dc-dc dual-active-bridge (DAB) converter, and dc-ac inverter. This work addresses the controller design issue for a dc-dc DAB converter when driving a regulated single-phase dc-ac inverter. Since the switching frequency of the inverter stage is much higher than that of the DAB stage, the single-phase inverter is modelled as a 120-Hz current sink. The effect of 120-Hz current by the single-phase inverter is studied. The limitation of PI-controller, low gain at 120 Hz, is investigated. Two methods are proposed to improve the regulation of the output voltage of DAB converters. The first one uses a bandstop filter and feedforward, while the second method uses an additional proportional-resonant controller in the feedback loop. Theoretical analysis, simulation and experiment results are provided

Parameter Determination Of Photovoltaic Cells From Field Testing Data using Particle Swarm Optimization

This paper presents a swarm intelligence approach to extract equivalent circuit parameters of Photovoltaic (PV) Cells. The circuit model of a PV cell is non-linear and transcendental, which makes it difficult to solve using conventional numerical methods. Particle swarm optimization (PSO) was applied to extract the solar cell parameters. It has been confirmed that the proposed approach can obtain good parameter precision under the variations of solar insolation and environmental temperature

A Comparative Efficiency Study of Silicon-Based Solid State Transformers

Solid state transformers (SSTs) have lower physical profiles than traditional 60 Hz transformers and provide active control of power flow. However, they are not as efficient as traditional 60 Hz transformers because of the presence of power electronic converters. This work presents an analytical loss calculation model of an SST. It evaluates conduction loss, switching loss and transformer loss in four candidate SST topologies. Loss breakdown under both rated-load and light-load conditions are compared for all four configurations

AC-AC Dual Active Bridge Converter for Solid State Transformer

This work investigates the application of an ac-ac dual active bridge converter for solid state transformer. The proposed converter topology consists of two active H-bridges and one high frequency transformer. Four-quadrant switch cells are used to ensure bi-directional power flow. The advantages of direct ac-ac conversion include fewer power conversion stages and minimized passive components. The ac-ac dual active bridge converter is controlled with phase shift modulation. Operating modes for both power flow directions are described and zero-voltage switching criteria are analyzed. One design example is presented. Simulation results verify the theoretical analysis

Generalized Average Modeling of Dual Active Bridge DC-DC Converter

Full-order continuous-time average modeling and dynamic analysis of bidirectional dc-dc dual active bridge (DAB) converters are studied. The transformer current in DAB converter is purely ac, making continuous-time modeling difficult. The proposed full-order continuous-time average model uses the dc terms and first order terms of transformer current and capacitor voltage as state variables, resulting in a third-order model, if capacitor equivalent series resistance (ESR) is not considered, and a sixth-order model if ESR is considered. A control-to-output-voltage transfer function is derived for DAB converters. Experimental results confirm that the proposed model correctly predicts the small-signal frequency response and an even more accurate prediction can be obtained if capacitor ESR is taken into account

Solid-state Transformer Architecture using AC-AC Dual-Active-Bridge Converter

Modern development of semiconductor power-switching devices has promoted the use of power electronic converters as power transformers at the distribution level. This paper presents an ac-ac dual-active-bridge (DAB) converter for a solid-state transformer. The proposed converter topology consists of two active H-bridges and one high-frequency transformer. Four-quadrant switch cells are used to allow bidirectional power flow. Because power is controlled by the phase shift between two bridges, output voltage can be regulated when input voltage changes. This paper analyzes the steady-state operation and the range of zero-voltage switching. It develops a switch commutation scheme for the ac-ac DAB converters. Experimental results from a scaled-down prototype are provided to verify the theoretical analysis

Closed-Loop Control of DC-DC Dual-Active-Bridge Converters Driving Single-Phase Inverters

A solid-state transformer (SST) is a high-frequency power electronic converter that is used as a distribution power transformer. A common three-stage configuration of an SST consists of ac-dc rectifier, isolated dc-dc dual-active-bridge (DAB) converter, and dc-ac inverter. This study addresses the controller design issue for a dc-dc DAB converter when driving a regulated single-phase dc-ac inverter. Since the switching frequency of the inverter stage is much higher than that of the DAB stage, the single-phase inverter is modeled as a double-line-frequency (e.g., 120 Hz) current sink. The effect of 120-Hz current by the single-phase inverter is studied. The limitation of a PI-controller, low gain at 120 Hz, is investigated. Two methods are proposed to improve the regulation of the output voltage of DAB converters. The first one uses a bandstop filter and feedforward, while the second method uses an additional proportional-resonant controller in the feedback loop. Theoretical analysis, simulation, and experiment results are provided

Particle Swarm Optimization of High-Frequency Transformer

A high frequency transformer is a critical component in a dual active bridge converter (DAB) used in a power electronics-based solid state transformer. Operation of a DAB converter requires its transformer to have a specific amount of winding leakage inductance. The demand for high efficiency requires minimization of transformer copper loss and core loss. Furthermore, available window area limits the winding arrangement of transformer. A hybrid of particle swarm optimization (PSO) and differential evolution (DE) is proposed to solve this multi-objective problem with constraints. DE provides diversity to pbest of each particle, which is missing in standard PSO. The differential evolution particle swarm optimization (DEPSO) algorithm is applied to find optimal transformer designs for DAB converters. Results show the DEPSO method is a generic effective way to find optimal high frequency transformer design

Identifying Techniques, Topologies and Features for Maximizing the Efficiency of a Distribution Grid with Solid State Power Devices

The FREEDM grid utilizes solid state transformers (SST) and solid state fault interruption devices (FID) which may lead to unfavorable operating losses as compared to a conventional grid. Various SST topologies and switching techniques are identified for minimizing losses and a performance evaluation is made to determine the efficiency of the FREEDM distribution network to improve the overall efficiency. Losses include conductors, SSTs, FIDs, and conventional distribution transformers tested under various loading levels. Compared to a conventional distribution network, the FREEDM grid has a slight reduction in losses. By choosing the proper distribution line configuration, conductor type, switching devices and switching techniques, the power losses on the system may be minimized further