Yuxiang Shi

High-Frequency-Link-Based Grid-Tied PV System With Small DC-Link Capacitor and Low-Frequency Ripple-Free Maximum Power Point Tracking

This paper proposes a grid-tied photovoltaic (PV) system consisting of modular current-fed dual-active-bridge (CF-DAB) dc-dc converter with cascaded multilevel inverter. The proposed converter allows a small dc-link capacitor in the three-phase wye-connected PV system; therefore, the system reliability can be improved by replacing electrolytic capacitors with film capacitors. The low-frequency ripple-free maximum power point tracking (MPPT) is also realized in the proposed converter. First of all, to minimize the influence resulting from reduced capacitance, a dc-link voltage synchronizing control is developed. Then, a detailed design of power mitigation control based on CF-DAB dynamic model is presented to prevent the large low-frequency voltage variation propagating from the dc-link to PV side. Finally, a novel variable step-size MPPT algorithm is proposed to ensure not only high MPPT efficiency, but also fast maximum power extraction under rapid irradiation change. A downscaled 5-kW PV converter module with a small dc-link capacitor was built in the laboratory with the proposed control and MPPT algorithm, and experimental results are given to validate the converter performance.

Optimized Operation of Current-Fed Dual Active Bridge DC–DC Converter for PV Applications

The current-fed dual active bridge (CF-DAB) dc-dc converter gains growing applications in photovoltaic (PV) and energy storage systems due to its advantages, e.g., a wide input voltage range, a high step-up ratio, a low input current ripple, and a multiport interface capability. In addition, the direct input current controllability and extra control freedom of the CF-DAB converter make it possible to buffer the double-line-frequency energy in grid-interactive PV systems without using electrolytic capacitors in the dc link. Therefore, a PV system achieves high reliability and highly efficient maximum power point tracking. This paper studies the optimized operation of a CF-DAB converter for a PV application in order to improve the system efficiency. The operating principle and soft-switching conditions over the wide operating range are thoroughly analyzed with phase-shift control and duty-cycle control, and an optimized operating mode is proposed to achieve the minimum root-mean-square transformer current. The proposed operating mode can extend the soft-switching region and reduce the power loss, particularly under a heavy load and a high input voltage. Moreover, the efficiency can be further improved with a higher dc-link voltage. A 5-kW hardware prototype was built in the laboratory, and experimental results are provided for verification. This paper provides a design guideline for the CF-DAB converter applied to PV systems, as well as other applications with a wide input voltage variation.

A single-phase grid-connected PV converter with minimal DC-link capacitor and low-frequency ripple-free maximum power point tracking

This paper presents a novel single-phase grid-connected photovoltaic (PV) converter based on current-fed dual active bridge (CF-DAB) dc-dc converter. The proposed converter allows a minimal DC-link capacitor with large voltage variation without low-frequency effect on maximum power point tracking (MPPT). An advanced control system is developed based on the converter operation principles. Minimized transformer peak current is capable to be achieved through a symmetrical dc voltage control. Simulation results are presented to validate the performance of the proposed PV converter. A 5kW laboratorial prototype is also implemented and the results will be presented later.

Modular multilevel dual active bridge DC-DC converter with ZVS and fast DC fault recovery for battery energy storage systems

This paper proposes a novel modular multilevel dual-active-bridge (MMDAB) dc-dc converter for spilt-battery energy storage system (BESS) in medium-voltage dc (MVDC) grid application. The proposed converter can be derived from conventional DAB converter and therefore exhibits favorite characteristics of galvanic isolation, inherent zero-voltage-switching (ZVS) condition and small passive components. In addition, compared to cascaded DAB converter, the proposed converter has limited dc fault current rising and fast dc fault recovery due to the leakage inductance and the kept cell energy during the fault. A new modulation method with minimal passive components requirement and low dv/dt has been proposed, and the corresponding control system is also developed to realize the power flow control and balancing control. A case study of 500 kW, 20 kHz MMDAB-based BESS is performed in simulation to validate the performance.

Isolated Modular Multilevel DC–DC Converter With DC Fault Current Control Capability Based on Current-Fed Dual Active Bridge for MVDC Application

In this paper, a current-fed modular multilevel dual-active-bridge (CF-MDAB) dc–dc converter is proposed for medium-voltage dc (MVDC) application. The proposed converter inherits favorite characteristics of DAB circuits including soft switching and small passive components. Thereby, high efficiency and high power density can be achieved. Moreover, with direct input and output dc current control, the CF-MDAB is suitable for a breakerless MVDC system since it can realize dc fault ride-though operation. In addition, the dv/dt in the converter is mitigated with the quasi-three-level modulation. In this paper, the proposed converter is applied to integrate the battery energy storage to a MVDC grid as an example to illustrate its operation principles and fault current control capability. The operation principles are presented for both normal and dc fault conditions; the dynamic models are also derived not only under normal operation mode but under dc fault operation mode as well. The control systems under different operation modes are designed, respectively, based on the developed mathematical models. A downscaled 40-kHz 3-kW CF-MDAB prototype was built in the laboratory. The experimental results under both normal condition and dc fault condition verified the analysis as well as the control performance of the proposed converter.

A 60-kW 3-kW/kg Five-Level T-Type SiC PV Inverter With 99.2% Peak Efficiency

A silicon carbide (SiC) T-type LCL inverter can achieve smaller device loss than two-level topology, however, its improvement on power density is limited by current ripple loss on magnetic components as switching frequency increases. This paper presents a five-level T-type (5LT2) photovoltaic (PV) inverter that achieves better utilization of SiC devices than the traditional three-level T-type LCL topology at higher switching frequency. The operation principle of the SiC 5LT2 PV inverter has been presented. The key design aspects including magnetic balancing, short-circuit protection, and digital controller computation time have been discussed and methods are developed. A 60-kW PV converter including boost stage and inverter stage has been built in the laboratory, which achieves a power density of 27 W/in3 and 3 kW/kg, and measured peak efficiency of 99.2%.

Ground Leakage Current Analysis a Suppression in a 60-kW 5-Level T-Type Transformerless SiC PV Inverter

In this paper, ground leakage current suppression in a 60-kW 5-level T-type (5LT2) transformerless SiC photovoltaic (PV) inverter has been presented. The common mode (CM) equivalent circuit is analyzed based on a high frequency (HF) CM loop and a low frequency (LF) CM loop, respectively. In the 5LT2 inverter, the derived HF CM voltage (CMV) is found to have 86% reduction compared to that of a 3-level T-type (3LT2) inverter. The simulation and experiment results are provided to demonstrate the advantages of 5LT2 inverter on HF leakage current suppression. In addition, LF CMV caused by neutral point (NP) voltage oscillation has been analyzed in this paper. It is shown that the LF CMV is nearly proportional to the NP voltage oscillation. Furthermore, an LF CMV compensation method is proposed to suppress the LF CMV by 64%, which is verified by simulation and experiment results. Finally, the leakage current shows 79% reduction in the 5LT2 inverter compared to the conventional 3LT2 inverter in the experiment. A further 52% leakage current reduction is achieved by the LF CMV compensation in the 5LT2 inverter.

A 50kW high power density paralleled-five-level PV converter based on SiC T-type MOSFET modules

SiC T-type LCL inverter can achieve smaller device loss than two-level topology, however its improvement on power density is limited by current ripple loss on magnetic components as switching frequency increases. This paper presents a paralleled-five-level (P5L) PV inverter which achieves better utilization of SiC devices than traditional T-type three-level (3L) LCL topology at higher switching frequency. Comparison of the SiC P5L PV inverter with SiC T-type 3L LCL PV inverter has been presented. The design challenges and methods to solve magnetic balancing, short circuit protection, and digital controller computation time issues are analyzed. A 50kW PV converter including boost stage and inverter stage has been built in the laboratory, which achieves a power density of 22.7 W/in3 and 2.5 kW/kg, and measured peak efficiency of 99.2%.

Ground leakage current suppression in a 50 kW 5-level T-type transformerless PV inverter

In this paper ground leakage current suppression in a 50 kW 5-level T-type transformerless PV inverter has been presented. Compared to a 3-level T-type PV inverter, this topology allows a simple modulation method such as carrier-based (CB) PWM can be used to suppress the leakage current without the penalty of the traditional methods. Phase disposition (PD) and phase opposition disposition (POD) based CB PWM method has been applied to this 5-level topology. The spectrum analysis has demonstrated that PD and POD will generate the same phase voltage spectrum. In addition, the common-mode (CM) voltage of a 5-level T-type PV inverter has been derived and the CM choke has been designed. The value of the CM choke is a 73% reduction compared to that of a 3-level T-type PV inverter. The simulation and experimental verification have been provided.

A novel modular dual-active-bridge (MDAB) dc-dc converter with dc fault ride-through capability for battery energy storage systems

This paper proposes a novel modular dual-active-bridge (DAB) dc-dc converter for spilt-battery energy storage system (BESS) in medium-voltage dc (MVDC) grid application. Compared to modular cascaded DAB converter, the proposed topology has current-fed DAB characteristics, with directly dc current control to achieve dc fault ride-through capability. In addition, the proposed converter exhibits favorite features of DAB converter such as galvanic isolation, soft-switching condition and small passive components. The operating principle is described in details, and the corresponding control system is developed to realize the power flow and balancing control. A case study of 500 kW, 20 kHz BESS based on proposed converter is simulated for validation.