Yaosuo Xue

Decoupled Active and Reactive Power Control for Large-Scale Grid-Connected Photovoltaic Systems Using Cascaded Modular Multilevel Converters

Large-scale grid-connected photovoltaic (PV) systems significantly contribute to worldwide renewable energy growth and penetration, which has inspired the application of cascaded modular multilevel converters due to their unique features such as modular structures, enhanced energy harvesting capability, scalability and so on. However, power distribution and control in the cascaded PV system faces tough challenge on output voltage overmodulation when considering the varied and nonuniform solar energy on segmented PV arrays. This paper addresses this issue and proposes a decoupled active and reactive power control strategy to enhance system operation performance. The relationship between output voltage components of each module and power generation is analyzed with the help of a newly derived vector diagram which illustrates the proposed power distribution principle. On top of this, an effective control system including active and reactive components extraction, voltage distribution and synthesization, is developed to achieve independent active and reactive power distribution and mitigate the aforementioned issue. Finally, a 3-MW, 12-kV PV system with the proposed control strategy is modeled and simulated in MATLAB and PSIM cosimulation platform. A downscaled PV system including two cascaded 5-kW converters with proposed control strategy is also implemented in the laboratory. Simulation and experimental results are provided to demonstrate the effectiveness of the proposed control strategy for large-scale grid-connected cascaded PV systems.

An Improved Control System for Modular Multilevel Converters with New Modulation Strategy and Voltage Balancing Control

Modular multilevel converter (MMC) has become one of the most promising converter topologies for future high-power applications. A challenging issue of the MMC is the voltage balancing among arm capacitors. A good overall control system is also vital for the MMC, which should be based on sound mathematical model, readily adaptable for different applications, and capable of high performance. This paper presents a general control structure for MMC inverters, which is suitable for both voltage-based and energy-based control methods, and includes voltage balancing between the upper and lower arms. A new method for voltage balancing among arm capacitors, which is based on an improved pulse-width modulation, is also presented. The proposed method avoids some major disadvantages found in present voltage balancing methods, such as dependence on computation-intensive voltage sorting algorithms, extra switching actions, interference with output voltage, etc. Furthermore, all switching actions are evenly distributed among power devices. The proposed control system as a whole can serve as a promising solution for practical applications, especially when the number of submodules is fairly high. Simulation and experimental results verify the effectiveness of the proposed methods.

Analysis of phase-locked loop low-frequency stability in three-phase grid-connected power converters considering impedance interactions

Synchronous reference frame (SRF) phase-locked loop (PLL) is a critical component for the control and grid synchronization of three-phase grid-connected power converters. The PLL behaviors, especially its low-frequency dynamics, influenced by different grid and load impedances as well as operation mode have not been investigated yet, which may not be captured by conventional linear PLL models. In this paper, we propose a state-feedback quasi-static SRF-PLL model, which can identify and quantify the inherent frequency self-synchronization mechanism in the converter control system. This self-synchronization effect is essentially due to the converter interactions with grid impedance and power flow directions. The low-frequency nonlinear behaviors of the PLL under different grid impedance conditions are then analyzed, which forms the framework of evaluating the impacts of the large penetration level of distributed generation units, weak grid, microgrid, and large reactive power consumption in terms of the frequency stability of PLL. Specifically, the PLL behavior of the converter system under islanded condition is investigated to explain the PLL instability issues and the related islanding-detection methods in early publications and industry reports.

Reactive Power Compensation and Optimization Strategy for Grid-Interactive Cascaded Photovoltaic Systems

Cascaded multilevel converter structure can be appealing for high-power solar photovoltaic (PV) systems thanks to its modularity, scalability, and distributed maximum power point tracking (MPPT). However, the power mismatch from cascaded individual PV converter modules can bring in voltage and system operation issues. This paper addresses these issues, explores the effects of reactive power compensation and optimization on system reliability and power quality, and proposes coordinated active and reactive power distribution to mitigate this issue. A vector method is first developed to illustrate the principle of power distribution. Accordingly, the relationship between power and voltage is analyzed with a wide operation range. Then, an optimized reactive power compensation algorithm (RPCA) is proposed to improve the system operation stability and reliability, and facilitate MPPT implementation for each converter module simultaneously. Furthermore, a comprehensive control system with the RPCA is designed to achieve effective power distribution and dynamic voltage regulation. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed reactive power compensation approach in grid-interactive cascaded PV systems.

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.

Frequency behavior and its stability of grid-interface converter in distributed generation systems

This paper presents a state-feedback model to predict the frequency behavior of the grid-interface converter which uses phase-locked loop (PLL) techniques to synchronize with the grid. The frequency positive-feedback mechanism in the converter system is proposed and quantified in the model. The nonlinear behavior of the PLL under the weak grid condition can be accurately predicted and the large-signal frequency stability region can be also estimated by the proposed model. It shows that large penetration of distributed generation (DG) units, large reactive power variation, and weak grid tends to destabilize the converter frequency operation. The proposed model can help study the electric power system or microgrid operation under a large penetration of renewable energy resources with the power electronic interfaces.

Three-level Active Neutral-Point-Clamped Zero-Current-Transition Converter for Sustainable Energy Systems

The widespread use of renewable energy resources brings forth a requirement for high-power, high-efficiency, high-control-bandwidth grid-tied converters. This paper proposes a three-level active neutral-point-clamped zero-current-transition (3L-ANPC ZCT) converter for the sustainable energy power conversion systems. The proposed multilevel soft-switching topology can effectively increase efficiency and switching frequency of the systems by almost eliminating turn-OFF losses and greatly reducing turn-ON losses of semiconductor devices. The soft commutations of the main devices are achieved with simple auxiliary circuit of two auxiliary switches and one LC resonant tank for each phase leg as in the case of a two-level ZCT converter. The voltage rating of the auxiliary switches is the same as main switches, and is clamped to half of dc voltage, while the current rating is much smaller. Furthermore, the auxiliary switches switch under zero-current condition and have no switching losses. Compared with existing three-level diode neutral-point-clamped zero-current-transition (3L-DNPC ZCT) converter, the number of components in auxiliary circuit is halved, so the volume and cost are reduced. In addition, the deterioration of the soft commutation due to the parasitic inductances in the 3L-DNPC ZCT inverter does not exist in the proposed topology. The operation principle and comparison with the 3L-DNPC ZCT are analyzed in detail in this paper. Eighty kilowatt half-bridge prototypes of the 3L-ANPC ZCT converter and the 3L-DNPC ZCT converter are built and their circuit operation and analysis are experimentally verified.

Reliability, efficiency, and cost comparisons of MW-scale photovoltaic inverters

This paper surveys the-state-of-the-art of high power photovoltaic (PV) inverters, and a novel quasi-Z source cascaded multilevel inverter (CMI) is proposed for application to MW-scale PV system. The proposed quasi-Z source CMI has a constant dc-link voltage and minimum KVA rating, compared to a traditional CMI with an unbalanced dc-link voltage and oversized KVA rating. Four MW-scale PV inverter topologies, including two 2-level inverters with and without transformer, traditional CMI, and quasi-Z source CMI, are compared in their reliability, power loss, and cost, by using an example of 1MW/4160V PV inverter. The quantified comparison shows that the traditional and quasi-Z source CMIs achieve better performance metrics in MW-scale PV inverter applications, than the 2-level inverters. Moreover, the quasi-Z source CMI is preponderant, when compared to traditional CMI, in its higher efficiency, lower cost, and one-third power modules saving. It is concluded that multilevel and modular topologies could become main structures for utility-scale PV inverters.

A novel active power decoupling method for single-phase photovoltaic or energy storage applications

Conventional single-phase inverters exhibit double line frequency power pulsating, which affects dc sources such as photovoltaic performance and battery lifetime. Bulky dc-link electrolytic capacitors are typically employed as transient energy buffer to decouple the pulsating ac power from constant dc power, but such passive components suffer from temperature and aging concerns. For high reliability and high power density, active power decoupling approach is preferred. This paper presents a novel active power decoupling method by using a six-switch single-phase inverter topology. A small film capacitor is used as pulsating energy buffer in ac side, which not only improves the reliability but also the efficiency. A novel vector PWM for this topology is also proposed to maximize the dc voltage utilization, and to achieve the independent controls of the inverter output power and power decoupling. The simulation results have verified the proposed power decoupling method.