Xiaozhong Liao

Bio: Xiaozhong Liao is an academic researcher from Beijing Institute of Technology. The author has contributed to research in topics: Inverter & Low voltage. The author has an hindex of 13, co-authored 67 publications receiving 701 citations.

A Frequency-Fixed SOGI-Based PLL for Single-Phase Grid-Connected Converters

Abstract: Second-order generalized integrator (SOGI) based phase-locked loops (PLLs) are widely used for grid synchronization in single-phase grid-connected power converters. Previously, the estimated frequency of the PLL stage is fed back to the front-end SOGI block to make SOGI-PLLs frequency-adaptive, which increases the implementation complexity, and makes the tuning sensitive, thus reducing stability margins. Alternatively, a frequency-fixed SOGI-based PLL (briefly called FFSOGI-PLL) is proposed to ensure stability and simple implementation in this letter. It is commonly known that the in-phase and quadrature-phase signals generated by the frequency-fixed SOGI are of different amplitudes in the presence of frequency drifts, which causes second-harmonic ripples in the estimated parameters of the PLL loop. To deal with this issue, a simple yet effective method is developed in FFSOGI-PLL. The standard SOGI-PLL is first introduced, followed by the working principle and small-signal model of FFSOGI-PLL. The FFSOGI-PLL is then compared with the SOGI-PLL in terms of stability and transient performance. Finally, experimental results are presented to demonstrate the effectiveness of FFSOGI-PLL.

Hybrid-Bridge-Based DAB Converter With Voltage Match Control for Wide Voltage Conversion Gain Application

Abstract: This paper proposes a voltage match (VM) control for hybrid-bridge-based dual active bridge (DAB) converter in wide voltage conversion gain applications. With the addition of an auxiliary half-bridge circuit, the topology becomes an integration of a half-bridge and a full-bridge DAB converter. Unlike other pulse width modulation generation method for DAB converters, this converter utilizes four-level voltage at one port of the transformer to obtain matched voltage waveforms within the range of twice the minimum conversion gain. Wide conversion gain, decoupling of the two power control variables and wide zero-voltage switching (ZVS) ranges can be achieved with the proposed VM control. Full load ranges of ZVS for the six main power switches can be achieved and the two auxiliary switches can also operate in a wide ZVS range. In addition, the power control is done only using two control variables and its implementation is very simple, only needing a divider and a conventional voltage regulator. These characteristics and benefits of the proposed control are verified by experimental results from a 1-kw converter prototype.

Unified Boundary Trapezoidal Modulation Control Utilizing Fixed Duty Cycle Compensation and Magnetizing Current Design for Dual Active Bridge DC–DC Converter

Abstract: The unified boundary trapezoidal modulation (TZM) control utilizing fixed duty cycle compensation and magnetizing current design for dual active bridge dc–dc converter is proposed in this paper. The fixed duty cycle compensation and magnetizing current design are first introduced to achieve the zero voltage switching (ZVS) of the power switches, which cannot be ensured with the conventional TZM control. As a result, all the power switches of dual active dc–dc converter can achieve ZVS and four switches can be turned off with very low current. Besides, based on the revealed power transfer characteristic, the power control variables including the duty cycles and phase-shift ratio can be unified without lookup tables or operation region division. With the proposed boundary TZM control, circulating current losses can be reduced and nonactive power is significantly suppressed according to the mathematic analysis, resulting in decrease of the conduction loss. A 1.6-kW laboratory prototype is built to verify the theoretical analysis and effectiveness of the proposed control.

Dual-Transformer-Based DAB Converter With Wide ZVS Range for Wide Voltage Conversion Gain Application

Abstract: A control strategy is proposed for the dual-transformer-based dual-active-bridge converter to achieve wide zero-voltage switching (ZVS) range for wide voltage conversion gain range application The phase-shift control is adopted for the dual-transformer-based converter with minimum power switches employing half bridge output, and a control law is proposed to achieve wide ZVS range and reduce the current-related loss With the proposed method, four switches of the converter can achieve full range of ZVS The other two switches can achieve full-range ZVS under positive power flow, while a slightly reduced ZVS region under reverse power flow Unlike the methods employing three control degrees of freedom, the proposed method only utilizes two decoupled control variables, making the controller easy to be implemented, more reliable, and independent of the converter parameters In addition, the design of turns ratios for the two transformers is also optimized based on the control law and loss analysis The effectiveness of the converter with the proposed control is verified by experimental results from a 1-kW prototype

Switching Technique for Inductive Power Transfer at High- $Q$ Regimes

Abstract: Inductive power transfer employing high quality factor (high- $Q$ ) resonators is an effective method to extend the transfer range of the wireless power transfer system. However, the overenhanced loading effect on the transmitter side exacerbates the degradation of power transfer capability and the phenomenon of frequency splitting at a short coupling distance. Currently, range adaptation techniques compensate and maximize the power transfer capability at the cost of power transfer efficiency, which leads to the power plateau and power transfer efficiency bound of 50% for voltage-fed inductive power transfer (IPT) system. In this paper, a switch-mode operation is proposed to improve the transfer characteristics of the high- $Q$ voltage-fed IPT system at a short distance. By employing the resonators as an energy storage element rather than a loosely coupled transformer, the proposed method takes advantage of the transient process of energy exchange between resonators, which decouples the load with the TX circuit and maximizes the transferred power without the need of reducing efficiency of the system. The proposed operation is demonstrated by the experiment. The results show that the switch-mode operation significantly enhanced the power transfer capability of the system used in the experiment. Meanwhile, the power transfer efficiency and the transferred power of the experiment circuit are independent with each other; both of them increase with coupling monotonously.

Single-Phase PLLs: A Review of Recent Advances

Abstract: Single-phase phase-locked loops (PLLs) are popular for the synchronization and control of single-phase grid-connected converters. They are also widely used for monitoring and diagnostic purposes in the power and energy areas. In recent years, a large number of single-phase PLLs with different structures and properties have been proposed in the literature. The main aim of this paper is to provide a review of these PLLs. To this end, the single-phase PLLs are first classified into two major categories: 1) power-based PLLs and 2) quadrature signal generation-based PLLs. The members of each category are then described and their pros and cons are discussed. This paper provides a deep insight into characteristics of different single-phase PLLs, and therefore, can be considered as a reference for researchers and engineers.

Centralized Control Architecture for Coordination of Distributed Renewable Generation and Energy Storage in Islanded AC Microgrids

Abstract: The coordinated operation of distributed energy resources such as storage and generation units and also loads is required for the reliable operation of an islanded microgrid. Since in islanded microgrids the storage units are commonly responsible for regulating the voltage amplitude and frequency in the local power system, the coordination should consider safe operating limits for the stored energy, which prevents fast degradation or damage to the storage units. This paper proposes a centralized control architecture, applicable for local area power systems such as a small-scale microgrid. The centralized architecture is based on three supervisory control tasks which consider: active power curtailment of generation for avoiding overcharge of the storage units, load shedding actions for preventing deep discharge of the storage units, and equalization of the state of charge (SoC) among distributed storage systems for avoiding uneven degradation. The proposed equalization method has proved to be effective for equalizing the SoC of distributed energy storage systems and for ensuring uniform charge/discharge ratios regardless of differences in the capacity of the storage units. Additionally, the strategy is complemented with an optimal scheduling of load connection, which minimizes the connection and disconnection cycles of the loads within a time horizon of 24 h. The proposed architecture is verified experimentally in a lab-scale prototype of a microgrid, which has real communication between the microgrid and the central controller.

Wireless Power and Data Transfer via a Common Inductive Link Using Frequency Division Multiplexing

Abstract: For wireless power transfer (WPT) systems, communication between the primary side and the pickup side is a challenge because of the large air gap and magnetic interferences. A novel method, which integrates bidirectional data communication into a high-power WPT system, is proposed in this paper. The power and data transfer share the same inductive link between coreless coils. Power/data frequency division multiplexing technique is applied, and the power and data are transmitted by employing different frequency carriers and controlled independently. The circuit model of the multiband system is provided to analyze the transmission gain of the communication channel, as well as the power delivery performance. The crosstalk interference between two carriers is discussed. In addition, the signal-to-noise ratios of the channels are also estimated, which gives a guideline for the design of mod/demod circuits. Finally, a 500-W WPT prototype has been built to demonstrate the effectiveness of the proposed WPT system.

A Grid Synchronization PLL Method Based on Mixed Second- and Third-Order Generalized Integrator for DC Offset Elimination and Frequency Adaptability

Abstract: The second-order generalized integrator (SOGI) has been widely used to implement grid synchronization for grid-connected inverters, and from grid voltages, it is able to extract the fundamental components with an output of two orthogonal sinusoidal signals. However, if there is a dc offset existing in the grid voltages, the general SOGI’s performance suffers from its generated dc effect in the lagging sine signal at the output. Therefore, in this paper, a mixed second- and third-order generalized integrator (MSTOGI) is proposed to eliminate this effect caused by the dc offset of grid voltages. A detailed theoretical analysis on the proposed MSTOGI is presented to reveal the mechanism of eliminating the dc offset. After that, the MSTOGI is applied to a phase-locked loop (PLL) and thereby establish an MSTOGI-PLL which is more adaptable to various grid conditions and power quality. Moreover, a frequency-adaptive control scheme is added to the proposed MSTOGI-PLL to eliminate the phase difference between the PLL output and the grid in grid-connected applications where the grid frequency may vary. Finally, the experimental results from a laboratory prototype are given to demonstrate and verify the effectiveness of the proposed MSTOGI-PLL in terms of steady-state performance, dynamic response, and frequency adaptability.

An Overview of Resonant Circuits for Wireless Power Transfer

Abstract: With ever-increasing concerns for the safety and convenience of the power supply, there is a fast growing interest in wireless power transfer (WPT) for industrial devices, consumer electronics, and electric vehicles (EVs). As the resonant circuit is one of the cores of both the near-field and far-field WPT systems, it is a pressing need for researchers to develop a high-efficiency high-frequency resonant circuit, especially for the mid-range near-field WPT system. In this paper, an overview of resonant circuits for the near-field WPT system is presented, with emphasis on the non-resonant converters with a resonant tank and resonant inverters with a resonant tank as well as compensation networks and selective resonant circuits. Moreover, some key issues including the zero-voltage switching, zero-voltage derivative switching and total harmonic distortion are addressed. With the increasing usage of wireless charging for EVs, bidirectional resonant inverters for WPT based vehicle-to-grid systems are elaborated.