Furong Xiao

Bio: Furong Xiao is an academic researcher from Beijing Institute of Technology. The author has contributed to research in topics: Phase-locked loop & AC power. The author has an hindex of 5, co-authored 10 publications receiving 237 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.

A Novel Open-Loop Frequency Estimation Method for Single-Phase Grid Synchronization Under Distorted Conditions

Abstract: In this paper, a new open-loop architecture with good dynamic performance and strong harmonic rejection capability is proposed for single-phase grid synchronization under distorted conditions. Different from previous single-phase grid synchronization algorithms based on the phase-locked loop technique, the proposed method is to estimate the frequency and phase angle of the grid voltage in an open-loop manner so that fast dynamic response and enhanced system stability can be achieved. First, an open-loop frequency estimation algorithm is introduced under ideal grid condition. Then, it is extended to distorted grid voltages through the combination of the developed frequency estimation unit and a prefiltering stage consisting of a second-order low-pass filter and a cascaded delayed signal cancellation (DSC) module. In addition, a transient process smoothing unit is designed to achieve smooth frequency transients in cases where the grid voltage experiences fast and large changes. The working principle of the new frequency estimation algorithm and the developed single-phase grid synchronization approach is given in detail, together with some simulation and experiment results for verifying their performance.

A Smooth LVRT Control Strategy for Single-Phase Two-Stage Grid-Connected PV Inverters

Abstract: Based on the inherent relationship between dc-bus voltage and grid feeding active power, two dc-bus voltage regulators with different references are adopted for a grid-connected PV inverter operating in both normal grid voltage mode and low grid voltage mode. In the proposed scheme, an additional dc-bus voltage regulator paralleled with maximum power point tracking controller is used to guarantee the reliability of the low voltage ride-through (LVRT) of the inverter. Unlike conventional LVRT strategies, the proposed strategy does not require detecting grid voltage sag fault in terms of realizing LVRT. Moreover, the developed method does not have switching operations. The proposed technique can also enhance the stability of a power system in case of varying environmental conditions during a low grid voltage period. The operation principle of the presented LVRT control strategy is presented in detail, together with the design guidelines for the key parameters. Finally, a 3 kW prototype is built to validate the feasibility of the proposed LVRT strategy.

A single-phase grid-connected PV inverter with improved grid-connected current

Abstract: This paper presents an effective method, which can address the existing dc-link double-line-frequency voltage ripples in single-phase grid-connected PV inverters, to improve the grid-connected current. The instantaneous power pulsations at grid side cause double-line-frequency fluctuations of the dc-link voltage in single-phase grid-connected PV systems. A PI controller is generally used to regulate the dc-link voltage and its output is used to generate the grid-connected current reference. Since the PI controller cannot effectively attenuate the double-line-frequency components, the generated grid-connected current reference suffers from harmonic components. As a consequent, the actual current injected to the grid also contains the undesirable harmonic components, which leads to a high total harmonic distortion (THD). To improve the grid-connected current, an adaptive filter, which can filter the double-line-frequency voltage ripples, is inserted into the dc-link voltage feedback control loop in this research work. In this way, the generated grid-connected current reference can be free from the negative effects of dc-link double-line-frequency voltage ripples. The principle of the proposed approach is presented in detail, and the simulation as well as the experimental results corroborates the theoretical analysis.

Fast voltage detection method for grid-tied renewable energy generation systems under distorted grid voltage conditions

Abstract: A fast voltage detection method to assist with the low-voltage ride-through operation of grid-tied renewable energy generation systems is proposed in this study It is designed to detect every phase voltage, so that it can be applied in both three-phase and single-phase applications The whole voltage detection approach consists of two stages, the pre-filtering stage and the voltage detection stage In the pre-filtering stage, a cascaded delayed signal cancellation (CDSC) module and a low-pass filter are connected in series to filter low-order harmonics and high-frequency noises For eliminating the low-order harmonics of interest, different types of CDSC methods are studied in detail Subsequently, a new orthogonal signal generator is built to calculate the voltage amplitude in the voltage detection stage Finally, the proposed voltage detection method is verified by experimental results

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.

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.

Grid Integration of Small-Scale Photovoltaic Systems in Secondary Distribution Network—A Review

Abstract: The relative share of renewable energy, specifically the solar photovoltaic (PV), is increasing exponentially in the world electric energy sector. This is a cumulative result of reduction in the cost of solar panels, improvement in the panel efficiency, and advancement in the associated power electronics. Among different types of PV plants, installation of small-scale rooftop PV is growing rapidly due to direct end-user benefits and lucrative governmental incentives. There are various standards developed in regards to grid integration of PVs and other distributed generations (DGs). Different power converter topologies are developed to interface the PV panel with the utility grid. To keep up with the stringent regulations imposed by the standards, various control strategies and grid synchronization methods have been developed. This review article amalgamates and summarizes all of the aforementioned aspects of a grid-integrated PV system including various standards, power stage architectures, grid synchronization methods, operation under extreme events, and control methodologies, pertaining to small-scale PV plants. This article will help freshman researchers to gain some familiarity with the topic and introduce them to some of the key issues encountered in this field.

DC Offset Rejection Improvement in Single-Phase SOGI-PLL Algorithms: Methods Review and Experimental Evaluation

Abstract: DC offset in the input of phase-locked loops (PLLs) is a challenging problem since it will result in fundamental frequency oscillations in the estimated phase and frequency. In this paper, a comprehensive analysis and performance evaluation of several advanced second-order generalized integrator (SOGI)-based PLL methods in enhancing the dc offset rejection capability for single-phase grid-connected power converters is presented. These methods include the cascade SOGI, modified SOGI, $\alpha \beta $ -frame delayed signal cancellation (DSC), complex coefficient filter, in-loop dq-frame DSC, notch filter, and moving average filter-based SOGI-PLL. Main characteristics and design aspects of these methods are presented. Main performance indexes, such as the setting time, frequency or phase errors are defined and these methods are systematically compared under various scenarios with both numerical and experimental results.

Disturbance Observer Based Fuzzy Sliding Mode Control of PV Grid Connected Inverter

Abstract: In this paper, a disturbance observer-based fuzzy sliding mode control (DOBFSMC) strategy is proposed for a single-phase PV grid-connected inverter. In the fact that the uncertainties caused by the inverter component parameter variations and the changes of climatic conditions may seriously affect the control performance of the inverter, a disturbance observer is designed to estimate these disturbances in real time and a sliding mode controller designed with the output information of the disturbance observer is employed to control the output voltage of the DC–AC inverter, and a fuzzy system is used to approximate the upper bound of the observation error between the actual disturbance and its observation value in order to improve the performance of the control system. The inverter has strong robustness since the disturbances can be adaptively compensated, and the chattering is greatly reduced since the switching gain can be very small as the upper bound of observation error is estimated by the fuzzy system. Finally, simulation results verify the effectiveness of the proposed method and demonstrate it can work reliably under different conditions.