Due to the inherent double-frequency (2 f 0) ripple in single-stage single-phase photovoltaic grid-connected inverters, the maximum power point tracking (MPPT) will inevitably be affected. To improve the MPPT performances, a passive LC power decoupling circuit with a robust second-order sliding-mode control (SOSMC) is thus proposed in this article. With the passive LC decoupling path, the double-frequency pulsation on the dc link is effectively cancelled out. Thus, the MPPT accuracy is significantly enhanced, and the utilization of a small dc-link capacitor becomes possible. However, resonance between the LC circuit and the main dc-link capacitor may appear, which can be damped through an active damping method. Additionally, the proposed SOSMC ensures good steady-state, dynamic performance (voltage fluctuation and settling time), and the robustness of the dc-link voltage, which is also beneficial to MPPT control in terms of high accuracy and fast dynamics. The systematic design of SOSMC is presented, and a detailed parameter optimization design of LC decoupling circuit is discussed. Experimental tests are performed on a 2.5-kW single-stage single-phase grid-connected inverter, and the results validate the effectiveness of the proposed strategy.

/pdf/optimization-design-and-control-of-single-stage-single-phase-4gj3yqblet.pdf

Optimization Design and Control of Single-Stage Single-Phase PV Inverters for MPPT Improvement

In a two-stage single-phase photovoltaic (PV) grid-connected inverter, the second harmonic current (SHC) in the PV panel will affect the maximum power point tracking operation and degrade the energy harvesting efficiency. In order to solve this problem, a flying capacitor clamped (FCC) boost three-level (TL) converter is adopted in this article, and the flying capacitor is employed to compensate the SHC for removing a large electrolytic capacitor. The propagation mechanism of the SHC is analyzed from the perspective of impedance, and a virtual impedance in parallel with the intermediate dc bus is introduced, achieving a perfect SHC compensation performance. The implementation of the virtual parallel impedance is illustrated, and the key parameters design of the FCC boost TL converter is given. A 3-kW two-stage single-phase PV grid-connected inverter is fabricated and tested, and the experimental results verify the validity of the theoretical analysis and the proposed control scheme.

Second Harmonic Current Reduction for Flying Capacitor Clamped Boost Three-Level Converter in Photovoltaic Grid-Connected Inverter

In order to reduce the capacity of the active power filter (APF), improve its dynamic tracking speed and anti-disturbance ability of harmonic currents, so as to better solve the problem of harmonic pollution in the medium and high voltage distribution network, this article is suitable for medium and high voltage distribution networks. The three-phase three-wire injection hybrid active power filter (IHAPF) of high voltage distribution network is the research object, and a fuzzy linear active disturbance rejection controller (Fuzzy-LADRC) suitable for IHAPF voltage outer loop control is proposed. The controller is composed of a fuzzy proportional controller, a linear extended state observer (LESO), and a total disturbance compensation link. The introduction of fuzzy logic control solves the difficult problem of controller parameter tuning, and uses Lyapunov stability definition to prove the stability of the system. Finally, the control performance of IHAPF under the control of Fuzzy-LADRC is simulated and verified by MATLAB&SIMULINK simulation platform, and compared with the traditional PI controller. The results show that the Fuzzy-LADRC controller is better than the traditional PI controller and has good tracking and anti-disturbance capabilities.

/pdf/fuzzy-linear-active-disturbance-rejection-control-of-3cdlmfmi7j.pdf

Fuzzy Linear Active Disturbance Rejection Control of Injection Hybrid Active Power Filter for Medium and High Voltage Distribution Network

For the two-stage single-phase power factor correction (PFC) converter, its instantaneous input power pulsates at twice the line frequency, generating the second harmonic current (SHC) at the dc-bus port. The dc–dc stage is used to regulate the output voltage or the dc bus voltage in different applications, and the SHC reduction schemes for the dc–dc stage with different control objectives are discussed in this article. When the dc–dc stage regulates the output voltage, it is pointed out that the control bandwidth of the dc–dc stage is required to be high enough and the dc bus capacitor should be large enough to suppress the SHC, and the design of the dc bus capacitor is given. When the dc–dc stage regulates the dc bus voltage, a virtual impedance is added in series with the input/output of the dc–dc stage to suppress the SHC, and a virtual impedance is added in parallel with the dc bus to improve the dynamic performance. The selection and design of the virtual impedances are also given. Based on that, three specific control schemes are proposed for the dc–dc stage to suppress the SHC, and the parameters design method is also presented. Finally, a 3.3-kW two-stage single-phase PFC converter is fabricated and tested in the lab to verify the effectiveness of the proposed SHC reduction schemes.

Second Harmonic Current Reduction Schemes for DC–DC Converter in Two-Stage PFC Converters

The instantaneous output power of the two-stage single-phase inverter pulsates at double-line frequency, generating a large amount of second harmonic current (SHC) in the front-end dc-dc converter and input dc source. The SHC is harmful to the system performance. To address the issue, a control strategy based on port-impedance editing, where inductor current feedforward (ICFF) and bus voltage feedforward paths are employed, is proposed to reduce the low frequency ripple in the article. Due to only bandpass filter employed in each feedforward path, the proposed control scheme is relatively easy to be implemented. Under the control strategy, the dc-bus port-impedance of dc-dc converter is increased rapidly at double-line frequency, while staying low impedance for other frequencies. What is more, different ICFF paths are taken into insight to obtain optimized path. Meanwhile, the small signal model of front-end dc-dc converter is established and the design principle of key parameters is provided for the derived control strategy. Furthermore, the dynamic response with bandwidth limitation is also discussed in this article. Finally, a two-stage single-phase inverter prototype was fabricated and tested. The experimental results verify the performance of the proposed strategy with SHC reduction.

Low Frequency Current Ripple Suppression for Two-Stage Single-Phase Inverter Based on Impedance Editing

The instantaneous output power of the two-stage single-phase grid-connected photovoltaic (PV) inverter pulsates at twice the line frequency ( $2f_{{\text{o}}}$ ), generating second harmonic current (SHC) in the front-end dc–dc converter and PV panel, which will affect the maximum power point tracking operation and deteriorate the overall conversion efficiency. The generating mechanism of the SHC is analyzed in this paper and it is pointed that in order to eliminate the SHC in the front-end dc–dc converter and PV panel, the voltage loop gain of the front-end dc–dc converter should be high enough at $2f_{{\text{o}}}$ . Since there is a −180° phase abrupt at the resonant frequency of the input side filter capacitor and the inductor, the system may be unstable. To cope with this problem, the inductor current feedback active-damping scheme (ADS) is adopted. For further improving the loop gain at $2f_{{\text{o}}}$ , proportional–integral–resonant regulator with ADS (PIR + ADS) is proposed in this paper. Besides, a step-by-step closed-loop parameters design method is presented. Finally, a 3-kW two-stage single-phase grid-connected PV inverter has been fabricated and tested, and the experimental results verify the feasibility of the proposed control schemes.

Second Harmonic Current Reduction in Front-End DC−DC Converter for Two-Stage Single-Phase Photovoltaic Grid-Connected Inverter

The second harmonic current compensator (SHCC) can be added into the single-phase converters for compensating the second harmonic current (SHC) and thus removing the undesired electrolytic capacitor. In this paper, a virtual impedance, which is in the form of a capacitor and a resistor connected in parallel, is introduced to be in parallel at the bus-side port of the SHCC for effectively compensating the SHC while guaranteeing the system stability. This virtual parallel impedance is realized by feeding forward the bus-side port voltage of the SHCC, and thus the SHCC can be modular designed. The closed-loop parameter design of the SHCC is also presented. A 3.3-kW prototype of a two-stage single-phase power factor correction converter is built and tested in the laboratory, and the experimental results verify the effectiveness and feasibility of the proposed control method.

A Virtual Impedance Based Control Scheme for Modular Electrolytic Capacitor-Less Second Harmonic Current Compensator

In the two-stage single-phase photovoltaic (PV) grid-connected inverter, the flying-capacitor-clamped boost three-level converter is adopted to achieve maximum power point tracking of the PV panel. To restrain the second harmonic current (SHC) in the PV panel and remove the undesired electrolytic capacitors, the flying capacitor is employed to compensate the SHC. To achieve this goal, the flying capacitor average voltage control incorporated bus voltage ripple limitation scheme is proposed in this letter. The closed-loop parameters design for the bus voltage ripple limitation loop is given. Finally, a prototype of 3-kVA PV grid-connected inverter is fabricated and tested in the lab, and the experimental results verify the validity of the proposed scheme.

Compensation of Second Harmonic Current Based on Bus Voltage Ripple Limitation in Single-Phase Photovoltaic Grid-Connected Inverter

This book provides complete design methodologies to design controllers for the dc-dc converter applied in two-stage single-phase power converters 

Shiqi Kan

Papers

Second Harmonic Current Reduction in Front-End DC−DC Converter for Two-Stage Single-Phase Photovoltaic Grid-Connected Inverter

Second Harmonic Current Reduction for Flying Capacitor Clamped Boost Three-Level Converter in Photovoltaic Grid-Connected Inverter

A Virtual Impedance Based Control Scheme for Modular Electrolytic Capacitor-Less Second Harmonic Current Compensator

Compensation of Second Harmonic Current Based on Bus Voltage Ripple Limitation in Single-Phase Photovoltaic Grid-Connected Inverter

Second Harmonic Current Reduction Techniques for Single-Phase Power Electronics Converter Systems