Jingyang Fang

A Magnetic Integrated LLCL Filter for Grid-Connected Voltage-Source Converters

High-order passive filters, such as the well-known LCL filters, are normally employed in the grid-connected power conversion systems to effectively attenuate the switching frequency harmonic introduced by the modulation of power converters. Although much more compact than the conventional single-inductor L filters, such passive filters are still bulky and expensive when compared with their active counterparts, e.g., the semiconductor switches. In order to improve the system power density and reduce its cost, the magnetic integration technique has been widely adopted so that the discrete inductors of passive filters are replaced by the integrated inductor, resulting in smaller magnetic cores and, therefore, the decreased volumes of passive filters. For conventional magnetic integrated LCL filters, the converter-side inductor and the grid-side inductor are integrated together and their coupling coefficient is intentionally minimized. In this letter, this coupling coefficient is fully utilized and properly designed, and the resulting coupling effect is equivalent to inserting an additional inductor into the filter capacitor branch loop. The integrated inductor and the filter capacitor can form an integrated LLCL filter, which exhibits the advantages of both the LLCL filter and magnetic integration, e.g., enhanced harmonic attenuation, reduced filter inductances, and system volume without adding the extra trap inductor. Finally, experimental results obtained from a single-phase grid-connected voltage-source converter interfaced by the proposed integrated filter are presented to validate its effectiveness.

An Integrated Trap-LCL Filter With Reduced Current Harmonics for Grid-Connected Converters Under Weak Grid Conditions

High-frequency current harmonics injected into the point of common coupling, mainly at the switching frequency and its multiples introduced by the modulation of grid-connected converters, should be limited to comply with grid codes. This requirement can be satisfied by using trap filters with LC-traps tuned at the switching frequency and its multiples in replacement of conventional L filters. However, all existing trap filters are subject to certain problems, e.g., sensitivity to grid impedance variation and decreased high-frequency roll-off rate. In view of this, this paper presents a trap-LCL filter with its parallel resonant LC-trap located on the converter side, named as LT-C-L filter. The modeling and analysis presented in this paper show that the LT-C-L filter can be the most promising trap filter due to its advantages such as high roll-off rate, robustness against parametric variations of grid impedance and passive damper, satisfactory harmonic attenuation at the switching frequency, reduction of converter current total-harmonic-distortion, and flexible design of filter components through magnetic integration. A single-phase ac/dc grid-connected converter prototype has been built and tested in the laboratory, and experimental results are presented to verify the superiority of the proposed filter.

Capacitor-Voltage Feedforward With Full Delay Compensation to Improve Weak Grids Adaptability of LCL-Filtered Grid-Connected Converters for Distributed Generation Systems

LCL-filtered grid-connected converters are widely used for distributed generation systems. However, the current regulation of such converters is susceptible to weak grid conditions, e.g., grid impedance variation and background harmonics. Paralleling multiple harmonic compensators (HCs) is a commonly used method to suppress the current distortion caused by grid background harmonics, but the control bandwidth should be wide enough to ensure system stability. In order to enhance the adaptability of LCL-filtered grid-connected converters under weak grid operation, this paper proposes an improved capacitor-voltage-feedforward control with full delay compensation. When used with converter-side current feedback, the proposed control can keep system low-frequency characteristic independent of grid impedance and provide a high-harmonic rejection capability without using additional HCs. Moreover, it completely avoids the design constraints of an LCL filter, i.e.,

A Battery/Ultracapacitor Hybrid Energy Storage System for Implementing the Power Management of Virtual Synchronous Generators

\omega _{r}< \omega _{s}/ 6

Grid-connected power converters with distributed virtual power system inertia

is required for single-loop converter-side current control. Therefore, a higher resonant frequency can be designed to achieve a wider control bandwidth and to lower the current distortion caused by the paralleled filter capacitor branch. Experimental results are finally presented to verify the proposed control, which are also in good agreement with theoretical analysis.

An Optimal Digital Pulse-Width-Modulated Dither Technique to Enhance the Resolution of High-Frequency Power Converters

Renewable energy sources (RESs) have been extensively integrated into modern power systems to meet the increasing worldwide energy demand as well as reduce greenhouse gas emission. As a result, the task of frequency regulation previously provided by synchronous generators is gradually taken over by power converters, which serve as the interface between the power grid and RESs. By regulating power converters as virtual synchronous generators (VSGs), they can exhibit similar frequency dynamic response. However, unlike synchronous generators, power converters are incapable of absorbing/delivering any kinetic energy, which necessitates extra energy storage systems (ESSs). Nonetheless, the implementation and coordination control of ESSs in VSGs have not been investigated by previous research. To fill this research gap, this letter proposes a hybrid ESS (HESS) consisting of a battery and an ultracapacitor to achieve the power management of VSGs. Through proper control, the ultracapacitor automatically tackles the fast-varying power introduced by inertia emulation while the battery implements the remaining parts of a VSG and only compensates for relatively long-term power fluctuations with slow dynamics. In this way, the proposed HESS allows reduction of the battery power fluctuations along with its changing rate. Finally, experimental results are presented to validate the proposed concept.

Robust Design of LCL Filters for Single-Current-Loop-Controlled Grid-Connected Power Converters With Unit PCC Voltage Feedforward

Renewable energy generators, e.g. wind turbines and photovoltaic (PV) arrays, have been increasingly employed to replace conventional gas/steam turbines so as to reduce greenhouse gas emission. However, such generators are normally coupled to the power grid through fast-response power converters without any inertia, leading to decreased power system inertia. As a consequence, system frequency deviations may easily go beyond the acceptable range under frequency events or contingencies, resulting in undesirable load-shedding or even blackouts. In order to address the inertia issue, this paper proposes a concept for all the grid-connected power converters to realize distributed virtual inertia. This can be achieved through regulating the dc-link voltages of power converters so that their dc-link capacitors are aggregated into an extremely large capacitor for frequency support. Moreover, the decisive factors of virtual inertia are identified and analyzed. Simulation and experiment results indicate that more than 10% frequency deviation reduction and 50% improvement of rate of change of frequency (ROCOF) can be expected with the proposed concept.

Distributed Power System Virtual Inertia Implemented by Grid-Connected Power Converters

Wide bandgap semiconductor switches have been increasingly utilized to improve the power density and efficiency of power converters, as such devices are able to operate at very high frequencies, e.g., up to 100 MHz, with reduced power losses. However, such a high-frequency operation may impose a challenge to the digital control system, and the system clock frequency should be up to 100 GHz in high-precision applications, which is difficult to realize in low-cost microprocessors. Instead of using extremely high-frequency clocks, preprocessing-based solutions that utilize digital pulse-width-modulated (DPWM) dither techniques can also enhance the DPWM resolution with moderate frequency clocks. Unfortunately, this is usually achieved at the expense of introducing low-frequency harmonics, which may complicate system controller and output filter design. In this paper, an optimal dither technique is proposed to enhance the resolution of DPWM power converters. The concepts of positive dither and negative dither are first proposed in this paper. Furthermore, vector-diagram-based analysis indicates that with proper utilization of positive dithers and negative dithers and carefully selected dither sequences, the lowest order harmonic introduced by the conventional dither technique can be completely eliminated when the dither period is multiples of six switching periods. In other cases, the proposed optimal dither technique can produce minimized lowest order harmonic. Finally, experimental results obtained from a Gallium Nitride (GaN) devices-based synchronous buck converter validate the feasibility of the proposed dither technique.

Pulse Density Modulation for Maximum Efficiency Point Tracking of Wireless Power Transfer Systems

The point of common coupling (PCC) voltage feedforward control is widely employed for LCL-type grid-connected converters to prevent large inrush current during startup, reduce steady-state tracking error, and suppress disturbances of grid voltage. Generally, this feedforward loop is ignored in the system modeling and stability analysis for simplicity. However, the grid impedance would be introduced to the control system through the feedforward loop under practical power grids, leading to a different current loop model from the original one, then conventional LCL filter design for single-current-loop control may be inappropriate. In this paper, the detailed modeling of current control loop with unit PCC voltage feedforward is presented. Based on the established model, it is found that unstable open-loop poles may appear under grid impedance variation, which thus may cause system instability. Therefore, the robust design regions of LCL filters against grid impedance variation are proposed for single-current-loop control schemes. Moreover, another interesting finding is that the feedforward control can help maintain a high current control bandwidth under grid impedance variation. This feature is highly desirable if multiple resonant controllers are employed for harmonic compensation. Experimental results are finally presented to verify the theoretical analysis and robust design presented in this paper.

Robust LCL filter design for grid-side current single-loop controlled grid-connected converters under weak power grids

Renewable energy sources (RESs), e.g., wind and solar photovoltaics, have been increasingly used to meet worldwide growing energy demands and reduce greenhouse gas emissions. However, RESs are normally coupled to the power grid through fast-response power converters without any inertia, leading to decreased power system inertia. As a result, the grid frequency may easily go beyond the acceptable range under severe frequency events, resulting in undesirable load-shedding, cascading failures, or even large-scale blackouts. To address the ever-decreasing inertia issue, this paper proposes the concept of distributed power system virtual inertia, which can be implemented by grid-connected power converters. Without modifications of system hardware, power system inertia can be emulated by the energy stored in the dc-link capacitors of grid-connected power converters. By regulating the dc-link voltages in proportional to the grid frequency, the dc-link capacitors are aggregated into an extremely large equivalent capacitor serving as an energy buffer for frequency support. Furthermore, the limitation of virtual inertia, together with its design parameters, is identified. Finally, the feasibility of the proposed concept is validated through simulation and experimental results, which indicate that 12.5% and 50% improvements of the frequency nadir and rate of change of frequency can be achieved.