Hongchang Li

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.

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

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.

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

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.

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

Maximum efficiency point tracking (MEPT) control has been adopted in state-of-the-art wireless power transfer (WPT) systems to meet the power demands with the highest efficiency against coupling and load variations. Conventional MEPT implementations use dc/dc converters on both transmitting and receiving sides to regulate the output voltage and maximize the system efficiency at the expense of increased overall complexity and power losses on the dc/dc converters. Other implementations use phase-shift control or on–off control of the transmitting side inverter and the receiving side active rectifier instead of dc/dc converters but cause new problems, e.g., hard switching, low average efficiency, and large dc voltage ripples. This paper proposes a pulse density modulation (PDM) based implementation for MEPT to eliminate all the mentioned disadvantages of existing implementations. Delta-sigma modulators are used as an example to realize the PDM. A dual-side soft switching technique is proposed for the PDM. The ripple factor of the output voltage with PDM is derived. A 50 W WPT system is built to validate the proposed method. The system efficiency is maintained higher than 70% for various load resistances when the power transfer distance is 0.5 m, which is 1.67 times the diameter of the coils.

A High-Bandwidth Integrated Current Measurement for Detecting Switching Current of Fast GaN Devices

Gallium nitride (GaN) devices are suitable for high-frequency power converters due to their excellent switching performance. To maximize the performance of GaN devices, it is necessary to study the switching characteristics, which requires measuring the switching current. However, GaN devices have a fast switching speed and are sensitive to parasitic parameters, so the current measurement should have a high bandwidth and should not introduce excessive parasitic inductance into the power converters. Traditional current measurements are difficult to meet these requirements, especially for fast GaN devices. This paper presents a high-bandwidth integrated current measurement for detecting the switching current of fast GaN devices. By effectively utilizing the parasitic inductance in the circuit, a single-turn coil is embedded in the printed circuit board. This coil could pick up a sufficiently strong voltage signal, which is then processed to reconstruct the switching current. Moreover, corrections are carried out to further improve the accuracy. The current measurement has a small insertion impedance and a high bandwidth with a small influence on the parasitic inductance of the converter. The accuracy of the current measurement is experimentally verified by a 40 V GaN-based double pulse test circuit with a load current up to 25 A.

Delta-sigma modulation for maximum efficiency point tracking of wireless power transfer systems

The maximum efficiency point tracking (MEPT) control strategy has been adopted in state-of-the-art wireless power transfer (WPT) systems to meet power demand with the highest efficiency against coupling and load variations. Most of the MEPT implementations use dc/dc converters to regulate the output voltage and maximize the efficiency at the expense of increased system complexity. Other implementations replace the function of dc/dc converters by a phase-shift inverter or an active rectifier but introduce new problems, e.g. hard switching and large dc voltage ripples. This paper proposes a novel MEPT implementation that utilizes the delta-sigma modulation in both the transmitting side inverter and the receiving side active rectifier so that the functions of MEPT can be realized while soft switching is guaranteed and the voltage ripples are minimized. Simulation results are present to verify the proposed method.