Grid-forming inverter plays a pivotal role as the increase of the capacity of renewable energy. Existing literature tends to use small signal modeling method to linearize the inner control loop including the voltage loop and current loop in dq coordinate with proportional integral controller, which is complicated. To simplify the analysis, this article chooses to use virtual admittance control strategy from the perspective of αβ coordinate. By using such method, the inner control loop can be strictly regarded as a linear system. Moreover, this article uses classical control theory by simplifying the control diagram step by step, with the help of the Bode diagram, the critical value of the virtual admittance can be located based on the stability analysis. Simulation results and Bode diagram verify the accuracy of the modeling method and the impact from the outer power loop to the system is discussed under different grid condition, revealing that the stability of the system is more likely to be affected by the outer power loop under weak grid condition.

In high power application, a single active power filter (APF) limited by its capacity cannot compensate for whole harmonics. APF parallel system is thus adopted to solve this problem. Centralized control scheme for the parallel system features good current sharing ability, but subject to communication fault. On the other hand, decentralized control schemes cannot share current properly. This paper proposes a decentralized control scheme for modular APF parallel system in which all APF modules have equal status and share the current in proportion to their own capacities without central controller. What is more, plug and play of an APF is achieved, and current sharing can be done automatically. The correctness and effectiveness of the scheme have been testified by simulations.

The differential boost inverter has been widely used as a microinverter for the renewable distributed generations as it achieves an ac voltage with amplitude larger than the dc input. However, the unlimited voltage gain of the boost inverter is practically unachievable due to the existence of the parasitic resistance of the input inductor, which narrows the input voltage range and distorts the output voltage waveform as the turn-on duty ratio of the power switches approaches unity. To solve this problem, a novel coupled-inductor-based differential boost inverter is proposed, which uses a two-winding or three-winding coupled inductor to enhanced voltage gain. As a result, the input voltage range of the proposed inverter is significantly widened, facilitating grid-connected applications with a very low dc input voltage, compatible to the outputs of most PV panels. Moreover, the leakage inductance energy of the coupled inductor is recycled with voltages across the power switches clamped to the dc link capacitor by using a lossless diode. A prototype with 30 V dc input, 155 V ac output, nominal power 200 W, was built, which achieves a high voltage-boost, while keeping relatively low voltages stresses on the semiconductor devices.

LCL-filtered three-level inverters have been widely used in PV applications because of their high efficiency, high power density, and low cost. In practice, the inverter-side current feedback (ICF) control has dominated the PV market due to its low-cost property. However, the grid-side current becomes more sensitive to the grid distortion owing to its indirect regulation in an ICF controlled system. To address this issue, this paper proposes a new control structure to improve the power factor and eliminate the low-order harmonics of the grid-side current simultaneously. By combining the feedforward and proportional feedback of the capacitor current, this method can effectively eliminate the reactive power induced by the capacitor of an LCL-filter. In addition, the low-order harmonics can also be eliminated if the capacitor current is sent back to the resonant controller. Effectiveness of the proposed method has finally been verified with a 5kW LCL-filtered three-phase T-type PV inverter.

Single-phase split-source inverter is an attractive single-stage topology, as it can realize buck-boost function in a very compact way. However, this topology suffers from higher voltage stresses on dc-link than two-stage structure, especially under low ac gain conditions. This paper therefore analyzes the substantial reason of this problem. Then, based on the analyzing result, a general structure is proposed to solve the problem, and one practical implementation based on reverse-blocking IBGT is presented. Besides, a very simple SPWM modulation scheme is proposed together to improve its performance. Finally, compared with the existing SSI and its extensions, the proposed circuit can achieve higher ac gain with the same dc-link voltage. Its working principle and merits are explained in detail and also verified by experimental results.

Power talk is a novel ultra narrow-band powerline communication (UNB-PLC) technique for communication among control units in MicroGrids (MGs). Unlike the existing UNB-PLC solutions, power talk does not require installation of additional dedicated communication hardware and, instead, uses only the power electronic converters through which the control units interface the common bus. This way the communication system has practically the same reliability as the power system. The information is transmitted by modulating the parameters of the primary control, incurring subtle power deviations that can be detected by other units. In this paper, we develop power talk communication strategies for direct-current (DC) MG systems with arbitrary number of control units that carry out all-to-all communication. We investigate two multiple access strategies: time-division multiple access, where only one unit transmits at a time, and full duplex, where all units transmit and receive simultaneously. We apply the concepts of signaling space, where the power talk symbol constellations are constructed, and detection space, where the demodulation of the symbols is performed. The proposed communication technique is challenged by the random changes of the bus parameters due to load variations. To this end, we investigate the performance of power talk when a solution based on training sequences that re-establishes detection spaces is employed. The presented evaluation shows that power talk has a potential to offer an effective and inexpensive solution for reliable communication among units in DC MGs.

We introduce a novel communication strategy for DC Micro Grids (MGs), termed power talk, in which the devices communicate by modulating the power levels in the DC bus. The information is transmitted by varying the parameters that the MG units use to control the level of the common bus voltage, while it is received by processing the bus measurements that units perform. This implies that the communication does not require a dedicated modem, but instead it is piggybacked on top of the power electronics. The communication is challenged by the random fluctuations of the voltage level due to the random load variations in the MG. We develop the corresponding communication model and address the random voltage fluctuations by using coding strategies that transform the MG into some well- known communication channels. The performance analysis shows that it is possible to mitigate the random voltage level variations and communicate reliably over the MG bus.

Constant power load converter will introduce unstable factor to the cascaded system. Exchanging the position of constant power load converter as the source converter can improve system stability. Active solutions to modify the negative impedance of constant power load converter can also make the system more stable. The concept of coordinative front-to-end impedance control is proposed in this paper, which can assist the cascaded system with better stability. This paper compares converter impedance behaviors between different control methods, and shows that the front-to-end impedance controller can make the system more stable. The conclusions have been validated by simulation results.

This paper presents the theoretical background and studies on the supply side current interharmonics in a double-stage Adjustable Speed Drive (ASD), when they are initiated by motor current imbalance. Some useful approaches are then proposed to compensate for these interharmonic currents in partially and/or fully controlled ASD, resulting in undistorted grid currents. The simulation studies verify the effectiveness of the proposed schemes.

A compact quasi-Z-source DC/DC converter is presented with high voltage gain, isolated output, and improved efficiency. The improvements in size and performance were achieved by using a square wave inverter with only two output switches driving an isolating transformer in push-pull mode, followed by a voltage doubling output rectifier. The converter is well-suited to applications requiring a high voltage gain, especially renewable energy sources such as photovoltaic and fuel-cell power supplies. To demonstrate the converter's performance a prototype designed to output 400 V at 500 W was constructed and tested. The efficiency at full load varied from 89.0% to 97.4% as the input voltage changed from 44 V to 82 V.

This review focuses on faults in Modular Multilevel Converter (MMC) for use in high voltage direct current (HVDC) systems by analyzing the vulnerable spots and failure mechanism from device to system and illustrating the control & protection methods under failure condition. At the beginning, several typical topologies of MMC-HVDC systems are presented. Then fault types such as capacitor voltage unbalance, unbalance between upper and lower arm voltage are analyzed and the corresponding fault detection and diagnosis approaches are explained. In addition, more attention is dedicated to control strategies, when running in MMC faults or grid faults. This paper ends up with a discussion of other opportunities for future development.

Converters with reduced switch counts usually face some performance tradeoffs, which make them suitable for some applications but not others. The same applies to the nine-switch converter, which is a reduced-switch version of the back-to-back twelve-switch converter. The nine-switch converter has since been shown to experience a higher voltage stress, which can be lowered in some cases. A corresponding evaluation of its current stress is however lacking, and is hence addressed now by computing its switch currents when used for ac-ac, ac-dc, dc-ac and dc-dc energy conversions. Relevant expressions, application requirements and simulation results are presented for identifying cases, where the nine-switch converter can have an improvement in performance despite its reduced switch count.