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As the electronic interfaces between distributed energy resources and the electrical network, power converters play a vital role in voltage stabilization and power conversion. So far, various power converter control methods have been developed. Now it is urgently needed to compare and understand these approaches to support the smart microgrid pyramid. This article provides an overview of the state-of-the-art of parallel power converter control in microgrid applications. The most important control schemes to address existing challenges, including concentrated control, master–slave control, droop mechanism, virtual synchronous generators, virtual oscillator control, distributed cooperative control, and model predictive control, are highlighted and analyzed in detail. In addition, the hierarchical control structure, as well as future trends, are reviewed and discussed. [ABSTRACT FROM AUTHOR]

Unlike a synchronous generator that could withstand a large overcurrent, an inverter-based distributed generation (DG) has low thermal inertia, and the inverter is likely damaged by overcurrents during grid faults. In this article, a new strategy, namely positive- and negative-sequence limiting with stability enhanced P-f droop control (PNSL-SEPFC), is proposed to limit the output currents and active power of droop-controlled inverters in islanded microgrids. This strategy is easy to implement in the inverter controller and does not require any fault detection. Inverter stability is analyzed mathematically, which gives guidelines to design the parameters of the PNSL-SEPFC strategy. PSCAD/EMTDC simulation based on a four-DG microgrid shows that the proposed PNSL-SEPFC can limit inverter output currents and powers with better performance under both symmetrical and asymmetrical faults. Furthermore, hardware experiments demonstrate that the proposed PNSL-SEPFC can ensure the inverters riding through grid faults safely and stably. (A video of experimental waveforms is attached.) [ABSTRACT FROM AUTHOR]

Three-phase four-wire grid-forming (GFM) inverters are promising to interface distributed energy resources into low-voltage networks. However, these inverters are prone to overcurrent under grid faults. Physically increasing the inverter current capacity is not cost-effective to cope with complicated fault conditions. In this article, an adaptive fault ride-through (FRT) scheme based on instantaneous saturators and virtual negative- and zero-sequence resistances is proposed. It features not only overcurrent limitation by modifying voltage references, but also seamless transition between normal and grid fault conditions. The proposed FRT scheme is first analyzed from different aspects, including the virtual sequence resistances, grid short-circuit ratio, fault types, and fault levels. The virtual sequence resistances are then designed to be adaptive to ensure high voltage quality at the healthy phase. The proposed FRT scheme is verified by MATLAB/Simulink simulations under asymmetrical faults. A laboratory platform with a grid-connected 3kW GFM inverter is further constructed to demonstrate its effectiveness (a video of the experimental results under three asymmetrical faults is attached). [ABSTRACT FROM AUTHOR]

In microgrids, intermittency of renewable energy sources (RES) and uncertain state-of-charge (SoC) of energy storage systems (ESS) can cause power deficiency to some distributed generation units (DGs). In this case, DGs with power deficiency may not meet the power demand, resulting in voltage collapse or frequency divergence. Unfortunately, this is seldom considered in inverter control design in existing literature. Thus, in-depth investigation into the microgrid performance under renewable energy resource fluctuations and appropriate control methods are urgently needed. In this article, an adaptive droop and adaptive virtual impedance control strategy is proposed. Unlike conventional droop control where the droop coefficients are fixed by assuming the DGs can always meet the load demand, the droop coefficients here are adjusted according to actual solar PV power output. In this way, proper power sharing among DGs can be achieved under renewable energy variation. Furthermore, the impact of varying DG capacities on system stability is mathematically investigated. An adaptive virtual impedance is then incorporated into the adaptive droop method to deal with the system instability caused by renewable energy variations. The proposed strategy is analyzed theoretically and validated in MATLAB/Simulink simulation and laboratory experiments. The results demonstrate the advantages of the proposed method over conventional approaches under various scenarios. [ABSTRACT FROM AUTHOR]

Electric vehicle (EV) has been widely used in our life, one of the key technologies is the batteries, power accumulator battery test system (PABTS), which is initiated for evaluating the performance of the EV batteries, has been used in many battery-manufacture companies. The parallel operation of the PABTS forms a dc-microgrid, owing to the low inertia of the dc-link capacitance, the charging and discharging tests of the batteries can easily cause dc-bus voltage fluctuations, which may jeopardize the system stability. To increase the system inertia and achieve good system stability, a fractional-order model-predictive-control (FOMPC) and fractional-order virtual-inertia-control (FOVIC) strategy (namely, FOMPC-FOVIC method) is proposed for the bidirectional grid-connected converter. First, the fractional-order virtual inertial link is used to replace the integral-order virtual inertial link, which significantly improves the system stability. Second, combined with fractional-order model-predictive-control, virtual dc current compensation is proposed to further suppress the dc-bus voltage fluctuations. Finally, the fractional-order discrete state-space equation of the virtual inertial link is derived, and the cost function design and its optimal solution are elaborated. To demonstrate the effectiveness of the proposed FOMPC-FOVIC scheme, experimental results indicate that the proposed method is superior to the existing methods in terms of inertial support and dc-bus voltage regulation. [ABSTRACT FROM AUTHOR]

Traditional wireless power transfer (WPT) systems usually adopt the triple-phase-shift control method to maintain a constant output voltage, track the maximum system efficiency point (MEPT), and achieve zero-voltage-switching (ZVS) operation for various applications. However, these three targets are achieved at the cost of high reactive power on both primary and secondary sides, especially under light-load conditions, leading to low efficiency. This has become one of the challenges that hinder a further deployment of WPT technologies in practice. To address this vital problem, in this article, how the reactive power lowers the system efficiency is revealed based on a mathematical model established. Then, a hybrid modulation control strategy based on a proper selection between the full-bridge and half-bridge modes of the inverter and active rectifier is developed. An experimental prototype is constructed to verify the effectiveness of the proposed control method. Experimental results show that the proposed method can reduce the reactive power, maintain a constant output voltage, and realize the MEPT and ZVS operation, with high efficiencies up to 94.29% in a wide load range. [ABSTRACT FROM AUTHOR]

Efficiency remains a key challenge in wireless charging in academia and industry. In this article, a new wireless power transfer (WPT) system based on a double-half-bridge (DHB) inverter with two variable inductors (VIs) is proposed. Compared with conventional full-bridge (FB) inverters, the DHB inverter can reduce the current through the mosfets under the same output power and thus, reduce the conduction loss. Next, by adjusting the inductances of the VIs instead of using phase shift (PS) method, the output voltage or current can be controlled, while the circulating current can be eliminated and wide-range zero voltage switching operation can be achieved. Consequently, the power loss can be further reduced. Circuit analysis, VI design, as well as hardware implementation, are provided in detail. A laboratory prototype is built to verify the feasibility of the proposed method. Close agreement is obtained between simulation and experimental results. The maximum efficiency can reach 92.4%, which is 3.65% higher than traditional PS control. [ABSTRACT FROM AUTHOR]

To maintain a stable output voltage under various operating conditions without introducing extra dc/dc converters, phase-shift (PS) control is usually adopted for wireless power transfer (WPT) systems. By using this method, however, zero-voltage switching (ZVS) operation cannot be guaranteed, especially in light-load conditions. To achieve high efficiency and reduce electromagnetic interference, it is significant for WPT systems to achieve ZVS operation of all switching devices in the whole operation range. In this article, an auxiliary variable inductor, of which the equivalent inductance can be controlled by adjusting the dc current in its auxiliary winding, is designed for series–series-compensated WPT systems under PS control to mitigate the loss arising from hard switching. As a result, a wide ZVS operation range of all switching devices can be achieved. A laboratory prototype is built to verify the theoretical analysis. The experimental results show that, under load and magnetic coupling variations, ZVS operation at fixed operation frequency as well as a constant dc output voltage can be maintained. Compared to the conventional method with only PS control, the proposed WPT can achieve higher overall efficiency in a wider load range owing to the wide ZVS operation range. [ABSTRACT FROM AUTHOR]