Three-level converters feature with low switching loss and small filter size. In order to realize high power density design for three-level converters, SiC MOSFETs may be selected instead of using Si IGBTs. However, all SiC MOSFETs based converters suffer from extreme high total cost. In this paper, a SiC MOSFET & Si device hybrid ANPC converter is proposed. It consists of four Si active switches and only two SiC MOSFETs. Thus, it has lower total cost compared to all SiC MOSFETs based ANPC converter. Furthermore, a dedicated modulation scheme is proposed to completely move all the switching events from Si devices to SiC MOSFETs by using redundant switching states. As a result, the switching losses are significantly reduced and extremely high efficiency is achieved. The proposed converter has fully utilized the low switching loss advantage of SiC MOSFET and the low-cost advantage of Si devices, which shows significant superiority in high-end grid-connected inverter and rectifier applications.

Due to multifeedback of state variables, a discrete state-space controller offers outstanding control bandwidth as well as control stability for popular LCL-type grid-connected inverters, while the grid current is still vulnerable to the grid-voltage harmonics. The feedforward of grid voltage, usually employed in continuous controllers to solve such a problem, is too complicated to be applied for the discrete state-space controller. By means of continuous transformation, a full grid-voltage feedforward (GVFF) decoupling strategy is successfully proposed in this paper to make the feedforward possible for the discrete state-space controller. Based on the transfer function analysis and comparison of discretized and continuous system, comprehensive verification is also provided to verify the effectiveness of the derivation of the GVFF path. Subsequently, the robustness analysis of the proposed strategy to the grid impedance is also performed. Moreover, a grid-voltage estimator instead of the measured voltage is employed for the full GVFF, which not only retains the important information of grid voltage but also eliminates the influence of accompanied noises. The distinct features of the proposed feedforward controller plus implementation strategy are the super steady waveform, dynamic response, and robustness to the variation of grid impedance. Besides, the complexity of the algorithm is moderate and the computational burden is not significantly increased. Finally, simulation and experimental results are provided to verify the feasibility and validity of the proposed strategy.

By regulating the inverter output current every switching cycle, instantaneous current-sharing control strategies are usually employed in paralleled modular uninterruptible power supplies (UPSs). In this paper, virtual impedance, which is usually utilized in the droop method, is firstly introduced to the instantaneous current-sharing control strategy to achieve good load current-sharing performance. The instantaneous circulating current model of the paralleled system is developed and the circulating impedance is derived as well to clearly identify the intrinsic nature of unequal sharing of load current. By inserting the virtual impedance to the circulating impedance with the help of proper control strategy, good current-sharing capability can be obtained. The parallel inductor, e.g., coupled inductor, which is generally added at the output of each inverter to reduce the circulating current among modules, can be, therefore, eliminated, contributing to reduced weight, volume, and cost. In the meantime, the system output-voltage regulation performance, e.g., amplitude and waveform quality, is not affected by the introduction of the virtual impedance. The designation of the virtual impedance is provided and its implementation is simple but rather effective. Experimental results are also provided to verify the feasibility of the proposed method.

Islanding detection is a mandatory function for grid-connected converters. The popular slip mode frequency shift (SMS) and auto phase shift active islanding detection methods are investigated and an improved (IM)-SMS strategy is proposed in this study. In the proposed method, additional phase shift is introduced to help in stimulating the action of the islanding detection and the algorithm is simplified as well. When the utility grid is disconnected, the algorithm keeps the frequency of the converter output voltage deviating until the frequency protection relay is triggered. The working principle of the method is introduced and the guidance of parameters selection and optimisation is also provided. The islanding detection performance is evaluated through theoretical analysis and verified by digital simulation and experimental results. The IM-SMS method exhibits features of simplicity, easy implementation and high reliability.

In this paper, a single-phase single-stage isolated zero current switched (ZCS) current-fed full-bridge ac/dc converter is proposed for IGBT-based high-power power factor correction (PFC) applications. By adding an additional commutation path and few resonant components, ZCS operation is realized for all IGBTs. The conduction loss is also lowered by the additional path. Furthermore, the control strategy for the proposed converter is compatible with that developed for traditional PFC converter. In this paper, the topology derivation and circuit operational analysis are given. The control strategy is introduced. A 3-kw prototype is developed and tested to verify the circuit functionality. Finally, the efficiency performance is investigated and comparison is made to show the advantages of the topology.

High-performance phase-locked loops (PLLs) are critical for power control in grid-connected systems. This paper presents a new method of designing a PLL for single-phase systems based on derivative elements (DEs). The quadrature signal generator (QSG) is constructed by two DEs with the same parameters. The PLL itself is realized by using the DE-based QSG. It avoids errors due to the overlap and accumulation that are present in PLLs based on integral elements, such as a PLL based on a second-order generalized integrator. Additionally, frequency feedback is not needed which allows the proposed PLL to achieve high performance when the grid frequency changes rapidly. This paper presents the model of the PLL and a theoretical performance analysis with respect to both the frequency-domain and time-domain behavior. The error arising from the discretization process is also compensated, ensuring this PLL method is suitable for implementation in a digital control system. Simulation and experimental results show that the proposed PLL achieves good performance in both harmonic rejection and dynamic response.