With the high proportion of distributed energy resources with randomness and intermittency penetrating the distribution network, traditional centralized optimization methods face problems such as communication packet loss, frequent failures, and low reliability and are difficult to apply to large-scale DC microgrids with wide-area dispersion effectively. Therefore, distributed optimization methods have attracted widespread attention due to their superior scalability and robustness. This paper investigates a convex relaxation-based distributed control strategy for DC microgrids with constant power loads (CPLs) and MPPT-controlled distributed generations (MPPT-DGs) to achieve global optimization. First, an optimal power flow (OPF) problem model for large-scale DC microgrids under a distributed framework is established, and a convex relaxation method taking exactness into account is proposed to transform the non-convex original problem into a new convex problem with the same optimal solution. Then, the Karush–Kuhn–Tucker (KKT) condition and its equivalent consistency-based condition are derived based on convex relaxation, and a distributed global optimization control method is proposed to achieve global optimal system operation, avoiding the solution of large-scale non-linear optimization problems. Finally, simulations and numerical experiments are presented to verify the correctness of the proposed strategy.

The continuous conduction mode (CCM) totem-pole bridgeless power factor correction (TPBPFC) converters are feasible because of the better reverse-recovery performance of the wide band gap (WBG) devices. However, practical applications encounter challenges such as low power density, high cost, and high-power loss. This paper proposes an improved hybrid Si/SiC CCM TPBPFC rectifier. A buck-type active power decoupling (APD) circuit is used to remove the bulky capacitor and increase the power density. A SiC-based bidirectional switch is connected between the Si-based PFC and APD bridges to achieve the soft switching of Si-based switches and decrease the power losses. The operation principle is given and then the analysis of soft switching conditions and power loss are provided. Finally, the verification of proposed circuit is experimented on a 500 W prototype and a 2% efficiency improvement is demonstrated.

Multi-energy flow analysis is the fundamental work to study the state estimation, safety analysis, and optimal control of integrated smart energy system (ISES). However, the randomness of renewable energy can easily lead to no static equilibrium in ISES, making its static stability a huge challenge. In addition, the coupling effect between multiple energy flows makes the analysis of the static stability of ISES more complicated. In this paper, the existence of equilibrium of ISES under multi-energy flow coupling is analyzed, and the analytical sufficient conditions for the static stability of the system are given. Firstly, the power flow equation of the ISES is derived. Then, based on Brouwer's fixed-point theorem, the analytical sufficient conditions for the power flow equation of the ISES are derived. Finally, a simulation model based on Matlab/Simulink is established, and the simulation results verify the correctness and effectiveness of the proposed conclusions.

Based on extended phase shift (EPS) modulation with voltage matching control and the DC blocking capacitor, an improved modulation method for dual-active-bridge (DAB) DC-DC converters with the blocking capacitor is proposed in this paper, which can realize full load range zero voltage switching (ZVS) under wide voltage range with low root-mean-square (RMS) current. In the proposed scheme, the volt-second products of all windings are kept equal to achieve the full load range ZVS, and the secondary side of the converter is switched between the full bridge mode and the half bridge mode to switch the optimal matching point of the voltage and then reduces RMS current and enlarges the ZVS region. The operation principle of the proposed method is analyzed, and ZVS region is illustrated and compared with conventional modulation to demonstrate the performance. A 1kW experimental prototype with 200V input and 100 - 400V output was built to verify the feasibility and effectiveness of the proposed method.

A hierarchical fault detection and fault location framework consisting of two levels is proposed for DC ring bus microgrids. In the primary level, a decentralized fault detection scheme based on back propagation (BP) artificial neural network (ANN) and discrete wavelet transform (DWT) is proposed to classify the pole-to-pole (P-P) and pole-to-ground (P-G) faults under measuring noises and load disturbances only relying on the information needed by the local double closed-loop controller, and the diagnosis speed and reliability of system are guaranteed. In the secondary level, a distributed line fault location method is presented by synthesizing the detection results of local and neighbor DG units with low bandwidth communication, and the accuracy of line faults is improved greatly without the central controller and additional sensors. Several simulation cases are analyzed under various fault configurations such as load mutations, different fault types, fault locations and fault impedance. The effectiveness and feasibility of the proposed method are verified.

In order to achieve low leakage currents, a symmetrical Z-source hybrid converter is proposed to simultaneously obtain a wide range of DC and AC output voltages. The proposed converter is derived from a traditional Z-source converter by replacing the control switch to a highly efficient and reliable inverter concept (HERIC) inverter. To be applied in PV applications, the impedance network is symmetrical, which can clamp the common-mode voltage of the proposed converter to be a constant (i.e., obtaining low leakage currents). A dedicated modulation scheme is introduced so that the symmetrical Z-source hybrid converter can achieve low leakage currents, a wide range of output voltages, high efficiency and reactive power injection. Simulations are performed on a 2-kW symmetrical Z-source hybrid converter to verify the effectiveness.

In photovoltaic (PV) grid-connected system based on series-connected power optimizer, the open-circuit fault of the inverter will seriously affect the security and reliability of system operation. To solve this problem, two kinds of open-circuit fault detection methods for the inverter based on series-connected power optimizer without communication are proposed. Firstly, the fault detection method based on charge change rate and power change rate is proposed separately. Then, the defects based on the charge judgment and the improvement based on the power change rate are analyzed. Finally, the effectiveness and reliability of the proposed method are verified by experiments.

This paper proposes a novel universal control for multi-bus AC microgrid with the capability to operate in both grid-connected (GC) and islanded (IS) modes. The control scheme is based on distributed manner, and the leader-follower consensus protocol is designed. Only few leader distributed generators (DGs) receive the correction signals from the unified mode controller, which is installed at the point of common coupling (PCC). The rest follower DGs would track the leader DGs, and exchange information with their neighbors. Under this control framework, the active and reactive power can be flexibly controlled to inject into the grid in GC mode. Meanwhile, the voltage/frequency support and active/reactive power sharing are guaranteed in IS mode. Moreover, the proposed method is able to offer seamless transition between GC and IS modes. Finally, simulations are presented to verify the satisfied transition process of different modes.

Cascaded H-bridge converters based islanded microgrids are applied to integrate the distributed generators (DGs). The accurate load sharing is an important task for the research of microgrids. Due to the characteristics of series connection, all DGs share a single common current. In this paper, the power factor of each DG is assigned to be consistent via regulating both the frequency and voltage. The real power and reactive power are shared accurately through the power sharing coefficients under the resistance-inductance and resistance-capacitance load. Meanwhile, an excellent load voltage quality is retained. The implementation of the proposed scheme only needs the local information in decentralized manners. The AC microgrid model is developed through simulations to verify the effectiveness of the proposed scheme.

This paper proposes a novel distributed control scheme for DC microgrids to acquire minimum generation cost and recover the global average voltage with consideration of capacity constraints. Our main focus is on constrained problems where the power of each distributed generator (DG) is restricted to lie in the feasible region. In this paper, a global load observer (GLO) is introduced that uses neighbor's data to estimate the total load across the microgird. To achieve minimizing generation cost, an alternative control strategy is proposed based on the specific characteristics of the optimum solution. When the total load which can be obtained by the GLO is less than the critical point, it generates power according to equal incremental cost rate (ICR) criteria; otherwise, it generates power maintain the maximum output point. To achieve voltage recovery, a voltage regulator is designed. The proposed controller on each DG communicates with its neighbor DGs on a sparse communication graph. The performance of the proposed scheme is validated and tested through MATLAB simulation.

Bulky capacitors are usually needed to buffer the double-frequency ripple power (DFRP) in single-phase power factor correction (PFC) rectifier. It is important to reduce the capacitance requirements for improving the system reliability and power density. In this paper a control method for capacitive idling converter based PFC (CIC-PFC) rectifier is proposed. The DFPR is diverted to the energy transfer capacitor and stored by swinging its voltage. Then, the required capacitance is reduced. Compared with other active power decoupling methods, the proposed is cost-effective as no extra hardware is added. The paper also discusses the selection of the energy transfer capacitor by comprehensively considering the voltage stress, cost, volume, current ripples, and power losses. Finally, experimental results are presented to show the effectiveness.

A predictive control scheme for the third-harmonic injection two-stage matrix converter (3TSMC) under unbalanced input voltages is presented. The proposed scheme is designed to predict the future behavior of the third-harmonic injection current and load currents respectively for each valid switching state. The control scheme selects the state that minimizes the error between the predicted currents and references. For the 3TSMC, the input currents are synthetized by the injection current. In order to keep sinusoidal input currents under unbalanced input voltages, the reference of the third-harmonic injection current is obtained by symmetrical component extraction. Firstly, this paper introduces the proposed control method, including establishing the discrete-time model and selecting cost functions. Then, the extraction for the reference of injection current is presented. Finally, this paper shows the simulation results to demonstrate that the proposed control method can generate good tracking of input and output currents under unbalanced input voltages.

This paper presents an active topology for power decoupling in single-phase ac-dc converters, featuring the effective suppression of low-frequency power ripple which is an inherent problem in single-phase energy conversion systems. This topology is composed of an H-bridge circuit and a ripple power-decoupling circuit. The ripple power-decoupling circuit shares one bridge arm with the H-bridge circuit so it has less additional components: just one IGBT, one diode, and an energy-storage inductor. Its modulation strategy is properly devised to coordinate the operation of decoupling circuit and H-bridge circuit. To enhance the power-decoupling performance, a direct ripple power-cancellation method based on energy-storage inductor is proposed. A multiresonant controller with feed-forward control is introduced for fast and precise current tracking. The effectiveness of this topology has been verified by detailed simulation studies as well as the laboratory prototype experiment results.