The DC solid-state transformer (DCSST) is a key component of the DC microgrid, and stability issues due to operation with constant power loads (CPLs) have become increasingly prominent. To stabilize the system of DCSST with CPLs, in this paper, a novel intelligent backstepping controller based on a nonlinear disturbance observer (NDO) is proposed, and the control coefficients of the backstepping control are automatically adjusted by an auxiliary deep deterministic policy gradient (DDPG) controller. More specifically, the NDO estimates the large-signal nonlinear disturbance of the load power variation, and a backstepping controller is designed to improve the stability of the system under the estimated large-signal disturbance. Then, an input voltage equalizing compensator is added to ensure the identical input voltage to each sub-module of DCSST. Furthermore, a DDPG sub-controller is introduced into the backstepping control to intelligently adjust the control coefficients to improve the operating efficiency of the system. The results of the simulation and experiment show that the system is more robust due to the learning ability of the proposed controller, and the proposed controller can improve the stability of the system under the large-signal disturbance and intelligently ensure fast dynamic response with accurate tracking over different operating environments.

Three phase AC/DC converter based on the traditional double loop control may act as a link bridge in hybrid micro-grid due to its bidirectional power flow with unit power factor on AC side. When the AC/DC converter works on the condition of unbalanced voltage on the AC side, a second order ripple voltage will be generated in the DC-link and some amount of third harmonic current will exist in the AC side, which could deteriorate the operation of the micro-grid. By analyzing of the power flow under unbalanced voltage conditions, a three-phase AC/DC converter system, based on super capacitor compensation, is presented. The system can guarantee the AC current sinusoidal and symmetrical, and can keep DC-link voltage fluctuating lightly around rating during the super capacitor working. As an energy buffer, the super capacitor mitigates the active power ripple. Simulation and experimental results are provided to verify the feasibility of the control system.

Hybrid AC/DC microgrids (HMG) are emerging as an attracting method for integrating AC/DC distributed energy resources (DERs). In the isolated hybrid AC/DC microgrid (IHMG), the key problem is how to balance power variation and regulate voltage and frequency. Various energy storage systems (ESS) and interlinking converter (IC) technologies are viable for this application. The present study proposes a novel unified power flow (PF) model which can be applied to compare and analyze the practical operation modes of the IHMG and, further, to evaluate and compare the abilities of the ESS with different connection topologies and ICs with different control approaches to maintain the voltage and frequency stability of the IHMG. Five operation modes of the IHMG are defined and explained. Then, a set of generic PF equations are derived. Moreover, three binary matrices are applied as input parameters of the unified power equations. These matrices enable a single operation mode of the IHMG at a time to be constructed in the power equation. Finally, the accuracy and effectiveness of the proposed scheme are verified against the time domain simulation result. The quasi-steady-state behaviors of multi-DC subgrids IHMG in different modes after a range of load fluctuation are investigated. The results show that the use of multiple grid-forming units in the AC and DC subgrids, when IC adopted normalized.

Cascaded inverters may suffer from multiple open-circuit faults (OCFs) that occur in a cascaded way, but most existing hardware-based fault-tolerant methods usually activate redundant submodules (SMs) after a long diagnosis process is finished, which leads to significant waveform distortion and power derating. This article proposes a seamless fault-tolerant model predictive control (SFTMPC) for cascaded full-bridge NPC inverters (CFNPCIs) to eliminate this problem and improve the utilization rate of remaining healthy devices. A finite state machine is designed to denote eight different operating stages of the CFNPCI with two cascaded OCFs. A redundant SM without bypass switches is directly activated as soon as an OCF is detected, and it will compensate for the current tracking error in diagnosis stages with higher control frequency than ordinary SMs. After diagnosis finishing, the allocated level and switching state selection method of the faulty SM are accordingly revised based on the diagnosis results, to provide power as much as possible utilizing the remaining healthy devices. The redundant SM is then switched to supplement the missing levels of the faulty SM with the same control frequency as ordinary SMs. Finally, experimental results are reported to validate the SFTMPC.

The real‐time monitoring technology of power line is an important guarantee for the digitization and intelligence of the modern power system. Traditional current transformers cannot provide continuous and stable energy when the power line current fluctuates in a large range. This paper proposes a comprehensive design method of an active electronic current transformer (AECT) based on a single‐phase full‐bridge voltage‐type PWM rectifier (VSR), which can acquire continuous and stable energy from the power line with a minimum magnetic core. The control of the AECT is divided into four different operation modes to adapt the fluctuation of primary current. Maximum power point tracking (MPPT) mode and minimum loss (ML) mode is designed to harvest the maximum power with lower iron core loss when the primary current is relatively low. Excitation mode and demagnetization mode are designed to harvest stable power when the primary current is higher than a certain current. The specific mode operation principles and control strategies are analysed in detail. Besides, the calculation method of the primary current is also presented for the primary current measurement. Finally, simulations and experiments verify that the iron core can obtain power steadily (9.91 W/kg) when the current fluctuation between 50 and 600 A.

The connection of single-phase AC loads to a DC microgrid not only large secondary currents flowing into energy storage units with droop control cause, but also lead to secondary voltage ripples appeared on the DC bus, which seriously affects the normal operation of DC microgrids. In this paper, the secondary currents and voltages can be suppressed simultaneously by means of reconstructing the output impedances of energy storage converter and active capacitor converter. The main characteristics, impedance model, control strategies, and system stability in terms of secondary ripples are comprehensively investigated. Particularly, it is shown that the output impedance of the energy storage converter with droop control is relatively small and the input impedance DC loads buck converter are relatively large, so the secondary currents mainly flow into energy storage units and DC bus capacitor. Based on this, the output impedance of the energy storage converter and active capacitor converter are reconstructed relatively larger and smaller respectively, so the secondary currents mainly flowing into the active capacitor. Therefore, the secondary ripple currents flowing into energy storage units and the secondary ripple voltages appeared on the DC bus can be suppressed. Finally, the proposed methods are varied through simulations results.