In this paper, a hybrid microgrid with series and parallel connected microconverters is proposed. A few series-connected low voltage (LV) microconverters are used to build a string converter with rated output voltage magnitude. Then, the parallel operation of string converters increases the power rating and redundancy of the microgrid. To realize both grid-tied and islanding microgrid control with proper power sharing, a hierarchical power regulation method is developed with simultaneous PCC voltage and micro-converter voltage control. First, the well-understood droop control is adopted to obtain PCC voltage reference for power sharing between parallel string converters. Second, a power balancing regulation through micro-converter local voltage control is used to re-distribute output power demand among series-connected microconverters in a string converter. As the string converter line current measured by all microconverters is the same, it is utilized to develop a novel synchronizer to interconnect the string converter central controller and the microconverter local controllers with only low bandwidth communications. Due to the coordination of string converter central control and microconverter local control, the proposed hybrid microgrid can operate in grid-tied and islanding modes with real and reactive power sharing for each microconverter.

In this paper, a three-terminal AC/DC hybrid microgrid with two DC terminals and one AC terminal is proposed. The proposed system consists of cascaded H-bridge (CHB) converters based AC grid interface and two dual active bridge (DAB) converters based DC subgrid interface that connects two isolated DC buses. In order to reduce the number of power conversion stages and power devices, the DAB converters are directly connected to CHB DC rails according to the system operation requirement. To overcome the imbalanced grid currents and DC rail voltages issues caused by this modified system configuration with only two power conversion stages, an improved method is proposed through the zero-sequence voltage injection in the CHB converters. In addition, to avoid the conflicts between zero-sequence voltage injection and the voltage/current regulation of the system, the impacts of the control parameters to the system stability and dynamic response are investigated. Evaluation results from both three-terminal and five-terminal hybrid AC/DC microgrids show that the generalized effectiveness of the proposed three-phase AC current and DC rail voltage balancing method.

In this paper, a modified double-loop voltage controller is developed to enhance the power sharing control of islanding microgrid, without the involvement of conventional droop control concept. Unlike the conventional double-loop voltage control where the inner and outer control loops are both implemented in the DG (distributed generation) unit local controller, the proposed method has an outer voltage control loop implemented in the microgrid central controller as secondary control to directly regulate the PCC (point of common coupling) voltage with reduced harmonics and magnitude/frequency variations. At the same time, an accurate power sharing is achieved by a modified inner control loop with internal impedance at DG unit local controllers. In contrast to the conventional series external virtual impedance for power sharing enhancement that can affect PCC voltage regulation, the internal impedance here can be very large but without affecting the control accuracy of PCC voltage. Furthermore, the overall control scheme has low computational load and the parameter design is straightforward. To reduce the communication burdens between PCC central controller and DG unit local controller, the DG unit fundamental line current is utilized as a novel synchronizer. With the use of abovementioned control method, a proper power sharing and nearly-zero PCC voltage control error can be achieved.

In this paper, a coordinated control method is proposed for three-phase AC/DC hybrid microgrid power electronic interlinking unit with dual converters sharing both DC and AC links. Unlike the conventional dual converters based system where the same control strategy is applied to both converters, converter1 in this proposed system is controlled by a closed-loop current regulation algorithm while converter2 employs voltage controller with coupled adaptive virtual impedance. With the flexible control of the coupled virtual impedance, the system performance is enhanced in the following three aspects. First, in grid-tied operation mode, the output power difference between dual converters is used as an input to tune the coupled virtual impedance of converter2 for a rapid dynamic power response. Second, the proposed coordinated control with inherent voltage support is able to achieve a seamless transfer without additional efforts. Third, during system islanded operation, a generalized power sharing and an active PCC voltage harmonic and unbalanced components mitigation are achieved via the adjustment of the coupled virtual impedance. Finally, the scalability of the proposed method in a system with more converters is briefly discussed and verified.

When a single-switch open-circuit fault occurs in the series resonant dual-active bridge (SRDAB) converter, the output DC voltage will drop by a half or rise by twice. To maintain continuous power supply, a fault tolerant control method based on the voltage single-loop control is proposed, where the rectifier-side output square voltage is regulated. Nevertheless, it may excite the resonance between the resonant inductors and DC capacitors, leading to severe low-frequency oscillations, (appearing as the envelope of the high-frequency current). This may trigger the over-current protection and the SRDAB fails to ride through the fault. To address this issue, low-frequency equivalent models are proposed for the bidirectional power-flow of the SRDAB, enabling frequency-domain analysis of the single-loop control. The analysis reveals that the oscillation depends on the duty-cycle and control parameters, and it is more likely to occur when the converter operates in the boost mode. However, it is not possible to suppress the oscillations by the voltage single-loop control. Thus, a dual-loop fault tolerant method is developed. The proposed control strategy includes an outer-loop voltage control, an inner-loop current envelope control and a non-linear correction unit. Experimental tests on a 1-kW SRDAB have validated the effectiveness of the proposed method.

Due to the nonuniform solar irradiance, different temperature, and inconsistent module features, the output power among each photovoltaic (PV) module in a single-phase cascaded H-bridge (CHB) inverter is unequal. The H-bridge module with higher power generation faces the risk of overmodulation. To further extending the operating range of the CHB PV inverter under unity power factor condition, in this article, a battery module is connected with the system in series to form a PV–battery hybrid CHB converter. Meanwhile, the corresponding flexible power control strategy is proposed. The injected power of the battery module is controlled adaptively according to the maximum modulation index of all PV modules. With the proposed approach, all PV modules are free from overmodulation even with severe power imbalance, and thus, more energies are harvested. The effectiveness of the proposed control method is verified by experimental results.