As an important form of distributed renewable energy utilization and consumption, the multi-parallel inverter microgrid system works in both an isolated and grid-connected operation mode. Secondary-frequency and voltage-regulation control are very important in solving problems that appears in these systems, such as the distributed secondary-frequency regulation real-time scheme, voltage and reactive power balancing, and the secondary-frequency regulation control under the disturbances and unbalanced conditions of a microgrid system. This paper introduces key technologies related to these issues, such as the consensus algorithm and event-triggered technique, the dynamic and adaptive virtual impedance technique, and the robust and self-anti-disturbance control technique. Research and design methods such as small-signal state-space analysis, the Lyapunov function design method, the impedance analysis method, μ-synthesis design, and the LMI matrix design method are adopted to solve the issues in secondary-frequency regulation and voltage regulation. As the number of inverters increases, the structure of the microgrid becomes more and more complex. Suggestions and prospects for future research are provided to realize control with low-communication technology and a distributed scheme. Finally, for the case study, the droop-control model and primary frequency/voltage deviation of a multi-parallel inverter microgrid system is analyzed, and a state-space model of a multi-parallel inverter microgrid system with a droop-control loop is established. Then, the quantitative relationship between the primary frequency/voltage deviation and the active and reactive power output in the system is discussed. The methods and problems of centralized and decentralized secondary-frequency regulation methods, secondary-frequency regulation methods based on a consensus algorithm and an event-triggered mechanism, reactive power and voltage equalization, power distribution, and small-signal stability of the multiple parallel inverter microgrid system regarding the virtual impedance loop are analyzed.
A conventional hybrid circuit breaker (HCB) is used to protect a voltage source converter-based high voltage direct current transmission system (VSC-HVDC) from a short circuit fault. With the increased converter capacity, the DC protection equipment also requires a regular upgrade. This paper adopts a novel type of HCB with a fault current limiter circuit (FCLC), and focuses on the responses of voltage and current during DC faults, which are associated with parameter selection. PSCAD/EMTDC based simulation of a three-terminal VSC-HVDC system confirms the effectiveness and value of HCB with FCLC, by using an equivalent circuit modelling approach. Laboratory experimental tests validate the simulation results. The peak fault current is reduced according to the current limiting inductor (CLI) increase, and can be isolated more quickly. By adopting parallel metal oxide arrester (MOA) with the main branch of HCB, voltage stresses across the breaker components decrease during transient and continuous operation, and less energy needs to be dissipated by the MOA. The remnant current for all cases is transmitted to power dissipating resistor (PDR) in the final stage, and the fault current is reduced to the lowest possible value. When the current from the main branch is transferred to the FCLC branch, transient voltage spikes occur, while smaller PDR is required to absorb current in the final stage.
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