A novel grid voltage modulated direct power control (GVM-DPC) strategy for a grid-connected voltage source converter is proposed to control the instantaneous active and reactive powers directly. The GVM-DPC method consists of a nonlinear GVM controller, a conventional controller (feedforward and PI feedback), and nonlinear damping. The proposed control strategy shows a relationship between DPC and voltage-oriented control methods designed in d-q frame. The main advantage is that the proposed method makes the system be a linear time-invariant system, which enables us to apply various control methods easily. The GVM-DPC guarantees not only the convergence rate but also the steady-state performance of the system. Moreover, it is ensured that the closed-loop system is exponentially stable. Finally, simulation and experimental results using a 2.2-kVA VSC are provided to validate the tracking performance and robustness of the proposed control architecture. In addition, the total harmonic distortion of the current is 1.9% which is much less than the requirement for grid operation.

Parallel VSG systems are emerging as an attractive solution for inertia support and frequency stability enhancement of the power grid. However, the dynamic frequency characteristic is more complicated due to the coupled interaction of line impedance, virtual inertia, and damping coefficient in parallel VSG systems. The problems for frequency stability, dynamic performance, and frequency/power oscillation have raised much more attention. In this paper, a systematic analysis of the influence of virtual inertia and damping coefficient on the dynamic frequency characteristic among parallel VSG systems was presented and a quantitative assessment for dynamic frequency performance based on the 2-norm H 2 was proposed. Furthermore, a coordinated control for virtual inertia and damping was proposed to improve frequency stability and achieve a better dynamic performance of parallel VSG systems. The effectiveness and superiority of the proposed method were verified by simulation and experimental results.

The increase in greenhouse gas emissions from the transportation sector together with the continued depletion of fossil fuels in general has encouraged an increase in the use of energy storage systems and renewable energy sources at seaports and also on short route yachts and ferries. At present, most seaports, particularly smaller ones, are not provided with cold-ironing facilities—shore-based power facilities, which provide electric power to ships from the national grid. Because of the lack of cold-ironing facilities at most ports, auxiliary diesel engines and diesel generators on ships must be kept operating and online while at berth to supply auxiliary loads of ship. To address these requirements, one possible solution would be to provide cold-ironing facilities at all ports. However, in many circumstances, this is not cost-efficient as a port might be far from the national grid. To overcome these limitations, a seaport microgrid can be formed through the integration of multiple shipboard microgrids (SMGs) with decentralized control together with a charging infrastructure that is located on-shore. This integration of multiple SMGs and port-based charging stations is termed a ship-based seaport microgrid. Typically, power is shared among different microgrids using data communication techniques, which adds to the cost and the complexity of the overall system. This article proposes a communication-less approach based on multimode, decentralized droop control that enables power sharing among several SMGs in both charging and discharging modes based on the state of charge of battery banks—electric power is either supplied or consumed. The proposed approach would be potentially useful for future autonomous ships and also for islands where port electrification is either not technically feasible or an economically viable solution. A simulation and hardware-in-the-loop results are provided to verify the control robustness of the proposed control strategy.

A dynamic consensus algorithm (DCA)-based coordinated secondary control with an autonomous current-sharing control strategy is proposed in this paper for balancing the discharge rate of energy storage systems (ESSs) in an islanded AC microgrid. The DCA is applied for information sharing between distributed generation (DG) units to regulate the output power of DGs according to the ESS capacities and state-of-charge (SoC). Power regulation is achieved by adjusting the virtual resistances of voltage-controlled inverters with an autonomous current-sharing controller. Compared with existing methods, the proposed approach can provide higher system reliability, expandability, and flexibility due to its distributed control architecture. The proposed controller can effectively prevent operation failure caused by over-current and unintentional outage of DGs by means of balanced discharge rate control. It can also provide fast response and accurate current sharing performance. A generalizable linearized state-space model for n-DG network in the z-domain is also derived and proposed in this paper; the model includes electrical, controller, and communication parts. The system stability and parameter sensitivity have been analyzed based on this model. To verify the effectiveness of the proposed control approach, this study presents simulation results from a ten-node network and a comparison between experimental results obtained from the conventional power sharing control and the DCA-based SoC coordinated control in a setup with three 2.2 kW DG units.

When connected to a distorted grid utility, droop-controlled grid-connected microgrids (DCGC-MGs) exhibit low equivalent impedance. The harmonic and unbalanced voltage at the point of common coupling (PCC) deteriorates the power quality of the grid-connected current (GCC) of DCGC-MG. This paper proposes an active, unbalanced, and harmonic GCC suppression strategy based on hierarchical theory. The voltage error between the bus of the DCGC-MG and the grid's PCC was transformed to the dq frame. On the basis of the grid, an additional compensator, which consists of multiple resonant voltage regulators, was then added to the original secondary control to generate the negative fundamental and unbalanced harmonic voltage reference. Proportional integral and multiple resonant controllers were adopted as voltage controller at the original primary level to improve the voltage tracking performance of the inverter. Consequently, the voltage difference between the PCC and the system bus decreased. In addition, we established a system model for parameter margin and stability analyses. Finally, the simulation and experiment results from a scaled-down laboratory prototype were presented to verify the validity of the proposed control strategy.

In this letter, a novel control scheme is proposed by combining the fractional-order control and the virtual synchronous generator, whereby more degrees of freedom are involved in the controller design. A well-damped power control without oscillations is achieved. At the same time, a significant improvement in the inertial effect is attained. Both theoretical and experimental results have validated the feasibility.