In this paper, a new consensus-based distributed secondary control (DSC) strategy is proposed for frequency control, dc-voltage regulation, and optimal dispatch (OD) in isolated hybrid ac/dc-microgrids (HMGs). At the same time, all the units are maintained within limits. The proposed control scheme dispatches the distributed generators (DGs) within the microgrid in compliance with the Karush-Kuhn-Tucker (KKT) conditions of a linear optimal power flow (OPF) formulation. Furthermore, the power through the interlinking converters (ICs) is also dispatched to help minimize the total operation cost of the microgrid. The controllers rely on local measurements and information from neighbouring devices at both sides of the microgrid (DGs and ICs). Thus, unlike conventional methods, the microgrid is considered a single entity and not as three independent systems interacting with one another, and the OD is calculated considering both the ac-DGs and dc-DGs. Extensive simulations demonstrate a good performance of the controller amid load step changes and unit congestion, driving the system to an optimal economic operation.

This paper proposes a generalized and efficient power-flow algorithm for islanded hybrid ac/dc microgrids. The algorithm considers the microgrid operational aspects, i.e., absence of a slack bus, unbalanced ac subgrid, droop-controlled ac and dc voltages and ac frequency, and coupling between the ac frequency and dc voltage through interlinking converters (ICs). To attain high computational efficiency, the algorithm adopts three features. First, it models the ac subgrid elements in sequence components, thereby dividing the subgrid's set of equations into three smaller sets for faster parallel solution. This approach also accurately represents the different types of ac distributed generators (DGs). Second, the algorithm sequentially solves for the power-flow variables of the ac and dc subgrids, thus reducing number of equations to be solved simultaneously, once again for further computational cost alleviation. Third, the algorithm implements the quadratically-convergent Newton-Raphson technique to solve the decoupled sets of equations. The proposed algorithm is validated through comparisons with time-domain simulations, in MATLAB/Simulink, for test hybrid ac/dc microgrids of different configurations. Moreover, three case studies are introduced to examine the proposed algorithm's effectiveness in solving large-scale microgrids, to investigate its limits-enforcement capabilities, and to evaluate its performance as compared to conventional methods.

Wind farms featuring a series-connected DC collection system have been shown to offer advantages in terms of total conversion efficiency and amount of offshore-deployed equipment. To ensure proper exploitation of these benefits, it is necessary to determine the ratings of wind turbine converters. These ratings must be sufficient to cover the expected operating conditions over the life of the wind farm, without unnecessarily oversizing the equipment. As shown in this work, the variable string current results in an interdependence of operating points among wind turbines that has to be considered for sizing these converters. This paper proposes a generic sizing framework for such single-string, series-connected DC wind farms. This framework is applied to three wind farm configurations featuring differential power processing, voltage source converters, and diode-bridge rectifiers and buck converters. Lastly, a case study for a 450 MW offshore wind farm demonstrates the implementation of this sizing methodology and quantifies the design trade-offs between converter ratings and energy production enabled from those converters. In two of the studied wind farm configurations, significant rating reductions are achievable while still avoiding energy curtailment for >99.7% of the energy production of an equivalent AC wind farm.

Modular multilevel converters (MMCs) based on half-bridge submodules (HBSMs) are unable to prevent the AC side contribution to DC side fault currents, thus necessitating circuit breakers (CBs) for protection. A solution to this problem is using submodules (SMs) that are capable of blocking the flow of current from the AC grid to feed the DC side fault. The full-bridge submodule (FBSM) is one type of fault blocking SM where the presence of two extra switches ensures that in the event of a DC fault, the reverse voltage from the FBSM capacitor is placed in the path of the AC side current feeding the DC side fault through the antiparallel diodes. However, the additional semiconductor switches in the FBSMs increase the converter cost, complexity, and losses. Several SM configurations have been proposed in recent years that provide DC fault blocking capability with lower losses and device counts than those of FBSMs. Besides, many of the proposed hybrid converter configurations that combine different topologies to optimize converter performance are also capable of providing DC fault blocking. Furthermore, certain SM topologies are capable of riding through DC faults by remaining deblocked and operating in static synchronous compensator (STATCOM) mode to provide reactive power support to the AC grid. In this paper, noteworthy SM and MMC configurations capable of DC fault blocking and ride-through are reviewed and compared in terms of component requirements, semiconductor losses, and DC fault handing capability. The review also includes a discussion on control strategies for MMC arm/leg energy balancing during STATCOM operation.

Wind and Solar energy sources cause frequency and voltage-related power quality issues, especially at high penetration levels. Battery energy storage systems help reduce fluctuations by absorbing excess power and delivering power deficits. However, batteries suffer from capacity fading and degradation due to frequent cycling. Storing energy in the form of a chemical fuel helps in overcoming these drawbacks. Since ammonia fuel cells address issues associated with hydrogen fuel cells, such as high flammability, poor volumetric density, and high storage costs, in this study the application of ammonia fuel cells for solar and wind power leveling is investigated. Excess wind/solar power is used in an electrochemical ammonia synthesizer (EAS) to produce ammonia, and a direct ammonia fuel cell (DAFC) converts the ammonia to electric power and supplies the power deficit. Smoothing filter concepts, such as moving average, moving median, Savitzky-Golay, moving regression (MR), and Gaussian filters, are developed to assess EAS and DAFC capacity requirements. Simulation results show that MR filter overall performance is superior to other smoothing filter approaches, resulting in reduced capacities for ammonia production and fuel cell output, lowering system costs. The developed energy storage system is an effective compensation method for solar and wind power fluctuations.