Hydrogen electrloyzers are power-to-gas storage devices that can facilitate large-scale integration of intermittent renewable sources into the future power systems. Due to their fast response and capability to operate in different loading conditions, they can be used as responsive loads providing support to AC grid during transients. This paper suggests taking one step further and using hydrogen electrolyzers to provide grid-forming services to the grid. As a result, the electrolyzer's role is elevated from supporting the grid (responsive load) to actively participating in forming voltage and frequency of the grid. The grid-forming capability of electrolyzer is linked to its hydrogen production constraints, which can potentially pose limitations on the grid-forming services. Besides the grid-forming mode, two additional operating modes, i.e., DC voltage mode and constant power mode, are proposed to ensure a safe operation of the electrolyzer in case of adversary interaction between grid-forming operation and hydrogen production constraints. This paper also studies the impacts of grid-forming services on the electrolyzer's physical features such as hydrogen stack temperature and efficiency. Comprehensive simulations are conducted on a low-inertia test network whose topology is inspired by a portion of the transmission grid in South Australia to confirm the effectiveness of the proposed concept under various operational conditions of the electrolyzer and upstream AC grid. Moreover, the practical feasibility of the proposed control system is experimentally validated by conducting hardware-in-the-loop tests.

Modern power systems are facing the tremendous challenge of integrating vast amounts of variable (non-dispatchable) renewable generation capacity, such as solar photovoltaic or wind power. In this context, the required power system flexibility needs to be allocated in other units that can include energy storage, demand management or providing reserve from renewables curtailing the output power. The present paper proposes the new concept of grid-forming load, which can be considered a totally flexible concept of demand. The concept is not only ensuring the load is supporting the grid stability by adapting the load to the overall system balancing, but also ensures that the load is actually contributing to form the grid and to provide synchronization power to the overall system. In this sense, the new concept allows running a system powered only by renewables operating at maximum power (or operator defined set-point) in grid-following mode, while the overall system control is ubicated in the demand side. This is an important change of paradigm as it considers that all the flexibility, synchronism and stability provision is on the demand side. This concept can applied either to isolated systems or also to future power systems, where millions of loads steer the power system while the renewables are operating at full power. The paper proposes the concept, suggests possible control implementations of the grid-forming load and analyses the concept in four simulation case studies.

VSC-HVDC systems are being increasingly employed in the power systems. The recently installed HVDC systems have a power capacity similar to traditional power plants. Hence, they are expected to have a similar behaviour as traditional synchronous generators during faults in AC grid, within their limits of course. Recent grid codes require HVDC converter stations to incorporate fault ride-through (FRT) capabilities in order to avoid HVDC converter station disconnection from AC grid for certain fault characteristics. In this paper, two FRT mechanisms are suggested for the two converter stations of an HVDC system. One FRT mechanism is added to the DC voltage control loop of the master converter station, while the other FRT mechanism is added to the active power control loop of the slave converter station. The objective is to ensure the stable operation of the HVDC system during faults that may occur in AC grids located on both sides of the HVDC system. The performance and stability of the suggested FRT mechanisms are tested considering the pre-fault power flow direction and all possible types of balanced and unbalanced faults. Simulation results confirm the effectiveness of the FRT mechanisms and revealed the critical modes during FRT operation.

This article addresses the design of the DC voltage droop control in modular multilevel converter (MMC)-based multiterminal HVDC grids. First, two energy-based control approaches, namely classic and cross control, are explored for the implementation of the voltage-power droop controller. The cross control, as the better solution for droop implementation, is further improved, making it more robust against disturbances. Then, a methodology is derived to select the droop gain combinations considering the AC grid, DC grid and MMC dynamics and their limitations. The methodology is based on a linear analysis to identify the valid droop gains which comply with the limits imposed on: the transient power sharing among MMCs, the DC grid voltage, the MMC AC and DC currents, the total MMC stored energy, and the stability margin of the complete multiterminal HVDC grid. Finally, time-domain simulations are conducted using the nonlinear model to validate the dynamic performance of the selected droop combinations obtained from the suggested methodology.

This study presents an improved version of bipolar dc microgrids in which a distributed voltage‐balancing strategy supplants the central voltage balancer. The improved bipolar dc microgrid is interlinked to the ac grid using a trans‐z‐source inverter for which a new voltage control system is adopted. The shoot‐through duty cycle, d ‐axis current reference, and voltages of dc‐side capacitors are controlled by the proposed voltage control system. A bipolar, bidirectional dc/dc boost converter is designed to fulfil the voltage regulation and voltage‐balancing functions in case the trans‐z‐source inverter is disconnected from the ac grid. A power‐sharing scheme is also proposed in which two droop gains are employed to modify voltage control loops of each converter. Simulations are done for a bipolar dc microgrid involving a trans‐z‐source interlinking converter while local distributed generations (DGs) are interfaced to the microgrid by bipolar boost converters.