This work focuses on grid-forming (GFM) control of Interconnecting Power Converters (IPCs) that are used to interconnect multiple HVAC and HVDC subgrids to form a grid of grids. We introduce the concept of dual-port GFM control that leverages the ability of Modular Multilevel Converters (MMCs) to simultaneously form its AC and DC terminal voltage and present two dual-port GFM MMC controls. We provide analytical results and high-fidelity simulations that demonstrate that (i) dual-port GFM control is more resilient to contingencies (i.e., line and generator outages) than state-of-the-art single-port GFM control, and (ii) unlike single-port GFM control, dual-port GFM control does not require assigning grid-forming and grid-following (GFL) roles to the IPC terminals in grids of grids. Finally, we provide an in-depth discussion and comparison of single-port GFM control and the proposed dual-port GFM controls.

An analysis of control structures for Modular Multilevel Converters (MMC) used in High-Voltage Direct Current (HVDC) applications is addressed. In particular, the study focuses on the case of a point-to-point link with master-slave control, considering an energy-based scheme (also known as closedloop or energy-controlled) for the MMC, meaning that the internal energy of the converter is explicitly controlled. With such an approach, the MMC internal energy can be controlled independently from the energy of the HVDC link, and whereas the internal capacitance of the MMC depends on the converter’s rating, the capacitance at the DC terminal depends on the cable length. Therefore, several possibilities regarding the outer control structure (internal energy and DC voltage) arise, affecting the overall dynamics differently. Whereas for a long link the classic control structures should perform well, for shorter links the transient performance might not be acceptable and other alternatives shall be used instead. Different control structures are presented and evaluated in this paper through small-signal and frequency-domain analysis, and validated through time-domain simulation with Matlab® Simscape Power SystemsTM.

With an increasing number of renewable energy resources being integrated into the power system through power electronics, conventional power plants based on large synchronous machines tend to be reduced in the upcoming future. With the Modular Multilevel Converter as the state-of-the-art power electronics solution for power systems, it will be necessary to operate some of these converters in grid-forming mode, in order to either preserve the stability and robustness of the ac grid or as the only solution to generate the ac grid in cases with no available synchronous generation. This paper aims to understand how the internal energy control of the MMC can be used to effectively mitigate the propagation of ac power disturbances to the dc side, thanks to its buffering capability. Different energy control structures are explored, and a comprehensive discussion and comparison among them is provided, including stability analysis, transient performance and tuning guidelines. Simplified and linear models are used to address the analysis, whereas detailed non-linear models implemented in MATLAB Simulink are used to verify the results via time-domain simulations.

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.

The concept of segmented AC power systems interconnected by means of HVDC is becoming more and more significant for many cases worldwide. Such systems already exist and will become more complex with the increasing emergence of HVDC links and grids. The concept of a grid composed of multiple subgrids motivates the need to define the governing roles of Interconnecting Power Converters (IPC) which connect the different subgrids. Such IPC can have various operation modes which are presented and analyzed in this paper. In addition, the principles of operation needed for each individual subgrid and for the overall system are introduced and discussed. The resulting fundamental equations are outlined and combined in a power flow formulation of the overall system. A simple case study is presented to illustrate the power flow formulation in a grid of multiple subgrids, also introducing example dynamic simulations of transitions between different operating points.

The penetration of renewable energy resources is changing the way the power system is understood and operated. With an increasing number of power electronics devices integrated into the power system, the MMC is arising as one of the key technologies for HVDC applications. While several control strategies for MMCs have been proposed in the last years, designing controls for MMCs remains a challenging and non-trivial problem due to its large number of degrees of freedom. In this paper, a general multivariable control structure is proposed that generalizes and improves upon previously reported approaches. Moreover, to fully exploit the degrees of freedom offered by this control, a model-based tuning methodology is presented that extends results from optimal H2 structured control design to MMC applications. A case study using an HVDC link is used to illustrate and analyze the performance of the MMC of the proposed approach in different scenarios.