  Citation: Chen, J.; Wang, S.; Ugalde-Loo, C.E.; Ming, W.; Adeuyi, O.D.; D’Arco, S.; Ceballos, S.; Parker, M.; Finney, S.; Pitto, A.; et al. Demonstration of Converter Control Interactions in MMC-HVDC Systems. Electronics 2022, 11, 175. https:// doi.org/10.3390/electronics11020175 Academic Editor: Nikolay Hinov Received: 10 December 2021 Accepted: 5 January 2022 Published: 6 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). electronics Article Demonstration of Converter Control Interactions in MMC-HVDC Systems Jinlei Chen 1 , Sheng Wang 1 , Carlos E. Ugalde-Loo 1, * , Wenlong Ming 1 , Oluwole D. Adeuyi 2 , Salvatore D’Arco 3 , Salvador Ceballos 4 , Max Parker 5 , Stephen Finney 6 , Andrea Pitto 7 , Diego Cirio 7 and Iñigo Azpiri 8 1 School of Engineering, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, Wales, UK; ChenJ111@cardiff.ac.uk (J.C.); WangS9@cardiff.ac.uk (S.W.); MingW@cardiff.ac.uk (W.M.) 2 SSE Renewables, 1 Waterloo Street, Glasgow G2 6AY, Scotland, UK; Oluwole.Adeuyi@sse.com 3 SINTEF Energy Research, Strindvegen 4, NO-7465 Trondheim, Norway; Salvatore.darco@sintef.no 4 Tecnalia Basque Research and Technology Alliance (BRTA), 48160 Derio, Spain; salvador.ceballos@tecnalia.com 5 Department of Electronic and Electrical Engineering, Institute for Energy and Environment, University of Strathclyde, 16 Richmond Street, Glasgow G1 1XQ, Scotland, UK; max.parker@strath.ac.uk 6 School of Engineering, University of Edinburgh, Sanderson Building, Robert Stevenson Road, The King’s Buildings, Edinburgh EH9 3FB, Scotland, UK; Stephen.Finney@ed.ac.uk 7 Ricerca sul Sistema Energetico (RSE), Via Rubattino Raffaele 54, 20134 Milano, Italy; Andrea.Pitto@rse-web.it (A.P.); Diego.Cirio@rse-web.it (D.C.) 8 Renewables Business Unit, Iberdrola Tower, Plaza Euskadi 5, 48009 Bilbao, Spain; iazpiri@iberdrola.es * Correspondence: Ugalde-LooC@cardiff.ac.uk Abstract: Although the control of modular multi-level converters (MMCs) in high-voltage direct- current (HVDC) networks has become a mature subject these days, the potential for adverse in- teractions between different converter controls remains an under-researched challenge attracting the attention from both academia and industry. Even for point-to-point HVDC links (i.e., simple HVDC systems), converter control interactions may result in the shifting of system operating volt- ages, increased power losses, and unintended power imbalances at converter stations. To bridge this research gap, the risk of multiple cross-over of control characteristics of MMCs is assessed in this paper through mathematical analysis, computational simulation, and experimental validation. Specifically, the following point-to-point HVDC link configurations are examined: (1) one MMC station equipped with a current versus voltage droop control and the other station equipped with a constant power control; and (2) one MMC station equipped with a power versus voltage droop control and the other station equipped with a constant current control. Design guidelines for droop coefficients are provided to prevent adverse control interactions. A 60-kW MMC test-rig is used to experimentally verify the impact of multiple crossing of control characteristics of the DC system configurations, with results verified through software simulation in MATLAB/Simulink using an open access toolbox. Results show that in operating conditions of 650 V and 50 A (DC voltage and DC current), drifts of 7.7% in the DC voltage and of 10% in the DC current occur due to adverse control interactions under the current versus voltage droop and power control scheme. Similarly, drifts of 7.7% both in the DC voltage and power occur under the power versus voltage droop and current control scheme. Keywords: HVDC; MMC; control; interaction; experimental demonstration 1. Introduction High-voltage direct-current (HVDC) transmission systems based on voltage source converter (VSC) technology are suitable for the grid-connection of offshore wind farms and for the development of DC grids. In general, the three main types of VSC topologies are two-level, three-level, and multi-level; however, from 1997 until 2010, VSC-HVDC Electronics 2022, 11, 175. https://doi.org/10.3390/electronics11020175 https://www.mdpi.com/journal/electronics