AC fault ride through in MMC-based HVDC systems

© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksVSC-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.Peer ReviewedPostprint (author's final draft

Dc voltage droop control design for mmc-based multiterminal hvdc grids

© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution toThis 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.Peer ReviewedObjectius de Desenvolupament Sostenible::7 - Energia Assequible i No ContaminantPostprint (published version

Grid-forming services from hydrogen electrolyzers

© 2023 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksHydrogen 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.Peer ReviewedPostprint (author's final draft

Interaction Assessment and Stability Analysis of the MMC-Based VSC-HVDC Link

This paper investigates the dynamic behavior of a modular multi-level converter (MMC)-based HVDC link. An overall state-space model is developed to identify the system critical modes, considering the dynamics of the master MMC and slave MMC, their control systems, and the HVDC cable. Complementary to the state-space model, an impedance-based model is also derived to obtain the minimum phase margin (PM) of the system. In addition, a relative gain array (RGA) analysis is conducted to quantify the level of interactions among the control systems of master and slave MMCs and their impacts on stability. Finally, with the help of the results obtained from the system analysis (eigenvalue, phase margin, sensitivity, and RGA), the system dynamic performance is improved

Mutual interactions and stability analysis of bipolar DC microgrids

© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksThis paper presents an Multi-Input Multi-Output (MIMO) analysis to investigate the mutual interactions and small-signal stability of bipolar-type dc microgrids. Since bipolar dc microgrid is replete with power-electronic converters, its dynamics can not be understood unless the interactions among control systems of converters are properly investigated. To tackle the challenge, each converter in microgrid is modeled via an MIMO transfer matrix. Then, the MIMO models are combined together based on the interactions among the control systems of source and load converters. From this integrative MIMO model, the mutual interactions between various input-output pairs are quantified using Gershgorin Band theorem. Also, Singular Value Decomposition (SVD) analysis is carried out to estimate the frequency of unstable poles. Test results not only successfully validate the effectiveness of the MIMO method but also show that the control system of voltage balancer has a major impact on the overall stability of bipolar dc microgrid, making it a suitable location for applying damping systems.Peer ReviewedPreprin

Methodology for interaction identification in modular multi-level converter-based HVDC systems

This paper suggests a methodology for the identification, classification, and evaluation of various types of interactions that may occur in an HVDC link based on modular multi-level converters (MMC). The methodology incorporates the most suitable analytic tools for the frequency-domain study of each interaction category. To do so, a detailed nonlinear model of an MMC-based HVDC link that consists of master and slave MMCs, AC grids, and the DC transmission system is derived. Then, it is linearized to obtain a multi-input multi-output (MIMO) linear model that represents the dynamics of the complete MMC-based HVDC link. Based on the control loops of interest, interactions are classified as (1) state variable interactions, (2) disturbance interactions, (3) control loop interactions, and (4) overall system interactions. Then, through the application examples, the mentioned four categories of interactions are studied in frequency domain via the relevant analytic tools. The results obtained from the frequency-domain analysis are validated by time-domain simulation.Peer ReviewedPostprint (author's final draft

Fault ride‐through control based on voltage prioritization for grid‐forming converters

Abstract This paper discusses that the existing grid codes on the fault ride‐through (FRT) operation have certain complications to be implemented in the grid‐forming (GFM) converters. In particular, they do not provide a clear guidelines on the suitable prioritization of the positive‐ and negative‐sequence currents needed for a desired voltage profile during FRT operation. To address this challenge, two FRT controls which are based on (i) voltage balancing priority, and (ii) voltage magnitude priority are investigated for GFM converters. The former gives priority to the elimination of negative‐sequence voltage, while the remaining converter's capacity is used to increase the positive‐sequence voltage. On the contrary, the latter prioritizes the increase of positive‐sequence voltage, while it tries to reduce negative‐sequence voltage by using the remaining converter's capacity. The dynamic performances of these two FRT controls are thoroughly discussed, and the simulation results show that depending on the desired voltage profile during fault, one of them can be implemented

Interaction assessment and stability analysis of the MMC-based VSC-HVDC link

This paper investigates the dynamic behavior of a modular multi-level converter (MMC)-based HVDC link. An overall state-space model is developed to identify the system critical modes, considering the dynamics of the master MMC and slave MMC, their control systems, and the HVDC cable. Complementary to the state-space model, an impedance-based model is also derived to obtain the minimum phase margin (PM) of the system. In addition, a relative gain array (RGA) analysis is conducted to quantify the level of interactions among the control systems of master and slave MMCs and their impacts on stability. Finally, with the help of the results obtained from the system analysis (eigenvalue, phase margin, sensitivity, and RGA), the system dynamic performance is improved.Postprint (published version

Optimal H-infinity control design for MMC-based HVDC links

© 2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksThe modular multilevel converter (MMC) has emerged as the preferred choice for voltage source converter (VSC)-based high voltage direct current (HVDC) systems due to its low losses, low harmonic distortion, modularity, and redundancy. These advantages come at the expense of a complex control system with the strong coupling among its control variables, which complicates the design procedure of both its individual control loops and the DC link controllers. In fact, the performance improvement of a control loop could lead to degraded performance of other loops. Hence, to deal with such a complex dynamic system, this article suggests adopting multivariable H optimal control techniques in order to ensure stability and optimized performance of a VSC-HVDC link. First, a full-order, centralized multi-input-multi-output (MIMO) controller is derived based on H optimization and used as a benchmark for the system performance level. Then, a fixed-structure, decentralized MIMO controller is synthesized to make a compromise between the optimal performance and feasibility of practical implementation. Finally, simulations are conducted to evaluate and compare the performance and robustness degradation due to the migration from a high-order, centralized controller towards a low-order, decentralized controller.Peer ReviewedPostprint (author's final draft