Modeling and enhanced control of hybrid full bridge–half bridge MMCs for HVDC grid studies

Modular multilevel converters (MMCs) are expected to play an important role in future high voltage direct current (HVDC) grids. Moreover, advanced MMC topologies may include various submodule (SM) types. In this sense, the modeling of MMCs is paramount for HVDC grid studies. Detailed models of MMCs are cumbersome for electromagnetic transient (EMT) programs due to the high number of components and large simulation times. For this reason, simplified models that reduce the computation times while reproducing the dynamics of the MMCs are needed. However, up to now, the models already developed do not consider hybrid MMCs, which consist of different types of SMs. In this paper, a procedure to simulate MMCs having different SM topologies is proposed. First, the structure of hybrid MMCs and the modeling method is presented. Next, an enhanced procedure to compute the number of SMs to be inserted that takes into account the different behavior of full-bridge SMs (FB-SMs) and half-bridge submodules (HB-SMs) is proposed in order to improve the steady-state and dynamic response of hybrid MMCs. Finally, the MMC model and its control are validated by means of detailed PSCAD simulations for both steady-state and transients conditions (AC and DC faults)

Robust local controllers design for the AC grid voltage control of an offshore wind farm

Ponencia presentada en IFAC PapersOnLine 53-2 (2020) 12751–12756In this paper we deal with the problem of voltage control of the AC grid in an offshore wind farm by means of several converters, all of them connected to the AC offshore grid at the Point of Common Coupling (PCC) through different transmission lines. We propose to control the voltage simultaneously by all the connected converters, i.e., we have multiple actuators with the same goal. However, the number of operative converters can change during the wind farm operation and dynamics changes. Thus, it is necessary to assure the stability of the whole system in all different scenarios. In order to achieve a global design and implementation strategy, we propose the use of same controller parameters for all converters. For this kind of wind farm topology, we address the design of controllers as an optimization problem where we seek to maximize the robustness against uncertainty in the model of transmission lines and changes in the number of connected wind turbines guaranteeing the stability of the whole system in all different scenarios as well as a given settling time. Due to the proposed design strategy it is not necessary communication between different converters and controllers do not need to be re-tuned when the number of connected converters changes

Full-Bridge Modular Multilevel Converter for the Four-Quadrant Supply of High Power Magnets in Particle Accelerators

Ponencia presentada en 2022 24th European Conference on Power Electronics and Applications (EPE'22 ECCE Europe), 5-9 September 2022, Hanover, Germany.Many particle accelerators require to supply chains of magnets with high quality, high magnitude, cycling currents. To do this, the power converters need to provide high output voltages, reaching in some cases tens of kilovolts. Additionally, converters are required to store the magnet energy during de-magnitasion cycles. For such application, Full-bridge Modular Multilevel Converters (FB-MMC) could be used given their capacity to store energy, their inherent reliability and their good harmonic performance. This paper studies how this converter topology could be used for this application, proposing a method to recover and store the energy of the magnet using the converter submodules

Modular Multi-level Converter Hardware-in-the-Loop Simulation on low-cost System-on-Chip devices

Comunicació presentada a IECON 2018 - 44th Annual Conference of the IEEE Industrial Electronics Society (October 21-23, 2018 Washington D.C., USA.)System-on-Chip (SoC) devices combine powerful general purpose processors, a Field-Programmable Gate Array (FPGA) and other peripherals which make them very convenient for Hardware-in-the-Loop (HIL) simulation. One of the limitations of these devices is that control engineers are not particularly familiarized with FPGA programming, which need extensive expertise in order to code these highly sophisticated algorithms using Hardware Description Languages (HDL). Notwithstanding, there exist High-Level Synthesis (HLS) tools which allow to program these devices using more generic programming languages such as C, C++ and SystemC. This paper evaluates SoC devices to implement a Modular Multi-Level Converter (MMC) model using HLS tools for being implemented in the FPGA fabric in order to perform HIL verification of control algorithms in a single low-cost device

Sizing and Short-Circuit Capability of a Transformerless HVDC DC-DC Converter

This work aims at optimizing the converter design of the double-T MMC DC-DC converter in terms of transmitted power per submodule and also in terms of transmitted power per silicon area, while, at the same time, providing the capability to block dc faults. Firstly, the converter operation is described and the optimal values of the inner ac and dc voltages that minimize device power rating are derived. Next, the submodule topology is analyzed and a thorough study on the converter capability for blocking fault currents is carried out, showing that the converter is able to isolate dc faults both at the input and at the output of the converter. Finally, the previous analytical study is verified by means of detailed PSCAD simulations

Modular multi-level DC-DC converter for high-power and high-voltage applications

A transformer-less DC-DC Modular Multi-level Converter (DC-DC-MMC) topology based on H-bridge cells is proposed in this paper. The suggested DC-DC-MMC can be used to either control the power flow in a HVdc grid or interconnect HVdc lines with different voltage levels. Branch energy and current control loops are presented as well as a cell capacitor voltage balancing strategy. Finally, the operation of the converter is validated by means of PSCAD simulations. Results for the operation of the DC-DC-MMC controlling the power flow between two HVdc grids with different voltage levels are presented.The present work was supported by the Spanish Ministry of Economy funds under Grant DPI2014-53245-R and by Universitat Jaume I under Grants P1·1B2013-51 and E-2014-24. The support of Fondecyt Chile under grant 1151325, CONICYT/FONDAP/15110019 is also kindly acknowledged

Protection Strategies for the Connection of Diode Rectifier-Based Wind Power Plants to HVdc Interconnectors

The connection of diode rectifier (DR)-based wind power plants (WPPs) to existing or planned high-voltage dc (HVdc) interconnectors can lead to important savings on cost and system robustness. Since the DR station usually operates in a bipolar configuration, its connection to symmetric monopoles is particularly challenging. However, there are no published detailed studies on the protection of DR connection WPPs to symmetric monopole interconnectors or even to bipolar interconnectors. This article includes the comparative study of five different protection strategies for such systems, including both solid and resistive DR station grounding and strategies with and without the use of dc-circuit breakers (dcCBs). An analytical study allows for the calculation of fault current during fault onset for both half-bridge and hybrid modular multilevel converter (MMC) stations. Using detailed electromagnetic transient (EMT) simulation studies, the different protection strategies are evaluated in terms of current, voltage, and isolation requirements of each element, as well as the need for dcCBs, fast communication, or larger surge arresters. Moreover, a distance fault detection algorithm is included for the wind turbine converters to distinguish between local ac-grid and dc-cable faults. From the simulation results, it is possible to conclude that DR high-impedance grounding, together with wind turbine distance protection, can be used for the protection of DR-based offshore WPPs connected to symmetric monopole interconnectors without requiring dcCBs

Analysis of the Performance of MMC Under Fault Conditions in HVDC-Based Offshore Wind Farms

This paper analyzes the behavior of a modular multilevel converter-high-voltage direct-current (MMC-HVDC)-connected offshore wind power plant (WPP) during dc faults. For that purpose, detailed models of the dc cable, MMC stations, and transformers have been used in order to obtain reliable results. The influence of the WPP control method in the short-circuit behavior of the HVDC link has also been studied. Results show that the dynamics of the WPP contribution to pole-to-ground faults are slightly slower than those of the wind turbines current control loops. Therefore, the wind turbine front-end converters can be used to reduce the peak and average value of the fault current in such a system. Moreover, it has been found that ferroresonant oscillations can appear in the offshore ac grid when the WPP delivers constant power during faults.This work was supported in part by the Spanish Ministry of Economy funds under Grant DPI2014-53245-R, in part by Universitat Jaume I under Grants P1·1B2013-51 and E-2014-24, and in part by CONICYT/FONDAP/15110019 and Fondecyt/1151325. Paper no. TPWRD-01518-2014

Power flow control using a DC-DC MMC for HVdc grid connected wind power plants

This paper proposes the use of a transformer-less DC-DC Modular Multilevel Converter (MMC) topology, based on cascaded H-bridge converters, for power flow control in High Voltage Direct Current (HVDC) grids used to connect off-shore wind power plants to on-shore grids. An energy based approach is used to regulate the DC voltage of H-bridge modules. Results for the operation of the DC-DC MMC supplying energy to a DC network and controlling the power flow in a HVDC system are presented.The support of Fondecyt grant 1151325, CONICYT/FONDAP/15110019, the Spanish Ministry of Economy Grant DPI2014-53245-R, University La Frontera grant DIUFRO09-0037 and Universitat Jaume I grants P1ā1B2013-51 and E-2014-24 is kindly acknowledged

Integrated control of offshore wind farms and HVDC links with MML converters

This thesis analyzes the integration of large quantities of offshore wind power into power systems through point-to-point or multiterminal HVdc grids that use modular multilevel converters (MMC). Firstly, a simplified method to simulate MMCs is developed with the objective of reducing the simulation times. Secondly, a new control strategy for offshore wind power plants connected through MMC-HVDC links is developed in order to meet the requirements of the new ENTSO-E grid codes (for instance, island operation or black-start). Next, a mathematical study is carried out to estimate the values of the DC fault currents in HVDC grids and the response of several wind power plant control strategies are analyzed in the event of DC faults. Finally, a new DC-DC MMC topology is proposed for HVDC grids.En la tesis se analiza la integración de grandes cantidades de energía eólica en los sistemas eléctricos de potencia a través de redes HVDC punto a punto y multipunto que usan convertidores modulares multinivel (MMC). En primer lugar se desarrolla un modelo que permite reducir los tiempos de simulación a la hora de modelar los MMCs. A continuación se propone una nueva estrategia de control para parques eólicos marinos que permita cumplir con los nuevos requisitos exigidos en los nuevos códigos de red desarrollados por ENTSO-E (por ejemplo, funcionamiento en isla o arranque sin fuentes de energía externas). A continuación se desarrolla un modelo teórico para el estudio de las corrientes de cortocircuitos en redes HVDC y se analiza la respuesta de distintos tipos de controles de parques eólicos ante cortocircuitos en la red DC. Finalmente se propone una nueva topología de convertidor DC-DC MMC para redes HVDC