Design, control and testing of modular multilevel converter prototype

This Master Thesis presents the design, control and testing of a Modular Multilevel Converter (MMC) prototype. This Voltage Source Converter (VSC) topology provides several advantages compared to previous generations for High-Voltage Direct Current (HVDC) applications, such as reduced harmonic distortion and lower switching losses. Today, most of the planned VSC-HVDC projects are based on the MMC technology. First, a mathematical model of the MMCs is obtained. Based on the analytical description of the converter, two di erent control strategies for MMCs are studied and its performance is evaluated by means of time domain simulations developed in Matlab Simulinkr. The second part describes the construction of a 5 kVA MMC prototype, focusing on the converter submodule design. A procedure to de ne its main circuits, including the components' selection, is detailed. Then, the layout of the submodule PCB is designed using Altium Designerr. Finally, an experimental MMC prototype is built based on the developed hardware design. Then, the studied control strategies are implemented in order to analyze their performance in a real setup. The control and modulation of the MMC is programmed in a Digital Signal Processor (DSP). The obtained experimental results show an adequate behavior of the prototype for the implemented controllers. Carlos Collado

Analysis of equilibrium points and optimal grid support of grid-forming modular multilevel converter for balanced and unbalanced faults

This short-communication presents a steady-state analysis of a grid-forming Modular Multilevel Converter (MMC) providing optimal voltage support to the AC network under normal and constrained conditions. The analysis is performed based on a multi-objective function (OF) optimization problem which prioritizes to maximize the positive-sequence and to minimize the negative- and zero-sequence voltage components at the point of common coupling (PCC) while it also considers the minimization of the arm impedance losses, respectively. If the voltage condition at the PCC is satisfied, the optimization attempts to reduce the arm impedance losses; otherwise, the algorithm prioritizes the PCC’s voltage components in order to minimize the error. Different network voltage and internal fault scenarios are evaluated, where it is shown that the suggested problem formulation can be used to obtain the optimal MMC’s quantities, providing voltage support during the faults.This work was supported by the Ministerio de Ciencia e Innovación (Proyecto Equired) under Grant PID2021-124292OB-I00. This work was also supported by the Agencia Estatal De Investigación, Spain under Grant PID2021-127788OA-I00. Eduardo Prieto-Araujo is an associate professor of the Serra Húnter programme. The work of Oriol Gomis-Bellmunt was also supported by the Institució Catalana de Recerca i Estudis Avançats (ICREA), Spain Academia Program.Peer ReviewedPostprint (published version

Optimization-based reactive power control in HVDC-connected wind power plants

One application of high–voltage dc (HVdc) systems is the connection of remotely located offshore wind power plants (WPPs). In these systems, the offshore WPP grid and the synchronous main grid operate in decoupled mode, and the onshore HVdc converter fulfills the grid code requirements of the main grid. Thus, the offshore grid can be operated independently during normal conditions by the offshore HVdc converter and the connected wind turbines. In general, it is well known that optimized reactive power allocation might lower the component loading and power losses. This paper aims to propose and assess a reactive power allocation optimization within HVdc–connected WPPs. For these systems, the offshore converter operates the adjoining grid by imposing frequency and voltage. The reference voltage magnitude is used as additional control variable for the optimization algorithm. The loss function incorporates both the collection grid and the converter losses. The use of the proposed strategy results in an effective reduction of losses compared to conventional reactive power dispatch strategies alongside with improvements of the voltage profile. A case study for a 500 MW–sized WPP demonstrates an additional annual energy production of 6819 MWh or an economical benefit of 886 k€yr-1 when using the proposed strategy.Postprint (author's final draft

Interaction analysis in islanded power systems with HVDC interconnections

Postprint (published version

Influence of the oxygen-containing surface functional groups in the methane hydrate nucleation and growth in nanoporous carbon

Petroleum pitch-derived activated carbon (PP-AC) has proved to be an excellent platform to promote the methane hydrate formation in milder condition than nature, even though the water-to-hydrate yield at the threshold formation pressure of 3.3 MPa is rather low (ca. 13%). Herein, we report that the presence of oxygen-containing surface functional groups in the oxidized carbon analogue (PP-AC_Ox) plays a significant role in the nucleation and growth in the low-pressure region. High-pressure methane adsorption/desorption isotherms revealed an enhancement in the water-to-hydrate yield up to ca. 51% around 3.3 MPa and 2 °C, in an extremely narrow working pressure window, with no hysteresis associated.Authors acknowledge financial support from MINECO Projects: MAT2016-80285-P and CONCERT Project-NASEMS (PCIN-2013-057), and Generalitat Valenciana (PROMETEOII/2014/004). MEC thanks Alexander von Humboldt foundation for financial support

Stability and interaction analysis in islanded power systems including VSC-HVDC and LCC-HVDC power converters

Islanded power systems are often connected to larger mainland power systems using HVDC cables. These interconnections are used to import power at lower cost compared to local generation and improve the security of supply. The increase of HVDC interconnectors in islanded systems will allow the reduction of local synchronous generation, which might lead to new interaction and stability problems due to the low inertia and short-circuit power available in the system. Traditionally LCC-HVDC technology has been used to connect island grids, but recently VSCs are presented as an alternative solution that offers more controllability to the islanded grid. Therefore, in order to increase the power transfer to the islands multi-infeed hybrid HVSC systems with VSCs and LCCs might become a common solution. The introduction of VSCs in islanded systems will allow operations in weak grids, but possible interactions with LCCs must be analysed in detail. This paper introduces the potential interactions in multi-infeed HVDC systems with LCCs and VSCs. An initial benchmark model of an islanded power system with a LCC and a VSC-HVDC link is presented to analyse new interaction phenomena between the converters and the islanded AC grid. Simulation results in PSCAD/EMTDC are presented to validate the benchmark model for voltage stability and commutation failure analysis.Postprint (published version

Design, control and testing of modular multilevel converter prototype

This Master Thesis presents the design, control and testing of a Modular Multilevel Converter (MMC) prototype. This Voltage Source Converter (VSC) topology provides several advantages compared to previous generations for High-Voltage Direct Current (HVDC) applications, such as reduced harmonic distortion and lower switching losses. Today, most of the planned VSC-HVDC projects are based on the MMC technology. First, a mathematical model of the MMCs is obtained. Based on the analytical description of the converter, two di erent control strategies for MMCs are studied and its performance is evaluated by means of time domain simulations developed in Matlab Simulinkr. The second part describes the construction of a 5 kVA MMC prototype, focusing on the converter submodule design. A procedure to de ne its main circuits, including the components' selection, is detailed. Then, the layout of the submodule PCB is designed using Altium Designerr. Finally, an experimental MMC prototype is built based on the developed hardware design. Then, the studied control strategies are implemented in order to analyze their performance in a real setup. The control and modulation of the MMC is programmed in a Digital Signal Processor (DSP). The obtained experimental results show an adequate behavior of the prototype for the implemented controllers. Carlos Collado

Optimización del flujo de potencia en parques eólicos marinos

El present projecte presenta una eina per a l'optimització del flux de potència en parc eòlics marins. Consta de dues parts diferenciades: la primera d'elles es basa en la realització d'un programa d'optimització amb diferents funcions objectiu que genera les referències de les potències activa i reactiva que deuen generar cada turbina per a complir amb el flux de potències òptim. La segona part tracta de validar els resultats obtinguts en l'optimització mitjançant una simulació dinàmica, on les entrades son les referències generades pel programa de optimització. L'algorisme d'optimització ha estat programat en MATLAB®, emprant la seva llibreria d'optimització y la eina fmincon. Per a la simulació dinàmica s'ha emprat Simulink®, on s'ha utilitzat la llibreria de SimPowerSystems

Reactive power management in an offshore AC network having multiple voltage source converters

Many offshore wind power plants are being developed every year at North and Baltic Sea. From the prospective of environment and integrated European power, combined power transmission from several offshore wind power plants using VSC-HVDC transmission system to different onshore grids are suitable instead of connecting each wind power plants individually. Offshore AC hub is beneficial for the wind power plants that are far from shore but close to each other within the vicinity of 20 km. This paper presents a method of controlling reactive power flow in the offshore AC grid to minimize the power losses and voltage deviation. In the proposed scheme, offshore grid frequency and voltage are controlled through more than one converter. Using frequency and voltage droop schemes, the active and reactive power sharing is achieved among converters. Furthermore, the optimization algorithm is developed to acquire the set points for the wind power plants and VSC-HVDC droop gains for the optimum operation of the network.Postprint (author's final draft

Modulation techniques applied to medium voltage modular multilevel converters for renewable energy integration: A review

Modular multilevel converters (MMC) inherent features are gaining more attention for dc voltage transmission systems. One of the main research paths regarding the converter performance deals with its voltage modulation. Specifically, for medium voltage applications with relatively small number of submodules, the voltage modulation techniques impact on the MMC performance needs to be studied. This work provides an extensive review of the carrier-based pulse with modulation (CB-PWM) techniques proposed to be applied on previous multilevel inverter versions. The CB-PWM methods were adapted to be compatible with an additional cell ranking and selection algorithm to ensure equal energy distribution on the arm cells. The state-of-the-art of zero sequence signals (ZSS) applied on three-phase inverters is also reviewed. The alliance between the ZSS with the CB-PWM, as well as the nearest level modulation (NLM), has an important impact on the MMC harmonic content, efficiency and voltage ripple of its cells capacitors. A 15 MW 28-cell-based MMC is used to investigate each particular combination between the modulation method and the common mode ZSS.Postprint (author's final draft