Integrated control of offshore wind farms and HVDC links with MML converters
- Autores: Ricardo Vidal Albalate
- Directores de la Tesis: Enrique Belenguer Balaguer (dir. tes.), Ramón Blasco Giménez (codir. tes.)
- Lectura: En la Universitat Jaume I ( España ) en 2017
- Idioma: inglés
- Número de páginas: 294
- Tribunal Calificador de la Tesis: Rubén Peña Guiñez (presid.), I. Peñarrocha (secret.), Eric Monmasson (voc.)
- Programa de doctorado: Programa Oficial de Doctorado en Tecnologías Industriales, Materiales y Edificación (Time)
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
- español
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.
- español
Conventional power plants are being replaced by renewable energy sources in many countries. In particular, wind power is the source that presents the largest deployment. However, this kind of power plants should also take part in the control of the electrical system in order to not jeopardize it. On the other hand, since offshore wind power plants are located far from the shore in many cases, it is necessary to use HVdc links to transmit the power. Furthermore, the electricity will have to be transported though long distances in power systems with a high penetration of renewable energy sources due the inherent variability of these sources. In this regard, HVdc grids present some advantages over HVac networks such as lower losses and ease of control. However, there are still some obstacles that hinder the development of large HVdc grids, for instance, clearing dc faults or the power flow control in large meshed grids. It is expected that MML converters will play an important role in overcoming the previous drawbacks in the future multiterminal meshed HVdc grids.
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. Firstly, a simplified method to simulate MMCs is developed with the objective of reducing the simulation times. The model allows an efficient and accurate simulation during both steady-state and transient conditions caused by ac or dc faults. Moreover, this model, which is based on the equivalent Thevenin circuit of each branch, is valid for different MMC topologies. Hence, it is used to model both ac-dc and dc-dc MML converters.
Secondly, a new control strategy for offshore wind farms connected through MMC-HVdc links is developed with the objective of allowing this kind of power plants to offer those ancillary services that are currently provided by conventional power plants (black-start capability, islanded operation, fault-ride-though, power flow control). In this way, WPPs can help enhance the system security and reliability at the time that the share of renewable energy sources can be boosted. The strategy is based on a distributed control in which the offshore ac grid is created jointly by all wind turbine converters and the rectifier MMC station of the HVdc link.
One of the main hindrances to the development of multiterminal HVdc grids is dc-fault clearing. DC faults should not be cleared by ac breakers in order to avoid de-energizing the whole grid. In this sense, the behaviour of MMC based HVdc links in the presence of dc faults is analyzed. A mathematical model, which can help select to select adequate protections systems, is developed to easily estimate the evolution of the fault currents. It is found that the wind power control has also a great influence on the evolution of the fault currents. In this regard, two WPP control strategies are analyzed: i) the offshore MMC creates the offshore ac grid and the wind turbines deliver the optimal power and ii) the converters of the wind turbines create the offshore ac grid using a distributed control. In the event of a dc fault, the control of the WPP grid is lost and overvoltages and overfrequencies can appear when the first strategy is used. However, the WPP ac grid is under control during the whole transient with the second control strategy. In both cases, the dynamics of the wind turbine converters are fast enough to allow for a wind power reduction which reduces the peak fault current by a 30% approximately. Moreover, the fault current fed by the wind farm can be drop to zero within 30 ms.
Finally, meshed HVdc grids will have more converter stations than lines in order to comply with the n-1 safety criteria and the operation of large offshore wind parks will lead to variable active power being delivered to the HVdc grids. If no additional measures are taken, the power flow will not be fully controlled and the power sharing between lines will largely depend on the different line resistances and on the variable wind power plant production. On the other hand, current HVdc point-to-point links are paving the way to future multiterminal grids. However, the variety of voltage levels makes impossible their interconnection. A novel MML dc-dc converter is proposed to overcome the previous drawbacks. Its modular multilevel structure is two fold. First, each branch is composed of N submodules, therefore, it is possible to connect in series as many cells as needed to achieved the voltage level of the dc grid. Secondly, several sections can be connected in parallel to achieve the desired power. The dc-dc converter and its control have been thoroughly validated by means of PSCAD simulations for different scenarios: i) interconnection of HVdc grids with different voltage levels and ii) power flow control within a meshed HVdc grid. Moreover, in the event of dc faults, the converter is able to block the dc fault currents. Finally, an exhaustive analysis of the converter (losses, optimal power rating, rated power) is carried out and some design guidelines are provided.
- español
