A review on DC collection grids for offshore wind farms with HVDC transmission system

Abstract: Traditionally, the internal network composition of offshore wind farms consists of alternating current (AC) collection grid; all outputs of wind energy conversion units (WECUs) on a wind farm are aggregated to an AC bus. Each WECU includes: a wind-turbine plus mechanical parts, a generator including electronic controller, and a huge 50-or 60-Hz power transformer. For a DC collection grid, all outputs of WECUs are aggregated to a DC bus; consequently, the transformer in each WECU is replaced by a power converter or rectifier. The converter is more compact and smaller in size compared to the transformer. Thus reducing the size and weight of the WECUs, and also simplifying the wind farm structure. Actually, the use of offshore AC collection grids instead of offshore DC collection grids is mainly motivated by the availability of control and protection devices. However, efficient solutions to control and protect DC grids including HVDC transmission systems have already been addressed. Presently, there are no operational wind farms with DC collection grids, only theoretical and small-scale prototypes are being investigated worldwide. Therefore, a suitable configuration of the DC collection grid, which has been practically verified, is not available yet. This paper discussed some of the main components required for a DC collection grid including: the wind-turbine-generator models, the control and protection methods, the offshore platform structure, and the DC-grid feeder configurations. The key component of a DC collection grid is the power converter; therefore, the paper also reviews some topologies of power converter suitable for DC grid applications

OFFSHORE WIND ENERGY STUDY AND ITS IMPACT ON SOUTH CAROLINA TRANSMISSION SYSTEM

As one of the renewable resources, wind energy is developing dramatically in last ten years. Offshore wind energy, with more stable speed and less environmental impact than onshore wind, will be the direction of large scale wind industry. Large scale wind farm penetration affects power system operation, planning and control. Studies concerning type III turbine based wind farm integration problems such as wind intermittency, harmonics, low voltage ride through capability have made great progress. However, there are few investigations concerning switching transient impacts of large scale type III turbine based offshore wind farm in transmission systems. This topic will gain more attention as type III wind generator based offshore wind farm capacity is increasing, and most of these large scale offshore wind farms are injected into transmission system. As expected to take one third of the whole wind energy by 2030, the large offshore wind energy need to be thoroughly studied before its integration particularly the switching transient impacts of offshore wind farms. In this dissertation, steady state impact of large scale offshore wind farms on South Carolina transmission system is studied using PSSE software for the first time. At the same time, the offshore wind farm configuration is designed; SC transmission system thermal and voltage limitation are studied with different amount of wind energy injection. The best recommendation is given for the location of wind power injection buses. Switching transient also impacts is also studied in using actual South Carolina transmission system. The equivalent wind farm model for switching transient is developed in PSCAD software and different level of wind farm penetration evaluates the transient performance of the system. A new mathematical method is developed to determine switching transient impact of offshore wind farm into system with less calculation time. This method is based on the frequency domain impedance model. Both machine part and control part are included in this model which makes this representation unique. The new method is compared with a well-established PSCAD method for steady state and transient responses. With this method, the DFIG impact on system transients can be studied without using time-domain simulations, which gives a better understanding of the transient behaviors and parameters involved in them. Additionally, for large scale offshore wind energy, a critical problem is how to transmit large offshore wind energy from the ocean efficiently and ecumenically. The evaluation of different offshore wind farm transmission system such as HVAC and HVDC is investigated in the last chapter

Power system planning methods and experiences in the energy transition framework

In recent years, the unbundling of the electricity market together with the profound “energy landscape” transformation have made the transmission network development planning a very complex multi-objective problem. The climate and energy objectives defined at the European level aim for a deepening integration of the European power markets and the electricity sector is recognized as one of the main contributors to the energy transition from a thermal-based power system to a renewable-based one. In the deregulated framework, network planners have to satisfy multiple different objectives, including: facilitating competition between market participants, providing non-discriminatory access to all generation resources for all customers, including green resources, mitigating transmission congestions, efficiently allocating the network development actions, minimizing risks associated with investments, enhancing power system security and reliability and minimizing the transmission infrastructure environmental impact. Further complexities are related to the significant uncertainty about future energy scenarios and policy rules. In particular, the increasing distributed renewable energy source integration dictated by the European energy targets, raises several issues in terms of future power flow patterns, power system flexibility and inertia requirements, and cost-effective development strategies identification. The thesis aims to investigate various aspects concerning the transmission network planning, with particular reference to the Italian power system and the experience gained working in the “Grid Planning and Interconnections Department” of Terna, the Italian Transmission System Operator. One of the main topics of this work is the use of the series compensation to exploit operating limits of underused portions of the HV – EHV transmission network in parallel to critically loaded ones, in order to control and provide alternative paths for power flows. The purpose is to extend the allowable transmission capacity across internal market sections. To this aim, a specific application of series compensation (together with reconductoring) to exploit the transfer capacity of a 250 km long, 230 kV-50 Hz transmission backbone spanning the critical section Centre South – Centre North is illustrated. The results are validated by means of static assessment and similar applications could be hypothesized for grid portions in the South of Italy where the primary network is mainly unloaded whereas the sub-transmission network reaches high levels of loading because of the huge renewable generation capacity situated there. A further characteristic of modern power systems is the need to integrate high levels of renewable energies while fulfilling reliability and security requirements. The offshore wind farms perspectives in the Italian transmission system are evaluated, considering policies, environmental and technical aspects. Furthermore, the adoption of the HVDC technology in parallel to the AC traditional system topic is addressed: planning static and dynamic studies involving a real HVDC Italian project are proposed. In particular, the impact of the planned HVDC link on the loadability and the dynamic performance of the system is investigated in medium and in long-term future planning scenarios. The evaluation of the thermal performance of a specific grid portion in the South of Italy affected by significant increase of power generation by variable energy sources is proposed both in the current situation and in the future scenarios in order to highlight the benefits related to the presence of the planned network reinforcements. Finally, some issues of the prospective reduced inertia systems are illustrated and a possible methodology to evaluate the economic impact of inertia constraints in long-term market studies is proposed. In the light of the emerging concept of power system flexibility, traditional planning evolved to assess the ability of the system to employ its resources when dealing with the changes in load demand and variable generation. Flexibility analyses of the Italian power system, carried out in terms of some market studies-based metrics and grid infrastructure-based indexes, are provided. The flexibility requirements assessment in planning scenarios are of interest to evaluate the impact of network development actions and have been included in the yearly National Development Plan. The last research topic involves the cost-effective target capacity assessment methodology developed by Terna in compliance with the Regulator directives presented together with the results yielded by its application to each significant market section of the Italian power system. The methodology has been positively evaluated from academic independent expert reviewers, and its outputs are relevant for the policy makers, regulatory authority and market participant to assess and co-design the energy transition plan of a future European interconnected power system

Technical-economic analysis, modeling and optimization of floating offshore wind farms

The offshore wind sector has grown significantly during the last decades driven by the increasing demand for clean energy and to reach defined energy targets based on renewable energies. As the wind speeds tend to be faster and steadier offshore, wind farms at sea can reach higher capacity factors compared to their onshore counterparts. Furthermore, fewer restrictions regarding land use, visual impact, and noise favors the application of this technology. However, most of today's offshore wind farms use bottom-fixed foundations that limit their feasible application to shallow water depths. Floating substructures for offshore wind turbines are a suitable solution to harness the full potential of offshore wind as they have less constraints to water depths and soil conditions and can be applied from shallow to deep waters. As several floating offshore wind turbine (FOWT) concepts have been successfully tested in wave tanks and prototypes have been proven in open seas, floating offshore wind is now moving towards the commercial phase with the first floating offshore wind farm (FOWF) commissioned in 2017 and several more are projected to be constructed in 2020. This transition increases the need for comprehensive tools that allow to model the complete system and to predict its behavior as well as to assess the performance for different locations. The aim of this thesis is to analyze from a technical and economic perspective commercial scale FOWFs. This includes the modeling of FOWTs and the study of their dynamic behavior as well as the economic assessment of different FOWT concepts. The optimization of the electrical layout is also addressed in this thesis. The first model developed is applied to analyze the performance of a Spar type FOWT. The model is tested with different load cases and compared to a reference model. The results of both models show an overall good agreement. Afterwards, the developed model is applied to study the behavior of the FOWT with respect to three different offshore sites. Even at the site with the harshest conditions and largest motions, no significant loss in energy generation is measured, which demonstrates the good performance of this concept. The second model is used to perform a technical-economic assessment of commercial scale FOWFs. It includes a comprehensive LCOE methodology based on a life cycle cost estimation as well as the computation of the energy yield. The model is applied to three FOWT concepts located at three different sites and considering a 500MW wind farm configuration. The findings indicate that FOWTs are a high competitive solution and energy can be produced at an equal or lower LCOE compared to bottom-fixed offshore wind or ocean energy technologies. Furthermore, a sensitivity analysis is performed to identify the key parameters that have a significant influence on the LCOE and which can be essential for further cost reductions. The last model is aimed to optimize the electrical layout of FOWFs based on the particle swarm optimization theory. The model is validated against a reference model at first and is then used to optimize the inter-array cable routing of a 500MW FOWF. The obtained electrical layout results in a reduction of the power cable costs and a decrease of the energy losses. Finally, the use of different power cable configurations is studied and it is shown that the use of solely dynamic power cables in comparison to combined dynamic and static cables results in decreased acquisition and installation costs due to the avoidance of cost-intensive submarine joints and additional installation activities.El sector eólico marino ha crecido significativamente durante las últimas décadas impulsado por la creciente demanda de energía limpia. Los parques eólicos en el mar pueden alcanzar factores de capacidad más altos en comparación a los parques eólicos en la tierra debido a que las velocidades del viento tienden a ser más altas y constantes en el mar. Ademas, existen menos restricciones con respecto al uso de la tierra, el impacto visual y el ruido. Sin embargo, la mayoría de los parques eólicos actuales utilizan subestructuras fijas que limitan su aplicación factible a aguas poco profundas. Las subestructuras flotantes para turbinas eólicas marinas (FOWTs en inglés) son una solución adecuada para aprovechar todo el potencial de la energía eólica, ya que tienen menos restricciones para las profundidades del agua y el fondo marino. Dado que varios prototipos de FOWTs se han probado con éxito en el mar, la industria ahora esta entrando a la fase comercial con el primer parque eólico flotante (FOWF en inglés) operativo y se proyecta que se pondrán en marcha más en los próximos anos. Esta transición aumenta la necesidad de herramientas integrales que permitan modelar el sistema completo y predecir su comportamiento, así como evaluar el rendimiento para diferentes lugares. El objetivo de esta tesis es analizar desde una perspectiva técnica y económica los FOWFs a escala comercial. Esto incluye el modelado de FOWTs, el estudio de su comportamiento dinámico, y la evaluación económica de diferentes conceptos. La optimización del diseño eléctrico también se aborda en esta tesis. El primer modelo desarrollado se aplica para analizar el rendimiento de un FOWT tipo Spar. El modelo se prueba con diferentes tipos de carga y se compara con un modelo de referencia. Los resultados de ambos modelos muestran una buena concordancia. Posteriormente, el modelo se aplica para estudiar el comportamiento con respecto a tres lugares diferentes. Los resultados muestran que incluso en el sitio con las condiciones más severas, no se mide ninguna pérdida significativa en la generación de energía, lo que demuestra el buen rendimiento de este concepto. El segundo modelo se utiliza para realizar una evaluación técnico-económica de los FOWF a escala comercial. Esto incluye una metodología integral del costo nivelado de energía (LCOE en ingles). El modelo se aplica a tres conceptos de FOWTs ubicados en tres lugares diferentes y considerando un parque eólico de 500MW. Los resultados indican que los FOWTs son una solución altamente competitiva y que la energía se puede producir con un LCOE igual o inferior en comparación con los parques eólicos con subestructuras fijas o las tecnologías de energía oceánica. Asimismo, se realiza un análisis de sensibilidad para identificar los parámetros claves que tienen una influencia significativa en el LCOE y que pueden ser esenciales para reducciones de costos. El último modelo se aplica para optimizar el diseño eléctrico en función de la teoría de optimización por enjambre de partículas. Inicialmente el modelo se valida contra un modelo de referencia y luego se utiliza para optimizar la conexión de los cables entre los FOWTs. El diseño eléctrico obtenido da como resultado una reducción de los costos de cables y una disminución de las pérdidas de energía. Finalmente, se estudia el uso de diferentes configuraciones de cables y se demuestra que el uso de cables únicamente dinámicos en comparación con los cables dinámicos y estáticos combinados da como resultado una disminución de los costos de adquisición e instalación debido a que evitan la necesidad de juntas submarinas costosas y costos adicionales de instalación.Postprint (published version

Learning, future cost and role of offshore renewable energy technologies in the North Sea energy system

The pace of cost decline of offshore renewable energy technologies significantly impacts their role in the North Sea energy transition. However, a good understanding of their remains a critical knowledge gap in the literature. Therefore, this thesis aims to quantify the future role of offshore renewables in the North Sea energy transition and assess the impact of cost development on their optimal deployments. The following findings were observed in this thesis, 1) Fixed-bottom offshore wind is well established in the North Sea region and is already competitive with onshore renewables 2) Floating wind is emerging and their current costs are high, but it can reach about 40 EUR/MWh by early 2040 and would require 44 billion EUR of learning investment.3) Grid connection costs will become a major factor as wind farm moves further away. Policy actions and innovation is needed in this space to avoid increasing integration costs. 4) Offshore wind (fixed-bottom and floating) can play a significant role in the North Sea energy system, comprising 498 GW of deployments in 2050 (222 GW of fixed-bottom and 276 GW of floating wind) and contributing up to a maximum of 51% of total power generation in the North Sea power system. 5) The role of the investigated low-TRL offshore renewables, including the tidal stream, wave technology, and bioethanol, was limited in all scenarios considered, as they remain expensive compared to other mature technologies in the system

Power control, fault analysis and protection of series connected diode rectifier and VSC based MTDC topology for offshore application.

A multiterminal high-voltage dc (MTDC) system is a promising method for transmitting energy generated from an offshore windfarm (OWF). The creation of MTDC systems became easier by the introduction of voltage source converter (VSC) due to the flexibility and controllability it provides. This technology is newer than the line-commutated converter technology (LCC). Power systems can include any number of windfarms together with converters for both offshore and onshore power conversion. Therefore, this thesis suggests a three-terminal MTDC model of two offshore windfarms and one onshore inverter. The electric energy generated by the two windfarms is rectified into dc and transmitted to the shore using dc cable. Although a VSC or a diode rectifier (DR) can convert ac to dc, a series connection of a VSC and two DRs was proposed at the windfarm side to convert the generated power to achieve controllability of the uncontrollable diode rectifiers and reduces the high cost of badditional VSCs. The proposed topology converts the ac power by dividing the windfarm power so that one-third is the share of the VSC and two-thirds is the share of the DRs. The same topology is used to convert the power produced from the other windfarm. Then, the dc power is transmitted via an undersea dc cable to the onshore location, and is then inverted into ac before it is supplied to the neighbouring ac grid using a grid-side VSC. The proposed topology has many advantages, including a significant save in windfarm VSC (WFVSC) capital cost and a significant reduction in the loss of power of the converter without losing the overall controllability. However, although this topology is suitable for windfarm applications, it might not be suitable for high-voltage direct current (HVDC) that requires bidirectional power flow unless making changes to the topology such as disconnecting the diode rectifiers. Furthermore, fault analyses were investigated, including dc faults and ac faults. Ac faults are categorised as symmetrical or unsymmetrical faults. For comparison purposes, a Simulink model was designed, implemented, and simulated as a reference model. The reference model can operate as VSC-, DR-based MTDC, or a mix of both in a way that any component can be added to or removed from the model at any time during the simulation run. The contribution to the dc fault current from various parts such as dc capacitor and the adjacent feeder was investigated thoroughly, and detailed mathematical formulae were developed to compute fault current from these contributors. In addition, the results of the system response due to both fault types are illustrated and discussed. Both symmetrical and unsymmetrical ac faults were initiated on the onshore grid side, and the system response results are presented for those faults. A generalised control scheme (GCS) was proposed in this thesis, which add the ability the model to control the reactive power and is suitable for both balanced and unbalanced ac faults conditions. A protection against faults was investigated and implemented using dc circuit breakers. The protection system was built to ensure safe operation and to fulfil the grid code requirements. Many grid codes are available and presented in the literature, such as Spanish, British, and Danish; however, a grid code by E.ON was chosen. The protection scheme in VSC-based MTDC networks plays a vital role during dc faults. It is vital that this protection be sensitive, selective, fast, and reliable. Specifically, it must isolate the fault reliably from the system within a short time after the fault occurrence, while maintaining the remaining components of the system in a secure operational condition. For optimal performance, the protection scheme discussed in this thesis employs solid-state circuit breakers. A literature survey relevant to the tasks mentioned above was conducted.PhD in Energy and Powe