Deep neural learning based distributed predictive control for offshore wind farm using high fidelity LES data

The paper explores the deep neural learning (DNL) based predictive control approach for offshore wind farm using high fidelity large eddy simulations (LES) data. The DNL architecture is defined by combining the Long Short-Term Memory (LSTM) units with Convolutional Neural Networks (CNN) for feature extraction and prediction of the offshore wind farm. This hybrid CNN-LSTM model is developed based on the dynamic models of the wind farm and wind turbines as well as higher-fidelity LES data. Then, distributed and decentralized model predictive control (MPC) methods are developed based on the hybrid model for maximizing the wind farm power generation and minimizing the usage of the control commands. Extensive simulations based on a two-turbine and a nine-turbine wind farm cases demonstrate the high prediction accuracy (97% or more) of the trained CNN-LSTM models. They also show that the distributed MPC can achieve up to 38% increase in power generation at farm scale than the decentralized MPC. The computational time of the distributed MPC is around 0.7s at each time step, which is sufficiently fast as a real-time control solution to wind farm operations

Offshore Wind Farms

The coastal zone is the host to many human activities, which have significantly increased in the last decades. However, sea level rise and more frequent storm events severely affect beaches and coastal structures, with negative consequences and dramatic impacts on coastal communities. These aspects add to typical coastal problems, like flooding and beach erosion, which already leading to large economic losses and human fatalities. Modeling is thus fundamental for an exhaustive understanding of the nearshore region in the present and future environment. Innovative tools and technologies may help to better understand coastal processes in terms of hydrodynamics, sediment transport, bed morphology, and their interaction with coastal structures. This book collects several contributions focusing on nearshore dynamics, and span among several time and spatial scales using both physical and numerical approaches. The aim is to describe the most recent advances in coastal dynamics

An Offshore Wind Farm Featuring Differential Power Processing

Offshore wind farms are a rapidly growing technology used to harvest wind energy on the open seas where wind speeds are significantly higher and steadier than onshore. Current wind farms located far away from shore (e.g., 50 km or more) require a large amount of equipment to be deployed in order to transport generated energy to shore most cost-effectively. In these cases, energy is transmitted to shore using High-Voltage DC (HVDC) transmission connected to wind turbines with AC voltage output. During the past decade, research has studied alternate arrangements to reduce the amount of equipment deployed offshore and increase conversion efficiency. The redesign of offshore collection systems between wind turbines from AC to DC voltages is seen as a key tool to achieve the research objectives. The presented research is focused on the design of offshore wind farms with DC collection system and series-connected wind turbines based on partial power processing converters (PPPCs). This wind farm configuration significantly improves conversion efficiency compared to AC wind farms with HVDC link, since PPPCs are only required to process output power differences among wind turbines in a wind farm to achieve maximum power point (MPP) operation, and other wind farm components are operated at variable operating points, improving low-load efficiency. Furthermore, PPPCs can be of reduced size to realize MPP operation. To find the best variable operating points, a loss minimizing HVDC link current scheduling scheme has been derived and a comprehensive sizing framework was developed to inform the best choice of PPPC ratings. The presented work addresses major design considerations at wind farm, wind turbine, and PPPC levels. An efficiency, size and economic evaluation has been conducted for a 450 MW wind farm located 100km from shore, confirming significant annual loss reductions and economic advantages compared to a conventional AC wind farm with HVDC link, as well as two other series-connected DC wind farm configurations. A generic converter sizing framework for single-string series-connected DC wind farms has been developed and applied to the 450 MW wind farm. Challenges in wind turbine startup with this configuration have been identified and schemes were developed to enable successful wind turbine startup without the need of significant adidtional hardware

AC Voltage Control of a Future Large Offshore Wind Farm Network Connected by HVDC

The offshore wind resource around the seas of the UK is a very large renewable energy resource. Future offshore wind farm sites leased by the Crown Estate for Round 3 development will need high power capacity grid connection, but their remote location presents a challenge for the electrical connection to the grid. Long distance AC cable transmission is not practical due to the large cable capacitance which leads to reactive power loss. This thesis considers the voltage source converter and high voltage direct current (VSC-HVDC) technology as the future grid connection for the offshore wind farm network, which is more controllable and has better transmission efficiencies for long distance and high power cable transmission applications. The offshore AC network is weak with very little inertia and has limited rating at the HVDC converter substation. The dynamics in key variables in the offshore wind farm AC network and how they affect certain components in the system were studied. Without proper control, the offshore voltage and the frequency will be sensitive to change. The capacitor of the AC filter at the offshore VSC-HVDC station was found to be vulnerable to over-voltage, therefore a closed loop AC voltage controller was proposed here to maintain a constant capacitor voltage and to prevent tripping or over-voltage damage. The tuning of the control gains were optimised with a pole placement design method and small signal analysis for observing the system eigenvalue damping. The control parameters were then tuned for a fast and well damped controller. The AC voltage controller was evaluated and compared to an open loop system. The controller was able to limit the resonance in the LC filter that can be triggered by large and fast disturbances in the current, voltage and frequency. Current saturation could be implemented within the control structure for device protection from over-currents. Insight on how the wind turbine fully rated frequency converters and controllers may interact with the VSC-HVDC converter station is also discussed

Multiterminal HVDC transmissions systems for offshore wind

Offshore wind is emerging as one of the future energy vectors. Offshore wind power plants locations provide more strong and constant wind speed that allows to extract more power compared to onshore locations. In addition, as wind turbine components transportation is less restricted to terrestrial infrastructure, bigger and more powerful wind turbines can be installed offshore. In Europe, 1,567 MW of offshore wind power was installed in 2013. It represents the 14\% of the total wind power installed in Europe. Offshore wind power plants near the shore can be connected to the main grid by means of conventional AC technology. However, if these wind farms are installed further than 80-100 km, the use of AC equipment is economically infeasible due to reactive power issues. In these applications, HVDC system based on static converters can be used. The projects build and commissioned nowadays are based on point-to-point connections, where, each wind farm or wind farm clusters are connected to the terrestrial grid individually. Consequently, these lines might be understood as an extension of the AC system. If different offshore wind farms are interconnected between them and connected at the same time to different AC systems, for example, different countries, the DC grid is created. This scenario creates one of the most important challenges in the electrical power system since its creation, more than 100 years ago. The most relevant challenges to be addressed are the stability and operation of the DC grid and the integration and interaction with the AC grid. This thesis addresses various aspects related to the future Multiterminal-HVDC systems for transmission of offshore wind power. First, the voltage control and the system operations are discussed and verified by means of emulations using an HVDC scaled experimental platform built for this purpose. Voltage stability might be endangered during contingencies due to the different inertia time constant of the AC and the DC system. DC systems only have the inertia of the capacitors compared to synchronous machines rotating masses of the AC systems. Therefore, in faulty conditions the power transmitted through the DC system must be reduced quickly and efficiently. For this reason, in this thesis a coordinated power reduction algorithm taking advantage of Dynamic Braking Resistors (DBR) connected to onshore converter stations and the ability of the power plants to reduce the generated power is presented. From the AC and DC grids integration point of view, the connection point between the offshore grid and the AC grid might be located remotely leading to a connection with a reduced Short Circuit Ratio (SCR). In the literature, several issues regarding the connection of transistor-based power converters to weak AC grid have been reported. In this thesis am advanced control for Voltage Source Converters connected to weak grids is presented and tested by means of simulations. From the AC and DC grids interactions, the voltage stability is not enough to operate a DC grid. Transport System Operators (TSO) operates the power flow through the cables and the power exchanged between by the power converters. In this thesis, a novel hierarchical power flow control method is presented. The aim of the proposed power flow control is to obtain the desired power flows changing the voltage control set-points while the system stability is ensured. Finally, a control procedure for offshore wind farms based on Squirrel Cage Induction Generators connected to a single power converter is introduced.L'energia eòlica marina emergeix com un dels vectors energètics del futur. Les localitzacions eòliques marines proporcionen vens més forts i constants que les terrestres, cosa que permet extreure més potència. A més a més, els aerogeneradors marins poden ser més grans i més potents ja que es redueixen les limitacions de gàlib existent en les infraestructures terrestres. A tall d'exemple, l'any 2013 a Europa es van instal.lar 1.567 MW de potència eòlica marina, cosa que representa un 14\% de la potència eòlica instal.lada a Europa. Els parcs eòlics marins poden ser connectats a la xarxa elèctrica terrestre utilitzant emparamenta convencional de corrent alterna, però quan la distancia amb la costa excedeix els 80-100 km l'ús d'aquesta tecnologia es torna econòmicament inviable degut a l'energia reactiva generada en els conductors. Per solucionar aquest problema, s'emparen els sistemes en corrent continua basats en convertidors estàtics. Els projectes construïts o projectats a dia d'avui es basen en esquemes de connexió punt-a-punt, on, cada parc eòlic o agrupació de parcs eòlics es troba connectat a la xara terrestre individualment. En conseqüència, l'operació d'aquestes línies es pot considerar com una extensió de la xarxa d'alterna. Però, si s'interconnecten diferents parc eòlics amb diferents xarxes terrestres d'alterna (per exemple, diferents països) en corrent continua, s'obtenen xarxes en corrent continua. Aquest nou escenari crea un dels majors reptes des de la creació dels sistema elèctric de potencia, ara fa més de 100 anys. Entre aquests reptes hi ha l'estabilitat i l'operació dels sistemes en corrent contínua i la seva integració i coexistència amb les xarxes en corrent alterna. En la present tesis s'han estudiat diferents aspectes dels futurs sistemes multi terminal en alta tensió en corrent contínua (HVDC, en anglès) per la transmissió de potencia generada mitjançant parcs eòlics marins. Primerament, es descriu el control de tensió i els modes d'operació dels sistemes multi terminal i es verifiquen en una plataforma experimental construïda per aquest propòsit. L'estabilitat de tensió dels sistemes en corrent continua, es pot veure afectada durant una falta a la xarxa d'alterna degut a la reduïda inèrcia dels sistemes multi terminal, només formada pels condensadors dels convertidors i els cables. Així la potència que no pot injectar a la xarxa ha de ser reduïda de forma ràpida i eficient. Per això, en aquesta tesis es presenta un sistema coordinat de reducció de potència que utilitza la resistència de frenat dels convertidors de connexió a la xarxa i els mètodes de reducció de potència dels parcs eòlics. Des del punt de vista de la integració de les xarxes en continua i en alterna, el punt d'interconnexió pot estar localitzat llunys dels grans centres de generació, la qual cosa implica tenir una potència de curtcircuit molt reduïda. En la bibliografia científica s'han descrit diverses problemàtiques a l'hora de connectar un convertidor de commutació forçada a les xarxes dèbils. Per tal de pal.liar aquests inconvenients, en aquesta tesis es presenta un algorisme avançat de connexió de convertidors a xarxes dèbils basat en control vectorial. Des del punt de vista de les interaccions i interoperabilitat dels sistemes en corrent alterna i continua, no n'hi ha suficient en garantir l'estabilitat, ja que el propòsit finals dels operadors de xarxa és fer fluir una potencia a traves de la xarxa per tal de satisfer la demanda. Per aquest propòsit en aquesta tesis es presenta un control jeràrquic de control del flux de potència que fixa el flux de potència a traves d'una xarxa multi terminal canviant les consignes del control primari, tot assegurant l'estabilitat del sistema. Per tancar la tesis, es presenta un nou controlador per parcs eòlics basats en aerogeneradors de gàbia d'esquirol controlats per un sol convertidor

Loss allocation in a distribution system with distributed generation units

In Denmark, a large part of the electricity is produced by wind turbines and combined heat and power plants (CHPs). Most of them are connected to the network through distribution systems. This paper presents a new algorithm for allocation of the losses in a distribution system with distributed generation. The algorithm is based on a reduced impedance matrix of the network and current injections from loads and production units. With the algorithm, the effect of the covariance between production and consumption can be evaluated. To verify the theoretical results, a model of the distribution system in Brønderslev in Northern Jutland, including measurement data, has been studied

Supervisory Control of Full Converter Wind Generation Systems to Meet International Grid Codes

This thesis proposes a new supervisory control scheme for full converter wind generators (FCWGs) in compliance with the latest international grid codes. Intermittent behaviour of wind turbines and maximum converter capacity are taken into account in determining the reactive power injection to the grid following severe disturbance. Detailed simulations show that the proposed controller can improve the fault-ride-through capability of FCWGs while also providing support to the network as required by the grid codes