Operation and control of transmission systems for offshore wind power plants

This thesis deals with grid integration of oshore wind power plants through HVDC (High Voltage Direct Current) or HVAC (High Voltage Alternating Current) transmission . The behaviour of wind farms and their transmission systems in normal operation and under faults is analyzed. On the eld of HVDC transmission , a control scheme based on an optimum voltage algorithm is proposed and compared to voltage droop control. The dierences between the proposed scheme and droop control in terms of losses are analyzed, in steady state as well as dynamically. This new control scheme is enhanced, being able to perform secondary and tertiary control strategies simultaneously for DC grids. Concerning HVAC transmission, the operation of AC connected wind power plants equipped with full power converter wind turbines is analyzed under deep voltage sags on the main AC grid. Standard control schemes, based on the strict application of grid codes, can lead to instability problems when this kind of severe disturbances occurs. A coordinated control scheme is proposed to operate the system, ensuring fault ride through capability. An index alerts of instability proximity and allows to activate active power and reactive power regulation to guarantee safe operation during faults. For enabling the optimal operation of transmission systems, an optimal power flow tool is described for hybrid HVDC-HVAC systems, for different objective functions. This tool is tested in a scaled platform. Finally, the secure and optimal operation of these systems is analysed for a scenario with high penetration of oshore wind, proposing a methodology to evaluate the cost of operation and wind energy curtailed. La present tesi tracta la integracio a xarxa de plantes eoliques marines a traves de transmissio en HVDC (Alta Tensio en Corrent Continu) o HVAC (Alta Tensio en Corrent Altern). S'analitza el comportament dels parcs eolics i del seus sistemes de transmissio en condicions normal i en situacions de pertorbacions electriques. En el camp de l'HVDC, es descriu un esquema de control basat en un algorisme de tensions optimes i es compara amb el control de tensio droop. Les diferencies entre l'esquema de control proposat i el droop s'analitzen, des del punt de vista de perdues, en estat estacionari i dinamicament. L'esquema de control proposat es millora, oferint la possibilitat de realitzar estrategies de control secundari i terciari en xarxes DC. Dins de la transmissio en l'HVAC, s'analitza l’operacio de parcs eolics equipats amb convertidors de plena potencia quan s'esdevenen sots profunds a la xarxa AC. Els esquemes de control estandards, basats en l’aplicacio estricta dels requisits de connexio, poden comportar problemes d'estabilitat transitoria quan s'esdevenen pertorbacions severes. Es proposa un esquema de control coordinat per operar el sistema, assegurant el fault ride through. Un index alerta de a de la proximitat d'una inestabilitat i permet activar la regulacio de potencia activa i reactiva per assegurar l’operacio segura durant la falta. Tenint en compte que l’expansio del sistema de transmissio presenta tant enllacos HVDC com HVAC, es desenvolupa una eina per determinar fluxos de potencia optims en xarxes hibrides HVDC-HVAC, per diferents funcions objectiu. Es realitzen proves experimentals en una plataforma de baixa tensio per comprovar el funcionament d'aquesta eina. Finalment, s'estudia l’operacio optima i segura d'aquests sistemes, per un escenari amb gran penetracio d'energia eolica, proposant una metodologia per avaluar el cost d’operacio i la reduccio d'energia eolica produida.