Έλεγχος διασύνδεσης ΕΡ/ΣΡ/ΕΡ με μετατροπείς πηγής τάσης με σκοπό τη βελτιωμένη απόκριση αιολικού πάρκου που τροφοδοτεί ασθενές ΣΗΕ

Abstract

In this thesis is studied the connection of an offshore Wind Farm (WF) with induction generators to a weak ac grid. This case presents several difficulties, as a result of the low short circuit ratio between the two interconnected ends and the fluctuating and unstable nature of the wind power. In order to constrain these difficulties and taking into account the long distance between the two interconnected ends (which necessitates the use of a dc cable), it was chosen to use the technology of the High Voltage Direct Current (HVDC) link based on Voltage Sourced Converters(VSCs) for the connection of the WF to the weak ac grid. This technology presents the advantages of dc transmission and due to the high switching capability of the VSCs’ valves, it can instantly regulate the reactive power and consequently the ac voltage, independently of the real power flow. So, the short circuit ratio between the two ends of the link does not have to be high. Furthermore, through this link, the dynamic reactive power compensation of the induction generations can be achieved. However, despite the benefits which are obtained through the technology of the HVDC link based on VSCs, the control of the link is still a demanding procedure. The design of the control system demands a comprehensive knowledge of the system behavior and accurate tuning in order to achieve the desirable output. The presence of highly fluctuating and noisy input signals can make the selection of the control parameters which ensure proper behavior in any operating conditions very difficult. Taking into account the above requirements, the control theory that was implemented to the control system of the above mentioned link is that of computational intelligence. Computational intelligence-based controllers do not require precise mathematical modeling of the system nor complex computations. They rely on the human ability to understand the system behavior and are based on qualitative control rules. In addition, they have inherent abilities to deal with imprecise or noisy data. The categories of computational intelligence that were used in the control system design are fuzzy systems and hybrid neuro-fuzzy systems. More precisely, three alternative versions of the control system of the link between the WF and the weak ac grid were designed: 1. a simple, manually tuned fuzzy control system , 2. a hybrid neuro-fuzzy control System 3. an adaptive, on-line tuned fuzzy control system. In order to test the performance of the control system and compare its three versions, the test of the system includes three parts. In the fist part was studied the performance of the system under a step increase of the wind speed. The step increase was chosen to be very steep, in order to test the system under extreme conditions. In the second part was studied the response of the system under a respective step decrease of the wind speed, in order to test the system performance under any operating condition. In the third part was examined the harmonic content of waveforms of the current and voltage at the point of common coupling, according to the limits of the international standard IEEE 519. Through extended simulation results it was shown that the proposed control system quickly reacts to the step changes of the wind power and the HVDC link based on VSCs manages to feed the weak ac grid with the power from the wind under almost stable ac voltage. In addition, due to the ability of the control system to adjust the stator frequency of the induction generators in relation to the wind velocity, maximum power absorption of the WF is achieved, without monitoring the wind speed or the rotor speed.