TWENTIES for IEEE ISGT Gothenburg 2010

1 a supplementary inertia control loop to be

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1) a supplementary inertia control loop to be introduced in each off-shore wind turbine control system. Such a methodology will exploit the kinetic energy stored in the rotating mass of wind turbines. 2) power exchanges/support with ac neighbouring systems through the exploitation of the dc offshore network. b) Voltage support In normal conditions, voltage support has to be offered at the DC/AC connecting points using VSC-based injectors. In addition, in the event of a fault in the AC transmission network, the generators may experience large over currents and power swings both during the fault and after its clearance by the appropriate switchgear. The type of response of the generators in an AC grid during and in the moments subsequent to voltage sag depends on its type (fixed speed generator, double fed induction generator, full converter generator). In the case of DC connections to shore, ride through fault performance must be focused on the development of innovative technological solutions in order for the DC link converter systems to be capable of providing fault ride though requirements in case of balanced and unbalanced faults through the injection into the AC grid of reactive current. Regarding unbalanced faults, it is important to develop innovative control techniques such that reactive power is independently injected in each phase of the system in order to avoid over-voltages in the non-faulted phases. c) Contribution for the damping of system oscillations When connecting massive amounts of wind power to a system, there is the need to guarantee that transient performance is not reduced and problems of small signal stability do not arise. The most frequent cause of this kind of instability is the lack of damping of the so-called electromechanical modes of oscillation, which are related with low frequency (0.1-2Hz) power oscillations that occur among the rotors of synchronous machines. One of the cases where a reduction of damping can take place is related with a situation where the connection of wind power in one area of the grid, replacing conventional production located in other areas, increases the power flows through weak interconnection lines. This situation may occur in the future, for example, in the north of Europe with the construction of large offshore wind farms. Power system stabilizers (PSS) installed in the excitation of synchronous generators is still one of the most cost effective solution for providing supplementary damping. However, in a system with large share of wind generation these devices may no longer be able to provide the necessary additional damping. Therefore, it may be necessary to implement damping functions in the HVDC grid control- command. IV. P ROTECTIONS AND COORDINATED CONTROL PLANS Unlike AC networks, the establishment of power flows inside a DC grid are the direct consequences of the actions of the control of its converter stations. This coordinated control is also the key element for providing ancillary services as described in section III and a high level of robustness for the HVDC grid, when facing internal or external disturbances.
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