Robust Wind Turbine System Against Voltage Sag with Induction Generators Interfaced to the Grid by Power Electronic Converters

Abstract

The emergence of new grid codes will pose wind turbine developers a new challenge: the ride through capability during voltage sags. This means that with a high penetration of wind power in the network, the wind turbines/wind farms should be able to continuously supply the network during voltage sags. One of such grid code voltage sag profile for ride through is shown in Fig. 1 for illustration. These new grid codes which are in progress both in Norway and several other countries will most likely influence the type of electrical system (generator and network interface) of future wind turbines. To cope with this new challenge several industries have already initiated research efforts oriented at developing ride through capability technology.Among the technology choices, squirrel cage induction machines are a very attractive choice for wind power generation because they are robust, inexpensive and have low maintenance requirement and cost. As network interface, the use of back-to-back converters for variable speed operation is extensively reported in the literature showing their capability to achieve maximum energy capture in a wide range of wind conditions. There is however little reference to the control of back to back converters for low voltage ride-through In this paper we analyze an electrical system for wind turbines with cage induction generators interfaced to the grid with back to back converters, and experimentally evaluate the ride through capability under a symmetrical short circuit in the power system.A 55 kW motor-generator set of squirrel cage induction machines with a wind emulator to control the torque is used as model for the wind farm. This set-up is connected to the grid through back to back converters, as schematically shown in Fig. 2. The rating of the machines used in this experiment is of relevance in this paper because at this rating the p.u. values are close to those of real wind turbines and thus the performance is representative of real cases.The purpose of this experiment is to evaluate the role of the power electronics converter as a grid interface under short circuit conditions for the most common and inexpensive type of generator: the squirrel cage induction generator. Standard vector control techniques are implemented, aimed at keeping the DC link voltage constant to ensure power balance during the short circuit. Experimental results show a very stiff DC link control which proves the robustness of the back to back converters against voltage sags. MemberBjarne Naess * Non-memberWilliam Gullvik * Non-memberTore Undeland * Non-member An electrical system for wind turbines using cage induction generator with back to back converters for connection to the network is experimentally tested under short circuit conditions in a 55 kW generator set-up. 50% voltage sag is realized in the laboratory set-up to investigate the performance of the power electronic converters control under the newly introduced grid codes. Standard vector control techniques are implemented with the aim to maintain bal...