Network Frequency Regulation with DFIG Wind Turbines compliant with Italian standards

Abstract

As a consequence of the increasing renewable energy sources penetration, Transmission System Operators are facing critical stability challenges, e.g. lack of rotating inertia, frequency regulation and voltage control. In fact, majority of the recently installed renewable power plants are decoupled from the grid and are, therefore, unable to contribute to the grid stability in the case of network perturbations. In the world, wind energy represents one of the most interesting Renewable Energy Sources in terms of new installations. Two main categories of wind turbine exist: fixed speed and variable speed. In the second category, which is the market standard nowadays, Permanent Magnet Synchronous Generator and Doubly Fed Induction Generator are possible technologies. In particular, while the first one is fully decoupled from the grid, the second one is directly connected to the grid via the stator circuit and includes a power electronic converter to drive both magnitude and frequency of rotor currents. Differently from a fixed speed wind generator, the power converter enables the wind turbine to regulate the output power over a wide range of wind speeds to maximize the wind-to-electricity conversion efficiency. With the aim of preserving grid stability, recently connected wind turbines are required to contribute in the frequency and voltage regulation. In detail, the recently applied Italian connection standard for new type D wind power plants (i.e. rated power higher than 10 MW) imposes controllers for providing frequency and voltage support. The paper discusses the positive role that wind generators could have in terms of network stability if recent standards are correctly applied, making use of suitable simulations realized in the DIgSILENT PowerFactory environment. Doubly Fed Induction Generators have been considered, since this technology is currently the most widely used.