Wind power is increasingly being applied as a mainstream energy supply. Wind turbines traditionally connect to the power grid through a doubly-fed induction generator (DFIG), which efficiently ensures that AC power is at line frequency (50 or 60 Hz), regardless of the actual rotating speed of the wind turbine. After the doubly-fed induction generator, a step-up transformer is generally used to increase the voltage.
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Increasingly, alternative generator and drivetrain systems are being used to connect wind turbines to the grid. Full-scale power converters, permanent magnet (PM) generators, and direct drive systems are quickly becoming commercially-viable alternatives to doubly-fed induction generator systems. These systems have better overall efficiency and low voltage ride through (LVRT) capabilities, (also known as fault ride through or FRT).
At the heart of all these designs is the need to provide the grid with reliable AC power, at the correct frequency with low harmonics, in the most efficient means possible. Regardless of the generator and drivetrain system, efficient, reliable AC power conversion requires some sort of efficient AC-to-AC converter. However, due to switching semiconductors, the AC-AC converter introduces harmonic distortion, causing the power generation system to exceed harmonic distortion limits set forth in IEEE 519. Harmonic distortion also decreases system efficiency. By inserting a CTM grid-tie filter, harmonic distortion to the grid (or step-up transformer) can be minimized, leading to compliance with IEEE 519 standards and increasing the efficiency of the entire wind power generation system. For additional efficiency, a liquid cooled filter system can be incorporated for superior thermal management and maximized energy efficiency.