Nowadays, power electronics is an essential technology for enabling variable speed operation of wind turbines with increased energy capture and reduced mechanical stress. The commonly used doubly-fed induction generator only needs a small rated converter to enable variable speed operation and has a correspondingly low cost. On the other hand, permanent magnet (PM) generators have advantages such as no slip rings, high efficiency and the possibility of gearless operation. Their drawback is the need for a large power electronics converter, as the full power must be converted into voltage of fixed frequency suitable for the connection to the grid. This drawback motivates the search for new configurations with reduced losses, costs and space requirements. In this article, a new configuration using a variable series capacitor is investigated. The configuration shows simple control, low losses and a reduced need for semiconductor rating, and thus contributes to the technology shift towards permanent magnet generator solutions.

Enercon develops and produces gearless wind power turbines that are a benchmark for the market in terms of power, reliability and lifespan. Therefore, research and development at Enercon determines the level of innovation. Enercon has chosen Moeller's XC100-FC modular programmable logic controller (PLC) with a fibre-optic CANopen interface for controlling and monitoring its aviation beacon system. This article describes first the controls needed in the wind turbines and then how the modular PLC from Moeller is integrated into such systems.

The Wind Power Drive Train Package

Combining a permanent magnet (PM) generator with a power converter in a wind power drive train is simple, but nobody integrates them into a totally optimised package as The Switch does. The development of magnetic materials and power switches has made the combination of a PM generator and a full-power converter a very competitive solution for the next-generation wind power systems.

By Jussi Vanhanen, Marketing Manager, The Switch, Finland

Structural Design of Sandwich Towers for Wind Energy Converters

A new kind of tower construction, called a sandwich tower, has been developed for wind energy converters. The tower consists of two steel shells, which are bonded together with a core material. Unlike a standard steel tower, the plate thickness is split into an inner and outer steel face. The core between the faces increases the stability of the shells. It works together like a sandwich shell. Different composite shell theories have been used to estimate the stability of such double-skin shell constructions. A model-scale test series has been carried out to analyse the influence of different core materials. The test specimens are loaded by uniform axial force to observe the shell buckling. The experimental results are compared to numerical simulations including measured geometrical imperfections. Within a numerical pre-design, the use of high-strength steels for the inner and outer faces is also considered to compare the various types of tower configurations. The goal is to find the best combination of steel faces with a core material in the ultimate limit state.

By Peter Schaumann and Christian Keindorf, ForWind, Germany