Control of doubly fed wind generators connected to the grid by asymmetric transmission lines

A speed controller operating on a doubly fed induction generator can be improved to compensate asymmetries on voltage waveforms feeding the machine´s stator. This is particularly interesting for variable-speed wind-power generators connected to an electrical network through weak transmission lines. Any asymmetry on the line causes the currents to be unbalanced, which will distort voltage waveforms at the beginning of the line, degrade the power quality injected into the network, lower the efficiency of energy transmission and increase the thermal losses in the generator. This paper presents the performance of three different compensation procedures based on the control of the waveform of a doubly fed generator´s output current. A brief description of the model of the wind power system developed using reference frame theory is first introduced, followed by the description of the three different control techniques proposed to maintain balanced currents: a linear PI controller, a nonlinear fuzzy controller and a mixed scheme consisting of a fuzzy control adjusting PI parameters on line. The performance of the three methods are then compared in terms of their efficiency in controlling the speed and compensating asymmetries. Simulations show that current asymmetries can be greatly decreased, by waveform control without sacrificing speed regulation.