Electrical transients of wind turbines in a small power grid

This paper presents field measurements and calculations on the disconnection and braking of wind turbines (WT) with rated power 60 kW and 100 kW in the grid of two Greek Islands. By means of a digital acquisition system with sampling frequency of 130 kHz, voltages and currents of the WT installations have been measured and analyzed just before and during the transient period. A simulation program has been used for this purpose. Results of simulation and measurements show in a wide range a good agreement. The simulation program was further used to investigate the disconnection process of WT under various circumstances. It is shown that during the WT disconnection, a sudden increase in stator currents may appear. So voltage dips may distort the power quality of the grid. Self-excitation of WT, with large induced voltage, occurs when they operate at low-load even with relatively small capacitive compensation. The amount of energy, that the brake of a WT absorbs during the disconnection, has been studied under various disconnection procedures. Suggestions are made for the proper moment of brake application, so that the energy absorbed is minimum.