Fault Ride-Through Characteristics of Small Wind Turbines

There is significant potential for an increase in the use of kilowatt-class small wind turbines (SWTs) in Japan due to reduced limitations with respect to installation, despite their high cost. At this stage, the Japanese grid code has not been considered sufficiently with respect to grid-connected SWTs, and the addition of fault ride-through (FRT) requirements for SWTs has been requested. Moreover, the FRT of SWTs is challenging to achieve owing to the low inertia constants when compared with those of large-scale wind turbines, which result in significant acceleration of the rotor speed and an increase in the input voltage of the power conditioning system (PCS) during FRT operation. In this study, FRT field tests were conducted on SWT systems against a voltage dip with a duration of similar to 1 s, and it was confirmed that the SWT systems satisfied the FRT requirements for photovoltaic (PV) systems connected to low-voltage distribution lines in Japan. The behaviors of the rotational speed of the SWTs and the PCS input voltage in an FRT operation were then analyzed, and it was noted that the increase in the PCS input voltage with the overspeed of the turbine can reach the upper limit and make the PCS cease operation, which indicates failure of the FRT. The overvoltage, therefore, requires restriction using a method such as pitch control, furling, and electrical and/or mechanical brakes.