The penetration of distributed generation (DG) in electrical power systems is rapidly increasing these days and more attention is drawn to maintaining a healthy distribution system. Islanding operation of DG is one of the biggest challenges. When Islanding occurs, the power supply from the main grid is interrupted which may cause a problem when distributed generators keep providing power into the distribution networks. This situation can damage both DG and power system equipment if the grid is suddenly reconnected to the system. This paper investigates improving the performance of rate of change of frequency (ROCOF) as used in embedded generation islanding detection of DFIG wind turbines. The factors of ROCOF relays which contribute to the diverse operating response are frequency measuring algorithms, measuring windows, and the different relay settings. It is found that an equivalent the impedance estimation can be used to avoid false operation of DFIG wind turbines during a non-islanding situation. This is demonstrated for a variety of loading conditions.