In this paper, the performance of a passive antiislanding method is experimentally tested for three phase (3φ) cogeneration systems. The tested method is based on determining the wavelet packet transform (WPT) high frequency sub-bands present in the d − q-axis components of instantaneous 3φ apparent powers (s d and sq), when evaluated at the point of common coupling (PCC). This passive anti-islanding method is founded based on the nature of instantaneous 3φ apparent powers that have components continuously exchanged between both sides of PCC. An islanding condition can be considered as a transient disturbance that creates non-periodic and non-stationary high frequency components in s d and sq. These frequency components can be parameterized by WPT high frequency sub-bands, which can provide accurate detection of the islanding condition. The d − q WPT-based anti-islanding method is tested for a 3φ co-generation system under various loading and power delivery conditions. Performance results reveal accurate, fast, and reliable detection and response to the islanding condition.Index Terms-3φ co-generation systems, anti-islanding protection, d − q-axis reference frame, and the wavelet packet transform.