In this paper, the performance of a passive anti-islanding method is experimentally tested for three phase (3φ) co-generation 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 s_q), 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 s_q. These frequency components can be parametrized 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.