The compensation performance of active power filter depends on harmonic detection mostly.Three methods of harmonic detection in three-phase four-wire system are introduced:d-q method based on instantaneous inactive power theory,active power separation method and equal power method based on power equilibrium.The basic principle of equal power method is emphasized,for which simulations with PSCAD / EMTDC are made in four combinatorial instances of distorted / normal voltage and symmetrical / dissymmetrical load.The compensation effect and real-time performance of three harmonic detection methods are compared based on the simulations,which shows that,the equal power method is the best in compensation effect,real-time performance and DC voltage utilization efficiency.It has less calculation,simple saftware design and hardwaree implementation,and high practicability.