The intermittent and volatility of the existing renewable energy power generation grid-connected system reduces the utilization rate of its equipment.The proposal of multi-function grid-connected inverter with power quality control and grid-connected functions has enhanced the utilization rate of grid-connected equipment and improved grid-connected power quality.In view of the detection and extraction of load current compensation components

which is one of the important aspects

in this paper the difference between the two current detection methods of three-phase three-wire system under unbalanced and non-line

ar loads is studied

and the computation complexity of these two methods is analyzed.The two methods are the i

p

-i

q

detection method based on the p-q theory and the detection method based on the CPT（conservative power theory） theory.The simulation model was carried out by the system model established by Matlab/Simulink.It is shown by the simulation result that the current detection method based on CPT theory can detect the components of the load current more flexibly under unbalanced and non-linear loads

but the computation needed by the i

p

-i

q

detection method is less.