The detection unit of a harmonic current is an important part of harmonic control. The mathematical model is built according to the worst operating state of the system. Through theoretical derivation, two harmonic current detection methods based on instantaneous reactive power theory, the p–q method and the ip–iq method, and their harmonic current detection errors are analyzed. An improved ip–iq method developed by removing the phase-locked loop is proposed and a detailed control block diagram is constructed. According to the different operating conditions of the system voltage, PSCAD is used to build a system simulation model with nonlinear and linear loads. Through simulation analysis, the correctness of the theoretical analysis and the effectiveness of the improved scheme are verified.