With the development of the High Voltage Direct Current (HVDC) technology, the audible sharp noise originated by the AC filter capacitors has become a serious noise pollution problem in HVDC converter stations. In this paper, in the interest of the accurate analysis of the noise characteristics for AC filter capacitors, a Multiple Harmonic Current Injection System (MHCIS) is proposed to simulate the actual working conditions of power capacitors in HVDC converter stations. Thus, how to accurately and quickly detect and control the harmonic currents is a vital important aspect for MHCIS. And the synchronization between the fundamental current and the harmonic currents is also a critical element for MHCIS. Firstly, for the sake of the synchronization between the fundamental current and the harmonic currents, a comparative survey about the performance among Single Phase Locked Loops based on the Orthogonal Signal Generation (OSG-SPLL) is presented, then a new SPLL strategy based on the Arbitrary Phase Delay (APD-SPLL) is proposed. A Multiple Synchronous Reference Frame (MSRF) combined with the Moving Average Filter (MAF) is applied to detect and control the selective harmonic current in MHCIS. Finally, the simulation and experimental results validate the performance of the proposed MHCIS in this paper.