# Improved PR Control Without Load Current Sensors and Phase-Locked Loops for APFs
); [EMAIL_ADDRESS] (J.Z.) Y. Zhang  
School of Mechanical and Electrical Engineering, China University of Mining & Technology-Beijing, Beijing 100083, China; [EMAIL_ADDRESS] (J.L. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (J.Z. ); [EMAIL_ADDRESS] (X.Z.) Y. Zhang  
School of Mechanical and Electrical Engineering, China University of Mining & Technology-Beijing, Beijing 100083, China; [EMAIL_ADDRESS] (J.L. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (J.Z. ); [EMAIL_ADDRESS] (J.Z.) Y. Zhang  
School of Mechanical and Electrical Engineering, China University of Mining & Technology-Beijing, Beijing 100083, China; [EMAIL_ADDRESS] (J.L. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (J.Z. ); [EMAIL_ADDRESS] (J.Z.) Y. Zhang  
School of Mechanical and Electrical Engineering, China University of Mining & Technology-Beijing, Beijing 100083, China; [EMAIL_ADDRESS] (J.L. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (J.Z. ); [EMAIL_ADDRESS] (J.Z.) Y. Zhang  
School of Mechanical and Electrical Engineering, China University of Mining & Technology-Beijing, Beijing 100083, China; zqf2300403077@student. APFs have the advantages of strong adaptability and fast response speed and are particularly suitable for industrial applications with frequent load changes [3]. At present, parallel APFs based on voltage source converters have become a research hotspot in the field of harmonic control and the preferred solution in engineering practice. Among them, parallel three-phase three-wire two-level APFs have been widely used in industrial scenarios due to their excellent filtering performance and economy [4]. To achieve excellent harmonic compensation performance in APFs, the design of an effective control strategy is essential. The dynamic response and steady-state accuracy of the control method directly affect the filter's compensation performance, particularly under sudden load changes or complex harmonic conditions [5]. Currently, research on APF control methods has branched into multiple technical directions. Among them, the traditional control scheme based on synchronous coordinate transformation is widely adopted due to its simple implementation. In the proposed method, the grid voltage phase is obtained using a PLL, which is then used to perform a synchronous coordinate transformation (such as the \(dq\) transformation) on the load current. This converts the fundamental component of the AC current into a DC signal. A low-pass filter is then used to extract the AC harmonic components, which are regulated by a proportional-integral (PI) controller to achieve harmonic compensation [6]. Although this approach offers fast dynamic response, its inherent bandwidth limitations reduce the accuracy of high-frequency harmonic compensation, making it difficult to meet performance requirements under complex harmonic conditions [7]. To address this issue, researchers introduced repetitive control strategies based on the internal model principle. This approach constructs an internal model of the periodic signal, enabling, in theory, zero-error tracking of all harmonic components and significantly enhancing steady-state accuracy. However, standalone repetitive control suffers from response lag, which can lead to transient oscillations during sudden load changes. To overcome this drawback, composite control strategies have been developed. The PI-repetitive control parallel structure (PIRC) proposed in [7] combines the fast dynamic response of PI control with the high-precision compensation of repetitive control, achieving improved performance through a time-frequency domain collaborative control mechanism. Despite its advantages, PIRC still has several limitations. First, it relies on the PLL, which introduces phase detection delays and can cause small-signal stability issues [8; 9; 10]. Second, PIRC involves high computational complexity, requires large memory resources, and exhibits limited effectiveness in compensating for non-periodic harmonics.