# Improved PR Control Without Load Current Sensors and Phase-Locked Loops for APFs
To tackle these challenges, PR control implemented in the stationary reference frame (\(\alpha\beta\) coordinate system) offers a simpler and more efficient alternative [11]. The PR controller directly constructs a complex domain resonator in the \(\alpha\beta\) coordinate system. In theory, there is no need for coordinate transformation or reliance on a PLL, which simplifies the control structure, reduces computational complexity, and improves the real-time performance and stability of the system. However, in practical applications, the PR controller is often implemented in combination with PLL and synchronous rotating coordinate transformation (\(dq\) transformation). By obtaining the phase information of the grid voltage through PLL and using the phase information for \(dq\) transformation, specific harmonic components can be converted into components of the same frequency, thereby reducing the number of PR controllers required. For example, the fifth and seventh harmonics will appear as sixth harmonic components in the \(dq\) coordinate system, so a PR controller for the sixth harmonic can be used to compensate for both harmonics at the same time. This approach not only significantly reduces the control algorithm's requirements for storage space and computing power, but its modular structure can also flexibly expand the compensation frequency band by connecting multiple resonators in parallel. Therefore, it is essential to further explore the application potential of PR controllers in APFs [12], particularly in conjunction with the emerging research trend of sensorless control strategies. In recent years, low-sensor schemes have gained increasing attention as a means to reduce system cost, enhance reliability, and avoid issues related to sensor noise, drift, and failure [13]. Reference [14] proposed a variable structure adaptive predictive control (VS-APPC) method based on pole placement theory and the internal model principle. This approach enables zero steady-state error sinusoidal tracking and accommodates system parameter uncertainties--without requiring harmonic detectors or current sensors. However, it demands high model accuracy and careful parameter tuning, making practical implementation complex. Reference [15] introduced a direct current control (DCC) method based on a predictive model, which relies solely on grid current measurements while eliminating the need for load and filter current sensors. Although the control structure is simple, its robustness is constrained by the accuracy of the filter model. Beyond reducing current sensors, the elimination of voltage sensors has also become a significant research focus [16; 17; 18; 19; 20; 21]. References [16; 17] propose voltage sensorless control strategies using electromotive force (EMF) estimation and model predictive control (MPC) algorithms. These methods simplify the system architecture considerably but depend heavily on accurate modeling and prediction and often involve high computational complexity. References [18; 19] utilize virtual flux estimation (VFO) and incorporate techniques such as cascade delay suppression (CDSC), second-order generalized integrator (SOGI), and resonant compensators to improve system robustness and high-frequency adaptability under unbalanced grid conditions. In addition, References [20; 21] employ time-domain symmetric component extraction (SCEA) to generate reference signals without requiring voltage measurements. By extracting the negative-sequence current and driving the inverter current to compensate for it, these methods effectively eliminate unbalanced components while simplifying the sensing configuration. Based on this line of research, References [22; 23] proposed a voltage sensorless control strategy for APFs that integrates a PR controller with the symmetric component extraction algorithm (SCEA). In the proposed method, SCEA derives the positive-sequence component of the three-phase load current to serve as the reference signal.