Abstract : For practical active power filters, strict requirements are placed on the DC-side capacitor voltage during both startup and normal operation. Therefore, modeling and analysis were carried out for DC-side voltage closed-loop control and voltage equalization closed-loop control, and a fuzzy PI hybrid controller was designed in this paper. The fuzzy PI hybrid controller differs from conventional PI control. Fuzzy control is essentially a nonlinear control, exhibiting better adaptability to parameter changes or nonlinearities in the controlled object, and stronger suppression of interference or noise, i.e., stronger robustness. However, fuzzy control also has shortcomings; its performance in eliminating system steady-state errors is relatively poor, making it difficult to achieve high control accuracy. Keywords : Active power filter; Fuzzy PI controller The DC capacitor connected to the DC bus of an active power filter (APF) serves as the power supply for the inverter, providing energy for its operation. Normal operation of the inverter requires a stable DC bus voltage. However, since energy is provided by the capacitor, the capacitor's terminal voltage is inevitably affected by the energy status of the power grid and load, changing with the operating conditions. To ensure the normal operation of the APF main circuit, its DC bus voltage must be maintained at a certain value to ensure that the main circuit can generate the correct compensation current in response to changes in the command current at any time. 1. Basic Principle of DC Bus Voltage Control The amount of active current absorbed or released by the inverter from the grid is controlled based on the difference between the actual value and the reference value of the DC bus voltage, thereby ensuring overall energy balance and stabilizing the DC bus voltage. The energy provided by the grid mainly supplies the load, with a small portion compensating for the inverter's energy consumption. The energy flow relationship is reflected in the active power of each part, as shown in Figure 1. For more details, please click to download.