Aiming at such issues as weak tracking ability and low control accuracy of traditional PI controllers in the unified power quality conditioner

a unified power quality conditioner (UPQC) with a parallel side active power filter (APF) as a double closed-loop nonlinear composite control method is proposed in this paper. A new dual-loop nonlinear composite control method of the APF at the parallel side of UPQC is adopted

a sliding mode PI composite nonlinear control algorithm is used for the voltage outer loop and a repetitive PI nonlinear control algorithm is used for the the current inner loop. Performance tests of the UPQC under static disturbances (such as grid voltage harmonics/imbalance and load current harmonics) and dynamic disturbances (such as voltage sags/surges and load variations) are performed through the experimental device. The results show that the proposed UPQC can

in case of load mutation and reference voltage jump

improve the response speed and steady-state accuracy of the DC-side voltage

thereby validating the correctness of the theory.