High-performance control of a UPS inverter through iterative learning based on zero-phase filtering

The purpose of a voltage controller for UPS inverters is to produce stable and regulated output voltage with low distortion under all loading conditions, especially under nonlinear loads. Since the loads which cause the tracking errors in the output voltage are typically periodic, iterative learning control is an obvious and excellent intuitive choice for achieving this close voltage tracking. In this paper, an iterative learning controller based on zero-phase filtering is directly combined with the open-loop UPS inverter. Due to the feedforward involved in the open-loop inverter operation, the overall dynamic response can be expected to be reasonably satisfactory. A zero-phase filter designed in frequency domain is applied to compensate the open-loop inverter so as to ensure error convergence. This acts to compensate the poor dynamics of the inverter owing to the presence of the L-C circuit. Furthermore, a forgetting factor is introduced in this control algorithm to increase the robustness of the scheme against measurement noise, initialization error and/or variation of system dynamics due to parameter drift. The experimental results show that the proposed controller can achieve very low total harmonic distortion and fast error convergence under different loads even with only one sensor. The proposed method can be an effective solution for UPS products where high-quality output voltage is more important than fast dynamic response.