A hybrid designed digital dual-loop control of high power ground power unit (GPU)

400 Hz inverters, known as Ground Power Units (GPUs), are widely used in aviation and marine industry. Due to their approximately eight times higher fundamental frequency, 400 Hz inverters are much more sensitive to practical delays such as sampling delays, when compared to traditional 50-60 Hz inverters. Conventional controllers designed for 50-60 Hz inverters, therefore, must be redesigned to properly address a different sampling rate issue. This paper proposes a modified dual cascade control loop applicable to 400 Hz inverters. The inverter topology has been modified by placing the filter inductor at the input side of transformer. Therefore, in the resulting configuration, the inductor current will serve as a new feedback variable with less harmonic content. The control system employs a proportional and a resonant controller designed in digital and analog domains, respectively, which is a new design approach compared to the existing methods resulting in enhanced performance of the 400 Hz GPU. In addition, a feedforward block has been added to the overall typical dual loop scheme to decouple the control variables from load current disturbances and improve the dynamic response of the inverter. An optimized smooth noise-robust derivative has also been introduced to improve the noise immunity. Simulations and experimental results on a 20 kVA prototype GPU show the validity of the proposed scheme to provide high-performance transient and steady state responses.