This paper presents a fully digital control method that can be used for inverters used in 400 Hz aircraft ground power units. The method presents an ideal tracking controller that incorporates the computational delay as well as the quantization effects. It uses an observer for the output current third, fifth and seventh harmonics in order to achieve decoupling between the voltage and current loops thus leading to a low THD content in presence of non-linear loads The implementation of the observer is reduced to IIR filters with normal architecture that have minimum round off errors. The control method is implemented on a 16-bit single chip DSP-based controller from Analog Devices (ADMC401) and tested on a single-phase 10kva IGBT-based inverter prototype showing almost complete elimination of the third, fifth and seventh harmonics from the output voltage. I.INTRODUCTION It is well known that non-linear loads can distort the output voltage of a voltage inverter even if instantaneous control is used. Thus, many advanced control strategies have been proposed over the last decade in order to maintain a low THD content in presence of nonlinear loads such as: sliding mode control, repetitive control, dead-beat control, selective harmonic control, dissipativity-based control, optimum state space control etc. However, the vast majority of them have dealt with 50/60 Hz inverters for UPS applications and few of them have focused on the 400 Hz inverters used in Ground Power Units (GPU) for aircraft applications [8], [12]. The main difference between the GPU and the UPS type inverters is that the frequency band between the fundamental and the harmonics generated by the inverter itself and by the nonlinear load is very low; therefore, it becomes very difficult to maintain a low THD content for a GPU inverter. The computational delay also cannot be ignored due to the high fundamental frequency where the effects are much more significant than in the case of 50/60 Hz inverters. Early ground power units (GPU) were designed using optimized modulation schemes that eliminated or reduced the amount of harmonic distortion, thus reducing the size of the output filter. In the very high power ranges the traditional method of choice was to use step wave inverters where multiple inverters working at low switching frequency are added together via special transformers to develop an output voltage of 12, 24 or even 48 pulses. The step wave inverters require a large number of semiconductors thus decreasing the reliability and increasing the cost. Regardless of the choice, these types of

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