This project presents a modified adaptive Kalman filter for sensor less current control of a three-phase inverter-based distributed generation system. Measurement and digital signal processor delays have been compensated by using a modified Kalman filter. The control variable states can be estimated one sample in advance with a reduced number of sensors. Two current control loops with the steepest descent adaptive control for grid voltage estimation have been employed to boost robustness. Stability and dynamic performance have been examined for different state variables using root locus. Simulation and experimental results validate the proposed control approach.
A PLL performs the transformation from a three-phase stationary reference frame to a two-phase synchronously rotating reference frame. The control system dynamics are investigated to assess system stability and to select feedback variables. Simulation and practical results verify the derived expressions and system performance. With the modified Kalman filter, the control state variables are predicted one sample in advance and the number of measurement sensors is reduced. The fluctuations are reflected to the dc side, as harmonic frequencies that are sourced from the dc-link capacitor. The harmonic impedance is determined by the proportional gain of the controller. From the Bode analysis, it is clear that the system with inductor voltage feedback is inherently more stable against low-order frequency harmonics than the other feedback control variables. The performance of the control algorithm described has been investigated using MATLAB/SIMULINK simulations. The proposed controller bandwidth is extended compared to the conventional controller; hence, the proposed delay compensation technique enhances the digital controller bandwidth. The experimental system was implemented with the same system parameters as the simulated studies. The results show that the currents are balanced and high quality sinusoidal. The switching frequency content is minimized with the presence of the –filter with the isolating transformer.
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