As distribution generations (DGs) gradually increase in the existing central power generation based grid system, various functions are required for the grid connected inverter for DGs to stabilize and maintain the grid system. Typical functions include system grid synchronization, current control, and DC voltage control. As in the special case of micro-grid, if the connection with the grid is disconnected, constant voltage control is performed like a UPS, but in most cases, constant current control is performed. The current control algorithm was proposed using a stationary reference frame PI (SRFPI) controller rather than the conventional synchronous reference frame PI controller or stationary reference PR controller. Since the park transformation for the synchronous reference frame GCI system can be omitted, the computational burden on the controller can be reduced, and the phase locked loop (PLL) can be omitted because the phase information, i.e, θ is unnecessary. In addition, the control system is simplified because the cross-coupling, in which the d-axis and q-axis system interface with each other, is omitted. However, steady state error and phase delay inevitably occur in the SRFPI controller using an AC signal.
In this paper, command feedforward (CFFC) and disturbance rejection control (DRC) algorithms were proposed to compensate for these drawbacks. To estimate the disturbance, a grid current observer is developed and the disturbance signal is decoupled to the controller input. In order to verify the proposed CFFC and DRC algorithms, steady state error and phase delay in the grid normal state, and total harmonic distortion (THD), which is an indicator of power quality, were confirmed. And overcurrent and response time was confirmed to improve the transient response characteristics in case of grid faults. The proposed algorithm is implemented and verified thorough simulation and experiment.