Repetitive current control of an interleaved grid-connected inverter

Repetitive control can be effective in improving current quality when used with grid-connected inverters. Mathematically, a repetitive controller is equivalent to a parallel combination of resonant controllers with high gain at the fundamental frequency and its harmonics, and accordingly good disturbance rejection can be achieved at these frequencies. To ensure stability, a low-pass filter needs to be incorporated within the repetitive controller to attenuate the high frequency resonant peaks of the controller gain (above the system's cross-over frequency), without significantly affecting the low frequency resonant peaks corresponding to significant grid harmonics that need to be rejected. The paper therefore argues that it is desirable that the system and the inverter's output filter should have a high bandwidth - higher than the most significant grid harmonics - which may be counter intuitive. The high bandwidth requirement dictates a high PWM switching frequency, which becomes challenging in high power systems as the maximum achievable switching frequency of power electronic devices reduces as their power rating increases. This limitation can be overcome in low voltage systems by using an interleaved inverter topology in which the power is shared between several 2-level half-bridge inverter legs connected in parallel. In addition to using low power devices capable of high switching frequency, the value of the LC output filter capacitance can be relatively very small thanks to the ripple cancellation feature of the interleaved inverter. Both of these features mean that the bandwidth of an interleaved inverter can be much higher than that achievable using a classical 2-level inverter with an LCL output filter. The paper discusses the design and practical implementation of a repetitive controller for an interleaved inverter with 6 half-bridge legs per phase. Simulation and experimental results are also presented to demonstrate the effectiveness of the proposed controller in improving the THD of the output current of the inverter.