A Power Ripple Compensator for DC Nanogrids via a Solid-State Converter

This paper presents a method for improving the reliability of DC nanogrids by decreasing the input capacitance requirement. The nanogrid DC bus capacitance requirement is reduced by utilizing the zero-sequence operation mode of the solid state converter (SSC). This SSC is often considered as the main DC nanogrid energy control unit that is linking the nanogrid with the main utility AC grid and managing the nanogrid elements. The proposed method injects a zero-sequence voltage in the SSC AC filter capacitors to compensate the low frequency power ripples that are imposed on the DC bus. The proposed method compensates power ripple due to non-linear loads, linear loads, distorted grids, and load variations. Moreover, the compensation method does not require additional components that are commonly used with the existing power ripple compensators. Furthermore, practical considerations of the proposed control are discussed in the paper regarding stabilization of the nanogrid in case of lack of critical damping. The analysis and results demonstrated that a 50 μF total capacitance is sufficient at the DC bus for 500V/5kW DC nanogrid to achieve negligible DC bus voltage ripple. Thereby, with such small DC nanogrid input capacitance, the DC link capacitor can be a film type capacitor to enhance the reliability of the nanogrid.