Contents I. Introduction
The Paris Agreement was adopted to reduce greenhouse gas (GHG) emissions in 2015 [1],and The International Energy Agency has reported that the total global GHG emissions will be approximately 35000MtCO2eq in 2022 [2]. With the increasing awareness of the need to reduce greenhouse gas emissions to combat global warming, there has been an increase in the adoption of renewable energy has increased. In Japan, the 6th Strategic Energy Plan was adopted, which sets targets for energy conservation and the introduction of renewable energy, including solar power [3]. AC distribution networks are used primarily for power transmission and distribution. Many loads have AC/DC, DC/DC, and/or DC/AC converters in AC distribution lines. Moreover, photovoltaic (PV) power generation system and battery energy storage system (BESS) are required DC/AC converters to an AC distribution network., Thus, DC distribution networks have attached attention owing to their energy saving capabilities [4] [5]. DC power distribution systems can be directly connected to DC devices such as PV power generation systems and BESS, which can save energy by reducing the number of power converters. Moreover, DC need not consider the frequency and reactive power, resulting in a simpler equipment configuration, lower failure rates, and improved reliability. Dual active bridge (DAB) converters, which are isolated DC/DC converters, are widely used in DC distribution systems. The DAB converter comprises two full-bridge converters and a high-frequency transformer. It exhibit large capacity, high efficiency, and bidirectional power control [6]. However, when a DAB converter is used to interconnect solar cells and storage batteries to a DC distribution line, the input and output currents of the DAB converter include a ripple current. This ripple current may effect the maximum power point tracking (MPPT) control of the PV cells and reduce the power loses. In addition, in the case of BESS and, lithium-ion batteries (LIB), LIB degradation is accelerated when the upper voltage limit of the LIB is exceeded owing to the ripple current [7]. Consequently, ripple reduction methods using active filters have been proposed. In [8], a shunt DC active filter (DCAF) was proposed. The shunt DCAF can compensate the ripple current by injecting the compensation current. However, the rating required for shunt DCAF is high. A series DCAFs was proposed in [9], The required rating for the series DCAF was lower than that for shunt DCAF. However, a matching transformer was required.
