A modular three-switch integrated equalizer for ultracapacitor strings in low-carbon electric power systems

• A novel modular three-switch integrated equalizer topology is proposed. • A family of derived topologies is obtained from the proposed equalizer. • The proposed equalizer can work as an equalizer or a DC-DC converter with the equalization function. • The experimental results show balancing is maintained with less than a 1% voltage difference. Equalizers, as an indispensable part of green energy-storage systems (GESSs), can prolong the practical usable time of the battery string or the ultracapacitor string (UCS). Unfortunately, most equalizers are only designed for balancing the energy of ultracapacitors (UCs) or batteries, limiting their circuit functionality while resulting in poor system integrity. Therefore, an integrated multifunctional three-switch modular equalizer and the topology-based derivation method based on it are proposed in this work. In the proposed equalizer, the equalization function and the voltage conversion capability are integrated into a single circuit using only three MOSFETs, preserving circuit simplicity while adding multifunctionality and system integration level. Moreover, the proposed equalizer achieves zero-voltage switching (ZVS) of the two high-frequency MOSFETs, which decreases the switch loss. The innovative design of the proposed equalizer significantly increases its functionality. To prove the effectiveness and feasibility of the proposed equalizer, the circuit principle and its derivation method were first analyzed. Next, a simulation model and a 60 W prototype of the equalizer were developed, and a series of equalization experiments along with a comparative study were conducted on a UCS composed of six UCs. Finally, the simulation, experimental, and comparison results collectively demonstrate the versatility and system-level integration advantages of the proposed equalizer.