Hybrid drive system of an industrial truck using a three-phase DC-DC converter feeding ultra-capacitors

The diesel-electric drive system of an industrial truck is extended with an energy storage subsystem utilizing ultra-capacitors thus establishing a reliable technical solution with a great energy saving potential. A bidirectional three-phase DC-DC converter is proposed to charge the ultra-capacitors during the regenerative braking of the electric drive and to discharge them when peak power is demanded. The developed DC-DC converter uses three-phase coupled inductors for filtering its phase and output currents. The presented current-control for the DC-DC converter is both effective and easy to implement. Practical results show that the DC-DC converter operates reliably in the laboratory as well as in the hybrid drive system of industrial trucks. Depending on the field of application for the industrial truck, total energy savings of ten to twenty percent are possible.