Modular multilevel converters (MMC) and series-connected IGBTs are two promising solutions to high-voltage dc (HVDC) transmission. It has been shown that, when integrated with series IGBTs, an MMC can benefit from a reduced number of sub-modules (SMs), simpler control, and improved reliability. In this paper, a MMC prototype, with the capability to integrate series IGBTs to replace single IGBTs in the sub-modules, is designed and developed. The MMC control is realized with a real-time simulator for versatile software implementation. The voltage sharing among the series IGBTs is regulated locally without the intervention of the central control. Furthermore, voltage balance under bi-directional current through the series strings are achieved. Experiments were conducted where single IGBTs in one sub-module were replaced with a series of three IGBTs each. The results verify that the MMC is functional since the local series IGBT control and the MMC central control are independent. It is also verified that both series strings can maintain voltage balance at the same time even though the string currents shift directions.