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Power electronic converters have evolved rapidly, both in terms of available electronic switching devices & converter topologies. The evolutions from thyristor-based 12-pulse converters to Voltage Source Converters (VSCs) to Modular Multilevel Converters (MMCs) have placed increasingly onerous demands on simulation technology, in particular real-time digital simulators. The relatively high carrier frequencies in PWM-based converters require time-steps in the range of 10 to 50 μs, typical of what may be achieved using standard INTEL/AMD multi-core processors. Today's MMC converters add to the challenge through large number of I/Os required to monitor the voltages of a large number of cells & to individually control a very large number of IGBT switches. In this paper the evolution of modern real-time digital power-system simulator is presented - the hardware/ software challenges & solutions, leading to development of a commercially available a COTS-based simulator. Its efficacy with a simulation of an MMC-based back-to-back HVDC bipolar interconnecting two AC networks is presented. Such AC-DC-AC converter system presents challenges in terms of both the requirement of a small time step, as well as a very large number if I/O channels.