A joint industry project (JIP) comprising ABB, Equinor, Total, and Chevron, is developing technologies for subsea power transmission, distribution, and conversion. The output will form a critical part of future advanced subsea-field developments. Begun in 2013, the project reached a major milestone in late 2017 when the first full-scale prototype of the variable-speed drive (VSD) passed a shallow-water test (Figure 1). Final preparations are now underway for a 3,000-hour test of the complete subsea power system with two VSDs in a parallel configuration combined with subsea switchgears and controls. The complete paper highlights elements of the technical development and an overview of the primary building blocks of the system, and presents in detail some of the challenges in developing, designing, and testing the control system.