Development and test of a medium voltage converter for ocean observatories (2009)

Power system engineering for ocean observatories has been an active area of research and development for the past 10 years. A number of architectures have been reported and discussed, including AC systems, DC systems, constant-voltage systems, constant-current systems, and various series and parallel cable topologies. A sequence of engineering analyses and prototype experiments led the U.S. MARS and the Canadian NEPTUNE programs to select a constant-voltage, branched DC architecture in their systems. A similar power architecture is in consideration for the U.S. Regional Scale Network (RSN). These regional power systems are generally comprised of shore-based power feed equipment (PFE), one or more long (10-400 km) single-conductor undersea telecom cables, one or more undersea nodes, and seawater electrodes to return currents from the nodes to the shore. The power conductors in these undersea cabled systems typically have resistances of 1 ohm/km, with operational currents of less than 2 Amps and operational voltages of less than 12 KVDC. High-voltage, low-current power transmission is essential to minimize power losses within the cables and to maintain stability under dynamic, negative impedance loads. A critical component in regional scale undersea power systems is the medium voltage converter (MVC). The MVC is a DC-DC converter in the primary network infrastructure that receives a medium voltage power input from the PFE via the telecom cable, typically at 1-10 KVDC, and provides down-conversion to one or more lower voltage outputs, typically 300-600 VDC, to feed power to science nodes and instrumentation in the secondary network infrastructure. The development of a reliable undersea MVC has been a difficult engineering obstacle in ocean observatories, causing delays and overruns in several projects. Unlike fiber optic timing and data networks, the ocean observatory power system has almost no equivalent terrestrial system market from which to draw mature, affordable, commercial components such as MVCs. The marine technology community has been at various stages of MVC design, development, and prototyping since 2000, trying to solve the difficult electrical, mechanical, and ocean engineering challenges found in ocean observatory systems. These challenges include high-voltage corona and arcing, operational availability, system cost, cable impedance dynamics, instrument load dynamics, thermal dissipation, safety issues, cable and connector faults, installation limitations, and a limited palette of high-quality commercial components for MVC design and engineering. In this paper, we report the successful development, test, and subsea installation of a 3KV, 3 kW MVC based upon the Vorperian modular stacked architecture. The MVC provides 3000 VDC to 625 VDC conversion for primary undersea networks. The MVC design consists of sixteen (16) DC-DC subconverters wired in a series input configuration, with each subconverter input operating at approximately 187 V (3000V/16). The sixteen subconverter outputs are wired in an 8?2 series-parallel configuration, with each subconverter output operating at approximately 78 V (625V/8). A feedback control loop monitors the MVC output and pulse-width modulates the duty cycle of the subconverter switching to maintain precision output regulation.