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 shore-based power feed equipment (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 Primary-Node Housing solid model with the MVC on the left and the optical payload on the right. The MVC chassis is mechanically mounted, electrically isolated, and thermally conducted to the left end cap. An optical payload is mounted in a similar fashion to the right end cap.
The development of a reliable undersea MVC has been a difficult engineering obstacle in ocean observatories, causing delays and overruns in several projects. 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.

This work reports on 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 16 DC-DC subconverters wired in a series input configuration, with each subconverter input operating at approximately 187V (3000V/16). The 16 subconverter outputs are wired in an 8 × 2 series-parallel configuration, with each subconverter output operating at approximately 78V (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.

The MVC consists of 16 low-voltage (LV) subconverter modules wired together in series at the inputs, an input plane that provides input protection, input monitoring, and converter startup, and an output plane that provides output filtering and PWM feedback control. The MVC converts a 3 KVDC cable input to a 625 VDC output at loads of circa 3 kW. The MVC system also includes additional output modules for control, switching, conversion, regulation, filtering, protection, and monitoring of power outputs to the internal housing payloads and to the external undersea instrumentation sites.

The MVC architecture was designed for modularity with the following building blocks: a thermally-conductive aluminum chassis with module slots; an input card with the input plane circuitry; a quad-subconverter card with four subconverters; a backplane card with monitoring, output plane, and PWM controller; one or more output cards; and power modules for internal payloads (e.g., optical amplifiers). The MVC is packaged in a 21-inch-diameter primary node housing. The chassis is mechanically mounted, electrically isolated, and thermally conducted to the left end cap. An optical payload is mounted in a similar fashion to the right end cap. The optical payload includes eight erbium-doped fiber amplifiers (EDFAs), optical add/drop multiplexers, optical attenuators, fiber management, and redundant Gigabit Ethernet telemetry units for primary node supervisory control and monitoring. The right end cap also includes undersea cable terminations to the primary and secondary power cable infrastructure, as well as connections to undersea electrodes for seawater power returns.

The subconverter stack, which dissipates a majority of heat in the MVC, is located near the end cap for conduction cooling. Additional card slots above the subconverters hold the input and output modules. Power converters for the primary node internal payloads are mounted to the top of the chassis. The MVCs were tested with a commercial off-the-shelf 3 KVDC power supply (a.k.a. power feed equipment or PFE) and a custom 160-km cable simulator. Kilowatt power loads with negative impedances and 20-km cable simulators were developed, built, and used during lab tests to simulate the secondary power infrastructure. Power supply efficiency was measured to be approximately 90% across the load range of 2-3 kW. Similar efficiency was obtained across the 2.5-3.5 kV input range. The efficiency dropped to about 82% at light load (725 Watts).

The output voltage regulation typically varied less than ±1 volt from no load to full load under static load conditions. The output voltage ripple was measured to be typically less than 200 mVpp across the same load range. The converter was exceptionally stable under dynamic loads, holding the output steady to within a few volts droop during 2-Amp off-on step load changes. The MVC output was also shown to hold load regulation under the failure of a subconverter module, demonstrating fault tolerance. The MVC architecture maintains operation with up to two failed subconverters for most common fault modes.

This work was done by Joseph Key of the Naval Surface Warfare Center, Carderock Division, and John Walrod of SAIC Advanced Systems Division. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. SAIC-0001

This Brief includes a Technical Support Package (TSP).
Development and Test of a Medium Voltage Converter for Ocean Observatories

(reference SAIC-0001) is currently available for download from the TSP library.

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This article first appeared in the February, 2011 issue of Defense Tech Briefs Magazine.

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