A research program has made progress toward the development of schemes for protection against faults in next-generation DC zonal shipboard electrical systems (DC SESs) and for configuration management in these systems to enable surviving parts of the systems to continue to distribute electric power to critical loads. Computational simulations of a representative prototype DC SES have shown that these schemes could afford protection at a system level (in contradistinction to a local level), thereby imparting, to the DC SES, an intended capability for self-healing.

Within a Zone, each of the non-critical loads would be connected to either the main or the alternate secondary DC bus, but not both. The critical loads would be connected to both buses via auctioneering diodes. During normal operation, the voltage on the main secondary bus would be somewhat higher than that on the alternate secondary bus, so the diodes would steer power from the main secondary bus to the critical loads. In the event of low voltage on the main secondary bus, the diodes would steer power from the alternate secondary bus to the critical loads

One basic principle of this research is that the DC SESs in question will incorporate voltage-source converters (VSCs) and other power electronic building blocks (PEBBs) so as to realize the potential advantages of DC power distribution over AC power distribution. Another basic principle is that many or perhaps all of the PEBBs could incorporate or be associated with computer-based electronic control subsystems that would cause the PEBBs to function as agents in an agent-based protection scheme. In the emerging discipline of agent-based design, which borrows heavily from the discipline of artificial intelligence, “agents” signifies, more specifically, agents that act autonomously in such a manner as to achieve a desired effect (e.g., zonal or system-wide protection).

The research program encompassed three parts: one focused on protection devices, one focused on the agent-based protection scheme, and one focused on the configuration-management scheme.

Protection Devices

In this part of the program, the focus was on the development of new protection devices that can rapidly limit fault (overload or short-circuit) currents and/or rapidly interrupt the distribution of power in zones containing faults. Two types of protection devices were found to be suitable:

VSCs could be used for limiting and interrupting fault currents. With suitable revisions of prior VSC designs, VSCs could be made to limit and interrupt fault currents within response times as short as a few microseconds.

New solid-state-based protection devices — current limiters and circuit breakers — could be employed to limit and interrupt fault currents. It has been shown that these devices should be of a hybrid type [e.g., incorporating high-speed mechanical circuit breakers in addition to solid-state electronic components] in order to satisfy the protection and operation requirements peculiar to DC SESs.

Protection Scheme

In simplified terms, what is needed is to isolate a damaged or otherwise faulty part of a power-distribution system or a power-consuming system connected to it. It was shown that the agent-based scheme could provide effective protection of a DC SES, in that the protection agents associated with the protection devices could autonomously detect and isolate any disturbance that could occur in the system, and the response times of the agents would be very short — less than 1 ms for disturbances on the DC primary bus, which is the most vulnerable section of a DC SES.

Configuration Management

The focus in this part of the program was on the development of configuration- management schemes in which the power-distribution circuitry of a DC SES would be reconfigured automatically so as to minimize the adverse effects of a fault or a disturbance upon the loads. The major accomplishment of this part of the program was the development of a scheme in which the protection agents would collaborate in performing reconfiguration in stages as needed:

  1. Automatic Load Transfer and Bus Reconfiguration

    The protection agents would be utilized to localize the fault rapidly and then assure continuity of power to critical loads, even in the presence of battle damage. First, the critical loads would be transferred to an alternate bus (see figure) without any disruption of power. Next, sectionalizing switches on the main DC bus would be operated to reconfigure that bus to limit the duration of interruption of power to the non-critical loads to an interval of about 10 to 20 ms.

  2. Load Management

    For extreme contingencies in which the demand for power exceeds the supply, a central agent at the power generator would curtail the loads when the generator becomes overloaded. The use of distributed generation as a means of enhancing load management was also investigated.

This work was done by Mesut Baran of North Carolina State University for the Office of Naval Research.

This Brief includes a Technical Support Package (TSP).
Protection Schemes for Advanced Shipboard Electrical Systems

(reference ONR-0005) is currently available for download from the TSP library.

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This article first appeared in the February, 2008 issue of Defense Tech Briefs Magazine.

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