N21A-T005 TITLE: Ground Fault Detection System
RT&L FOCUS AREA(S): Directed energy
TECHNOLOGY AREA(S): Ground / Sea Vehicles
OBJECTIVE: Develop a ground fault detection system to quickly detect and localize all ground faults on 440 VAC 3 phase shipboard distribution systems that are either ungrounded or high resistance grounded.
DESCRIPTION: U.S. Navy shipboard electrical distribution systems typically include 440 VAC 3 phase power distribution systems where the power system neutral is either ungrounded or high resistance grounded. MIL-STD-1399-300-1 provides characteristics of the 440 VAC power. These systems enable continued operation with one of the three phase conductors grounded. However, if a second ground fault on another phase were to occur, then two phases of the power system will be grounded, resulting in a line-to-line fault, circuit breakers tripping and loss of power to loads. Fixing line to ground faults quickly is critical to avoiding a double ground fault and loss of service to loads.
Commercial distribution systems use solidly grounded systems so that when a single ground fault occurs, the circuit breaker trips and power is lost to loads. In a well-designed commercial power system, the circuit breaker that is on the source side of the ground fault and nearest to the ground fault will trip, thus providing a crude approximation of where the ground fault is located. Navy ungrounded and high resistance grounded power systems enable continued operation with a single line-to-ground fault, improving the overall survivability of the electrical plant. However, because a circuit breaker does not trip, localizing the ground fault is much more difficult.
Ungrounded and high resistance grounded power systems are used in commercial medical facilities, in process control industries, and where continued operation with a single line fault is of significant value. These applications have not typically employed ground fault localization methods; they have instead focused on inspection and preventative maintenance to reduce the risk of a ground fault. These systems are not expected to continue operation following battle damage, or can schedule system down time to locate the ground fault manually.
Commercial power transmission systems do employ ground detection methods because in high voltage transmission systems, circuit breakers can be hundreds of miles apart. These transmission systems are generally point-to-point and do not resemble the radial or zonal distribution seen on naval ships. Much of the academic research on ground fault localization has focused on these systems. Naval power systems are considerably different.
While existing ground fault detection circuits onboard naval ships can identify that a ground fault exists and identify which phase is experiencing the ground fault, identifying the specific feeder cable and location on that feeder cable is currently difficult. A typical process for identifying which feeder cable is faulted is to open the circuit breaker for the feeder cable and seeing if the "upstream" ground fault detection circuit no longer identifies a ground fault. This method however, requires a time and labor intensive process of sailors tagging out equipment and securing power to loads. Often operational necessity prevents securing loads and locating the ground fault is deferred to a later less critical time. Furthermore, this method will fail if two different feeders have the same phase grounded. Additionally, the method will usually fail for intermittent ground faults, such as one that depends on the roll-angle of the ship (loose bolt in an electrical connection box for example).
The Navy seeks to develop a system capable of correctly identifying which feeder cable is ground-faulted at least 90% of the time, and identify the location of the ground fault on that faulted cable within 3 meters at least 75% of the time. The shorter lengths of cables on shipboard systems differentiates them from terrestrial power systems for which most ground fault localization method research has concentrated on. The ability to achieve these requirements should be demonstrated first in dynamic simulation, then in a shipboard representative land-based system with a three phase generator and multiple feeders and loads.
Technologies that have been proposed in the past for localizing ground faults in terrestrial systems include common mode current monitoring, common mode current transient feature extraction, and travelling wave identification. None of these technologies have been incorporated into products for naval power systems. Applying these or other technologies to reliably detect and localize ground faults in shipboard systems should enable a significant reduction in time that a ground fault exists and exposes the ship to risk of a double ground fault and loss of power to mission critical equipment. [Refs 1,3,4,5]
PHASE I: Develop a design for an intermittent and persistent ground fault detection and localization system for naval shipboard 440 VAC three phase radial power systems. Identify risks and knowledge gaps and conduct analysis and/or experiments to eliminate the knowledge gaps and mitigate the risks. Prepare a dynamic simulation of a naval shipboard 440 VAC system and the ground fault detection and localization system to validate the concept. The Phase I Option, if exercised, will include the initial design specifications and capabilities description for the ground fault detection and localization system. Develop a test plan and test procedures for the prototype to be developed in Phase II.
PHASE II: Develop a functional prototype of the intermittent and persistent ground fault detection system for a naval shipboard 440 VAC three phase radial power system. Test the prototype and validate the dynamic model created in Phase I. If necessary, the dynamic model shall be updated and validated with new test data. The validated model shall be used to demonstrate performance of the system in a representative naval system. Work with the Navy to develop the draft specifications. After completing the company testing, deliver the prototype to the Navy to be tested at a Government or academic facility. These Navy conducted tests will be in addition to the company conducted tests and potentially much more comprehensive The Navy will provide results of Navy conducted tests to the company.
PHASE III DUAL USE APPLICATIONS: Support the Navy in transitioning the technology to Navy use. Develop and test a production ready intermittent and persistent ground fault detection system in accordance with the draft specification. Update the draft specification and dynamic model based on lessons learned from the production and testing of the system. Deliver the production ready system to the Government for testing at a Government or Academic facility.
Explore non-Navy markets for the ground fault detection system. Ungrounded and high resistance grounded power systems are used in medical facilities, in process control industries, and where continued operation with a single line fault is of significant value.
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KEYWORDS: Ground fault localization; Ground fault detection; Ungrounded power system; High resistance grounded power system; Common mode current; Feeder cable