RT&L FOCUS AREA(S): General Warfighting Requirements
TECHNOLOGY AREA(S): Electronics; Human Systems; Information Systems
OBJECTIVE: Develop a solution, consisting of hardware and software, to detect and locate ground faults in a high-resistance, wye grounded, pulsed power system in real time.
DESCRIPTION: An existing system will benefit from increased ability to detect and locate ground faults. Although, solutions will ultimately integrate with equipment already available in said system’s current health monitoring infrastructure, it is understood that new software and (in all likelihood) additional hardware will be needed to achieve the objective.
Ground faults occur due to insulation breakdown. A high-resistance, wye grounded, electrical power system is tolerant of one ground fault on any phase, but not multiple ground faults in different locations on the same phase. Since the existing system does not indicate when a first line-to-ground fault occurs, cables are regularly inspected using an insulation resistance tester. Although this process enables manual detection of ground faults, determining location for corrective action is more difficult. A less arduous, real time solution will assure that all ground faults are being detected and reported within milliseconds of occurring, which will increase overall safety of the system. The goal is to detect and locate the first ground fault virtually immediately, and correct it as soon as possible, so that there is never an instance in which two ground faults occur in different locations on the same phase.
Insulation breakdown in a particular location may result in a single line-to-ground fault. This line-to-ground fault causes very low fault currents, on the order of .01% of load current, and must be detected, located, and isolated before another ground fault occurs on the same phase. Shipboard ground faults can be located anywhere in runs of several hundred feet of hard-to-reach cable. Fault currents are in the milliamp (mA) range in a system that nominally carries several kiloamps (kAs). Thus, solutions must reliably and accurately detect and locate ground faults that generate signals orders of magnitude smaller than operational currents, which may be alternating or direct (AC/DC) depending on cable section. Operating voltage levels are also in the kV range.
Insulation breakdown in a second location may result in undesired large current flow between the two fault locations, resulting in catastrophic damage to the power system, its equipment, and possibly other high-power equipment.
The solution must be capable of detecting ground faults of 10,000 Ohms or less. False negatives should not occur below the 10,000-Ohm threshold, and false positives should be minimized as searching for non-existent cable faults would prove burdensome and decrease confidence in the detection system. A false positive rate of 1% or below is considered appropriate at this time, but an official requirement has not yet been established. Measuring the exact resistance value (in Ohms) of the fault is not as vital as simply identifying that a ground fault is present, so accuracy, resolution, and sensitivity of the measurement are not defined at this time. Location should be determined with reasonable accuracy and resolution (e.g., ±10s of feet) to decrease mean time to repair (MTTR). Solutions that significantly narrow down location of faults are preferred, since they will decrease the time required to find and fix the damaged cable.
In summary, an innovative approach is needed to indicate the presence and location of an active ground fault in real time, so that it can be remedied before a second ground fault occurs. Additional capability may include prognostics that detect/predict the formation of ground faults before they occur.
PHASE I: Develop a concept for detecting and locating ground faults with minimal impact to existing power architecture. Validate the concept and demonstrate feasibility utilizing modeling and simulation and other software/hardware tools. The Phase I effort will include prototype plans to be developed under Phase II.
PHASE II: Develop a prototype to validate/verify the technological approach. Demonstrate that a line-to-ground fault on a single phase, or formation of said fault, can be detected and located by the prototype system. The goal is to detect, locate, and correct the first fault before a second occurs in a different location on the same phase; therefore, a safe method for insertion of faults at known locations may be required for testing.
Determine if the solution will be effective at the voltage and current levels required. Fault detection and location results will be verified against requirements to confirm that the technology can reliably sense faults and estimate their position(s). Include preliminary calculation of false positive and false negative rates using the prototype system. Accuracy, resolution, sensitivity and other metrics will be assessed as deemed necessary.
Consider human factors, including how to best illustrate the presence and location of faults on a display so that maintainers understand where to go to resolve the issue. The graphical user interface should be easy to read, interact with, and understand. Validate that faults are indicated in an acceptable manner that may be integrated into existing systems.
PHASE III DUAL USE APPLICATIONS: Integrate solution at NAWCAD Lakehurst test site using a representative model that meets actual power requirements. Conduct extensive testing that includes all viable fault modes and locations, i.e., test ground faults in pertinent cable sections as detailed by SMEs (Subject Matter Experts). After detecting faults, use a secondary method (e.g., insulation resistance tester) to determine actual fault location and calculate percent error/accuracy of location measurement. If a fault is not present near the location specified within a certain distance threshold, it must be recorded as a false positive. Additionally, regular insulation resistance testing must continue to determine if any ground faults are going undetected. If so, these must be recorded as false negatives. Integrated Product Team (IPT) will determine accuracy and resolution requirements necessary for transition.
This SBIR topic may benefit private sector companies working with high-power electricity in the energy, industrial and transportation sectors. This may include power generation, transmission and distribution, including both AC and DC (e.g., photovoltaic) applications, large manufacturing/industrial plant operations, and high-power railroad applications. Any commercial application that utilizes high power and experiences relatively low-fault currents, in comparison to operational currents, may benefit.
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