In the near future, the U.S. Navy plans to construct the next generation of aircraft carriers and destroyers – the CVN-21 and DD(X) classes, respectively. Construction of the CVN-21 class is slated to begin in 2007. This class of aircraft carriers will use the basic hull design of the USS Nimitz-class carriers, but the similarities end there. The DD(X) class remains in the design phase and will be unlike any other surface combatant vessel at sea today, utilizing new revolutionary composite materials and integrated power systems. These vessels will have the newest, state-of-the-art power and electronics systems, incorporating the latest features – electromagnetic catapults, permanent magnet motors, advanced induction motors, new solid-state power electronic transformers, and stealth features that reduce radar and electromagnetic signatures. The Navy believes that with the advent of modern electrical equipment and systems, along with extensive automation technologies, they will be able to significantly reduce the total construction costs and manpower requirements to run and maintain these vessels.1,2

The new technologies will undergo rigorous military specification testing and qualification to ensure that they can withstand harsh environments. In order to determine appropriate test standards, long-term performance criteria must first be established. The tests must ascertain the length of time these systems will function at their original design levels, and system time-to-failure must be confirmed.

To better manage new technology testing, the Department of Defense (DoD) has implemented Condition Based Maintenance Plus (CBM+). The objectives of CBM+ are three-fold. The first goal is to decrease maintenance and logistics support costs. The second aim is to increase maintenance efficiency and productivity. The final objective is to integrate the information environment that supports maintenance and logistics operations.

Many current maintenance systems rely on time or operational intervals (or availabilities) for servicing. Time-based maintenance requires that machines be examined and repaired offline, either according to a time schedule or running hours. Such maintenance schemes fail to address specific equipment conditions that could enable maintenance based on evidence of need.3 Unexpected equipment failures occur between time-based maintenance intervals. Manpower, time, and money are wasted because periodic maintenance is performed with little awareness of the current equipment condition.

To eliminate the problems associated with time-based maintenance, equipment condition monitoring (CM) can be applied at the system level. CM can detect changes and trends in the equipment characteristics and will therefore predict the need for maintenance before serious deterioration or breakdown occurs. CM can also estimate the length of time the system will continue to function as intended. This CM technique applied to CBM+ will keep operators informed of the status of the ship’s many systems. Maintenance needs will be clearly indicated, reducing manpower waste and averting unplanned downtimes.4

Several capabilities are necessary for the CM system to function properly. The system must be able to monitor equipment that is in use. The CM system cannot be adversely affected by electrical interference, and the system must be immune to single-point failures. Furthermore, if the CM system fails, the monitored equipment should continue to function as designed.

The CM system should contain the following four parts:
1) sensors; 2) data acquisition equipment; 3) fault detection; and 4) diagnosis. The sensors convert a physical quantity (i.e., voltage, current, and partial discharge) into an electrical signal. Detectable changes in these physical quantities can reveal incipient faults long before catastrophic failures occur.

Selection of sensors will rely on the monitoring methods required and the failure mechanisms of the equipment. These sensors should be suitable for online measurements while meeting the sensitivity requirements of the application. In addition, the sensors must be competitively priced and meet the application criteria. Finally, they should be as non-invasive to the system as is practical.

Data acquisition equipment must be able to segregate, amplify, and pre-process the output signals from the sensors. For example, the equipment will be able to convert analog signals into digital signals and correct sensor imbalances. A microprocessor computer and data communication software may be needed to implement the system.

The primary purpose of fault detection is to discover incipient faults within the system equipment. If faults are revealed, an alarm will signal the ship’s watch stander who can then determine whether the fault impedes the mission. Further analysis of the fault will provide an informed equipment maintenance decision. Diagnosis of these abnormal signals by the CM system will be a powerful tool for the crew and provide a clear indication and direction of any necessary maintenance.4

Several failure mechanisms for transformers aboard naval ships can be prevented by using condition monitoring systems. For example, windings and their insulation are one of the biggest problems that affect the life of a transformer. Two wear-indicators are temperature and partial discharge. Hot-spots affect the transformer load capability and adversely affect transformer life via thermal aging of the insulation. These hot-spot temperatures caused by overloading or local overheating can be monitored with fiber optic temperature sensors in the CM system. Also, transformer dielectric failures involve partial discharges (PD) in the initial stages of the failure process; hence, early detection of PD sources in the insulating system is important to CBM+. The use of piezo-electric acoustic emission PD sensors has been a favored approach in current transformers.4 Voltage and current fiber optic sensor systems can analyze transformer input and output trends to better predict transformer failure. Using these sensors to feed information to the CM system will enhance the capabilities of CBM+.

Power generators and induction motors in naval vessels can also benefit from CM systems. Power generators and induction motors may have faults in their stators or rotors. Electrical winding insulation breakdown is a leading cause of failure. PD detection sensors can play a vital role in the CBM+ system. Also, eddy current losses in the rotor due to large negative-sequence current transients can lead to overheating and the initiation of serious fatigue cracking. Shorted and/or arcing end-turns due to carbon dust tracking may lead to winding failure. Fiber optic current sensors and air-gap magnetic flux monitoring can pinpoint the number and location of impending shorted turns and insulation breakdown that may lead to failure. Induction motor stator current signal analysis is another tool to discover stator winding fault conditions which may cause non-uniformity in the magnetic field in the air-gap. This non-uniformity results in harmonics in the stator current, which are indicative of fault conditions. Also, utilizing sensor technologies to monitor fluctuations in motor speed and frequency shifts in the supply current and axial fields could be CBM+ indicators of cracked or broken rotor bars.4

Conclusion
The EMPF at the American Competitiveness Institute can research and develop sensor equipment for transformer condition monitoring. EMPF staff can also identify correct sensor types and determine ideal sensor positioning on or within the equipment. Because transformers may feed vital loads that are critical to mission readiness and success, detecting and predicting transformer condition and life-expectancy will greatly benefit military personnel.

References
1. http://www.en.wikipedia.org/wiki/CVN-21
2. http://peos.crane.navy.mil/ddx/default.htm
3. http://www.acq.osd.mil/log/mppr/cbm+/POA&M%20v6_CBM+.doc
4. Y. Han, Y.H. Song, “Condition Monitoring Techniques for Electrical Equipment – A Literature Survey,” IEEE Transactions on Power Delivery, Vol. 18, No. 1, January 2003.
5. http://www.globalsecurity.org/military/systems/ship/cvx-gallery.htm