The U.S. Navy has put into motion a series of events which will revolutionize the U.S. Naval Fleet, namely, the introduction of the “all-electric” ship. For this vision to become reality, the electric ship will have all-electric drive and integrated power distribution and conversion systems. An estimate from shipboard designers at Naval Surface Warfare Center Carderock Division, Philadelphia, indicates that these ships could require at least 10,000 current sensors per ship to properly monitor and regulate electrical power flow. This simply is not achievable with conventional wire-wound toroidal or Hall-effect transducers due to weight, size, and safety limitations.

Fiber optic magneto-optic field sensors offer the enabling technology to meet the challenges posed by the high power. These sensor devices are positioned for use in switchgear, power distribution, electronics, and medium voltage applications (< 37 kV) where the conventional current and voltage transformers may be undesirable due to space and weight concerns. These fiber optic sensors use the magnetic and electrical fields surrounding conductors to change the properties of light traveling within specialized optical crystals, producing a very sensitive measurement in a very small physical device. Other advantages include: the intrinsic safety associated with fiber optics; no shock hazards; no ignition capability; rejection of ambient electromagnetic interference; and the ability to use a common device for a wide range of measurement ranges and system-voltage applications.

The small form factor and the passive operation of these transducers allows greater flexibility to the ship’s designers. First, the lightweight nature of the transducer package represents a 98% reduction in weight and footprint when compared to conventional instrumentation transducers. Next, the intrinsic isolation of the non-conductive optical fiber allows the same transducer to be used on 15 kV systems as well as 460 V systems. Finally, because these are field-monitoring transducers, they do not burden
the generating system, resulting in savings by eliminating the production of heat within the monitoring system.

Basic theory of operation of the optical current sensor
The method of using optical crystals to detect a change in a magnetic field is not new. More than 150 years ago, Sir Michael Faraday discovered that when linearly polarized light travels through flint glass, which is exposed to a magnetic field, its plane of polarization rotates. This property, now known as the Faraday Effect, is widely used in the optical telecommunications field to prevent reflected light energy from coupling back into a light source and changing source parameters. If the Faraday material is placed in the path of a magnetic field, such as one generated by current flow, the plane of polarization rotates in a known and predictable fashion. By monitoring the rotation of the incident polarization state, a direct measurement of the magnetic field intensity can be made and magnitude of the current can be inferred.

Currently, there are other types of optical current sensors on the market which use a radically different technology. These existing sensors use either bulk optical crystals or long lengths of optical fiber, coiled around a current-carrying conductor. When an optical path completely encircles a conductor, a numerical integration can be performed about the optical path that directly relates the Faraday rotation to the current flowing through the optical path. The measurement sensitivity is dependent on the number of optical fiber turns around the conductor being monitored.

The use of optical fiber as a sensor is impractical in many shipboard applications where size and installation procedure are of significant importance. It is not feasible to interrupt power by disconnecting the conductor, installing the fiber coil assembly, and then reconnecting the conductor. Similarly the all-fiber sensor with a 1000+ loop fiber coil assembly, which encircles the conductor, can not be smaller than 4-5 cm in diameter. This significantly limits application of the technology to larger conductors. Violation of this condition typically results in extremely high temperature sensitivity and impacts the long term reliability of the sensor.

The sensors utilizing bulk glass crystals take advantage of the Faraday effect exhibited by the bulk glasses. An advantage of the use of bulk glass is that the sensor can be fabricated from materials which are more sensitive to the influencing magnetic field than those used in normal optical fiber. These bulk crystals are annealed slowly to release the internal stresses. Reducing the internal stresses improves the optical and mechanical properties of the crystal, which is desirable for manufacturing as well as calibration. By themselves, bulk-glass sensors are stable both in temperature and mechanical handling and can be made relatively inexpensively, which portends well for mass production concepts using these sensors.

Despite these apparent advantages over coiled optical fiber sensors, the bulk-glass sensors suffer from their own set of limitations. The bulk glass optical transducers are relatively large, on the same order as the all-optical fiber sensors previously described. Bulk glasses are not ferromagnetic, which restricts their applications to extremely high current measurements. Additionally, obtaining multiple circular paths around a bulk-glass arrangement in order to increase the sensitivity of the sensor has been accomplished by some researchers, but there are limitations of using this configuration in applications that experience tremendous temperature fluctuations. Finally, assembly and alignment of bulk-glass sensors has historically been performed by hand, resulting in higher labor costs that preclude their widespread use.

As previously stated, the ferromagnetic materials used in the magneto-optical sensors are more sensitive than the materials used in simple fiber optic cable or bulk-optic crystals. The result of using ferromagnetic materials is that a sensor requires much smaller Faraday rotator to measure a given magnetic field strength, and offers versatility in physical size and measurement properties range. Methods to grow these ferromagnetic materials are well established and directly support other markets, specifically optical telecommunications, hence tremendous economies of scale are realized that surpass that of bulk-glass and rival the cost of optical fiber.

Installation of optical sensors (shown in Figure 5-1) differs significantly from both the optical fiber and bulk-glass technologies described above. Specifically, it is not necessary for a transducer to completely encircle the conductor being monitored. Rather than perform an integration of the magnetic field, the sensor samples a point in the magnetic field using a crystal at a location that is predetermined by the sensor design and construction. Furthermore, because the magnetic field is generally uniform in magnitude near the surface of a circular conductor, the sensor’s physical location on the conductor is irrelevant. The use of highly sensitive Faraday materials and the sensor configuration also enables interesting capabilities which are not available with either all-optical fiber or bulk-glass methods of current sensing.

Figure 5-2 shows the physical dimensions of the transducer. As shown, the aluminum sensor body is 25.4 mm long and the diameter is 12.7 mm. Support ferrules extend another 5 mm from each end of the aluminum sensor body. The device can be made smaller and lighter to fit into a number of possible mounting locations.

The innovative magneto-optical sensors described above are being developed for a wide range of shipboard power sensing applications under a Navy ManTech program that teams ACI, Airak, Inc., and NSWCCD-Philadelphia. This program includes prototype manufacture and testing of the current and the voltage sensors for high power applications and taking them from Technology Readiness Level 4 to TRL 7. At the conclusion of the program in January 2006, the ruggedized sensor system, which includes the sensors and an electro-optic driver module, will be ready for initial deployment on a test bed vehicle.