One of the operational requirements for new ships built for the US Navy is automatic control and monitoring of the ships' electrical systems. Such systems include power converters, generators, and load centers. At present, current and voltage sensors are heavy and require much space. Therefore, an evaluation of current sensor technologies is needed.

In recent years, optical current sensors have reached technological maturity and are now competing with more conventional current sensors. Optical current sensors are easily installed and integrated into existing systems. These sensors are appealing for measuring electrical current, electric fields, and magnetic fields. Optical current sensors provide galvanic isolation of the sensor head from surrounding electronics, in addition to overcoming several of the challenges posed by their conventional counterparts.

Electrowinning
Electrowinning is the process of using electrolysis to recover metals from solutions. The production of chlorine and metals like zinc, magnesium, copper, and aluminum requires DC currents as high as 500kA. These high currents are measured frequently in order to maintain process control and to protect equipment. The accuracy of these measurements should be within ±0.1%.

Hall effect current sensors are traditionally used to measure high DC currents, such as those associated with electrowinning. However, several disadvantages are posed by Hall effect current sensors. Often, the process of installing Hall effect current sensors is intricate and time consuming. The magnetic field distribution output from Hall effect current sensors must be analyzed to minimize cross talk and other errors. Conventional Hall effect systems can also weigh up to 2,000 kg and consume up to 10kW of power.

Interferometric current sensors (Figure 5-1) can replace conventional Hall effect current sensors in the electrowinning process. Interferometric current sensors can measure up to 500kA of current with an accuracy of +0.1%. Furthermore, electronic crosstalk does not degrade sensor performance. Large bandwidths enable detection of current ripples and those transients can be recorded. The sensor output is digital, which may inspire new data acquisition and processing capabilities. Interferometric current sensors are lighter and less complex than their Hall effect current sensor counterparts. Installation is straightforward and power consumption is negligible.

Power generation and distribution
Current sensors are used for power generation and power distribution systems. Within power generation systems, currents up to 150kA must be measured with an accuracy of a few amps. Current is measured at several locations within power distribution systems in order to protect expensive equipment. Spikes of current must be detected and eliminated within milliseconds to protect system circuitry. Current sensors also help measure energy flow for billing purposes.

Often, Current Transformers (CT) are used to measure current within power generation and distribution systems. Nevertheless, CT sensors require thousands of copper windings, making them large and costly. Furthermore, CT must be placed outside the power generators.

Polarimetric current sensors (Figure 5-2), on the other hand, can be installed within the generator and thus reduce the overall size of the power generation and distribution system. This size reduction can lead to decreased system cost as well. Furthermore, polarimetric sensors weigh only a few ounces and can measure up to 300kA of currents.

Fault detection
A common type of electrical failure occurs when electrical conductors come into contact with a ground potential. This type of fault, also called a short circuit, allows large electrical currents to flow from the energy source through all available ground paths and then back to the energy source. Power system faults must be quickly isolated in order to maintain the functionality and stability of the system.

Current transformers are often employed to protect the energy source. However, during high fault conditions, the resulting magnetic fields frequently cause the iron core within transformers to saturate. This saturation distorts subsequent current measurements. All current transformers will saturate unless built with excessive steel for prevention. However, incorporating exorbitant amounts of steel into each transformer is impractical.

Saturation can be avoided by using interferometric current sensors since optical sensors contain no magnetic components and no iron core. As a result, interferometers significantly reduce the system's complexity since saturation is not a factor. Interferometric sensors can detect faults quickly since their response time is less than five microseconds. Also, current faults reaching 500kA can be detected.

Advantages and disadvantages
The electro-optic Kerr effect (EOKE) can degrade the performance of both interferometric and polarimetric current sensors. The EOKE is a change in a material's refractive index in response to an electric field. All materials display a Kerr effect to an extent, but some materials display the effect more strongly than others. Current sensors operating in high electric fields will have distorted responses or altered characteristics because of the EOKE. To minimize the effect, several precautions must be implemented. The sensor should be strategically placed and the polarizer correctly aligned. Also, optical fiber current sensors on high voltage lines need screened, low electric field locations to minimize the effect.

In high electric field environments, interferometric current sensors are more stable and less prone to wave form distortion by the EOKE than polarimetric sensors. Nevertheless, polarimetric sensors have smaller temperature dependence than interferometric sensors.

When compared to polarimetric sensors, interferometric sensors are larger and have slower sampling rates. Nevertheless, interferometric sensors measure higher currents and operate at a wider temperature range.