Advanced sensors will play a vital role in monitoring the condition of present and future military power generation, distribution and protection systems. Presently available sensors are capable of basic measurements of current, voltage, and temperature within distribution switchgear and power generation control modules - as well as reporting on fault conditions for those power systems. They also tend to be large, heavy, and limited in capacity of measuring short duration or significantly high amplitude transients.

The currently utilized sensors are also susceptible to the effects of stray fields, electromagnetic interference, and most are electrically conductive. New fiber optic based advanced sensors are made of all-dielectric non-conductive materials, and have increased capability for detecting high speed / high magnitude transients, while weighing considerably less. In order to determine if these sensors are applicable to the U.S. Navy's integrated power system (IPS) platforms, ACI will develop testing scenarios to determine the full capacity and operation ranges of procured devices. In direct support of Navy shipbuilding and leveraging the EMPF National Center of Excellence's Navy ManTech thrusts, ACI will work with NAVSEA PMS500 in realizing the potential of these sensors.

ACI will evaluate the following sensors: a flexible optical current sensor, a fiber optic temperature/pressure sensor, and a partial discharge detector system. Presently, ACI is working with our partners at the Naval Surface Warfare Center, Carderock Division - Philadelphia, PA (NSWCCD-Phila) Land Based Test Site (LBTS) to monitor medium voltage switchgear. A multiple time frame recording system provided by NxtPhase for monitoring electrical power systems is being used to record events and anomalies within the switchgear. All of the sensors' control electronics assemblies and data recorder are installed in a NEMA 4 Advanced Sensor Cabinet enclosure adjacent to the switchgear. For current measurement, ACI is utilizing a single-phase flexible optical metering system also by NxtPhase which consists of an optical sensor unit and an optical electronics chassis unit. The optical sensor is a flexible, hermetically sealed, all-dielectric current sensing optical fiber cable loop of a variable length. The optical electronics chassis unit consists of sensor, status monitoring, alarm, low energy analog (LEA) output, and power supply modules. Also included with the chassis unit are 50 meters of fiber and electrical interconnecting cable assemblies. (See Figure 1-1).

The single-phase current sensing loop is measuring the load-side current of a switchgear compartment's breaker. The current is sensed on the measured conductors using interferometric optical technology. A light source sends light through a waveguide creating two linearly polarized light waves. Via an optical fiber, the light passes through a quarter wave plate creating right and left hand circularly polarized light from the two linearly polarized light waves. The two light waves traverse the fiber sensing loop around the conductor, reflect off a mirror at the end of the fiber loop, and return along the same path. While encircling the conductor, the magnetic field induced by the current flowing in the conductor creates a differential optical phase shift between the two light waves due to the Faraday Effect. The two optical waves then travel back through the optical circuit and are finally routed to the optical detector where the electronics de-modulate the light waves to determine the phase shift. The magnitude of the phase shift between the two light waves is proportional to the current, and an analog or digital signal representing the current is sent to the data recorder for real time or trending current measurement.

For partial discharge (PD) sensing, ACI is using a new RF-based predictive maintenance system by Eaton/Cutler-Hammer. Partial discharges generally occur when dielectric properties of an aged insulation system under high potential are not sufficient to withhold applied electrical fields. Analysis can detect the following forms of partial discharges:

• PD inside solid insulation systems (due to delamination - or voids in insulation)
• PD between a high-voltage conductor and insulation (if a void had developed between a high-voltage conductor and insulation, or due to bad impregnation)
• PD between the conductor insulation material and ground ("slot discharge" between winding and laminations)
• Surface Tracking (due to surface contamination of an isolator)
• Arcing and sparki

Therefore, a high level of measured PD can be an indication of insulation fatigue or arcing / sparking due to worsening electrical connections. It can also indicate surface tracking discharge due to particulate contamination of insulation. The basic PD sensing system is made up of integrated partial discharge 3-Phase coupling capacitor sensors (IPDS), and a radio frequency current transformer (RFCT) sensor. Integral to the system are additional sensors to monitor compartment temperature and humidity. One of the benefits of the coupling capacitors is that they can detect PDs in adjacent compartments along with PDs within the buss work of their own compartment. Each IPDS is bolted directly between each phase bus and ground and can be mounted to either - effective ground isolation meeting the ratings of the medium voltage switchgear allows this. The RFCT identifies PDs related to the feeder cabling and is placed around the common cable ground shield. All of the sensors are connected to the PD monitor mounted in the Advanced Sensor Cabinet, which displays readings and records data. This monitor is then interfaced with the data recording system enabling real time or trending data accumulation.

Over time within electrical connections, vibration and heat can loosen lug connections, and oxidation can build up between dissimilar metals. Both of these effects can cause an increase in the resistance at the connection, which in turn can lead to higher than designed current flow through those connections - effectively the development of "hot spots". With increased current and resistance comes increased temperature, and if these connection temperatures get too hot, reduced efficiency can be expected, or failures can occur as a worst case scenario. However, if the connection temperatures can be monitored for trending analysis, impending failures may be averted.

To aid this, ACI is utilizing an optical temperature sensor system by Prime Photonics to measure these electrical connections. A temperature sensor is bolted directly to the bus work and an optical fiber safely carries the signal out of the compartment and to the Advanced Sensor Cabinet. The interrogator generates a light wave, which is sent through the optical cable to the sensor. When the temperature is applied to the sensor, fundamental parameters of the light, such as intensity, or wavelength, are changed. The modified light is reflected back through the cable to the interrogator, where it is carefully measured to determine the amount of change in the light wave. Algorithms are then used to convert the optical signal to a calibrated electronics signal which is connected to the data recorder. When more points of measurement are realized in the future, a fourth component, a fiber optic switch will be utilized to sweep up to 32 points of measurement. Two fiber optic switches can be utilized per interrogator, and multiple interrogators can be linked together to make for an extensive fiber optic condition monitoring system. Additionally, due to extremely low optical losses during light transmission through the fiber, this photonic sensor is ideally suited for applications where the interrogator box must be separated from the sensor by large distances (up to several miles).

By completing rigorous adverse condition testing of these emerging technology sensors, and leveraging the efforts of the EMPF, ACI is providing advanced condition monitoring solutions to the U.S. Navy. Optical fiber sensors and advanced technology sensors have the tremendous benefits of light weight and small size. They are highly reliable with long operational lifetimes, and are uniquely suited for high temperature and high pressure applications, are corrosion resistant and self-compensating. All of these sensors have high sensitivity resolution and high frequency response - therefore, more accurate measurements. Additionally, the fiber optics are immune to electromagnetic interference (EMI) and radio frequency interference (RFI), and are intrinsically safe.