Researchers have perfected the optical technology in current measurement to measure currents and transmit the data from high voltage system to ground potential using state of the art Laser technology. The fundamental of this technology includes the idea of using fiber optic cables to isolate the current transformers from ground potentials. The advantages of the optically powered scheme compared to the conventional, high voltage, free standing magnetic CT include an environmentally friendly, light weight, non seismic critical composite signal column together with proven, conventional, low voltage rated ‘dry type’ CT technology.
The overall optical technology in current measurement functional blocks are demonstrated in figure. 2. The sensor itself is a low voltage rated CT together with burden resister. Both are certified for the desired protection and metering application. The voltage output of this sensor head is fed in to the Optically Powered Data Link (OPDL), which is the core of this technology. The fibers, which connect the ground and the HV remote OPDL units together, can be separated in to two links. The first is the composite insulator with the incorporated fiber, which is connected at the high voltage side to the remote unit of the OPDL. The second link comprises the ground side of the signal column, which is connected to the fiber cable wired to the control. The receiving unit in the existing control enclosure provides the signal output for either the protection relays or/and the metering and monitoring equipment. Figure. 2 show a picture of the complete OPCT system
The OPDL system can be divided in to a remote unit at high voltage potential and a local unit, which is based in the sub station control room or an existing control enclosure. This unit houses the laser with its associated laser driver and the data recovery circuitry. The laser system used for this application can couple a maximum optical power of 1.5 Watt in to the power link fiber. These lasers are not to be very reliable with a long life time (MTBF: >100,000 h). A self-check Function supervises all vital functions of the OPCT. An alarm will be initiated long before the laser reaches the end of its life time indicating necessary maintenance. A trip signal will be set if the system has identified a misoperation
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