Optical current sensors eliminate CT saturation

Optical current sensors are achieving increased acceptance and use in high voltage substations due to their superior accuracy, bandwidth, dynamic range and inherent isolation. Once deemed specialized devices intended for novel applications, optical sensors have risen to a performance level exceeding conventional magnetic devices. A specific area where optical current sensors outperform conventional iron core transformers is the measurement of very high currents that occur during a fault on the power system. Conventional instrument transformers utilize an iron core and windings ratio to step down the current measured in the primary to a more manageable current level for secondary devices such as meters and relays. This signal may be distorted due to saturation of the magnetic core. In a pure optical current sensor, no such mechanism for saturation exists. However, optical sensors must be used and applied properly to provide distortion free signal replication well into the hundreds of kiloamp region. This paper discusses the characteristics of optical current sensors, specifically for relaying applications where measurement of fault-level currents is required.