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 sensor1, 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 kilo amp region. This paper discusses the characteristics of optical current sensors, specifically for relaying applications where measurement of fault-level currents is required.
When faults on a power system occur, they must be isolated quickly to maintain the safe operation of the system, minimize damage to equipment, and maintain stability of the system. Therefore, the accurate measurement of fault current is a critical input to protection relays which monitor the current and/or voltage signals to determine whether the monitored portion is faulted and should be isolated, or whether conditions are normal and should remain closed to maintain the flow of power. If protection relays receive the “true” representation of current flowing on a transmission line, or into transformers, capacitor banks, or reactor banks, they will make decisions based on the current that is actually flowing, not based on a distorted representation of the current which the relay may need to compensate for. An undistorted view could improve the ability of the relay to trip when it should and to prevent false trips. Additionally, analyzing the power system as a whole, optical current sensors make design and analysis easy since no CT saturation will ever be encountered. Optical sensors behave in a simple and predictable manner known for every situation.
- immunity to electromagnetic interference (EMI)
- high electrical insulation
- large bandwidth
- potentially high sensitivity
- ease in signal light transmission
- being compact and lightweight
- potentially low-cost
- no danger of explosion
- ease of integration into digital control systems
- no saturation
- passive measurement
- the electronic circuit present may cause distrotions.
- the measurement is not much accurate
Optical current sensors provide a reliable method of measuring very high fault currents with significant DC offsets without any type of saturation, as is understood with conventional current transformers. Depending on the design of the sensor, several turns of fiber can be wound around the conductor to increase the signal to noise ratio of the sensor. This gain in signal to noise ratio is traded with the ability of the sensor to measure extremely high fault currents without fringe management algorithms. However, if desired, advanced processing techniques such as fringe management techniques can be implemented in sensors, and high signal to noise ratios and high fault current measurements can be achieved simultaneously.