Optical Instrument Transformers

Author: Farnoosh Rahmatian, NuGrid Power Corp., Canada

Measuring voltages and currents using light!  To some of us, with traditional electrical measurement technology background, it sounds like magic.  It definitely sounded intriguing to me as an undergraduate electrical engineering student about 30 years ago.  I had an implicit trust that there is science behind it, not just magic.  Once I read a bit more and learned about the Faraday Effect (magnetic field affecting the polarization of light) and the Pockels Effect (electric field affecting the speed of light in certain materials), it all seemed reasonable.  After all, these folks lived over a hundred years ago, (Michael Faraday, 1791 - 1867, and Friedrich C. A. Pockels, 1865 - 1913) and were well respected scientists, so it must be science.  It wasn’t until I worked on it (together with some very intelligent people) that I realized that making a good optical voltage or current sensor is indeed based on science, but it is also an art.

System Description
Optical instrument transformer systems typically consist of three major parts: the optical sensors or transformers (primary sensors), the sensor electronics (signal processing unit or the secondary converter), and the cabling system between the two (typically optical fibers).  Figure 1 shows a schematic of a typical optical sensor system. Figure 2 shows a picture of the optical transformers of an actual 550 kV class combined optical voltage and current sensor system.
The primary sensors, or optical transformers, as labeled by IEEE Std. 1601, are usually passive, consisting of glass and insulating material.  The electronics module, typically installed in the substation control house, or in a substation yard cabinet, contains the light source, photo-detectors, and digital processing unit.  Light is sent via the cabling system (usually optical fibers), from the electronic chassis to the sensor head (optical transformer), it is affected by current or voltage, and is returned via the cabling system back to the electronic chassis.  The return light is detected, analyzed, and deciphered to extract voltage or current present at the location of the sensor heads.

Optical Current Sensing through Faraday Effect
Most optical current transformers are based on the Faraday Effect, also known as the magneto-optic effect, discovered by British scientist Michael Faraday in 1845:
For a plane-polarized light wave propagating through the optical medium in a direction parallel to the applied magnetic field, the polarization plane of the light rotates.
The angle of that rotation is expressed by:  = VHdl where:
V = Verdet Constant (V ~5e-06 rad/A for fused silica)
H = magnetic field strength
l   = length (distance that light must pass through medium)

In summary, magneto-optic effect helps measure an integral of magnetic field over certain distance where the sensing fiber is aligned with the magnetic field.
Separately, Ampere’s law says integration of magnetic field around any closed path is equal to the current flowing through that path:  I = Hdl .

Combination of the two relationships here, using a magneto-optic medium (e.g., fused silica glass) to integrated magnetic field over a closed loop, will yield polarization rotation directly proportional to the current; i.e.,   = VNI
I    = current in the conductor
N = # turns (of sensing fiber coil or optical path in machined bulk glass)
Multiple turns of light around the conductor can help adjust sensitivity, easily achieved in fiber optic sensor heads.
Early optical current sensors were manufactured as machined glass blocks (bulk glass sensors) with the current carrying conductor passing through the middle.  Optical fibers are coupled to the glass block with a beam of light internally reflected to pass around the conductor exiting through a polarizer and returning to a photo detector.   The change in the amount of light received at the photodetector is proportional to the current in the conductor.   These earlier polarimetric sensors worked fine, but sensitivity was somewhat limited as only a turn or two of light could pass around the conductor.  Newer approaches offered more flexibility in use and better rejection of external effects such as vibration.

The new designs evolved with optical fibers replacing glass blocks.   Interferometric sensing, where two signals of opposite polarities are used, replaced polarimetric sensing.   The magnetic field influences the optical signals in opposite directions with the current being proportional to the difference between the signals.  Fiber optic sensing medium allowed multiple turns (sometimes hundreds) improving sensitivity and reducing noise while the interferometric technique allowed excellent vibration and temperature performance.  
High performance, linear interferometric fiber OCTs are very flexible and easily adapted to different applications with adjustments to performance and form factor as required.     The sensing head can be adjusted to any size with any number of turns.  The same design from a signal processing standpoint can be physically adapted to use for mounting on transformers, circuit breakers bushings, gas insulated apparatus or as a free-standing instrument transformer.  

BeijingSifang June 2016