Let us start from the very beginning. We must begin by establishing a clear distinction between current sensors and voltage sensors which are much more complex to manufacture.
For current sensors, we are looking for a physical optical effect capable of translating the flow of a current in a conductor. A flowing current induces a magnetic field, which the Faraday effect (or magneto-optic effect) describes as the interaction of a magnetic field in a transparent optical medium. The magnetic field modifies the electrons’ path within the medium and causes a polarisation change of the crossing light beam. The theory illustrates the way the light beam acquires a simple rotation of the linear polarization input state.
Michael Faraday's laboratory in London
This effect, discovered by M. Faraday in 1845, was the first real evidence of interactions between magnetism and light. We must also note that Faraday discovered the “induction” phenomena in 1831, and it took at least 30 years to get to the first “electromagnetism” theory by J.C. Maxwell.
An additional 20 years were required for H. Hertz to produce the electromagnetic waves theory, finally demonstrating that light can be influenced by an electromagnetic wave, which in turn can interact with the electron path in atoms. The Faraday Effect was discovered in 1845 but was modelized mathematically for the first time 50 years later by the classical “model of electron-linked elastically” in the theoretical atom model, given by Thomson in 1897.
In addition, according to electromagnetic theory, a current flowing in a conductor produces a magnetic field. If we can make a summation of this field all around the conductor with one or several closed loops, then we can obtain a value proportionate to the current. This well-known law, the “Ampere theorem”, must be adhered to for all current sensor specifications, including both conventional and optical sensor technology.
Using Ampere’s law provides a current measurement independent of:
- other nearby, non-encircled conductors with circulating currents
- the position of the conductor in the integrating closed loop
- the variations of the loop’s geometry, vibrations, and thermal expansion
There are two main categories of technical optical solutions capable of obtaining a closed loop around a conductor. We will focus here on the one called “Ring Glass” technology using a plate of glass as a solid element drilled with a hole for the conductor, with machined and polished edges made to reflect the light internally and create a closed loop around the measured conductor. This glass piece is known as “The Ring-Glass”, or RG.
The ring glass technology is applied to the CMO, CTO and VTO product range dedicated for intelligent AC substation.
Ring-glass technology applied to CMO, VTO and CTO LPITs
The choice of the ring glass solution for AC digital substations was made for various reasons, including:
- ease of manufacturing and industrialization (construction, mounting): today the ring glass can be made entirely through automatic machining centers, enabling ample production quantity and low prices
- use of multimode components, simplifying and reducing the costs of commissioning (simple ST connectors, multimode cables with large core fibers, etc...)
- ease of electronics’ make and signal processing
- ultimately, lower costs, quick delivery time, and reduced outage for commissioning.
The ring glass solution is the result of long-term studies and several small innovations, to achieve optimal performance specifications, including:
- quality-control of the ring glass piece
- development of test benches for component alignment
- the fibre holders (called a “pigtail”) for attaching the fibre to the RG
- effective packaging without constraints on the glass, the filters, etc…
By using this sensor principle, the development of a stand-alone device capable of measuring currents in high voltage networks from 72.5 kV to 800 kV is possible.
One single CTO phase unit includes a head with one or several optical sensors, all linked to the processing electronics by optical fibers. A stand-alone HV insulator isolates the sensors from the ground.
Schematics of the CTO
With respect to voltage sensors, the optimally sought outcome is a physical optical effect capable of translating the electric field, afforded by the difference of potential between the HV line and the ground.
Moreover, the “Pockels Effect” is an electro-optical effect that translates the influence of the electric field within a transparent crystal. In a solid crystal, transparent to light, electron clouds become small orientated dipoles along the electric field lines. Density variations within the medium leads to a “linear birefringence”, which modifies the polarization state of a monochromatic light beam.
A range of sensitivity levels can be obtained through use of varying crystal types, thanks to their crystalline orientations with respect to light polarization and the direction of electric fields. Several years of research and adaptations to numerous technology innovations have led to the present specification. As of today, Grid Solutions is one of the only optical VT providers in the industry. In addition to the “Pockels Effect” electric field sensitivity, the signal between the two potentials must be integrated to obtain a true voltage measurement. It is something equivalent to the Ampere law, as well as the Maxwell equations.
Schematics of the VTO
This application of electric law requires us to maintain a measurement system connected between the high voltage part of a device to the ground. It’s important to note the fact that the optical technology at hand cannot attain the requisite length of crystals that are required by the line voltage (about 1-meter air distance for every 100 kV). Consequently, a voltage divider must be used. The choice of using this “smart” capacitive divider has been an important innovation within the VTO device. The smart-divider is stable with respect to temperature, with long-term reliability and safety.
For CMO, one new advantage is the ability to provide combined measurement (current and voltage) very easily, without complex electromechanical parts.
Schematics of the CMO
The CMO is a combination of the CTO and VTO devices. This requires managing the fibres’ path in the insulator beside the capacitive divider and providing a reliable insulation system to support all dielectric requirements, as defined in the IEC 61869 Standard. Here again, several innovations have been proposed to avoid the use of dielectric oil or gas in the device. This industry challenge led Grid Solutions to master dielectric gel injection, which is a major component for building and helping to ensure the reliability of the CMO in the long-term.