Current transformers (CTs) and voltage transformers (VTs) are used extensively in substations for measuring and monitoring voltage and current. These measurements form an essential part of operations and protection functions in the substation, and thus reliability and accuracy of voltage and current transformers (VCTs) is important. A new generation of digital CTs and VTs, based on optical effects, offers significant advantages over conventional instruments.
Fibre-optic current and voltage sensors offer significant advantages over traditional current and voltage measurement technologies:
Optical VCTs are lightweight and of small size. This makes it possible to operate them not only as freestanding devices but one can easily integrate them into other power products. Substation footprint and installation costs are reduced. Other advantages are enhanced safety (no risk from open secondary CT circuits or catastrophic failure) and environmental friendliness (no oil). Optical current sensors are immediately compatible with modern digital substation communication, which helps to eliminate large amounts of copper cabling .
Optical fiber sensors are of particular interest for applications in the high-voltage environments of the electric power industry due to their characteristic properties including a dielectric nature, immunity to electro-magnetic interference, and small size and weight. The current sensor employs the Faraday effect in a thermally annealed coil of sensing fiber. Voltage sensors are based on the Pockels effect in electro optical crystals or the converse piezoelectric effect in a cylinder-shaped quartz crystal.Optical current transformers
The Faraday effect
The Faraday effect is a magneto-optical effect that causes a change of the state of polarisation of light in the presence of a magnetic field. It describes the rotation of polarisation of light propagating in the direction of a magnetic field. When a beam of light is sent through a material exhibiting the Faraday effect, the polarisation of the light will be rotated by the angle θ which is dependent on the magnetic field strength parallel to the direction of propagation of the light.
The Faraday effect is proportional to the magnetisation of the material, The rotation can then be described in terms of the magnetic field strength β and the Verdet constant V.
Which, in the case of a constant uniform magnetic field reduces to
The Verdet constant V is the specific rotation of a material and is defined as the angle over the magnetic field times the length. V is determined by the magnetic properties of the material.
β is the component of the magnetic flux density parallel to the light propagation direction.
Measurement of phase rotation
All Faraday effect detection principles are fundamentally based on intensity detection. The structure, materials and the path of light of the different Faraday sensors however, differ greatly. There are several methods used for measuring phase rotation. The simplest and most general makes use of a polariser as shown in Fig. 2 . The magnetic field in a Faraday medium can be measured by determining the rotation of polarisation θ, that occurs after a linearly polarised light beam passed the Faraday medium. This can be done by measuring the intensity of the light beam after passing a second polariser. The intensity of this light beam is a function of the angle of rotation and thus the magnetic field strength.The characteristic of the sensor is determined by the orientation of the two polarisers to each other. The angle between the transmission axes of the polarisers determines by how much the value of the transmitted intensity varies with a varying magnetic field. The angles can be chosen anywhere between 0° and 90° giving the same results for all other quadrants. The resulting intensities can be calculated employing Malus’ law: considering a linearly polarised light beam incident on a polariser, its perpendicular component of the beam is blocked. Therefore, the amplitude of the light transmitted by the polariser is shown in eqn. 3.
E(θ)=E0 cos (θ) (3)
And the intensity of the transmitted light is given by the formula in eqn. 4.
I(θ)= I0 cos2(θ) (4)
E0 is the electric field vector,
I0 is the intensity of the incident beam.
Other detection methods use polarisation separating prisms and two detectors. The two orthogonal linearly polarised beams are detected separately. This technique has the advantage that optical losses in the fibres and sensor material can be compensated. The rotation of the polarisation can directly be obtained by comparing the two sensor signals.
More advanced methods use interferometers to measure the degree of rotation . In the polarimetric detection scheme, the rotation of the plane of polarisation of linear polarised light was measured. This rotation can also be measured in terms of circular polarisation, corresponding to a phase difference between the two circular orthogonal modes (left-handed and right-handed circular polarisation). This can be done using an interferometric detection scheme where a modulation frequency carrier is generated, and the optical phase variation that is modulated by time delay induced between arms of the interferometer, will contain the electric current information.
Fig. 3 shows a current sensor based on an interferometer principles.
Optic fibre rings
In this application the fibre itself acts as a transducer mechanism. The magneto-optical effect is used to induce a rotation in the angle of polarisation of the light propagating in the fibre, which is proportional to the magnetic field. Usually, the fibre is coiled around the electrical conductor, making it immune to external currents and magnetic fields. Although the Verdet constant of a fibre is not very high, a measurable rotation can be achieved with a long fibre wound around the conductor many times. The sensitivity of the instrument can be changed by varying the number of turns of fibre. The fibres are typically single-mode silica fibres and do not require a precision matching or alignment. The sensitivity can be adjusted by adding dopants to the core or by varying the number of turns. In order to remain the state of polarisation in the fibre between the sensing region and the light source/detectors, polarisation-maintaining single-mode fibres are used. These fibres have a core of elliptical cross-section or index of refraction anisotropy introduced by dopants or uniaxial stress . Fig. 4 shows the typical construction.
This type of instrument has the advantage that the light transmission to and from the region of current measurement is done by optical fibers which are compatible with low voltage analogue and digital equipment used for metering and relaying applications.Bulk optical material
This type of OCT is analogous to an optical implementation of a conventional CT. It consists of a electro-optical material completely enclosing the conductor (Fig. 5). Numerous designs have been proposed with light beams encircling the current carrying conductor exactly once or several times. These sensors are fabricated from single-glass blocks with relatively low Verdet constants and do not suffer from bending induced problems occurring in optical fibre sensing elements. The optical material is assembled in a quadrangular format around the conductor as shown in Fig. 6.
Optical voltage transformers
Optical voltage sensors (OVT) differ from OCT in that the voltage is applied across the length of the sensor. In a similar fashion to the OCT, the voltage affects the properties of the light passing through the optical material, and this is used to provide the measurement.
Pockels effect: birefringence (birefraction)
Most optical voltage sensors are based on an electro-optic (EO) crystal and longitudinal Pockels effect. Light transmission to and from the region of voltage measurement is done by optical fibers which bring inherent immunity to electromagnetic interference and compatibility with low voltage analog and digital equipment used for metering and relaying applications. EO materials, usually crystals, change their refractive index under the influence of an electric field. The field may be applied along the direction of propagation or at right angles to it. The effect of a change in refractive index is to introduce a change in phase of the light beam passing through the crystal. This change in phase also introduces a change in polarisation of the beam. Electro-optic crystals are typically anisotropic.
Typical crystals used in pockels cells are Potassium Dihydrogen Phosphate (KDP), lithium niobate (LiNbO3), and Bisumuth germanganate (Bi4Ge3O12).
The Pockels cell alters the polarisation of a transmitted light beam when voltage is applied to the cell by causing a phase retardation between orthogonal polarisation components of the beam. In the absence of an applied field, there is no difference in the phase retardation between orthogonal polarisation components of the light beam because the refractive index is the same for both polarisation directions and so there is no polarisation change in the transmitted light. However, an applied electric field creates fast and slow axes at 90° to one another. The difference in velocity for beams with polarisation components along these two directions, with voltage applied, retards the phase of one polarisation component relative to the other thereby changing the polarisation state of the emerging beam.To be detectable polarisation must change by less than 90°, and this limits the maximum voltage that can be applied to a crystal of any particular dimensions. This maximum voltage is known as the half-wave voltage or half-wave field strength. Typical half-wave voltages for some EO materials range from 3 to 75 kV [3, 6].
The problem with using the Pockels effect for high voltages is the sensitivity of the EO crystal, which is usually too high in comparison with the measured voltage.
The conventional solution is to use capacitive dividers to obtain a small part of the total voltage on the optical voltage sensor However, the method limits the performance of the optical measurement technology due to the high cost and the stability problem of the capacitive dividers. Another method is to use the multi-segmented sensor which consists of crystal slices and spacers of dielectric material The half-wave voltage of the multi-segmented sensor is far larger than a single EO crystal in longitudinal modulation .
Piezo-optic voltage sensor
High voltage OVTs which make use of the piezo-electric effect have been developed. The system consists of a piezo-electric crystal which deforms mechanically under the influence of an electric field. The deformation is transferred to an optic fibre wound around the crystal, producing a phase and polarity shift in the light transmitted through the fibre, which is proportional to the voltage applied. Units with measurement ranges up to 420 kV have been produced. A typical 170 kV unit is shown in Fig. 6 .Combined composite insulator
One of the more useful applications is the combined current and voltage transformer, which combines an OCT with an OVT in a single insulator shell. The OCT is mounted at the top of the insulator and the OVT is inside the insulated shell. Fig. 7 shows one of the products on the market.
Application in the digital substation
OCVTs are available from most major switchgear manufacturers and, because they do not need to be stand-alone systems, are being incorporated into high and medium voltage switchgear as a standard feature. Available with digital outputs compliant with IEC 61850 and other standards, they are ready for integration into control systems of the future. OCTs and OVT have been incorporated into substation equipment and are in important element of the digital substation, such as disconnecting circuit breakers from ABB . OCVTs can also be retrofitted to existing equipment and are available for mobile measurements that would not be not possible with conventional CTs. Using fibre as a sensor allows the use of the OCT in applications that would preclude a conventional CT.
 R Silva, et al: “Optical Current Sensors for High Power Systems: A Review”, Applied Science, 2012.
 S Liehr: “Optical Measurement of Currents in Power Converters”, www.researchgate.net/publication/237212593_Optical_Measurement_of_Currents_in_Power_Converters
 K Bonhert, et al: “Fiber-Optic Current and Voltage Sensors for High-Voltage Substations” 16th International Conference on Optical Fibre Sensors, October, 2003.
 E Langford and J Swindlehurst: “Optical Current & Voltage Sensors” www.ieso.ca/Documents/imowebpub/200705/rm_pres-20070307-Ontario-IESO-Pres.pdf
 K Bonhert, et al: “Fiber-optic voltage sensor using fiber-gyro technology”, Proceedings Eurosensors XXIV, September, 2010.
 L Piheiro, et al: “Optical high voltage measurement transformer using white light interferometry”, Annals of optics, 2002.
Send your comments to firstname.lastname@example.org