Fiber-optic electric current transformers: physical bases and technical implementation. Part II
TECHNICAL IMPLEMENTATION OF FIBER-OPTIC CURRENT TRANSFORMERS
The classical transformers does not have sufficient reliability at voltages 110–750 kV, and they are prone to creating explosive and fire-dangerous situations . So a fiber-optic method for measuring current using the Faraday effect in an optical fiber has been actively developing [2–4]. The main part of the market of fiber-optic current transformers (FOCT) is represented by such leading global companies as General Electric (GE), ABB, Arteche. In Russia, only "Profotech" manufactures such devices. Their operating principles were considered in the first part of the review [5–8]. Let’s look at the technical implementation of fiber-optic current transformers.
2.1. FOCT measuring fiber interferometer
As follows from the first part of the review, optical measurement of the current is reduced to measuring the Faraday phase shift ΔϕF between orthogonal circularly (elliptically) polarized waves or the angle of rotation θF of the plane of polarization of linearly polarized light. In modern electron-optical current transformers, the interferometric method is most often used. Below we consider one of the variants of the widely used scheme of a all-fiber measuring interferometer (linear Sagnac interferometer) (Fig. 3) .
The scheme operates as follows. The radiation of a low-coherent fiber superluminescent source 1 is propagated through the coupler 2 and the fiber polarizer 3. The polarizer 3 converts unpolarized radiation into the linear polarized one. Next, the linear polarized radiation enters the fiber birefringence modulator 4. The birefringence modulator is a fiber that maintains linear polarization of the radiation (PM fiber) wound with a certain tension on the piezoceramic cylinder. The alternating voltage applied to the piezoceramic cylinder leads to the PM fiber tension modulation and, accordingly, fiber’s birefringence modulation. Due to the 45° alignment between the polarizer transmission axis and birefringence axes of the modulator input fiber, two coherent orthogonal linearly polarized waves (x and y polarization modes) of the interferometer with equal intensityare formed. The modulator provides a harmonic modulation of the waves phase shift. The presence of a modulator makes it possible to apply the modulation detection method widely used in fiber gyroscopy (see the features and advantages of this method below). Since the modulator is a reciprocal optical element, to obtain the required phase modulation between the light waves propagating through the modulator in the forward and reverse directions, a time delay is introduced by half the period of the modulation control signal. This delay is determined by the time of propagation of light through a long PM fiber line (delay line 5 and connecting line 5a). The waves with orthogonal linear polarizations, passing elements 5 and 5a, are then enter the quarter-wave plate.
Being initially coherent, linearly polarized waves of low-coherent radiation, propagating through a highly anisotropic fiber of the delay line and the connecting line, are depolarized (lose coherence). As a result of the depolarization of low-coherent light over a considerable length of the fiber optic path, the influence of a number of undesirable factors on the accuracy of the interferometer is reduced. The λ / 4 plate converts these incoherent linearly polarized waves into orthogonal circularly polarized waves, which after passing through a spun-optical fiber (sensitive circuit 8) sensitive to a magnetic field are reflected from the mirror at fiber’s end and propagate in the reverse direction. After mirror reflection, the polarization of each wave is converted into an orthogonal one (the left-circular polarization is converted into a right-circular polarization and vice versa). After the reverse passing the λ / 4 plate, waves are again converted to linearly polarized ones, but orthogonal to the original (x polarized wave propagating in the forward direction becomes y polarized wave propagating in the opposite direction along the PM fiber line, and vice versa). Conversion of the polarization states of the radiation to orthogonal to the initial ones during the reverse pass leads to the waves coherence restoration and the phase shift Δϕ between the linearly polarized waves arriving to the polarizer turns out to be zero for reciprocal effects and doubled for the nonreciprocal Faraday effect.
After propagation along the optical scheme in the forward and reverse directions the waves that pass through the polarizer interfere. Depending on the phase shift Δϕ between the waves, the power of the light detected by the photodetector changes. In the general case, the dependence of the light power on the phase shift Δϕ (output characteristic of the interferometer) can be represented as:
P(Δϕ) = P0 [1+K cos(Δϕ)]. (9)
Here P0 is the light intensity at the input, K ≈ 1 is the visibility (contrast) of the interference pattern, and the phase shift Δϕ between the waves is Δϕ = ΔϕF + ϕm cos(2πfmt), where fm is the modulation frequency, ϕm is the amplitude of the modulation of the phase difference of the waves, ΔϕF = 4VNI, where V is the Verdet constant, N – the number of fiber turns in the sensing coil, I – measured current. Relation (9) is the basis for calculating the current by the modulation method. It is also used in modulation-free schemes where Δϕ = ΔϕF + Δϕ0, here Δϕ0 is the initial phase shift of the operating point selection.
If there is harmonic phase modulation, the output signal P(t) (9) of the interferometer is the sum of harmonics of the modulation frequency (see. Fig. 4). In the absence of flowing current (ΔϕF = 0), only even harmonics are present in the output signal, with the predominance of the second harmonic (in the figure, the functions Δϕ(t) and P(t) are represented by solid lines). If ΔϕF is not equal to zero, odd harmonics also appear in the signal, with the predominance of the first harmonic (the functions Δϕ(t) and P(t) are shown by dotted lines). In this case, the amplitudes of the harmonics depend on the measured Faraday shift Δϕ F. The magnitude of the phase shift ΔϕF is calculated from the ratio of the amplitudes of the harmonics, then the current can be calculated using (6a). This modulation method provides high accuracy and independence to the variations of optical elements parameters (power of the radiation source, modulation amplitude) and the influence of low-frequency noise of electronics.
An important role in achieving the high accuracy of modern FOCT is played by the usage of a low-coherent radiation source. Usually, superluminescent optical radiation sources (fiber or semiconductor) are used at a wavelength of 1.55 microns or 1.3 microns, with a spectral width of at least 20 nm. When using low-coherent radiation, nonlinear effects in the fiber (e. g., the Kerr effect) are significantly reduced, the unwanted coupling of orthogonally polarized waves on the fiber inhomogeneities (in particular, due to the deformations of the fiber coating ) is reduced, and interference from parasitic reflections in the scheme is reduced due to the depolarization (loss of coherence) of the waves when the distance exceeds the depolarization length Ld. In the case of a fiber superluminescent source (λ = 1 550 nm and Δλ = 20 nm) according to (8) Ld ~ 5 m for a standard spun fiber (Lb = 10 mm, Ls = 3 mm) and Ld = Lb (λ / Δλ) ~ 0, 3 m for a typical PM fiber (Lb = 3 mm).
2.2. Other FOCT applications
The optical scheme of reflective interferometer with modulation considered above is based on measuring the Faraday phase shift using the entire output characteristic of the interferometer. Such scheme are referred to as the open-loop scheme. They possess an accuracy sufficient for metering of electricity consumption (accuracy class of 0.2s). At the same time, the scheme requires increased attention to the linearity of the output characteristic, and its dynamic range from above is limited by phase shifts ±π / 2 due to the periodicity of the characteristic. To increase linearity and expand the range, it is necessary to apply more sophisticated signal processing algorithms.
The highest accuracy and wide dynamic range are provided by compensation schemes, where the Faraday phase shift is zeroed using an additional sensitive coil connected in series with the measuring circuit. In the additional coil, a magnetic field is created by the compensating current, which is produced in the electronic unit and is the output of FOCT. Compensation schemes are referred to as the closed-loop schemes.
The non-modulator FOCT reflection schemes are also possible using the Faraday rotator to form a operation point in the middle of the characteristic of the interferometer P (Δϕ). The rotator can be either discrete (yttrium aluminum garnet) or fiber (on a spun fiber). Such schemes are less accurate than the modulator schemes, but they are more broadband.
The simplest scheme is the one-pass FOCT scheme which does not require a rotator. In this scheme, a direct measurement of the rotation of the plane of polarization of light in a magnetically sensitive optical fiber located between two polarizers is implemented. The disadvantage of the scheme is a relatively low accuracy, which is however sufficient for the protection of power equipment (high voltage lines).
FOCTs based on the magneto-optical Faraday effect in an optical fiber are a new high-precision device for measuring both direct and alternating electric current. Measured current range is from amperes to hundreds of kiloamperes. Currently, the main areas of FOCT application (both in Russia and abroad) are high-voltage electric power industry (110–750 kV) and non-ferrous metallurgy (control of the technological process for producing non-ferrous metals).
The FOCT sensing element is a fiber coil enclosing a bus with current, and measuring the current is reduced to measuring the Faraday phase shift between orthogonal circularly polarized light waves induced by the magnetic field. Usually, a linear Sagnac interferometer (reflective interferometer) with a modulation detection method are used to registrate the Faraday phase shift.
High accuracy of measurements of modern FOCT is achieved by fulfilling a number of special requirements. In particular, the necessary condition is a closed fiber loop, which, in accordance with the fundamental physical law, the theorem on the circulation of the magnetic field strength vector, ensures that external current bus and the shape of the sensitive loop do not affect the measurement result. The use of low-coherent radiation allows to minimize the parasitic effects leading to the appearance of an additional signal indistinguishable from the useful one. An important role is played by the depolarization of low-coherence radiation in the optical path. The noted approaches allow implementing FOCT with the highest accuracy classes (0.2s, 0.1) for practical use, which together with practical advantages (safety of operation, less installation and maintenance costs, etc.) makes this device a real alternative to traditional current transformers.