Patent Application
20250189564
Voltage sensing in low and medium voltage switchgear is required for power system protection and control. The voltage is specified on a per unit basis which defines the normal electrical system operating parameters. ANSI elements 27 and 59 monitor under and over voltage conditions which, when asserted, trip protective circuit controls into an open state, for protection and control of the electrical power system. Voltage sensors in 480V power distribution switchgear are comprised of #12 SIS wires tapped directly to the copper bus bars, with wire leads brought out directly to electromagnetic transformers which step down the voltage to a level which can be digitally sampled as an input to digital microprocessor relays. At medium voltage, such as 12.5KV, the voltage sensing requires draw-out potential transformers which connect to the copper bus bars on the primary H1/H2 side of the potential transformer, and where the stepped down voltage on the secondary X1/X2 terminals of the potential transformer (PT) are tapped with #12 SIS wire and stepped down to digital logic voltage levels which can be sampled with an analog to digital (A/D) conversion card as an input to the backplane of a digital microprocessor relay.
In the former case of #12 SIS wire tapped directly to 480V bus, these leads may be wired directly into meters or relays, where the step-down voltage transformation to analog voltage levels for digital sampling occurs, without intermediate fusing or circuit breakers, such that the leads always remain energized, which poses a safety issue for maintenance or troubleshooting, as the sensor wires remain hot when disconnected as long as the bus is live. In many instances it is required to troubleshoot SCADA circuits without taking an outage, and this presents a problem if the voltage sensing leads require testing or reconfiguration as they are unable to be de-energized independently of the main bus where the sensing leads are connected.
In the case of draw out potential transformers in medium voltage switchgear, the instruments are heavy, mechanically prone to jamming, and occupy valuable space in the switchgear cubicle layout which could be used by additional feeders or instrumentation if alternative voltage sensing means with reduced form factor were available.
It is therefor desirable to provide a means of observing voltage, in the form of electric field potential, which does not require analog induction transformers and which is galvanically isolated such that testing or troubleshooting the voltage sensing circuit does not risk electrical energy exposure from the main bus. The present invention teaches an optical voltage sensor for indoor low and medium voltage switchgear which is galvanically isolated from the electrical power distribution system and which does not require the large iron cores and copper windings of potential transformer (PT) drawers.
The Kerr effect describes the polarization dependent phase shift exerted on orthogonally polarized modes traversing a perpendicular path relative to an electric field. The S and P orthogonally polarized modes experience different indices of refraction which produces linear birefringence which varies exponentially based on the electric field potential as an inverse function of linear distance from the electric field source comprised of the bus bar conductor, as modulated by the Kerr constant which describes the magnitude of electric field induced birefringence, which for fused silica glass, such as standard SM telecom fiber, is 0.053 (10−14 mV−2) at a wavelength of 633 nm.
Where the birefringence induced by an electric field E is described as Δn, the difference between the observed refractive indices of the parallel and perpendicular polarized modes when traversing a perpendicular path to an electric field, E, and where the Kerr constant, K, correlates the degree of birefringence observed, based on the average of the S and P refractive indices, inversely proportional to the square of the electric field potential, E.
The present invention measures the phase shift experienced by two orthogonally S and P polarized modes of equal amplitude, after traversing a perpendicular path to the electric field radiating from a phase conductor bus bar.
In the present invention, an optical pulse linearly polarized at 45° is transmitted along an all dielectric fiber optic cable, routed within rigid insulating dielectric conduit, in parallel with a bus bar phase conductor, and in parallel with respect to the ground bus and switchgear ground potential, and where the phase shift resulting from the birefringence induced by the perpendicularly oriented electric field radiating from the phase conductor is analyzed at a receiver by splitting the polarized optical pulse equally and filtering through circular polarizers which measure the amounts of right and left circularly polarized light, and where the difference in the intensity measurements transduces the degree of phase shift experienced by the linearly polarized, in phase, equal amplitude orthogonal modes which traverse a perpendicular path through the electric field, E.
For linearly polarized light, the Stokes parameter S3, which describes the preponderance of right circularly polarized light over left circularly polarized light, is null, as the phase angle & between the modes, which describes the phase shift, is initially zero, for light which is linearly polarized at 45°, and only after a phase shift & occurs and the linear polarization becomes elliptical does S3 become observable:
From Equation 2, observations of S3 directly measure the phase delay, δ, between the orthogonally polarized modes, which is directly proportional to the electric field induced birefringence, and when modulated by the Kerr constant K, the observed intensity difference varies exponentially with the electric field potential, E, according to Equation 1 as an inverse function of observation distance from the electric field source.
An optical transmitter module, comprised of a single mode laser transmitter connected to a DC power supply, with adjustable modulation rate, is mounted on the front side of the interior of a switchgear cabinet, and transmits via single mode fiber optic patch cord to a linearly polarizing optical module mounted and installed proximate the phase conductor bus bar, on a back side of the switchgear cubicle where voltage is being measured, and the output of the linear polarizer optical module connects to an all dielectric single mode fiber optic sensor cable, housed within rigid insulating dielectric conduit, which is mounted parallel to the bus bar with an insulating bracket with integrated vibration dampers, and the dielectric single mode fiber optic sensor cable traverses a perpendicular path relative to the electric field radiating from the phase conductor bus bar, with respect to the ground potential established by the ground bus within the switchgear cubicle which connects to the switchgear ground grid network, and the phase-shifted orthogonally polarized modes which are outputs of the fiber optic voltage sensor cable routed within rigid conduit in parallel with the bus bar conductor connects with a polarization analyzer module, which splits the signal via a 1×2 polarization maintaining fused fiber coupler, or a PLC silicon splitter chip as used in passive optical networks (PON), with each half of the divided signal routed to polarizing filters with output intensities dependent on the amount of right and left circularly polarized light generated by the phase shift, and where the difference in the output intensities is a direct measurement of the Stokes parameter S3, from which the phase delay δ and electric field induced birefringence is directly calculated with Equation 2, which varies exponentially with the electric field potential, as an inverse function of electro-optical interaction distance from the electric field source, as modulated by the Kerr constant for the waveguide material of the single mode fiber optic cable sensor, based on Equation 1.
It will be obvious to one skilled in the art field of the present invention that the optical transmitter comprised of a single mode laser and photo-diode photo-detector modules may be integrated directly proximate to a SCADA I/O concentrator back-plane, while the polarizing filter which provides the linearly polarized 45° reference input, and the 1×2 polarization maintaining optical splitting and phase shift measuring polarization filtering modules, may be installed directly proximate the input and output of the fiber optic cable sensor which traverses a parallel path relative to the phase conductor bus bar and ground bus bar, perpendicular to the electric field with respect to ground potential, where the input and output transitions are demarcated by the dielectrically insulating and vibration dampening mechanical support brackets which secure the Kerr-effect sensing fiber optic cable and conduit perpendicular to the electric field radiating from the phase conductor bus bar, with respect to ground potential.
The fiber optic cable sensor will be jacketed and insulated mechanically to dampen any potential for vibration within the outer rigid conduit, while the mechanically insulating brackets incorporate damping to eliminate the transfer of any acoustic vibration from the environment to the cable sensor during normal operation, such that the linear birefringence of the Kerr effect fiber optic cable sensor is insulated from acoustic disturbances and therefor only transduces the birefringence induced by the electric field and which is described by Equation 1.
In the event of abnormal seismic events, such as would occur during an earthquake, or any other environmental event producing acoustic waves of magnitude which would render the sensor blind due to the overwhelming acoustic noise observed at the analyzer, the abnormal birefringence signal which is generated by such a seismic event shall be programmed to automatically trip all protective circuit devices via the connected SCADA controls, as though a fault had occurred, since in the case of environmental catastrophes such as earthquakes, where structures may be expected to fail, the de-energization of circuits is a necessary safety measure to prevent unintended incident energy exposure and electrocution hazards. As the polarization filtering occurs proximate the fiber optic cable sensor where the electro-optical phase shifting Kerr effect is observed in the presence of the electric field radiating from energized bus bar within the switch gear, the use of polarization maintaining fiber is not required, and single mode fiber may be used for all patch cord links between modules as shown in
The fiber optic cable sensor waveguide material may be fused silica, flint glass, or alternatively doped to manipulate the Kerr constant, however the present invention claims all such embodiments including standard fused silica glass with Kerr constant of 0.053 (10−14 mV−2).
The passive optical filtering modules connected to the input and output of the Kerr effect sensing fiber optic cable, which is mounted in parallel with respect to the phase conductor bus bar and ground bus bar, may be mounted on the back side of the switch gear cubicle panel, with direct outputs connecting via the insulating mounting brackets which position the fiber optic cable sensor and conduit relative to the bus bar, while the single mode laser transmitter and associated power supply, and the photo-diode optical photodetector where optical intensity measurements proportional to the electric field induced birefringence are measured, may be mounted and installed on the front side of the switch gear cubicle section where the bus voltage is being optically measured, in a preferred installation embodiment. With no voltage applied to the phase conductor bus bar, with respect to the ground bus potential, some characteristic amount of linear birefringence will exist in the single mode fiber optic Kerr-effect electric field sensing circuit, and this will be observed at a zero-energy condition for calibration. For alternating current measurements, this zero-voltage linear birefringence characteristic magnitude for the optical voltage sensing network will re-occur at every zero-crossing, with the observed phase delay resulting from linear birefringence induced by the electric field increasing and decreasing concomitantly with the rise and fall of the sinusoidal voltage waveform, and as the polarized optical pulses are modulated with a much higher frequency than the fundamental electrical power frequency, a hypothetical 100 megabit per second optical pulse rate samples each voltage cycle wave form at rate of 1,666,666 samples per 60 HZ electrical wave cycle, which more than guarantees accurate sampling of the 60 HZ fundamental frequency of the electrical field wave form being optically measured, per the 2× sampling rate requirement stipulated by the Shannon-Nyquist sampling theorem to avoid aliasing in the sampled signal. Such a well-oversampled optical sensor frequency also offers digital harmonic waveform analysis potential with the discrete Fourier transform.
