RU2419941C1 - Combined electric arc sensor - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: combined electric arc sensor containing fibre waveguide with light-permeable cover, photoreceiver also includes optic Faraday gate; end of fibre waveguide is connected through optic Faraday gate to optic input of photoreceiver the electric output of which is the output of sensor; at that, optic Faraday gate is normally closed.

EFFECT: increasing signal-interference ratio in relation to the ratio of light impacts of artificial and natural origin.

2 dwg

Description

The invention relates to electrical engineering and is intended for use in relay arc protection systems of a complete switchgear (KRU) to detect the occurrence of an electric arc.

Currently, optical arc shields (DLDs), which respond to a light flash of an electric arc, are widely used. These devices have the following basic requirements for reliability:

- ODZ should be triggered when an electric arc occurs inside the controlled switchgear cabinets;

- ODZ should not be triggered by light flashes inside switchgear cabinets that are not related to the occurrence of an electric arc;

- arc protection should not be triggered when short circuits (short-circuit) occur, including short-circuit arcs, outside the controlled switchgear cabinets.

It is known that the main factors of an electric arc are a light flash and an abnormal increase in electric current. However, a light flash can be associated not only with the appearance of an electric arc on the busbars inside the switchgear cabinet, but also with welding work inside the cabinet in the absence of light insulation of its compartments, for example, the open doors of the latter. Also, an emergency increase in current can be caused by a fault external to the controlled switchgear cabinets. Therefore, it has recently been recommended to supplement DLD with current blocking devices so that their operation is due to the simultaneous appearance of both factors of the electric arc. This is accomplished by the serial connection of the relay output ODZ with the relay output of the maximum current protection (MTZ). In this case, the current to the trip coil of the high-voltage circuit breaker in the event of an arc is supplied while closing the contacts of the relay ODZ and relay MTZ. The disadvantage of this construction is the decrease in the speed of arc protection, since the MTZ, which reacts not only to arc faults, but also external short-circuit, as a rule, has a time delay to ensure selectivity of relay protection as a whole.

A switchgear cabinet is known [1], in which, in order to provide arc protection, a magnetically controlled contact is placed in series with the photo-thyristors in the circuit of the trip coil of the drive of the high-voltage circuit breaker, which is located next to the current-carrying bus, which controls the magnitude of the magnetic field around the busbar and closes the circuit of the trip coil only at the same time triggering of at least one photothyristor and with an emergency increase in bus current.

This device responds to two factors of an electric arc - an emergency increase in electric current and a light flash.

The disadvantage of this device is the possibility of its false operation due to external short circuit. When an external short circuit occurs, an abrupt increase in electric current occurs, which will lead to the operation of a magnetically controlled contact. Also, an external short circuit leads to electromagnetic interference, including along the operational supply circuits, which can cause the photo-thyristor to trip.

The closest technical solution to the proposed one is an electric arc relay [2], which includes a sensor consisting of a fiber waveguide with a translucent sheath of a light-conducting core, the end of which is connected to a photodetector. This device is designed to detect the occurrence of an electric arc in several switchgear cabinets using a single sensor. A fiber optic cable is laid in all compartments of controlled switchgear cabinets, where an electric arc can occur. When it occurs, light from it falls on the side surface of the fiber, passes through the light guide sheath and is captured by the core. Then, through the same fiber, the captured light is transmitted to its end, to which a photodetector is connected, where the captured light is converted into an electric signal and from the sensor output this signal is transmitted to the relay arc protection to disconnect the circuits through which the electric arc is fed, thereby producing it cancellation.

The disadvantage of the prototype is the low signal / noise ratio in relation to the light effects of artificial and natural origin.

The optical signal P, which is formed at the end of the fiber connected to the photodetector, can be written as

where s, L is the coordinate along the fiber and its length; α is the extinction coefficient of optical power; k _s is the fraction of the extinction coefficient going to scattering; NA is the numerical aperture of the fiber; n _co is the refractive index of the core; E is the flux density of the irradiating radiation; S is the cross-sectional area of the core of the fiber.

When irradiating a small section of the fiber 1, as is the case in the case of an electric arc in the compartment of the switchgear cabinet, formula (1) transforms to

where s _arcs , Δs is the position of the electric arc along the fiber and the length of the irradiated section, E _arcs is the illumination created by the arc at the location of the fiber; P _arc - the optical power of the signal at the end of the fiber from the arc. The minimum signal value, obviously, will be at s _arcs = L:

When the fiber is irradiated along its entire length, as is the case from light sources, which is an obstacle, formula (1) is written in the form

where E _pom - illumination created by interference; R _pom is the optical power of the signal at the end of the fiber from interference.

The optical arc protection should work if the threshold level P _{then is} exceeded by the recorded optical power P _arcs

P _{arc min} > P _then

and do not work if the optical interference power P _pom does not exceed the threshold level P _pore

P _p <P _pore .

SNR can be defined as

Obviously, the greater the signal-to-noise ratio, the more the sensor noise immunity will be higher.

Modern DLD are designed to operate when the illumination created by the electric arc is equal to E _arcs = 10,000 lux. The illumination created inside the switchgear cabinet for maintenance work is equal to E _pom = 200 lux. The length of the fiber laid inside the switchgear must be at least L = 25 m. The fiber section laid in the compartment of the switchgear cabinet is Δs = 1 m. A polymer optical fiber with an extinction coefficient α = 0.046 1 / m is used as a fiber. Then the signal-to-noise ratio will be equal to SNR = 1, which should lead to frequent false alarms of the sensor.

The technical result provided by the claimed invention is to increase the signal-to-noise ratio with respect to light effects of artificial and natural origin.

The technical result is achieved in that a combined electric arc sensor containing a fiber light guide with a light-transmitting sheath, the photodetector further comprises a Faraday optical shutter, the end of the fiber light guide is connected through the Faraday optical shutter to the optical input of the photodetector, the electrical output of which is the output of the sensor, the Faraday optical shutter is normally closed.

The essence of the invention consists in the fact that to detect the occurrence of an electric arc, it is proposed to use a combined sensor that simultaneously responds to two arc factors - a light flash and an increase in electric current, and it is proposed to determine the increase in electric current by a non-contact optical method.

Functional diagram of the proposed sensor is shown in the drawing of Figure 1.

Figure 2 shows an embodiment of the optical shutter Faraday.

The sensor consists of a fiber waveguide 1 with a translucent sheath, the end of which is connected to the optical input of the Faraday optical shutter 2, the optical output of the shutter 2 is connected to the optical input of the photodetector 3. The electrical output of the photodetector 3 is the output of the sensor and is connected to the relay arc protection. The Faraday 2 optical shutter is normally open, i.e. in the absence of an abrupt increase in electric current, light does not pass through the shutter 3.

The light guide 1 is laid in the compartments of switchgear cabinets, where an electric arc can occur so that light from it falls on the translucent sheath of the light guide 1. The Faraday optical shutter 2 is put on the busbar so that it is magnetically connected to it.

The sensor operates as follows. In the absence of an arc in the optical fiber 1 there is no optical signal. The current flowing through the bus controlled by the optical shutter 2 does not exceed the nominal value, and the optical shutter 2 is locked. There is no optical signal at the optical input of the photodetector 3, and there is no electrical output signal at its output. There is no signal at the sensor output. The appearance of an electric arc is accompanied by a powerful flash of light, the light from which falls on the side surface of the fiber 1 and induces an optical signal in its light guide core that propagates through this fiber to the end. Also, the appearance of an electric arc is accompanied by an abnormal increase in current 5-10 times higher than the nominal value in the monitored conductive bus, this leads to an increase in the magnetic field around the conductive bus, with which the Faraday 2 optical shutter is magnetically connected, which leads to its opening, and the optical signal with the end of the fiber 1, passing through the shutter 2, enters the optical input of the photodetector 3, where, when the optical signal exceeds a predetermined level, an output electrical signal is generated, which from the output photodetector 3 enters the arc relay protection.

As the light guide 1, for example, a polymer optical fiber made of polymethylmethacrylate (PMMA) of IC POV LLC, in a sheath of white silicone rubber, translucent for arc radiation, can be used. This fiber is made of dielectric materials and is not affected by electromagnetic interference, which will eliminate false alarms caused by these interference.

As the photodetector 3, you can use a digital photodetector from the Avago HFBR-25X6 series.

The optical shutter Faraday 2 can be implemented according to the scheme shown in figure 2. The optical shutter 2 consists of an input polarizer 4, a fiber 5 made of magneto-optical material, such as lead glass, and an output analyzer 6. The polarization angle between the polarizer 4 and analyzer 6 should be 90 °. The light guide 5 should cover the conductive bus and form a closed loop. The relationship between the current I flowing through a controlled conductive bus and the transmittance k of the optical shutter is determined by the formula

k = ν sin ² (VI) + w,

where ν is the loss of optical power in the polarizer 4, the optical fiber 5 and the analyzer 6; V is the Verdet constant of the magneto-optical material of which the fiber is made; w - transmission fiber 5 in the absence of electric current.

The optical signal P _fp arriving at the photodetector 3 will be equal to

and the signal-to-noise ratio at the optical input of the photodetector 3 will be equal to

Thus, the signal-to-noise ratio on the photodetector 3 in comparison with the prototype will increase by an amount equal to

.

.

Lead glass has a Verdet constant equal to V = 1.46 · 10 ^-5 rad / A. The ratio ν / w can be at least 100. With an electric arc current of I = 10 kA, the coefficient K increases more than three times.

Thus, the proposed combined sensor of the electric arc can significantly increase the signal-to-noise ratio.

Information sources

1. Cabinet of complete switchgear. USSR author's certificate for invention No. 650143, 11/23/1976, MKI ² Н02В 13/00.

2. An Arc Relay. Patent WO 88/08217, 11.20.1987, MKI ⁴ Н02Н 7/26, Н02В 1/18.

Claims (1)

A combined electric arc sensor containing a fiber light guide with a translucent sheath, a photodetector, characterized in that it further comprises a Faraday optical shutter, the end of the fiber light guide is connected through the Faraday optical shutter to the optical input of the photodetector, the electrical output of which is the sensor output, and the Faraday optical shutter is normal closed.

Images