Optical CT apparatus and principle of operation

According to the general development strategy of our country energy system pretty soon all substations must reconstruct a digital format. And for that you want to replace the old (but robust) analog power transformers, current transformers and voltage transformers on their digital counterpart. In this article I will discuss the optical current transformers, that on what basis they are working and what strengths and weaknesses they have.

Why change analog equipment

In fact, the reason why the need to replace equipment rather banal. Analog current and voltage transformers quite difficult to integrate into a network monitoring and control and the accuracy of the measurement is no longer meet modern standards and requirements. Therefore, the replacement of analogue transmitters on optical is a matter of time and funding.

The principle of operation of the optical device and TT

The operating principle of the optical CT is based on the Faraday effect, according to which the propagation linearly polarized light in a magnetic field, there is rotation of the polarization plane Sveta. And using the Pockels effect, according to which there is a change of the angle of refraction and the polarization under the direct influence of the electric field.

If I say very simply, depending on the angle of beam deflection electronics calculates the current flow taking into account all the errors.

In most cases, optical CT are optical column, inside which are arranged: an optical sensor, which is a fixed number of fiber turns. They are housed perpendicularly on the bus and it passes the primary current. In this case there is no physical contact of the tire and the sensor. Further, the fiber passes through the optical isolator polymer cross (placed in the bottom of the column cavity). And yet, in the outer column, nothing more.

Then the signal is sent in digital form on a common bus in the electronics unit, which is mounted on the OPU.

In this case, the distance of the optical CTs from the electronics unit must not exceed a distance of 1,300 meters.

Advantages and disadvantages of optical CT

Compared to analog converters optical CTs have the following advantages:

pros

1. Sufficiently wide measuring channel at a high electrodynamic and thermal resistance.

2. High linearity.

3. The complete absence of such phenomena as: saturation, hysteresis, residual and irreversible change parameter after the acceleration (short circuit).

4. No response.

5. A wide frequency range, which allows to perform the voltage and current harmonic analysis in the high voltage circuit.

6. Completely exclude the impact of the load of the secondary circuit and the losses in them.

7. Increased resistance fiber optic channels to external electromagnetic interference.

8. The smaller size and weight compared with the analog samples.

9. Since the design no oil or gas, or paper, such CTs have enhanced safety performance.

Minuses

Perhaps the only drawback may be called is still relatively high cost of these products. But, and later all the flaws will be revealed during the trial operation.

conclusion

Undoubtedly, for the future of optical instruments and there will come a time when absolutely all stations and substations will be connected into a common single network. But when that time comes is not known, although there is a plan in 2030 to transfer all of the substation to the figure.

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