Electronic current and voltage transformers

Electronic current transformers were discussed in technical literature in the early 1960s, but their practical realization useable in modern power systems became pos-sible in the 1980s. In recent years there have been developments of electronic current and voltage transformers which are promising source of input signals for computer based relays. We will present a brief account of their principles of operation here and refer the reader to the references cited above for additional information.

2.6.3.1 Electronic current transformers¹²

The operating principle of these devices (also known as Magneto-Optic Current Transformers) is based on Faraday Effect, by which the plane of polarization of a polarized light beam is deflected by an angle which depends upon the magnetic field to which the beam is subjected.

The principle of operation of electronic current transformers is shown in Figure 2.21. A light beam is produced by a LED, and collated by a lens before

Light beam from LED

Lens

Polarizer and beam splitter

Faraday effect crystal block

Signal processor Current carrying conductor

Output

Figure 2.21 Schematic of the principle of operation of an electronic current transformer

it passes through a polarizer, where the beam is split into two parts. One part of the beam goes directly to a detector in the signal processing circuit, while the second part of the linearly polarized light beam is passed through a special crystal block with total reflections taking place at its three corners so that the light beam makes a complete circuit around the conductor through which the current to be measured is flowing. By Ampere’s law, a complete circuit through the magnetic field produced by the current leads to a closed path integral of the magnetic field, which is proportional to the current. The deflection of the plane of polarization of the light is thus proportional to the instantaneous value of the current. The beam with deflected plane of polarization is also sent to the signal processor, where the angle of rotation of the beam as it went through the magnetic field is measured by comparison with the first half of the beam, and an output voltage signal proportional to the instantaneous current value is produced. This signal can then be routed to appropriate application. It should be clear that the voltage signal is easily handled by computer relays, while traditional relays requiring heavy current inputs must be supplied through a current amplifier.

2.6.3.2 Electronic voltage transformers¹³

The electronic voltage transformer is based upon the electro-optic device known as Pockels cell, and is illustrated in Figure 2.22. A light beam produced by a LED and collated by a lens, as in the case of the electronic current transformer is passed through a polarizer and a quarter-wave plate, producing a circularly polarized light beam. This beam is passed through a Pockels cell which is subjected to an electric potential field produced by applying a voltage in a direction perpendicular to the direction of the light beam. The effect of the passage through the electric field on

Light beam from LED

Lens Polarizer

Pockles Cell

Signal processor

Voltage Input from PT or CVT

Output

Quarter Wave plate

Analyzer

Figure 2.22 Schematic of the principle of operation of an electronic voltage transformer

the light beam is to convert the circularly polarized light beam to an elliptically polarized light beam, with the degree of ellipticity being proportional to the strength of the electric field. An analyzer splits the elliptically polarized beam in two linearly polarized beams, with their planes of polarization perpendicular to each other. The relative intensity of each of the beams is compared in the signal processor, which measures the degree of ellipticity which is proportional to the instantaneous electric field (and by inference the instantaneous value of the applied voltage). As before, the measurement is converted to a voltage which reproduces the voltage applied to the Pockels cell.

2.6.3.3 Rogowski coils

Rogowski coil is a helical coil placed in a loop around the current carrying conductor with a return conductor placed in the center of the helical coil as illustrated in Figure 2.23. The coil has a non-magnetic core (for example air), and thus has no saturation effects. The coil has a voltage induced which is proportional to the rate

Current carrying conductor

Helical coil

Return conductor

Figure 2.23 Schematic of the principle of operation of a Rogowski coil

of change of its flux linkage. Since the flux is proportional to the current in the conductor in the center, the voltage in the Rogowski coil is proportional to the rate of change of current. This signal can be processed through an integrator (or first sampled and then integrated digitally) to provide a signal proportional to the current.

Clearly the output of such a system is not suitable for conventional relays which require large current signals to operate, but is convenient for computer relays.

The return conductor passing through the center of the coil cancels any induced voltage due to any other interfering magnetic field which may be present. Although this is not a main-stream current measuring circuit, versions of this design have been used in integrated sensing and protection applications.¹⁴

2.7 Summary

In this chapter, we have presented a classification of relay types, and have described their behavior with the help of an input-output relationship. We have pointed out the importance of the concept of a phasor during the occurrence of transient phenomena.

We have given a short account of some of the relay application considerations; how-ever, we have accepted that a thorough discussion of relay applications is beyond the scope of this book. Finally, we have pointed out the causes of errors created by current and voltage transformers. These errors ultimately affect the accuracy with which a relay can discriminate between faults internal to its zone of protection, and all other system transients. We have also described electronic current and voltage transformer principles, which eliminate some of these sources of errors. This very promising technology is still not in wide use in power systems. In the following chapter we will begin a discussion of the mathematical basis for relaying algo-rithms, and then take up an account of relaying algorithms which are in use at the present time.

Problems