GB2244142A - Current transformer measuring circuits - Google Patents

Abstract

A circuit including a current transformer (T) further includes means (14, 15) to produce a compensating voltage which reduces or substantially eliminates the e.m.f. tending to develop within the transformer secondary winding (12). As shown, an additional winding (14) and an amplifier (15) are used together with a power supply (16) and a load resistor (13). <IMAGE>

Description

CURRENT TRANSFORMERS The present invention relates to current transformers for use as current measuring devices.

Current transformers are particularly useful for isolating measuring equipment from a circuit having a current to be measured - especially where the circuit may be at a high potential or where direct insertion is difficult, inadmissible or temporary.

Current transformers of the type generally in use have certain disadvantages. They are limited in performance by the saturation flux of the core, which limits the V.t (voltage . time) product that can be sustained. The maximum time over which a valid pulse measurement can be taken, or the lower limit of operating frequency, is thereby fixed for a given design of core. The time and the lower frequency limit can be extended by increasing the saturation flux, but this has practical disadvantages in terms of size, stray reactive effects and upper frequency performance.

Current transformers are discussed in an article entitled "Current Transformers", by Rodney C. Cross, in the American Journal of Physics 54(12), December 1986, pages 1110-1113. Techniques for extending the performance of standard current transformers are described. At the end of the article, there is mentioned the possibility of using a transformer with zero load resistance to feed a commercial probe which would otherwise have saturated. Although this would increase the time to reach saturation it does not, of itself, change the mode of operating the current transformer and there would be no compensation for the resistance of the secondary winding itself.

According to the present invention, there is provided a circuit including a current transformer, which circuit further includes means for detecting and amplifying the e.m.f. tending to develop in a secondary winding to produce a compensating voltage which reduces or substantially eliminates the e.m.f. within the secondary winding. For a given limitation of the V.t product, the factor V is thus reduced, giving a corresponding increase in the factor t.

The present invention enables extension of the effective time or the frequency limit over which valid measurements can be made, thereby enabling smaller transformers, e.g. of a type designed for use with shorter pulses or at higher frequencies, to be used with longer pulses or at lower frequencies, with consequent savings in size and cost.

Preferably, said detecting and amplifying means comprises sensing means (e.g. an additional winding) on the secondary side of said transformer and an amplifier circuit having a high input impedance driven by said sensing means, the latter and the amplifier circuit detecting and amplifying respectively the e.m.f.

arising from any flux change in a core of the transformer and an output of the amplifier circuit being connected to provide a driving e.m.f. for a path carrying an output current from said secondary winding.

This has the advantage that it substantially eliminates not only that part of the secondary winding's e.m.f.

appearing across a load (e.g. a resistor) but also the e.m.f. used in driving the secondary current through the resistance of the secondary winding.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of a known form of circuit including a current transformer; and Figs. 2, 3 and 4 are embodiments of circuits according to the present invention.

In the circuit of Fig. 1, reference T designates a current transformer having a primary winding 10 and a core 11, a secondary winding 1 2 of the transformer T feeding a resistive load 13. If current flows in the primary winding 10 due to a current being monitored, a current is compelled to flow in the load 13 because the presence of current in primary winding 10 will cause a change in flux in the core 11 of the transformer T.

This flux change causes an e.m.f. in secondary winding 12 which in turn develops a current in the load 13. A current balance is maintained where the primary ampereturns (magnetising force) exceeds that of the secondary by an amount required to establish the flux in the core.

Accurate reproduction of the primary winding current is maintained in the secondary winding provided the core magnetising force remains a small proportion of the winding component. While primary current is maintained, the core component of the total magnetising force begins to rise to a value where the secondary current is no longer an accurate replica of the primary and eventually the secondary current falls to zero when the core saturates.

A first embodiment of the present invention is shown in Fig. 2, in which items which correspond with items in Fig. 1 have the same references as in Fig. 1.

The current transformer T has an additional winding 14 connected across the input of an amplifier 15, reference numeral 16 designating a power supply and reference numeral 17 designating a diode connected in parallel with the series connection of the load 13 and the secondary winding 12. Amplifier 15 is driven by the e.m.f. caused by the changing flux in the core 11 as sensed by winding 14, so that the necessary detection is effected by virtue of winding 14, which can be very closely coupled with winding 12 by making it one strand of a multi-stranded bundle, of which winding 12 is made up of the remaining strands connected in parallel.

An alternative mode of detection could be accomplished by using winding 12 itself, but this is affected by the presence of current in the resistance of winding 12 and reduces the apparent e.m.f., partially nullifying the benefits of this technique and resulting at best in the effective reduction of the external load to approach zero while leaving the resistance of the winding itself uncompensated.

In a modified form of the circuit of Fig. 2, shown in Fig. 3, the output of amplifier 15 is connected to a field effect transistor (FET) 18 which controls the output from power supply 16 in such a way that the voltage present across secondary winding 12 is of such a value that the e.m.f. in it is reduced very nearly to zero - the limitation being in the gain of the overall circuit which includes amplifier 15. This process is analogous to the insertion of a negative impedance in series with winding 12 which tends to cancel that of the load on winding 12 and the impedance of winding 12 itself.

In this way, the rate of change of flux in the core 11 is reduced considerably from the value that would be present in the circuit of Fig. 1. The time taken for the accuracy of the current balance between the primary winding and the secondary winding to reach an unacceptable level has been extended in proportion.

Until the amplifier and associated circuitry become effective, there is a time during which the output of winding 12 drives the load 13 and diode 17 in series. Apart from the additional voltage developed across the diode 17, the load on winding 12 is not substantially different from that found in the circuit of Fig. 1. The current transformer T performs normally. Thus the rise-time does not depend upon the amplifier and its associated circuitry.

The circuit of Fig. 3 includes a refinement to extend the useful operating time of the current transformer T further. A resistor 19 is used to provide a small bias to the flux in core 11 in the opposite direction to that caused by the presence of current in the primary winding 10. By this method, the available flux excursion is increased and still longer primary current pulses can be accommodated. In the event of current reversal in winding 10, a diode 20 holds the voltage across secondary winding 12 to the value present on the supply 16. This supply is bypassed by a capacitor 21, which is chosen to have a low impedance at high frequency so as to improve the effectiveness of power supply 16.

Fig. 4 is a circuit diagram of a modified form of the circuit of Fig. 3. The FET 18 is used to boost the output of an amplifier 22, whose output is prevented from reaching an excessive positive level by a diode 23. Diodes 24 and 25 prevent the input voltage to the amplifier from exceeding its design limits during the rise of fast pulses of primary current, when the secondary current flows around load resistor 13 and diode 17, or when the primary current is reversed.

Resistors 26, 27 and 28 determine the stability of the circuit. A capacitor 29 removes the d.c. component of voltage (due to the presence of the supply 16) from the feedback path. Reference numeral 30 designates screened co-axial cable for connecting additional winding 14 across the input to amplifier 22.

Claims (4)

1. A circuit including a current transformer, which circuit further includes means for detecting and amplifying the e.m.f. tending to develop in a secondary winding to produce a compensating voltage which reduces or substantially eliminates the e.m.f. within the secondary winding.

2. A circuit according to claim 1, wherein said detecting and amplifying means comprises sensing means on the secondary side of said transformer and an amplifier circuit having a high input impedance driven by sensing means, the latter and the amplifier circuit detecting and amplifying respectively the e.m.f.

arising from any flux change in a core of the transformer and an output of the amplifier circuit being connected to provide a driving e.m.f. for a path carrying an output current from said secondary winding.

3. A circuit according to claim 2, wherein said sensing means comprises an additional winding.

4. A circuit including a current transformer, substantially as herein described with reference to Fig. 2 or Fig. 3 or Fig. 4 of the accompanying drawings.