GB2388914A

GB2388914A - Current transformer with reduced resistance

Info

Publication number: GB2388914A
Application number: GB0210799A
Authority: GB
Inventors: Kaushik Ghosh
Original assignee: PRI Ltd
Current assignee: Secure Meters UK Ltd
Priority date: 2002-05-10
Filing date: 2002-05-10
Publication date: 2003-11-26
Anticipated expiration: 2022-05-10
Also published as: MY135177A; TR200300636A2; GB2388914B; ZA200303603B; GB0210799D0

Abstract

A current transformer 10, preferably of the clamp-on type, comprises a core split into two sections, a secondary coil 2 and a signal conditioning circuit. The signal conditioning circuit is connected to the secondary coil so as to improve the performance of the current transformer by reducing the total resistance of the current transformer 10. The signal conditioning circuit preferably comprises an operational amplifier 3 and at least one resistive element 1 arranged in the feedback loop of the amplifier 3. Positive and/or negative feedback loops may be employed.

Description

23889 1 4

CLAMP-ON CT

The present invention relates to a method for improving the repeatability and reproducibility of measurements when using a metering device. More 5 particularly, the present invention relates to a method of improving the repeatability and reproducibility of high accuracy power and energy measurements when using a clamp-on current transformer.

Clamp-on current transformers (hereinafter referred to as CT's) are well known to be used in the art of power metering. They offer an easy to use 10 method of measuring the power and energy consumed or dissipated by a circuit or system. The secondary current in a typical current transformer is proportional to the primary current and differs in phase from the primary current by a phase angle (a). The phase angle between the primary and secondary current of a CT can be expressed by: = {(Im Costs - Ie Sino) / N. Is}. (180/7t) Degree 0 = tan(XL/(Rs+Rb)) Where, Im = Magnetising component of exciting current 20 Ie = Loss component of exciting current N = Turns ratio Is = Secondary current = Angle between secondary induced voltage and secondary current XL = Reactance of secondary winding of the CT 25 Rs = Secondary winding of the CT Rb = Burden resistance on the CT In theory, from analysis of the above expression, it can be expected that if the above parameters remain constant then the phase angle will be constant.

( However, in reality, the inherent construction method of a clamp-on CT causes a variation in the value of the reactance of the secondary winding of the CT (XL) during each operation. Hence the phase angle varies with each operation which results in poor reproducibility.

5 The CT secondary winding reactance is determined by: X - 2..f.L L = us Ur A N2 And, g Where 10 f = Supply frequency L = Inductance of the secondary u0 = Permeability of air Ur = Permeability of the CT core material A = Cross-sectional area of the core 15 N = No. of turns on the winding g = the magnetic path length in the CT core For a normal ring CT all the above parameters remain constant at a given supply frequency ensuring a constant phase angle between the primary 20 and secondary current.

However, the case is different for clamp-on CT's as the core material is split in two sections which are assumed to be in close contact with each other at the time of measurement. But this close contact may not exist for each and every measurement as a small air gap in the CT's split core joints can introduce 25 a significant change in the phase angle. If the air gap is included in the above inductance expression, the inductance of the CT is given by: L = Un A N2 a + (glUr)

( 3 Where a = The length of the air gap in the CT,s split core joint. The air gap may be a result of dust or other small particles and variations in the tension in the CT's mechanical spring.

5 As there is no guarantee that the air gap will remain the same for each operation, the inductance value may vary accordingly. This changing inductance results in an inconsistent phase angle added by the CT in the power measurement. Hence the power or energy measured using the split core CT will show a poor repeatability, reproducibility and unreliable results.

10 In the light of the above problem, the object of the present invention is to improve upon the poor repeatability and reproducibility provided by the current state of the art clamp-on CT's.

The above problem is solved by the present invention by providing an electronic circuit which reduces the total resistance in a CT's secondary to a 15 negligible level so that the phase angle between the primary and secondary current of the CT is at a constant angle irrespective of the value of the inductance of the CT. This can be shown by examining the aforementioned phase angle formula: 20 0 = {(Im CosO - Ie Sino) / N. Is}. (180/) Degree Where, = tan-(-XL/(Rs+Rb)) If (Rs+Rb) is zero (or negligible) then = tan(-XL/O) => = tan (ax) 25 => o - 90 irrespective of the inductance value.

The phase angle error of the CT - {- Ie/N. Is} (180/) Degree = 0 degrees (assuming that the loss component "Ie" is negligible)

( As is evident from the above theory, a constant phase angle between primary and secondary currents of the CT ensures the reproducibility of the power and/or energy measurement.

5 From a first aspect, the present invention provides a current transformer with a primary and secondary coil, wherein an inverting operational amplifier is connected across the secondary coil of said transformer and wherein a burden resistance is arranged in the feedback loop of said amplifier, said arrangement improving the performance of the current transformer. This 10 improvement is achieved as the inverting terminal of the op-arnp is virtually at the same potential as the non inverting terminal; thereby providing a virtual zero ohm burden across the secondary coil of the current transformer.

It will be appreciated that as a secondary current flows through the burden resistor in the feedback loop, the signal amplitude level remains the 1 5 same.

From a second aspect, the present invention provides an electronic circuit to be electrically connected to the secondary of a current transformer to reduce the total resistance in the secondary of the current transformer to a negligible level, said circuit comprising: 20 an operational amplifier arranged in a non-inverting configuration with the inverting input connected to one end of a first resistive element and wherein the other end of said resistive element is connected to a reference ground terminal; a second resistive element arranged in the feedback loop of the 25 operational arnplifer and located between the inverting input and the output of the operational amplifier; a third resistive element connected beh.een the non-inveing input of the operational amplifier and the output of the amplifier, thereby providing positive feedback;

s wherein, the values of the first, second and third resistive elements are used to determine the value of a negative resistance provided by the circuit.

The circuit described in the second aspect of the present invention acts as a negative resistance circuit and the negative resistance so created is used to 5 nullify the secondary resistance of the current transformer. If the resistances on the inverting terminal in a preferred embodiment, are Rl and R2 and that on the non inverting terminal R3, then the input resistance of the circuit is: Rin = Vin / fin = - R3.(R1/R2) To ensure stable operation, the values of the first, second and third resistive elements are chosen so that Rin never exceeds the value of the resistance in the secondary coil of the current transformer.

In order that the present invention be more readily understood, 15 embodiments thereof will be described by way of example with reference to the accompanying drawings, in which: Figure I shows the circuit diagram for a state of the art current transformer with a burden resistance; Figure 2 shows the circuit diagram for a current transformer with a 20 burden resistance according to the present invention; Figure 3 shows a circuit according to the present invention which provides a negative resistance to counter the resistance of the secondary coil of a current transformer.

Figure I represents a conventional arrangement for a secondary of a 25 current transformer lO comprising a burden resistance l arranged in parallel with the secondary coil of the current transformer 10. The resistance of the secondary coil is denoted by reference numeral 2.

Figure 2 represents an arrangement provided by the present invention which improves upon the performance of the conventional arrangement. An

( operational amplifier 3 is arranged to form an inverting configuration by being electrically connected across the secondary coil of the current transformer 10 and a burden resistance 1 is arranged in the feedback loop of the amplifier 3.

The non-inverting input (+) is grounded to a reference and the secondary coil 5 offers a resistance 2.

Although this arrangement improves performance of the current transformer, it does not remove the effects of the resistance 2 in the secondary coil hence the phase angle between the primary and secondary coil of the transformer 10 continues to vary each time it is operated. The present invention 10 also provides a circuit 20 to counter the resistance 2 of the secondary coil and is shown in Figure 3.

The non-inverting input (a) of an operational amplifier 4 is electrically connected to the output of the CT. The non-inverting input is further connected to one end of a resistor R3 with the other end of the resistor R3 connected to 15 the output of the amplifier 4. The inverting input (-) is electrically connected to one end of a resistor R1 which has its other end grounded to a reference.

Further, one end of another resistor R2 is also connected to the inverting input of the amplifier 4 with the other end of the resistor R2 being connected to the output of the arnplifer 4.

20 The circuit 20 is arranged as above to provide a negative resistance to counter the resistance 2 in the secondary coil of the current transformer and the negative resistance is given by: Rin = Vin/Iin = -R3 (R1/R2) To ensure stable operation, the value of the three resistances R1, R2, R3 are chosen so as to never exceed the resistance 2 in the coil of the current transformer. Furthermore, the values of the resistor R1 and resistor R2 may be chosen to be same.

( 7 It will be appreciated that the circuit 20 may be provided for any type of split-core current transformer and is not only limited for use with a clamp-on current transformer.

Claims

( CLAIMS:

1. A current transformer comprising: a core formed from two sections capable of relative movement; 5 a signal coil; and a signal conditioning circuit; wherein the signal conditioning circuit is connected to the signal coil so as to improve the performance of the current transformer by reducing the total resistance of the current transformer.

2. The current transformer according to claim I, wherein the signal conditioning circuit comprises an inverting operational amplifier and at least one resistive element arranged in the feedback loop of the amplifier.

3. The current transformer according to claim 1, wherein the signal conditioning circuit provides a negative resistance to counter the resistance in the secondary coil of the transformer thereby reducing the total resistance of the current transformer to a negligible level.

4. The current transformer according to claim I or 3, wherein the signal conditioning circuit comprises an operational amplifier arranged in a noninverting configuration and at least three resistive elements, wherein at least one of the resistive elements is connected between the noninverting input of the amplifier 25 and the output of the amplifier thereby providing positive feedback.

(

5. The current transformer according to claim 4, wherein the value of the three resistive elements is chosen so as to never exceed the value of the resistance in the secondary coil of the transformer.

5

6. The current transformer according to any one of the preceding claims wherein the transformer is a clamp-on transformer.