CA2201968C - A current ratio device for use in forming a current transformer - Google Patents

Abstract

A current ratio device for use in forming a hand-held openable-core clamp-on type of current transformer. The device employs electronically-aided three-stage circuitry for forming a current transformer capable of accurate low current measurements over a wide range of operation. Such a current transformer is specifically adapted for use with highly accurate active/reactive power and energy meters, and harmonic power analysers. For example, a preferred form of clamp-on transformer can be used to measure currents in the range from 1A to 200A with ratio errors of less than 50x10-6 for both the in-phase and quadrature components.

Description

22u19o A Current Ratio Taevice for u~A ~n forming a Current Transforrr,er Field of the Tnvn This invention relates to a the field of current ratio devices for use in current transformers, and in a preferred form a clamp-on current transformer having ratio errors below SOxI O~ for use in the measurement of electric power and energy.
Backsround of the Inyention anri prior Art A current transformer refers to a transformer used to deliver to electrical measuring aad controlling devices a representation of the line current. Power and energy meters and harmonic power analyzers are currently available with accuracies in the range of approximately 0.01% and 0.1%. These accurate measuring instruments can be used for in situ calibration of power and energy measuring systems and on-site measurements of harmonics in distribution systems.
These measuring instruments require the use of a current transformer to transform the relatively high currents by a known ratio into lower current values that can be easily measured or applied to the measuring instruments.
Two forms of current transformer are cur-ently available. The first is a reference current transformer where the current carrying circuit must he opened in order to insert the transformer into the circuit. This is often inconvenient and in many cases impossible given the location of the circuit to be tested. This problem has been addressed by a second type of current transformer, namely a hand-held clamp-on current transformer incorporating passive two-stage circuitry. Passive two-stage (two cores) transformers will be discussed in more detail belov~i in conjunction with the drawings. However, traditional clamp-on current transformers have large errors in the order of several percent for both in-phase and quadrature components because the interruption of the magnetic core increases the reluctance of the core.
These high error margins render traditional clamp-on current transformers unsuitable for use in conjunction with the highly accurate measuring instruments discussed above.

221 ~b "' Various attempts have been made to improve the accuracy of current transformers. For example, Miljanic, P. et al. in an article "An electronically enhanced magnetic core for current transformers", published in IEEE Trans. Instrum. Meas., Vol. 40, No.
2, pp,410-414, April 1991 have described an electronically-aided two-stage technique that provides the magnetizing current from an electronic circuit and hence greatly increases the apparent permeability of the magnetic core of a current transformer, without a galvanic connection to either of the primary or secondary circuits.
The Miljanic electronically-aided two-stage technique has been successfully applied to an accurate 5000 A/S A operable-core current transformer; and a 1000 A/5 A hand-held clamp-on current transformer as described in So, E. et al., "High-current high-precision openable-core ac and ac/dc current transformers", IEEE Trans. Instrum. Meas., Vol. 42, No. 2, pp. 571-576, April 1993; and Ramboz, .1.D. "A highly accurate hand-held clamp-on current transformer", IEEE Trans. Instrum. Meas., Vol. 45, No. 2, pp.445-448, April 1996, respecti vely_ In the case of an operable core transformer, the primary limitation of both the passive two-stage arrangement and the electronically-aided two-stage an,angement is its operating linearity. Specifically with small magnetic cores (desirable for a small, light weight clamp-on transformer), it is di$icult to achieve a wide dynamic range of operation between 0.5% and 100% of rated current with errors of less than SOOxlO~ for both the in-phase and quadrature components.
For example, as reported in the Ramboz article when a 1000 AIS A clamp-on current transformer operates over a current range from 1 % to 100% of rated current (i.e. 2A to 200A
for a 200 A service} errors of less than 500x10'6 far the in-phase component and less than 1000x10'6 for the quadrature component are obtained. I3owever, when operated at currents between 0.5%-1% of rated current (i.e. 1 A to 2 A for a 200 A service), the errors increase to approximately 1000x10-6 for the in-phase component and approximately 3000x1()-6 for the quadrature component. These errom are unacceptable when highly accurate measurements are required such as in the calibration of electric power meters.

2~i~19f~8 "'-~ Consequently, there is a need for a current ratio device for use in forming a current transformer capable of (a) improving the linearity problem of prior art openable core devices, (b) using small magnetic cores and (c) reducing the overall error by preferably at least an order of magnitude. For example, it would be desirable to be able to produce a small size clamp-on current transformer capable of being operated over a current range of between 0.5%
and 100% of rated current (i.e. 1 A to 200 A) with ratio errars of less than SOxIO~ for both the in-phase component and for the quadrature component.
Summary of the I ve ' An object of the present invention is to provide a current ratio device for use in forming a current transformer, that improves the linearity of errors through a current range of between 0.5% and 100% of rated current.
An object of a preferred embodiment of the present invention is to provide a current ratio device for use in forming a current transformer that uses small magnetic cores suitable for use in hand-held clamp-on type transformers capable of operating in a current range of between 0.5% and 100% of rated current with ratio errors of generally less than SOxIO~ for both the in-phase component and for the quadrature component.
In accordance with one aspect of the present invention there is provided a current ratio device for use in constructing a current transformer, said device comprising: a first magnetic core;
a second magnetic core, a third magnetic core, a first measuring winding electromagnetically coupled W th the second core and the third core but not with the first core, a second measuring winding electromagnetically coupled with the third core but not with the first and second cores, at least one amplifier having an input and an output, the input of a said amplifier being connected to receive an alternating waveform appearing across the first measuring winding, the input of a said amplifier being connected to receive an alternating waveform appearing across the second measuring winding, outer winding means eleciromagnetically coupled with the first, second and third cores, said outer winding means including a secondary winding for connection across a measuring burden, and the output of a said amplifier being connected to said outer W nding means to provide a magnetizing current required to magnetize the first core when a primary winding is electromagnetically coupled with the first, second and third cores and an alternating current flows in said primary winding.
Detailed Descr~tion of the Drawings_ hmbodiments of the present invention will be described therein below in conjunction with the following drawings in which:
Figure 1 is a prior art passive two-stage circuit for use in a current transformer;
Figure 2 is a prior art electronically-aided two-stage circuit for use in a current transformer;
Figure 3A is an electronically-aided three-stage circuit for use in a current transformer according to an embodiment of the present invention;
Figure 3B is an alternative electronically-aided three-stage circuit for use in a current transformer according to another embodiment of the present invention;
Figure 4A is a small scale plan view of a toroidal assembly of cores and windings for implementing the circuit of Figure 3, according to an embodiment of a structural aspect of the present invention;
Figure 4B is a cross-section of the toroidal assembly of Figure 4A according to an e~.mbodiment of the invention;
Figure 4C is a cross-section of the toroidal assembly according to another embodiment of the invention; and Figure 5 is an electronically-aided three-stage circuit for use in a current transformer according to another embodiment of~th~e present invention.
Detailed Description of the Prior Art Passive Tivo-Stage 'the prior art circuit of Figure 1 shows a passive two-stage current transformer 10 with two cores C, and Cz. The operation of circuit 10 is described in detail in Brooks, H.B. and Holtz, lv.C., "The two-stage C.'urrent 'Transfornner," AIEE Trans. 41. 382 (1922).

In general, the core C, couples elecaromagnetically with Uvo windings that are outside it, namely a primary wllldlng WP, and a secondary mndlng W,. but does not couple electromagnetically with a third, sensing or measuring ~~'lildlll~ W~, that is wound on the core C2. ~-he core C, couples electromagnetically with all three windings (WP>WS,Wm). In the normal manner of a current transformer, an input or primary current I~, flows in the primary windin~~ W~" and an output or secondary current IS flows in the secondary winding WS and a burden Q (usually a very low impedance, current Ineasnrin~; instrument). This is only a theoretical result, since in practice the current throu~~h the burden B is the secondary current I, minus the an magnetizing current (an ewor current) 1",. It is this factor that introduces errors into the current transformer, that must be compensated for.
In other words since a component of the primary current I~ is required to magnetize the core C,, the secondary current I, is deficient by a corresponding amount, and the theoretical equality of ampere turns between the primary and secondary circuits contains an error, such error being expressed as the magnetizing current I". The flux in the core C, corresponding to this magnetizing current is sensed by the measuring wndlng Wm that acts with core C, to generate the current Ih, in the winding Wm that is equal to the magnetizing current.
Eleclrorlically-cricled Two-Stage The prior art circuit of Figure 2 shows an electronically-aided two-stage current transformer with two cores C, and C,. The operation of circuit 20 is described in detail in U.S. Patent 20 5,307,008 issued April 26, 1994 to 1=.. So, The circuit 20 has been successfully applied to high-cul-rent 5000 A/5 A openable-core and 1000 A/S A hand-held clamp-on current transformers as described in the above literature references to So and Ramboz.
The core C, couples electromagnetically with three windings that are outside it, namely a ~5 primar~~ winding WP, a secondary ~.vinding WS and a compensating or correction winding W~, but does not couple electromagnetically wlth a fourth, sensing or measuring winding Wm that is wound on the core C, The core C, couples electromagnetically with all four windings -b-(~'~p~ws=W~>W"~. T'he primary current Ip flows in the primary winding Wp, and the secondary current I~ Mows in the secondary winding WS and the burden B.
Since a component of the primary current IP is required to magnetize the core C,, the secondary current IS is deficient by a corresponding amount, and the theoretical equality of ampere turns between the primary and secondary circuits contains an error, such error being expressed as the magnetizing current. The flux in the core.C, corresponding to this magnetizing current is sensed by the measuring winding Wm that acts with core CZ to generate a current 1M in the winding Wm that is equal to the magnetizing current.
An amplifier A, (which essentially distinguishes circuit 20 from the passive circuit.l0) is used to reduce the burden on the core CZ so that the current in the measuring winding Wm is an accurate measure of the current that magnetises the core C, . The ends of the winding W, are connected to input terminals 22 and 24 of the amplifier A, so that such input receives the current in this measuring winding W,~. The amplifier A, has an external power supply (not shown) and a gain that is such that no appreciable voltage is permitted to remain across the terminals 22 and 24.
A pair of output terminals 26 and 28 of the amplifier A, form a series circuit through windings Wm and W~ so that current IM passing through winding Wm is, equal to the current h passing through winding W~. Therefore, in this case, the compensating current I~ equals the measuring current IM and becomes equal to the magnetizing current, thus supplying the necessary magnetizing current for the core C:,. This arrangement eliminates the need for this magnetizing current to be supplied from the primary current Ip. As a result, the true equality of the ampere turns in the primary and secondary windings is not upset by the need to supply a magnetizing current.
An induced voltage is still necessary to cancel the internal burden or leakage impedance of the measuring winding Wm. In order for the core CZ to be properly demagnetized (i.e. to reduce magnetic flux to a very small value) the induced voltage must be very small.

-~- 2201968 - T.o generate a small induced voltage, the core CZ must be made of cure material having high permeability with a sufficiently large cross section and a short magnetic path length or low reluctance. Also, the internal burden or leakage impedance of measuring winding Wm must be made as small as possible. However, these requirements contraindicate the inherent properties of a hand-held clamp-on current transformer (e.g. physically small size and light weight). In particular, the limitations imposed on the size of the magnetic cores, the size of the copper wires and the number of toms of the windings, make it di~cult for the measuring wording Wm to have a small internal burden or leakage impedance.
An additional problem is introduced by the openable split-core feature of a clamp-on current transformer, which increases the reluctance of the magnetic path or reduces the effective permeability of the cores.
In summary, for a small clamp-on current transformer, it is difFcult to have the magnetic flux in core CZ reduced to a very small value. This in turn imposes a limit on the effectiveness of the electronically-aided two-stage circuit 20 in reducing the overall ratio errors of a hand-held clamp-on current transformer. As described in the Ramboz reference, it is difficult to achieve a wide dynamic range of operation between 0.5% and 1 UU% of rated current with errors of much less than 500x10'6 for both in-phase and quadrature components.
Detailed r intion of Preferred Embodiments of the Present Invent,n"
Figure 3A illustrates an electronically-aided three-stage current ratio device 3-0 for use in ' forming a current transformer, that is an embodiment of the present invention.
The current ratio devices discussed below will be referred to as current transformer circuits for simplicity and consistency in terminology. The cores C, and C2 and the windings WP,VVS,W~,Wm operate in the same manner as described in conjunction with the circuit 20 of Figure 2. The circuit 30 introduces a core C, and a second measuring winding W~ ~ a third stage to the two-stage circuit 20 of Figure 2. The windings Wm and Worm have the same number of turns as each other in the embodiment shown in Figure 3A.

-g- 221968 As discussed in conjunction with Figure 2 the goal of circuit 30 is to reduce the magnetic flux in core CZ to a very small value, which requires the measuring winding Wm to have a small internal burden or leakage impedance. The introduction of a third stage to the circuit 20 to form circuit 34 functions to further reduce the magnetic flux of the core CZ, which is S proportional to its magnetizing current, thereby increasing its effective permeability.
The core C, couples electromagnetically with three windings that are outside it, namely the primary winding WP, the secondary winding W, and the correction winding W~, but does not couple electromagnetically with the two measuring windings W,~ Wmm that are wound on the cores CZ and C3, respectively. The core CZ couples electromagnetically with four windings (Wp,Ws,Wc,W"~, and the core C, couples eleetromagnetically with all five windings (Wa>Ws,W~,W""W~"") . The input or primary current Ip flows in the primary winding W~, and the secondary current I, flows in the secondary winding Wf and the burden B.
The amplifier A, is used to reduce the burden on the core CZ so that the current in the second measuring 'winding Wm,~ is an accurate measure of the current that magnetises the core C,.
The ends of the winding Wmm are connected to input terminals 32 and 34 of the amplifier A, so that such input receives the current in this winding Wmm. The amplifier A, has ar~ external power supply (not shown) and a gain that is such that no appreciable voltage is permitted to remain across the terminals 32 and 34.
A pair of output terminals 36 and 38 of the amplifier Ai form a series circuit through windings W~ and a parallel combination of windings Wm and W""" so that current IM passing in one direction through winding Wm and Wmm is equal to the current I~ passing through winding W~. The magnetizing current of the core CZ is further reduced by a factor of approximately Z"~/ZBD, where Z~ is the magnetizing impedance of core C, and Zp" is the leakage impedance of winding Wmm 2S Figure 3B shows an alternative circuit 31 in which the compensating winding W~ of Figure 3A has been omitted, The function of the winding W~ has been taken over by compensating means constituted by the combination of the secondary winding WS and both the measuring 2?~1 ~~~

-- windings, since the outputs 36 and 38 of the amplifier A, now passes a current Im equal to the magnetizing current through the two measuring windings in, parallel and the secondary winding W$ .
figures 4A-4C show preferred structural arrangements of cores and windings for the circuit 30 of Figure 3A. Figure 4A shows a toroidal core and winding assembly 40 defining a central hole 42 suitable for receiving a single turn primary {not shown}, such as a busbar, or alternatively for the passage of convolutions of a multiple turns primary winding there through. Outside the core C, there is an outer winding assembly Wo consisting of various windings, namely the correction winding W~ and the secondary winding Ws when the Figure 3A, 3B or 5 circuitry is used.
The outer winding assembly Wo is wound as two separate halves Woe and Wa, The inner windings Wm and Wmm are similarly each divided into two portions whereby between their ends to define a pair of diametrically opposite, unwound core sections 44.
After a complete toroidal core and winding assembly 40 of this construction has been formed, it is cut thmugh along a diametrical plane 33 that passes through the unwound sections 44 and contains the axis X of the toroidal assembly.
The toroidal assembly 40 consists of laminated core members that in cross-section (Figure 4B) consist of two spaced apart side members 50 and 52 and top and bottom members 54 and 56 to complete a closed periphery. The members 50, 52, 54, and 56 combine to form an outer core that constitutes the core C, that completely encloses and shields a pair of inner members 58 and 60 that constitute the second and third cores CZ and C3 , respectively.
The outer winding assembly Wo (i.e. W~ and Wf in Fig. 3A or WS in Fig. 3B) is wound around core C,. The fn~st measuring winding W,~ is wound around cores Cz and C; and the second measuring winding Wm,~ is wound around core C;. A copper shield 64 surrounds the entire assembly 40. A space between the cores and windings is filled with the usual insulation and epoxy that fill all parts of the space 62 that would otherwise be void.

~~lJ~ ~~~

Figure 4C illustrates an alternative embodiment (assembly 41) of the core structure in which both the outer first core C~ (having members 50, 52, 54, and 56) and an intermediate second core CZ (having members 70, 72, 74, and 76) axe divided into four components that enclose and shield the third inner core C3 that consists of member 60. The outer winding assembly Wo (i.e. W~ and WS in Fig. 3A or WS in Fig. 3B) is wound around core C,. The first measuring winding Wm is wound around core Ci and the second measuring winding W",m is wound around core C3. A copper shield 54 surrounds the entire assembly 40. A
space between the cores and windings is filled with the usual insulation and epoxy that fill all parts of the space 52 that would otherwise be void.
Details of a toroid construction is well known in the art. An example of a torpid core is shown in U.S. Patent 5,307,008.
Figure 5 illustrates an alternative embodiment of a three-stage electronically-aided current transformer circuit 100. 1'he circuit 100 introduces a second correction winding W~ and a second amplifier Az to the circuit 30 of Figure 3A. The windings W~ and Wm~, have the same I S number of turns as each other in the embodiment shown in Figure 5.
The core C, couples electromagnetically with three windings that are outside it, namely the primary winding Wp, the secondary winding W, and the correction winding W~, but does not couple electromagnetically with the two measuring windings Wm, W~ arid the second correction winding W~, where the second carrection winding Wu is wound on core CZ and the second measuring winding W~ is wound on core C3. The core CZ couples electromagnetically with five windings (W~,Ws,W~,Wm,~ra).~ ~d the core C3 couples electromagnetically with all six windings (W~"W"W~,W~,Wmm,Wcc) ~ The input or primary ctu~rent Ip flows in the primary winding Wp, and the secondary current Is flows in the secondary winding W, and the burden B.
The amplifier A, is used to reduce the burden on the core CZ so that the current in the measuring winding Wm is an accurate measure of the current that magnetises the core Cl.
The ends of the winding W~ are ~rmected to input terminals 110 and 112 of the amplifier A, so that such input receives the current in this measuring winding Wm. 'The amplifier A, has an external power supply (not shown) and a gain that is such that no appreciable voltage is permitted to remain across the terminals 110 and 112.
A pair of output terminals 114 and 116 of the amplifier A, form a series circuit through windings Wm and W~ so that current IM passing through winding Wm is equal to the current h passing through winding Vf~. Therefore, in this case, the compensating current h equals the measuring current IM and becomes equal to the magnetizing current, thus supplying the necessary magnetizing current for the cores C, and Cz.
The amplifier AZ is used to reduce the burden on the core C3 so that the current in the measuring winding Wmm is an accurate measure of the current that magnetises the core C2.
'fhe ends of the winding W,"," are connected io input terminals 102 and i 04 of the amplifier Az so that such input receives the current in this measuring winding W,~,~, The amplifier Az has an external power supply (not show) and a gain that is such that no appreciable voltage is permitted to remain across the terminals 102 and 104.
A pair of output terminals 106 and I08 of the amplifier AZ form a series circuit through windings W""n and Wu so that current I~ passing through winding Wmm is equal to the current Is~ passing through winding W~~. Therefore, in this case, the compensating current Iu equals the measuring current h,"~, and becomes equal to the magnetizing current, thus supplying the necessary magnetizing current for the core C~.
The circuit l0U enhances the effective permeability of the core CZ, which in turn is used to greatly enhance the effective permeability of the core C,. The overall result is further improvement in the linearity and the ratio accuracy of the openable-core clamp-on current transformer.
In the circuits 30, 31 and 100 it is necessary for the windings to having the same number of 2~ turns as each other. If it is preferred to avoid this requirement, which in some instances -l2-necessitates a larger number of turns than is convenient, a more general circuit can be used. An example of such a circuit incorporating re:>istors is shown in Figure 5 of U.S.
Patent 5,307,008.
Example A current transformer embodying they circuit 30 was built into a clamp-on current transformer S with a ratio of 200 A/I A for testing and performance evaluation. The construction of the cores and windings are summarized in Tables 1 and 2, respectively.

COItC SPCCIrICATIONS

C, - four 0. I rllll7 tape-wound supermalloy cores with an initial permeability greater than 40,000 ( 1 ) inner side .50: 19.1 mm x 34.3 mm x 39.4 mm (2) outer side :52: 19.1 mm x 66 mm x 71. I mm (3,4) upper and lower sides 54, 56: 6.4 mm x 34.3 mm x 71.1 mm -pre-cut in two halves, I 0 C, - one 0.1 mm tape-wound supermalloy core with an initial permeability greater than 40,000 (1)core58:6.4mmx45.7mmx61 mm - precut in two halves C; - one 0.1 mm tape-wound supermalloy core with an initial permeability greater than 40,000 (1)core60:6.4mmx45.7mmx61 mm - precut in two halves _13_ WINDING SPECIFICATIONS

two measuring windings- each winding has a total of 150 turns (a pair of separate 75-Wmpumm turn )calf windings) correction winding - winding has a total of 1 50 turns (a pair of separate 75-turn W~ half ~.vindings) secondary winding - winding has a total of 200 turns (a pair of separate 100-turn W, half ~,viodings) primary winding - a single-turn winding of the feed-through type IO WP

As previously discussed, in conjunction with assembly -t0 of Figure 4)3, the entire core assembly is enclosed in a copper box for mechanical protection and electrostatic shielding and filled with epoxy to prevent me>vc:ments of various purls of the core assembly. The resulting measuring head has dimensions of 44.5 II1111 \ 22.9 mm x 82.6 mm.
The handle is 1 S made of insulating material with a strong spring action to ensure good surface contact between the two halves of the core assembly. 'tests were performed to evaluate the~errors and linearity of the current transformer with single-stage, two-stage, and three-stage operation at power frequencies.
Errors and Linearity 20 The errors and linearity of the transformer were measured at frequencies of 50 and 60 Hz, with burdens of 0.1 SZ and ISZ, and the core operating in the single-stage mode, two-stage mode (as circuit 20), and three-stage mode (as circuit ~0) using a current-comparator-based current transformer test set. This test set is described in detail in Moore, W.J.M and Miljanic, P.N., "The current comparator", Inst. Elec. Cng. Clectrical Measurement Series, Vol. 4, 25 London, UK: Peter Peregrinus, 1988. Since the results at both frequencies are similar, only the results for 50 Hz are shown in Table A and Table I3.

22~J i 90 '' Note that the results of the testing using a passive two-stage passive (as circuit 10) would be similar to the single-stage results listed in Tables A and B.
An error formula is defined as follows:
a +.I~ - (mss - lP)~a. ... (1) where a represents the in-phase error, ~i represents the quadrature error, and n is the transformer turns ratio.
One might expect the performance of the circuit 30 to be approximately the combination of the advantages of the single-stage and the electronically-aided two-stage (20). However, as l0 the performance results of Tables A and B indicate the improvement by adding the third stage is a major performance enhancement over the both the single-stage and electronically-aided two-stage devices.
As summarized in Tables A and B, with the core operating in the three-stage mode and burdens of 0.1 S~ and i S~, at currents between 0.5% and 100% of rated, the clamp-on current transformer is excellent W th errors of less than 10x10-6 for the in-phase component and less than 50x10-6 for the quadrature component. This is a major improvement over the electronically-aided two-stage mode with a burden of lia, where ezxors are less than 250x10'6 for the in-phase component and less than 100x10'6 for the quadrature component.
The repeatability characteristic of the transformer was also evaluated by repeatedly opening and closing the clamp-on current transformer and measuring the errors again afterwards. The variations in the errors were consistent to better than SOx 10-6 for both the in-phase and quadrature components.

-lj-TABLE A
Burden 0.1~

Frequency50 Hz IS (A) Single-stage Two-stage Three-stage {20) (30) a ~ a ~ a 1 -1030 + j5280123 + j62 -4 + j2S

0.5 -800 + j5590122 + j61 -5 + j24 0.1 -600 + j6250150 + j61 -5 + j28 0.05 -440 + j6680154 + jd5 -4 -t j29 0.01 -240 + j7170166 + jb4 -4 + j31 0.005 -180 + j7270176 + j62 -4 + j31 To summarize the results of Table A:
( 1 ) with a burden of 0. ll~, the electronically-aided two-stage mode compared to the single-stage mode provides an improvement in the accuracy of the hand-held clamp-on current transformer by a maximum of about 8 times (10301123 at lA) for the in-I 5 phase component and by a maximum of about 117 times (7270/62 at 0.005A) for the quadrature component;
(2) in contrast, with a burden of 0.11;2, the three-stage mode compared to the single-stage mode provides an improvement in the accuracy of the hand-held clamp-on current transformer by at least 45 times ( 18014 at O. U05A) and as high as 257 times (1030/4 at lA) for the in-phase component and at least 211 times (5280125 at lA) and as high as 234 times (7270131 at 0.005A) as for the quadrature component;
and (3) in addition, with a burden of 0.1 s~, the three-stage mode compared to the electronically-aided two-stage mode provides further improvement in the accuracy of the hand-held clamp-on current transformer by at least 30 times {12314 at lA} for the in-phase component and at least two times (62131 at 0.005A) for the quadrature component.

~L~~~ '~~~

TABLE B
Burden 1~

Frequency50 Hz 1s (A) Single-stage Two-stage Three-stage (20) (30) a ~ a ~ a 1 -1500 + j7050176 + j83 -3 + j37 0.5 -1150 + j7740188 + j80 -4 + j35 0.1 -770 + j8080195 + j79 -4 + j35 0.05 -560 + j8620206 ~- j79 -4 + j36 0.01 -300 + j9290228 + j84 -2 + j37 0.005 -220 + j9410230 + j82 -2 + jag To sununarize the results of Table B:
( I ) with a burden of 1 S~, the electronically-aided two-stage mode compared to the single-stage mode provides an improvement in the accuracy of the hand-held clamp-on current transformer by a maximum of about 8 times (1500/176 at lA) for the in-I S phase component and by a maximum of about 115 times (9410/82 at 0.U05A) for the quadrature component;
(2) in contrast, with a burden of 1 S~, the three-stage mode compared to the single-stage mode provides an improvement in the accuracy of the hand-held clamp-on current transformer by at least 110 times (220/2 at O.OOSA) and as high as 500 times (150013 at lA) for the in-phase component and at least 190 times (7050137 at lA) and as high as 241 times (9410/39 at O.OOSA) as for the quadrature component;
and (3) in addition, with a burden of 1!?, the three-stage mode compared to the electronically-aided two-stage mode provides further improvement in the accuracy of the hand-held clamp-on current transformer by at least 59 times {17613 at lA) for the in-phase component and at least two times (82/39 at O.OOSA) for the quadrature component.

Claims (14)

1. A current ratio device for use in constructing a current transformer, said device comprising:
(a) a first magnetic core, (b) a second magnetic core, (c) a third magnetic core, (d) a first measuring winding electromagnetically coupled with the second core and the third core but not with the first core, (e) a second measuring winding electromagnetically coupled with the third core but not with the first and second cores, (f) at least one amplifier having an input and an output, (g) the input of a said amplifier being connected to receive an alternating waveform appearing across the first measuring winding, (h) the input of a said amplifier being connected to receive an alternating waveform appearing across the second measuring winding, (i) outer winding means electromagnetically coupled with the first, second and third cores, said outer winding means including a secondary winding for connection across a measuring burden, and a) the output of a said amplifier being connected to said outer winding means toprovide a magnetizing current required to magnetize the first core when a primary winding is electromagnetically coupled with the first, second and third cores and an alternating current flows in said primary winding.

2. The device of claim 1, wherein there is only one amplifier, the input of which receives the waveforms from both the first and the second measuring windings and the output of which is connected to said outer winding means.

3. The device of claim 2, wherein the number of turns of the first and second measuring windings are equal to each other.

4. The device according to claim 2, wherein the outer winding means includes a correction winding, the output of the one amplifier being connected to the correction winding in series with a parallel connection of the first and second measuring windings to pass said magnetizing current therethrough.

5. The device according to claim 2, wherein the output of the one amplifier is connected to the secondary winding in series with a parallel connection of the first and second measuring windings to pass said magnetizing current therethrough.

6. The device of claim 1, wherein there are two amplifiers, the input of a first of said amplifiers receiving the waveform from the first measuring winding and the input of a second one of said amplifiers receiving the waveform from the second measuring winding.

7. The device of claim 6, wherein the number of turns of the first and second measuring windings are equal to each other.

8. The device according to claim 6, wherein the outer winding means includes a correction winding, the output of the first of said amplifiers being connected to said correction winding in series with the first measuring winding to pass said magnetizing current therethrough.

9. The device according to claim 8, including a further correction winding electromagnetically coupled with the second core and the third core but not with the first core, the output of the second of said amplifiers being corrected to said further correction winding in series with the second measuring winding to provide a magnetizing current for the third core.

10. The device according to claim 9, wherein the number of turns of the two correction windings are equal to each other.

11. The device according to claim 1, wherein said cores are combined into a toroidal core assembly and each of said windings is divided into a pair of separate portions, each such portion extending around the toroidal core assembly for a major part of a respective opposite half thereof to define between said portions a pair of diametrically opposite unwound core sections devoid of any windings.

12. The device according to claim 11, wherein the device is cut through both said unwound core sections along a plane containing the axis of the toroidal core assembly to form a pair of separate sub-assemblies that can be clamped together to reform the device.

13. The device according to claim 12, wherein said toroidal core assembly comprises:
(i) a toroidal outer core having a cross-section comprising a closed peripheral portion to define a cavity surrounded by the outer core, said outer core forming said first core, (ii) one inner core in said cavity forming said second core, (iii) another inner core in said cavity spaced from said one inner core, the other inner core forming said third core, said second measuring winding being wound around said third core, (iv) said first measuring winding being wound around both said inner cores, and (v) said outer winding means extending around the outside of the peripheral portion of the outer core.

14. The device according to claim 12, wherein said toroidal core assembly comprises:
(i) a toroidal magnetic outer core having a cross-section comprising a closed peripheral portion defining a cavity surrounded by the outer core, said outer care forming said first core, (ii) an toroidal intermediate core in said cavity having a cross-section comprising a closed peripheral portion defining an inner cavity surrounded by the intermediate core, said intermediate core forming said second core, and said first measuring winding being wound around said second core, (iii) an inner core in said inner cavity and forming said third core, said second measuring winding being wound around said third core, and (iv) said outer winding means extending around the outside of the peripheral portion of the outer core.