SHUNT ASSEMBLY FOR CURRENT MEASUREMENT

Abstract: A shunt assembly for current measurement is formed of a shunt element (12) of substantially cylindrical form and of ZTC material having a pair of current connectors in the form of flat strips (10, 11) attached to its ends. Preferably sensing connectors (13, 14) of the same ZTC material are connected to the ends of the shunt element through holes in the connectors. The sensing connectors may form part of a continuous wire passing through an axial hole through the shunt element. Alternatively, they may be formed as integral extensions of the shunt element.

Description: Slnva.-ri.-t Assembl y * -Fo r- * Cu - r-ervfc Measu rement The present invention relates to current measurement, and more specifically to current shunts. The standard technique for measuring an electric current is to pass the current through a resistor of small known resistance and measure the voltage drop across the resistor. Thus the current is in effect measured by a voltage measuring device or ci rcuit which is shunted by the resistor, which is therefore termed a shunt. The shunt has 4 terminals or connections; 2 end connections through which the current to be measured flows, and 2 further connections (nor¬ mally located close to the current connections) for picking off the voltage resul¬ ting from the current flow. The latter 2 terminals are termed sensing or Kelvin terminals. The connection to a sensing terminal to the shunt will involve a thermo¬ electric effect (assuming the connector is of a different material to the shunt). The temperature of the shunt will be liable to vary, because of both ambient temperature variations and the heating effect of the current being measured (assuming that that current may be substantial); the current heating will of course be subject to a lag due to the thermal capacitance of the shunt. To avoid temperature sensitivity, the two connectors are therefore normally of the same material. For accurate current measurement, the shunt must be of known value. For some purposes, the value of the shunt must be accurately controlled. Often, however, it is sufficient for the value of the shunt to be measured accurately, with the voltage measurement being converted to the current value by suitable calculation. The shunt must also be stable. To achieve suitable low resistance, shunts are normally made of metal alloys having a resistance higher than that of good conductors but nevertheless fairly low. The main cause of instability in such shunts is temperature variation. To overcome this, ZTC (zero temperature coef¬ ficient) alloys have been developed which have a substantially zero temperature coefficient of resistance. The resistivity of a material plotted against temperature can be expressed as a polynomial function of temperature. The linear term is normally predomi¬ nant, its coefficient being the temperature coefficient of resistance of the material, with higher terms having progressively smaller coefficients (so that they are only significant if the temperature range is large). However, materials have been developed in which the coefficient of the linear term is substantially zero. The best-known of such alloys is termed manganin, which consists of 83-85% Cu, 10- 13% Mn, and 4% Ni, for which the leading term in its resistivity function is quadratic, giving a substantial temperature range over which the resistivity is substantially constant. Other materials such as zeranin are also available, giving even larger temperature ranges over which the resistivity is substantially constant; for zeranin, the l i near and quadratic terms of its resistivity function are both substantially zero, so the leading term in its resistivity function is cubic. A further requirement for accuracy is that the shunt should have a linear characteristic of voltage against current. It turns out that Ohm's law is only approximate for typical shunts, as the current distribution through the body of the shunt tends to vary slightly with the size of the current. This phenomenon is termed "current crowding". Known techniques for overcoming this effect include making the shunt element of sinuous or zig-zag shape, and making a number of sensing contacts to it to average out the voltage variations due to current crowding. The physical size of the shunt may also impose certain requirements. For sensing large currents, the current connections to the shunt will be large, with cross-sections of the order of several mm 2 , and it is desirable for the shunt to have a cross-section of roughly comparable size. It is also desirable for the shunt to have a length of some mm; a shorter length is difficult to control accurately and to form Kelvin connections to, while a longer length may result in an inconveniently large shunt. The general object of the invention is to provide an improved shunt. According to the invention there is provided a shunt assembly for current measurement characterized in that it comprises a shunt element of substantially cylindrical form and of ZTC material having a pair of current connectors in the form of flat strips attached to its ends. Preferably sensing connectors of the same ZTC material are connected to the ends of the shunt element through holes in the connectors. A particular application of current shunts is in electricity distribution boxes, where a number of output cables are fed from a number of input cables. The box typically contains a number of connectors or busbars in the form of flat strips, normally of copper, which connect the input terminals to the output ter¬ minals, often along routes which have angles or zig-zags in them. The inclusion of a current shunt in a busbar allows the current through it to be monitored. A conventional shunt element normally has the same section as the copper strips, and the strips are normally in the same plane. It is important to have good (ie stable) . connections between a shunt element and the copper strips to which it is connected, and it is usual to make such connections by electron beam welding. This places a practical lower limit (in the region of 2 mm) on the length of the shunt element. This in turn places a practical lower limit on the resistance of the shunt. For some applications, a very low shunt resistance (eg in the region of 70 μΩ) is desirable, but cannot be achieved with this technique. Further, if the copper strips are at right angles to each other, it may be desirable to locate the shunt element at their junction (in the form of an exten¬ sion on the end of one of the strips, connected to the side edge of the other strip at its end). However, the current distribution through the shunt element is skewed, with a greater current density at the inner part of the shunt element than the outer. This conventional arrangement therefore suffers from current crowding effects and changes of resistance with current. In the present invention, the strips forming the busbars are of course in planes which are offset by the length of the shunt element. This may require a crank in one of the strips if it is essential that the two strips should return to a common plane. However, there are many situations where it is desirable for the two strips to be in different planes. The present invention automatically introduces a change of plane between the two strips, and is therefore particu¬ larly suitable for such situations. The distance between the planes can be adjusted, within broad limits, by a suitable choice of the dimensions of the shunt element. Further, the attachments of the two strips to the ends of the shunt element are independent, so the two strips can therefore be set at any desired angle to each other, eg in-line or at right angles. The shunt of the present invention is easy to manufacture and of low cost, and has high accuracy. Further features of the invention will become apparent from the following description of a shunt assembly embodying the invention, given by way of example and with reference to the drawings, in which: Fig. 1 is a perspective sketch of a shunt assembly; Fig. 2 is a section through the shunt assembly of Fig. 1 ; and Fig. 3 is a perspective view of a modified shunt element. Referring to Fig. 1 , the shunt assembly comprises a pair of strips 10 and 1 1 with a ZTC shunt element 1 2 connected between them. As shown, the strips 10 and 1 1 are in different but parallel planes, with the axis of the shunt element 12 perpendicular to their planes. The angle between the two strips is shown as 90 * , but the angle between them can clearly have any value (including 0°, in-line). For a given desired shunt resistance, the distance between the two planes can be adjusted within reasonable limits by choosing the diameter of the shunt element appropriately. (To hold the resistance constant, the diameter of the shunt element should be increased as the square root of its length.) The voltage across the shunt element is sensed by two sensing connections 12 and 13. These two connections need to make contact with the ends of the shunt element. It is possible to form these contacts by using discs (with projec¬ ting tabs) which are held between the ends of the shunt element 12 and the strips 10 and 1 1. However, it is preferred to form these contacts by connections to the centres of the ends of the shunt element. The sensing connections are therefore connected to the ends of the shunt element 12 through holes 15 and 16 (Fig. 2) through the strips 10 and 1 1. The sensing connections could be of any suitable material, and could be attached to the ends of the shunt element. However, it is preferred to insert them into holes formed in the ends of the shunt element. It is also preferred to form them of manganin (ie the same material as the shunt element), and to form them from a single length of manganin wire which passes through an axial hole through the entire length of the shunt element, as shown in Fig. 2. The por¬ tion of the wire inside the shunt element in effect becomes a part of the shunt element. The use of the same material for the sensing connections as the shunt element also minimizes thermo-electric effects. Fig. 3 shows an alternative form of shunt element, which is formed as a cylinder 12' with integral connectors 13' and 14' projecting from its ends. The various components of the shunt assembly can be soldered together by suing a simple solder and heat, eg by spreading a solder paste over the appro¬ priate areas and heating the assembly. We have found that this results in an assembly with highly stable electrical characteristics. The surfaces which require soldering together are the ends of the shunt element and the facing areas of the strips forming the busar, and the hole through the shunt element and that part of the connector wire within the hole. We have also found that it does not matter whether the holes 15 and 16 in the strips are filled with solder or not, and that the arrangement is very tolerant to production quality problems, such as poor soldering. We have found that such shunt assemblies have highly stable electrical characteristics, over current ranges of typically 200 A to 100 A. The diameter of the manganin rod will normally be chosen to match the current range to be measured. Very low currents can be measured if the diameter is small; in the limit, the same manganin rod or wire can be used for both the shunt resistor and the sensing connections. For very large currents, multiple rods could be used in parallel; either one can be provided with sensing connections, or they can all be provided with sensing connections which are connected together (or which have their outputs averaged). The current distribution through the central (equatorial) parts of the shnt element is likely to be fairly uniform, but the distribution through the ends of the shunt element and the adjacent portions of the busbar will presumably have a fairly complicated pattern. However, either there is no significant current crowding or the effects of what current crowding there is are closely balanced. As noted above, the objective is to achieve a stable characteristic rather than one which is accurately determined. By cutting the shunt element to the appropriate length from a stock rod of manganin, an accuracy of 1-5% can be achieved. (It is desirable to measure the diameter of the manganin rod rather than use the nominal value for good accuracy, as manganin rods are normally made by extrusion and the diameter of the rod tends to increase as the extrusion die wears). A much greater accuracy can however be obtained if the diameter of the manganin rod is accurately controlled; the diameter can be controlled to a tolerance of 10 μm, giving a resistance accuracy of around 0.1%. Variations in the soldering, etc may have a slight effect on the accuracy. If desired, the shunt element can be turned or ground down gently after manufacture to trim its value. Normally, however, the value will be measured accurately and used to obtain accurate current values from the voltages measured across the shunt.

Claims: Claims 1 A shunt assembly for current measurement characterized in that it comprises a shunt element (12) of substantially cylindrical form and of ZTC material having a pair of current connectors in the form of flat strips ( 10, 1 1) attached to its ends. 2 A shunt assembly according to claim 12 characterized by sensing connectors ( 13, 14) of the same ZTC material connected to the ends of the shunt element through holes in the current connectors ( 10, 1 1). 3 A shunt assembly according to claim 2 characterized in that the sensing connectors ( 14, 15) form part of a continuous wire passing through an axial hole (17, Fig. 2) through the shunt element (12). 4 A shunt assembly according to claim 2 characterized in that the sensing connectors ( 14, 15) are formed as integral extensions of the shunt element (Fig. 3). 5 Any novel and inventive feature or combination of features specifically disclosed herein within the meaning of Article 4H of the International Convention (Paris Convention).

Inventors: AMEY KEITH ROBERT, BRADY DAVID, MOORE PAUL MARTIN, AMEY, KEITH, ROBERT, BRADY, DAVID, MOORE, PAUL, MARTIN
Applicants: AMEY KEITH ROBERT, BRADY DAVID, MOORE PAUL MARTIN, REMOTE METERING SYSTEMS LTD, AMEY, KEITH, ROBERT, BRADY, DAVID, MOORE, PAUL, MARTIN, REMOTE METERING SYSTEMS LTD.
Application year: 1996
Main classification: G01R1
IPC codes: G01R1, G01R15, G01R19