Busbar current transducer with suppression of external fields and gradients

Yokeless busbar current sensors are cheap and compact, but they are sensitive to external magnetic fields and field gradients [1], [2]. We have described current transducer based on a couple of microfluxgate sensors [3] inside the cylindrical hole in the busbar [4], [5]. The advantage of this transducer is high range and small size compared to similar transducer based on circular sensor array around the conductor [6]. Compared to Hall current sensors [7], [8], fluxgate current sensor has 10-times better stability. Solution based on AMR has 10-times lower range due to the small saturation field of AMR sensors [9]. We have suggested that a partial suppression of the magnetic fields generated by external currents can be achieved by using four microfluxgate sensors: two of them are active and they measure y-component of the field at points A and C(B
_Ay
and B
_Cy
) the other two measure both field components at point B(B
_Bx
and B
_By
) [10]. This approach can be effective for the case of known geometry of the external currents or when the external currents are also measured. However, in general case the position and size of the external current is not always known and the interference field can be generated by several currents simultaneously. Significant magnetic fields can also by generated by coils and by magnetic materials (both hard and soft in external field) in the vicinity of the transducer. In this paper we therefore suggest current sensor which is immune to external homogeneous fields and first order field gradients. This means that our new solution does not perfectly compensate the field from external conductors, but effectively suppresses their influence, as higher order gradients from distant currents are small. Fig. 1 shows the current busbar with fluxgate sensor array inserted into the hole and field distribution shown on a busbar cross section. The size of the copper busbar is 6 x 1 cm and the hole diameter is 19 mm. The DC current value for this simulation was 1000 A. The sensor array shown on the photograph consists of six sensors - three of them are visible as a row with 6 mm pitch, which measures By in positions A,B and C. The other 3 sensors are on the opposite side of the PCB and they measure Bx at the same points. By moving the sensor array in y direction we can scan the field values at other positions above the central plane and verify the simulations. In general the fit between our measurements and FEM simulations is within 5%. After performing the simulation and measurement exercise we fixed the sensor optimum position for the final transducer design. All previously published current transducers of this type such as [3] and [7] used sensors in the central line (positions A and C) measuring By. The fact that B
_Ay
and B
_Cy
have opposite sign was used to suppress homogeneous external field B
₀
. However, field distribution of By along the A-C line is linear (Fig. 2 left) and this means that external dBy/dx field gradient cannot be suppressed. Possible solution of this problem would be to change the busbar geometry in order to achieve significantly non-linear field profile and evaluate second-order gradient from the measured current. We have made extensive simulations with various geometries but we never achieved significant non-linearity. Instead of that we decided to find alternative sensor positions. The novelty of our approach is that we use Bx instead of By. Bx response to the measured current does not change sign when mirroring around the y axis. At H and I points the sensitivity is the same, 5mT/1000 A. Due to the 2 mT maximum range of the DRV425 sensor the actual current range will be 400 A. Summing B
_Hx
+ B
_Ix
therefore does not suppress homogeneous external field B
_0x
, but instead of that it suppresses dBx/dx field gradient. This type of gradient is not typical for long conductors, but for dipolar sources such as coils and permanent magnets. This setup is also completely insensitive to external currents in x direction. The dependence on B
_0x
can be suppressed by using the third x-sensor at the central point A. Remaining is the dependence on dBx/dy field gradient. Here we face the similar problem as before, because Bx response to the measured current change sign when mirroring around the x axis. Fortunately, field distribution along the DE line is strongly non-linear and thus the external dBx/dy field gradient can be suppressed. This is documented by the Bx profile along the DE line (Fig. 2 right). We achieve the required compensation by using additional sensor located at J and eventually also K point. While B
_Jy
- B
_hy
is insensitive to the measured current, this value depends only on dBy/dx field gradient. The resulting formula for the measured field B
_m
is B
_m
= B
_H
+ B
_I
- (B
_J
+ B
_K
)/2- B
_B
and in our example the lateral (x-direction) current is suppressed completely, while the superior (y direction) current in the vicinity of 20 cm is suppressed by the factor of 167. In conclusion, by using novel geometry of the fluxgate current transducer we achieve compensation of external fields and first order field gradients.