In switch gear current sensors are for example used to measure the amount of current in a conductor and provides measured current information to the protection relay to determine whether the current exceeds a predefined level, which could cause damage to the switch gear. If the said predefined current level is exceeded in the conductor, the protection relay switches off the current in the conductor.

In a fault condition, very high current flows through the switch gear current conductors. Reliable measurement of the current in each separate conductor is impeded by on the one hand the influence of the magnetic field of adjacent conductors and on the other hand by the magnetic saturation of the known current sensors, especially when the current range in a conductor is large.

Conventional current sensors have typically an annular core of a ferromagnetic material arranged around a conductor and a secondary coil wound around the annular core.

The current in the conductor produces a magnetic field, which magnetizes the ferromagnetic material, which in turn produces a current in the secondary coil. The current in the secondary is generally linear to the current in the conductor.

However, at a certain level of current in the conductor and accordingly at a certain level of magnetic field produced by this current, the ferromagnetic material of the annular core cannot be magnetized further. As a result the current generated by the magnetized core material in the secondary coil, does no longer follow the current in the conductor.

It is known to increase the size of such sensors, to be able to still measure the higher currents in the conductor. However, such bulky sensors are not desired. Especially, when the current is to be measured in three linked conductors, which is typical for busbars in switch gear, the large size of such sensors prevents the sensors to be arranged around the conductors and to be housed in the switch gear housing.

Recently, giant magnetoresistance sensors (GMR) have become available for measuring a magnetic field. These sensors comprise thin-film structures composed of alternating ferromagnetic and non-magnetic conductive layers, resulting in small sensors. The resistance of these structures change under the influence of a magnetic field. The sensor has accordingly a range in which the resistance substantially changes linearly on a change in magnetic field. The resistance decreases linearly on an increase of magnetic field. However, at a certain point of magnetic field intensity, the so-called saturation field, a further increase in the magnetic field does not result in a further reduction in resistance.

US 2012306487 discloses a current sensor using GMR sensors. In this publication it is proposed to arrange two pairs of GMR sensors at different distances from the conductor. By arranging the GMR sensors at a different distance the saturation point is reached at different magnetic field strength and as a result a wider sensing range is obtained.

However, also these current sensors are influenced by magnetic fields of adjacent conductors. Accordingly it is an object of the invention to reduce the above mentioned disadvantages.

This object is achieved according to the invention with a combination of at least three conductors and at least three current sensors, wherein each current sensor comprises:

- a body with a conductor reception space, wherein one of the at least three conductors extends through the conductor reception space;

- a first magnetic field sensor arranged in the body at a first distance from the reception space;

- a second magnetic field sensor arranged in the body at a second distance, different from the first distance, from the reception space;

- a sensor controller for combining the output of the first and second magnetic field sensor to provide an output signal correlated to the current of a conductor extending through the conductor reception space; and

wherein the combination further comprises: - a main controller connected to each sensor controller for receiving the output signals of each current sensor, wherein the main controller comprises

calibration means for gathering calibration data and for correcting the output signals of the current sensors for interfering magnetic fields.

With combination according to the invention, the size of the current sensors is kept limited by using at least two magnetic field sensors positioned on different distances from the conductor, such that despite the magnetic saturation problem a wide current sensing range is obtained. The relative small size of the current sensors allows for a positioning close to the conductors, such that the influence of the magnetic field of adjacent conductors is reduced.

The influence of the adjacent magnetic fields is further reduced by the provision of calibration means, which allows the combination to be calibrated, after the combination has been mounted and the relative position of the conductors and current sensors is fixed. The obtained calibration data is then used to correct the output signals of the current sensors, such that the influence of interfering magnetic fields is corrected and each output signal is representative for the current in the respective conductor.

Preferably, the first and second magnetic field sensors are giant magnetoresistance sensors. However, other magnetic field sensors like HALL, PCB based Rogowski coil could possibly be used.

In a preferred embodiment of the combination according to the invention the calibration means perform the following steps for gathering calibration data during calibration operation of the combination:

- using a defined current progress in each of the at least three conductors, recording the output signals of the at least three current sensors;

- comparing the defined current progress with the recorded output signals; and

- calculating a matrix of correction values based on the comparison.

After installing the combination according to the invention, the combination is put in a calibration mode, wherein a known defined current progress is applied on the conductors. For example, a varying current in a single conductor and then one after the other. This allows for the calibration means to gather calibration data, with which the influence of magnetic fields of adjacent conductors can be calculated. These influences on the output signals of the current sensors can then be corrected during normal operation of the combination with correction values resulting from the calibration step.

Preferably, these correction values are placed in a matrix, such that usual matrix calculations can be used for calculating the corrected output signals.

A further preferred embodiment of the combination according to the invention further comprises a protection relay interfacing circuit for interfacing with an existing protection relay, wherein the interfacing circuit is configured to activate the protection relay when the corrected output signals exceed a predefined limit. For example, the interfacing circuit amplifies the current feedback signal to a certain power level (VA) to power the protection relay. This is to make the current sensor according to the invention similar with the prior art current transformer based current sensors which also could power the protection relay. With the combination according to the invention, the accuracy of the current measurements is higher than with conventional methods. This allows for a better determination whether the current in a specific conductor exceeds a predefined limit, requiring that a protection relay is activated to cut off the power from the conductors.

Without the combination according to the invention, the influence of adjacent magnetic fields could increase the measurement of a current sensor, such that a limit would be exceeded even when the actual current is below said limit. It would also be possible that the actual current would exceed the limit, but that due to the influence of adjacent magnetic fields the sensor output indicates a lower current.

The invention also relates to a current sensor for a combination according to the invention, which current sensor comprises:

- a body with a conductor reception space configured for a conductor extending through the conductor reception space;

- a first magnetic field sensor arranged in the body at a first distance from the reception space;

- a second magnetic field sensor arranged in the body at a second distance, different from the first distance, from the reception space; - a sensor controller for combining the output of the first and second magnetic field sensor to provide an output signal correlated to the current of a conductor extending through the conductor reception space

A preferred embodiment further comprising at least one further magnetic field sensor arranged in the body at a unique distance from the reception space.

By providing additional magnetic field sensors, the sensing range can further be expanded and / or the accuracy of the measurements can further be improved by using the most accurate sensing range of each sensor.

Preferably, the first and second magnetic field sensors are giant magnetoresistance sensors.

In a further preferred embodiment of the current sensor according to the invention the body is an annular body, wherein the conductor reception space is provided by the center opening of the annular body.

An annular body provides a shape which corresponds to the known current sensors having an annular core of a ferromagnetic material. As a result such an embodiment of the invention can more easily replace older current sensors, without having to redesign the switch gear.

Preferably, the body is of an epoxy material and the first and second magnetic field sensors are embedded in the epoxy material. The epoxy material protects the magnetic field sensors for damage, for example due to heat.

Yet another embodiment of the current sensor according to the invention further comprises an energy harvesting device arranged in the body for powering at least the sensor and / or controller.

An energy harvesting device is able to convert for example the electrical field of the conductors into a power supply for the current sensor. Especially, when GMR sensors are used, the power consumption is typically low, such that an energy harvesting device can easily power the current sensor. As a result, it is possible to provide a self sustaining current sensor.

These and other features of the invention will be elucidated in conjunction with the accompanying drawings.

Figure 1 shows a schematic view of an embodiment of the combination according to the invention.

Figure 2 shows a schematic view of a current sensor according to the invention of figure 1.

Figure 3 shows a diagram of measured currents during the calibration operation.

Figure 1 shows a schematic view of an embodiment of the combination 1 according to the invention. This combination has three strip-like conductors 2, 3, 4. the conductors 2, 3, 4 extend through the central conductor reception space 5 of a respective current sensor 6.

The output signals 7 of the current sensors 6 are fed to a main controller

8, which has calibration and correction means 9, which correct the output signals 7 for influences of magnetic fields of adjacent conductors 2, 3, 4.

The corrected output signals 10 are then amplified using protection relay interfacing circuit 11 to a certain power level (VA) to power the protection relay. This is to make the current sensor according to the invention similar with the current transformer based current sensors according to the prior art which also power the protection relay.

Figure 2 shows a schematic view of a current sensor 6 with an annular body 13 and a central conductor reception space 5 through which a conductor 2 extends.

Embedded in the annular body 13, which is typically of epoxy material, are a first GMR sensor 14 and a second GMR sensor 15. The distance di of the first GMR sensor 14 to the conductor reception space 5 is smaller than the distance d ₂ of the second GMR sensor 15.

When the current in the conductor 2 increases, the first GMR sensor 14 will first reach the saturation point at which moment, the second GMR sensor 15 is still able to measure the current in the conductor 2. The sensor controller 16 combines the measurements of both GMR sensors 14, 15 and provides a single output signal which is related to the current in the conductor 2.

In order to calibrate the combination 1 according to the invention in a calibration mode, the conductors 2, 3, 4 are provided with a defined current progress , I ₂ , I3. As shown in figure 3, the first conductor 2 is provided with a linear increasing current h, while the second and third conductors 3, 4 are kept power free (I ₂ , 13) . The resulting sensor output Si, S ₂ , S3 measured by the current sensors 6 is also depicted in figure 3.

The sensor output Si will follow the current progress , but the adjacent current sensors 6 will measure an interfering magnetic field and accordingly register also a current S ₂ , S3. In this calibration mode, these registered currents S ₂ , S3 are used to calculate to correction values, as it is known in this calibration mode, that the resulting sensor output S ₂ and S3 should read no current. These correction values are used in the normal operation mode of the combination by the calibration and correcting 9 to correct the output signals 7.