EP1166131A1

EP1166131A1 - Current meter

Info

Publication number: EP1166131A1
Application number: EP00902617A
Authority: EP
Inventors: Thomas Pniok; Gerd Jodehl; Heinz Wollny
Original assignee: AEG Niederspannungstechnik GmbH and Co KG
Current assignee: AEG Niederspannungstechnik GmbH and Co KG
Priority date: 1999-03-31
Filing date: 2000-01-19
Publication date: 2002-01-02
Also published as: CA2368901A1; DE19914772A1; KR20010111285A; CN1353818A; WO2000060366A1; JP2002541460A; AU2438700A; CN1180266C; KR100681577B1

Abstract

The invention relates to a current meter which is based on the principle of surface field measurement. Hall sensors (1) are used which are arranged in such a way, that they are excited by a magnetic field which is proportional to the current. Two Hall sensors are arranged in such a way, that they detect the field to be measured quantitatively in a uniform manner, but with opposing polarity signs. By subtracting the output signals from the Hall sensors, the measured value of the current is amplified, whilst external parasitic induction is eliminated.

Description

CURRENT SENSOR

DESCRIPTION

The invention relates to a current measuring transducer for the potential-separated measurement of high direct currents in power distribution systems with nominal voltages up to several kV. For example, this sensor can be used with an overcurrent relay for DC high-speed switches.

So far, shunt isolating amplifiers, measuring devices with ferrite cores and Hall sensors and LEM converters have been used to measure currents. However, these known devices have considerable disadvantages.

For example, in the case of a current measurement using a shunt resistor, the current to be measured is passed through a measuring resistor and the voltage drop caused by the current is measured. This form of measurement unnecessarily consumes energy that contributes to heating the switch environment. Furthermore, this current measuring device is necessarily integrated in the circuit to be measured, as a result of which the current to be measured itself is influenced, which is why the measurement is more or less falsified depending on the size of the shunt resistor. Furthermore, a floating measurement at high voltages is not possible in this way.

In LEM converters, a ferrite core is arranged around a conductor whose current flow is to be measured. A second coil is arranged around this ferrite core, through which a

Current is controlled so that the resulting magnetic field is adjusted to zero. In this way, an isolated measurement is possible, but the costs for this type of measurement are high. Using a Hall sensor, a magnetic field can be measured relatively well using the Hall effect. A Hall sensor generates a voltage that is proportional to the magnetic field that acts on the Hall sensor. This Hall effect occurs to different extents depending on the material used, Hall sensors made of a semiconductor are the most advantageous. The Hall sensor can thus measure a magnetic field which is induced by a current flowing through a conductor. In this way, the measurement of the current is electrically isolated.

However, since the voltage generated by the Hall sensor during the measurement is low and the magnetic field is exposed to external influences under normal conditions, an amplification of the magnetic field is required, which is usually achieved by ferrite cores. In this device, however, a non-linearity occurs due to the saturation behavior of the magnetic cores used. This leads to the disadvantage that the current can only be measured with sufficient accuracy in a certain limited range, but the measured current outside this range deviates greatly from the actual current due to the saturation behavior. Furthermore, the costs for this measuring device are relatively high due to the ferrite core.

In summary, the devices according to the prior art have the disadvantages that a non-linearity occurs due to the saturation behavior, the devices only have a limited dielectric strength, the devices cause relatively high costs and also have a high level of self-consumption.

REPLACEMENT SHEET RULE 26 Therefore, the invention has for its object to provide an inexpensive current sensor that detects the current value isolated, has sufficient accuracy and has extensive immunity to interference.

This object is achieved by a current sensor as set out in the appended claim 1. This means that the current sensor according to the invention has at least two Hall sensors arranged on a conductor. The Hall sensors are arranged such that they detect the same amount of a magnetic field generated by a current flowing through the conductor and the same amount of an interference field and detect either the magnetic field or the interference field with different signs.

Further configurations are set out in the subclaims.

Accordingly, either adding or subtracting the current measurement value is amplified, but an interference field due to external interference is eliminated.

The current sensor according to the invention advantageously achieves that the current value acquisition can be carried out inexpensively, since no additional ferrite cores are required. Extensive immunity to interference and a high insulation voltage can be achieved.

According to the invention, a current sensor for a simple electronic trigger in the direct current range can be obtained in this way, which detects the current value with potential isolation for direct voltages up to 4 kV.

REPLACEMENT LEAF REGE The invention is described below using exemplary embodiments with reference to the accompanying drawings. Show it:

1 is a schematic representation of an overcurrent relay with a current sensor attached to a line according to a first embodiment;

2 is an enlarged partial view of the illustration of FIG. 1, showing the current sensor in more detail,

3 shows a current sensor according to a second exemplary embodiment,

Fig. 4 is a block diagram of an evaluation circuit for a current sensor according to the first or second embodiment, and

5 shows a block diagram of an evaluation circuit for a current sensor with four Hall sensors according to a third exemplary embodiment.

1 shows a schematic illustration of an overcurrent relay according to a first exemplary embodiment. The overcurrent relay has a base switch BS and

Arc extinguishing system LS, the exact function of which, however, is not important for this exemplary embodiment, which is why their detailed description is omitted.

A current sensor SMA is attached to a line 2. This SMA current sensor is based on the principle of surface field measurement or the Hall effect.

The current sensor SMA according to the first exemplary embodiment is shown in more detail in FIG. 2. Here are on part of a conductor 2 two Hall sensors la and lb arranged opposite each other.

Two Hall sensors la and lb are used since the magnetic field is relatively weak and due to interference from the environment, i.e. an interference field is disturbed. To eliminate these external interferences, the two Hall sensors 1 a and 1 b are arranged such that both Hall sensors measure the magnitude of the magnetic field generated by the current flow equally, but each measure the magnetic field with opposite signs. If the amount of the magnetic field generated by the current flowing through the conductor 2 is B, the Hall sensor la measures, for example, a magnetic field + B, whereas the Hall sensor 1b measures a magnetic field -B.

The output signals from the two Hall sensors la and lb are subtracted. This eliminates the interference field from the output signals and amplifies the measured value of the magnetic field. It is therefore not necessary to amplify the magnetic field itself with the aid of, for example, ferrite cores, as according to the prior art, since the interference field is largely eliminated by subtracting the signals and a strong measurement signal of the magnetic field to be measured is generated.

The measured value of the Hall sensor la is denoted by MWla and the measured value of the Hall sensor lb is denoted by MWlb. If the two Hall sensors are arranged close enough to one another, the interference field can be assumed to be the same on both Hall sensors. This results in the following for the measured values:

MWla = + B + S MWlb = -B + S

REPLACEMENT SHEET RULE 26 S here denotes the interference field. The subtraction of the two measured values thus leads to the total measured value MW:

MW = MWla - MWlb = + B - (- B) + S - S = 2B

This eliminates the interference field and doubles the useful measurement value, i.e. the measured magnetic field.

Alternatively, the two Hall sensors can also be arranged such that they each measure the entire measured magnetic field with different signs, i.e. So the useful field B with the same sign and the interference field S with different signs. In this case, the interference field is canceled by adding:

MWla = B + S MWlb = B - S MW = MWla + MWlb = 2B

As described above, the arrangement of the

Hall sensors to ensure that the probes are as short as possible from one another so that the interference field at the positions of the Hall sensors 1 is as equal as possible. It is also important that the field strength is not affected by current displacement influences. The arrangement of the Hall sensors on round conductors is advantageous. 2, for example, the conductor 2 in the Hall sensors is designed as a round conductor.

So that the two Hall sensors measure the same amount of magnetic field generated by the current flowing in the conductor 2, both Hall sensors should be arranged at the same distance from the conductor 2.

REPLACEMENT SHEET RULE 26 In addition, the Hall sensors 1 can be arranged such that the conductor 2 runs between the two Hall sensors 1, as shown in FIG. 2. This arrangement is one way of arranging the Hall sensors in such a way that they detect the magnetic field in the same amount, but with opposite signs. However, other arrangements are of course also conceivable, for example an arrangement in which the two Hall sensors 1 are arranged directly next to one another on one side of the conductor 2.

According to FIG. 1, the current measuring transducer consisting of the Hall sensors 1 is installed in a predetermined conductor configuration, such as, for example, according to FIG. 1 with an overcurrent relay with the conductor 2 and the return conductor 4.

Hall sensors 1 a and 1 b can therefore be arranged and calibrated in such a way that the influence of return conductor 4 is taken into account, thereby reducing known interference effects due to the conductor configuration. The current sensor according to the first embodiment can thus preferably be used in a known and rigid conductor configuration.

A current sensor according to a second embodiment is described below, which can also be used with an unknown conductor configuration.

This current sensor is shown in Fig. 3. According to FIG. 3, the two Hall sensors are surrounded by a tubular shield 3. This measure shields the external interference, which is why a more precise measurement is possible.

According to the embodiments described above, two Hall sensors, i.e. use a pair of Hall sensors

REPLACEMENT BLADE (RULE 26) det. However, any number of Hall sensor pairs can be used as long as they are connected in such a way that the interference field is eliminated and the measured value signals are added.

The distance between the resulting measured value MW and the interference field can be increased by increasing the number of Hall sensors, since the interference field is eliminated in each pair of Hall sensors while the measured signal is doubled. This means that a 2n-fold magnetic field is measured with n Hall sensor pairs.

An evaluation circuit for a current sensor according to the first or second embodiment with two Hall sensors is described below with reference to FIG. 4.

Two Hall sensors 11 and 21 are arranged opposite one another on a tubular conductor L. These Hall sensors are arranged opposite each other in such a way that the interference field is eliminated by subtracting the output signals from the two Hall sensors.

The output signal from the Hall sensor 11 is first passed to a temperature compensation sensor 12. The temperature influence on the measurement is eliminated by this temperature compensation sensor 12. The compensated signal is amplified by an amplifier 13, the amplified signal being fed to an offset compensation device 14, in which an offset of the signal is compensated.

In the same way, the output signal from the Hall sensor 21 becomes a temperature compensation sensor 22, an amplifier 23 and an offset compensation device 24 fed. The signals are compared with one another by the offset matching devices 14 and 24 so that they can be fed to a subtractor 5.

The subtractor 5 subtracts the two measurement signals from one another and outputs a resulting signal in which the interference field is eliminated as described above. The output signal from the subtractor 5 is amplified by an amplifier 6 and output to corresponding further processing units. An overcurrent release 8 and a signal converter interface 7 are shown here as an example. The overcurrent release can be a release as described in the first embodiment. The signal converter interface 7 outputs, for example, a current that is proportional to the measurement signal and varies, for example, between 4 and 20 mA. In addition, further interfaces can be connected, as indicated by dashed lines with reference number 9.

As already described above, the number of Hall sensors of a current measuring transducer is not limited to two, but any number of pairs of Hall sensors is possible.

An evaluation circuit for a current sensor with four Hall sensors according to a third exemplary embodiment is described below with reference to FIG. 5.

In the illustration, the same reference numerals correspond to the same components as in FIG. 4. This means that the two branches shown in the upper half of the illustration with the temperature compensation sensors 12 and 22, the amplifiers 13 and 23 and the offset compensation devices 14 and 24 of the arrangement corresponds to FIG. 4. From-

REPLACEMENT SHEET RULE 26 The output signals of these two branches are subtracted from one another by a subtractor 51, as a result of which the interference field is canceled. The output signal from the subtractor 51 is amplified by an amplifier 61 before being fed to an adder 15.

In addition to this arrangement, two further Hall sensors 31 and 41 are arranged on the conductor, which are spatially shifted, for example, by 90 ° with respect to the arrangement of Hall sensors 11 and 21. In a similar way to that described above, the output signal from the Hall sensor 31 is fed to a temperature compensation sensor 32, the temperature-compensated signal is amplified by an amplifier 33 and an offset or offset compensation is carried out by the offset compensation device 34. The output signal from the Hall sensor 41 is fed to a temperature compensation sensor 42, the temperature-compensated signal is amplified by an amplifier 43 and an offset or offset compensation is carried out by the offset compensation device 44. The output signals of the

Offset adjustment devices 34 and 44 are then subtracted from one another by a subtractor 52, the output signal from the subtractor 52 being amplified by an amplifier 62 before it is fed to the adder 15.

The adder 15 adds the resulting measurement signals from the two Hall sensor pairs 11 and 21 and 31 and 41. The sum signal is amplified by an amplifier 16 and then fed to the further units 7, 8 and 9 as in the evaluation circuit according to FIG. 4.

According to the third exemplary embodiment, two pairs of Hall sensors have been used. As already described above, a higher number of Hall sensor

SPARE BLADE (RULE 26) pairs can be used. The evaluation circuit for such an arrangement can be constructed similarly to that shown in FIG. 5, in which case a plurality of signals are fed to the adder 15.

4 and 5, an arrangement can be selected for the Hall sensors in which, as mentioned in the description of the first exemplary embodiment, the interference field is eliminated by adding the output signals. This means that in this case the Hall sensors must be arranged in such a way that they detect the magnetic field generated by the conductor with the same sign in each case, but the interference field with different signs. In the evaluation circuit, the subtractor 5 must then be replaced by an adder according to the third embodiment. In the variant of the evaluation circuit with two pairs of Hall sensors, the subtractors 51 and 52 according to the fourth exemplary embodiment must each be replaced by adders.

A current sensor based on the principle of surface field measurement has been specified above. The current sensor has at least two Hall sensors 1 a and 1 b arranged on a conductor 2. The Hall sensors are arranged such that they detect the same amount of a magnetic field generated by a current flowing through the conductor and the same amount of an interference field and detect either the magnetic field or the interference field with different signs. Accordingly, either the addition or subtraction amplifies the measured current value, but eliminates external interference caused by an interference field.

REPLACEMENT SHEET RULE 26

Claims

PATENT CLAIMS

1. Current sensor with at least two on a conductor (2) arranged Hall sensors (la, lb), the Hall sensors (la, lb) being arranged such that they have the same magnitude as a magnetic field generated by a current flowing through the conductor (2) and Detect an interference field with the same amount and detect either the magnetic field or the interference field, each with a different sign.

2. Current sensor according to claim 1, wherein the Hall sensors (la, lb) are arranged such that the magnetic field generated by the current flowing through the conductor (2) current is detected by both Hall sensors with different signs, and the output signals of the Hall sensors (la , lb) are subtracted from each other.

REPLACEMENT SHEET RULE 26

3. Current sensor according to claim 1, wherein the Hall sensors (la, lb) are arranged such that the magnetic field generated by the current flowing through the conductor (2) is detected by both Hall sensors with the same sign, and the output signals of the Hall sensors (la, lb) are added.

4. Current sensor according to one of the preceding claims, wherein two Hall sensors (la, lb) are arranged such that the conductor (2) runs between the two Hall sensors.

5. Current sensor according to one of the preceding claims with a shield (3) which is attached around the Hall sensors (la, lb) and the conductor (2).

6. Current sensor according to one of the preceding claims, wherein the conductor (2) is a round conductor.

7. Current sensor according to one of the preceding claims, wherein the Hall sensors (la, lb) have the smallest possible distance from one another.

8. Current sensor according to one of the preceding claims, wherein the Hall sensors (la, lb) each have the same distance from the conductor (2).

9. Current sensor according to claim 2, wherein a plurality of pairs of Hall sensors (11 and 21, 31 and 41) are provided, with each pair, the output signals are subtracted from each other by a subtractor (5, 51, 52) and the resulting output signals from the

REPLACEMENT SHEET RULE 26 Hall sensor pairs can be added by an adder (15).

10. Current sensor according to claim 3, wherein a plurality of Hall sensor pairs (11 and 21, 31 and 41) are provided, with each pair, the output signals are added by an adder and the resulting output signals from the Hall sensor pairs are added by an adder (15) .

11. Current sensor according to one of the preceding claims, wherein the output signal of a Hall sensor (11, 21, 31, 41) is supplied to a temperature compensation sensor (12, 22, 32, 42).

REPLACEMENT BLADE (RULE 26)