Technical field

The present invention relates to current measurement based on magnetic field sensors. Furthermore, the present invention relates to adapting a current measurement system including a magnetic field sensor to correct errors.

Background of the invention

Current measurement in medium- and high-voltage installations strongly requires galvanic isolation to protect the measurement equipment from high voltage exposure. Conventionally, magnetic field sensors are used which are located close at a power transmission line through which the primary current to be measured flows so that a magnetic field caused by the primary current can be measured and evaluated.

As magnetic field sensors magnetoresistive sensors can be used. Magnetoresistive sensors may include a GMR (giant magnetoresistance) sensor or the like and allow measuring of both DC and AC currents. Magnetoresistive sensors can be provided without any hysteresis, since no iron core is required which is conventionally used in other type of current measurement systems. Due to this advantage, magnetoresistive sensors can provide a high sensitivity (i.e. good detection level also for small signals) and a very high dynamic range, which facilitates detecting residual current or earth fault currents. Particularly, at high-frequency currents, the high sensitivity enables detecting control signals transmitted over the transmission line. Furthermore, transient events and harmonics present in the transmission line can be measured due to the high bandwidth of magnetoresistive sensors. However, magnetoresistive sensors exhibit pure accuracy due to a high temperature dependency and high manufacturing tolerances. Gain error, aging error and temperature error can be significant. It is therefore an object of the present invention to provide a method for measuring currents flowing in a transmission line using a magnetoresistive sensor with an increased accuracy.

Summary of the invention

This object has been achieved by the method for measuring a primary current in a transmission line using a magnetic field sensor device according to claim 1 and by a current measurement system according to the further independent claim.

Further embodiments are indicated in the depending claims.

According to a first aspect a method for measuring a primary current through a transmission line by means of a magnetic field sensor device is provided, comprising the steps of:

- injecting a reference signal through a portion of the transmission line associated with the sensor device;

- receiving a fed-through reference signal;

- extracting a sensor signal portion from an obtained sensor signal, wherein the sensor signal portion results from the injected reference signal flowing through the magnetic field sensor device;

- performing a comparison based on the fed-through reference signal and the extracted sensor signal portion to obtain a correction signal;

- providing a measurement signal depending on the sensor signal and on the correction signal.

Throughout this application and for clarity, reference signal means reference current signal, fed-through reference signal means fed-through reference current signal, and injected reference signal means injected reference current signal. Furthermore, fed-through reference current signal signifies that portion of the injected reference current signal, which is flowing through the portion of the transmission line associated with the magnetic-field sensor device.

One idea of above method is to inject a reference signal into the transmission line, so that a corresponding reference current is caused to flow through the sensed portion of the transmission line which is magnetically coupled / associated with a magnetic field sensor device. The reference signal has a known amplitude and frequency, wherein the frequency is substantially higher than the frequencies of the primary current on the transmission line.

The magnetic field sensor device provides a sensor signal (e.g. as a sensor voltage) related to the current flowing through the sensed portion of the transmission line.

The reference current fed-through the sensed portion of the transmission line is received from the transmission line and compared with a corresponding reference current sensor signal portion. The reference current sensor signal portion corresponds to the signal portion of the sensor signal caused by the fed-through reference current. To distinguish the reference current from the primary current to be measured the reference current has a reference frequency which is not represented in the primary current. A comparison between the fed-through reference current and the corresponding current reference sensor signal portion results in a correction signal which can be applied onto the sensor signal obtained by the magnetic field sensor device so that a current measurement signal is obtained. As the sensor signal portion effected by the reference current applied through the sensed portion of the transmission line is exposed to the same error factors as the sensor signal caused by the primary current to be measured, the obtained correction signal may be applied for a correction of the sensor signal for the primary current. Furthermore, the fed-through reference signal may be obtained by means of a coupling unit including a band-pass filter characteristic. Particularly, the fed-through reference signal may be amplified before applied to comparison. It can be provided that the fed-through reference signal is obtained by means of a coupling unit including a band-pass filter characteristic.

Moreover, in the step of comparing the correction signal may be obtained as a difference or relation between the fed-through reference signal and the extracted sensor signal portion.

According to an embodiment, the injected reference signal and/or the fed-through reference signal may be blocked from propagating into other portions of the transmission line.

Furthermore, the injected reference signal may be a sinusoidal AC signal having a frequency which is higher than frequencies in the primary current through the transmission line. According to another aspect a current measurement system for measuring a primary current in a transmission line is provided, comprising:

- a magnetic field sensor device to provide a sensor signal depending on a magnetic field caused by the primary current flowing through the transmission line;

- a reference signal source for providing a reference signal to be injected through a portion of the transmission line associated with the sensor device

- a means for receiving a fed-through reference signal;

- a means for extracting a sensor signal portion from the obtained sensor signal, wherein the sensor signal portion results from the injected reference signal flowing through the magnetic field sensor device;

- a comparator for performing a comparison based on the fed-through reference signal and the extracted sensor signal portion to obtain a correction signal; - a correction unit for obtaining a measurement signal depending on the sensor signal and based on the fed-through reference signal and the extracted sensor signal portion. It may be provided that the current measurement system further comprises:

- a comparator for performing a comparison based on the fed-through reference signal and the extracted sensor signal portion to obtain a correction signal; and

- the correction unit for obtaining a measurement signal depending on the sensor signal and on the correction signal.

Furthermore, the means for receiving a fed-through reference signal may include a band-pass filter.

According to another embodiment at least one terminal of the sensor device may be coupled with a choke to block the injected reference signal to propagate to another portion of the transmission line.

Furthermore, an amplifier may be provided to amplify the fed-though signal before applying to the comparator.

Moreover, the means for receiving the fed-through reference signal may comprise a coupling unit, particularly formed as a band-pass filter with a pass-through frequency corresponding to the frequency of the reference signal.

Brief description of the drawings

Embodiments are described in more detail in conjunction with the accompanying drawings in which:

Figure 1 shows a schematic diagram of a current measurement system;

Figure 2 shows a schematic diagram of a further current measurement system. Description of embodiments

Figure 1 shows a current measurement system 1 configured to measure a primary current l _p in an electrical power transmission line 2 which may be a transmission line for carrying and conveying medium or high voltage electrical power. On the transmission line 2, a sensor device 3 is applied on a sensing portion of the transmission line 2.

The sensor device 3 is galvanically isolated from the transmission line 2 and configured as a magnetic field detecting device. The sensor device 3 can be formed with one or more magnetoresistive sensors placed close to the sensing portion of the transmission line 2 to detect a magnetic field caused by a primary current l _p flowing through the transmission line 2 and providing a sensor signal (SS) related thereof.

Magnetic field sensor devices 3 may include a plurality of magnetoresistive sensors in a bridge circuit to improve the sensitivity of the sensor device 3. Configurations of the magnetic field sensor device 3 are well known in the art and therefore not described herein in more detail. Magnetic field sensor devices can be obtained as standard devices, such as Sensitec CMS3100 or the like.

In practice, the transmission line 2 provides a first and a second line impedance which is indicated as Zi for a first portion 2a of the transmission line 2 which is on one side of the sensing portion of the transmission line 2 and Z2 for a second portion 2b of the transmission line 2 which is on another side of the sensing portion of the transmission line 2, respectively.

A reference signal is provided. For instance a voltage signal can be generated as a reference which is coupled so that a reference signal \ _n \ as a reference current is injected on a first terminal Ti of the sensor device 3. The first terminal Ti is located between the first portion 2a and the sensing portion of the transmission line 2. The injected reference signal flows through the sensed portion of the transmission line 2 and is tapped/received from a second terminal T2 of the sensor device 3 as the fed- through reference signal Ift. The second terminal T2 is located between the second portion 2b and the sensing portion of the transmission line 2.

The reference signal may be provided by a reference signal source 4 which provides an AC voltage signal, preferably a sinusoidal AC voltage signal of a reference amplitude and a reference frequency which should be selected to be substantially higher than the frequency of the primary current l _p to be measured. For instance, the frequency of the voltage signal can be in a range of 1 to 10 MHz, particularly 2MHz.

The reference signal source 4 is may be coupled via a first coupling unit 5 with the first terminal Ti of the sensor device 3. The first coupling unit 5 may be formed as a passive filter with a band-pass characteristic which allows only the reference signal to be forwarded to the first terminal Ti, while due to the band-pass characteristic it is prevented any other frequency portion of the primary current l _p to flow through the reference signal source 4. Here, it can be easily understood that it is required that the reference frequency is selected as a frequency which is not present in the primary current l _p so as to allow a clear separation of the reference signal form any AC portions included in the primary current l _p .

The first coupling unit 5 may comprise a series L-C resonant circuit with a first inductance L1 and a first capacitance C1 and provides a very low impedance at the selected frequency of the reference signal. Other configurations for the band-pass filter which provide a low impedance for the reference signal can be applied as well.

At the second terminal T2 of the sensor device 3, the fed-through reference signal Ift is tapped/received by means of a second coupling unit 6. The second coupling unit 6 may comprise an L-C resonant circuit with a second inductance L2 and a second capacitance C2 and provides a very low impedance at the selected frequency of the reference signal.

The second coupling unit 6 may forward the filtered fed-through reference current signal to a current amplifier 7 to amplify the amplitude of the obtained fed-through reference signal which is a current signal. The amplifier gain may be preselected and calibrated in an initial calibration process.

Due to its configuration, the coupling units 5, 6 shall present a very high impedance at the frequencies of the primary current to be measured. Thus, the reference signal source 4 and the current amplifier 7 are completely decoupled from the primary current signal to be measured.

The sensor signal SS provided by the sensor device 3 is bandpass-filtered by a filtering means 8 with a pass-through frequency corresponding to the frequency of the reference signal. The bandpass-filtering can be performed e.g. by means of a Fourier transformation to obtain a sensor signal portion at the frequency of the reference signal. Other options for bandpass-filtering such as by means of a passive or active band-pass filter can also be applied.

The filtered sensor signal FSS at the output of the filtering means 8 and the amplified fed-through reference signal are applied to a comparator 1 1 which provides a correction signal SC as a difference or a relation between the filtered sensor signal FSS and the amplified fed-through reference signal FAS.

The correction signal SC is applied on the sensor signal SS in a correction unit 10 which can be configured as a multiplier, an adder or the like to obtain a measurement signal Smeas which corresponds to the corrected sensor signal. The sensor signal SS applied to the correction unit 10 might be unfiltered or filtered so that only the sensor signal portion related to the primary current is applied to the correction unit 10. Directly applying the sensor signal SS to the correction unit 10 is possible as long as the sensor signal portion which results from the injected reference current is neglectable. The sensor signal can be filtered in a low-pass filter 9, if the reference signal portion through the sensor device 3 is not neglectable with respect to the primary current portion. This current measurement system 1 makes use of the high dynamic range and large bandwidth of the magnetoresistive current sensors, as the sensor device 3 is capable to catch up both the primary current l _p and the fed-through reference signal lit. After separating the fed-through reference signal Ift from the primary current l _p to be measured, the filtered signal portion FSS of the sensor signal SS measured by the sensor device and extracted by the filtering means 8 is compared to the amplified fed-through reference signal FAS provided by the amplifier 7. The resulting correction signal SC may represent a drift of the sensor device 3 and can be used to correct the measured sensor signal SS related to the primary current l _p .

With respect to Figure 2, in addition to the current measurement system 1 of Figure 1 , a first choke 12 having a first choke inductance Uhokei is applied at or close the first terminal Ti of the sensor device 3, i.e. in the first portion 2a of the transmission line 2. The first choke 12 is required, if the first impedance Zi of the first portion 2a of the transmission line 2 is low compared to the impedance seen when looking into the first terminal Ti, not known and/or not constant. The first choke inductance Uhokei of the first choke 12 is selected to provide a high impedance at the frequency of the injected reference signal linj. This is to assure that a signal portion of the injected reference signal linj flows through the sensing portion of the transmission line 2. Furthermore, a third capacitor C3 can be connected in parallel to the first choke to provide a parallel resonant circuit to improve the filtering effect for the frequency of the reference signal.

The same might be applied to the second portion 2b of the transmission line 2 connected with the second terminal T2 of the sensor device 3. The second choke 13 may be connected at or close to the second terminal T2 of the sensor device 3, particularly if the second impedance Z2 is low compared to the impedance seen when looking into the second terminal T2, not known and/or not constant. Second choke impedance L _C hoke2 of the second choke 13 provides high impedance at the frequency of the reference signal. This is to avoid that a signal portion of the fed- through reference signal flows through the second portion 2b of the transmission line 2 and can therefore not be received by the second coupling unit 6. The first and second chokes 12, 13 therefore serve to guide the injected reference signal \ _n \ from the reference signal source 4 to the amplifier 7 without that any or significant signal portions of the reference signal might get lost through the first and second portions 2a, 2b of the transmission line 2, respectively. Hence, by using the chokes 12, 13, it can be ensured that the ratio of injected reference current to the received fed-through reference signal Ift remains constant when the impedances on/in the transmission line 2 change.

Reference list

1 Current measurement system

2 transmission line

2a, 2b first, second portion of the transmission line

3 sensor device

4 reference signal source

5 first coupling unit

6 second coupling unit

7 amplifier

8 filtering means

9 Low-pass filter

10 correction unit

1 1 comparator

Z1 , Z2 first, second impedance

SS Sensor signal

SC correction signal

lp primary current

lit fed-through current

FSS Filtered sensor signal

FAS amplified fed-through signal

meas measuring signal