Technical field

The disclosure relates to a sensor system for measuring electric currents of an electrical system that can be for example a multiphase electrical system such as a three-phase electrical system. Furthermore, the disclosure relates to a method for measuring electric currents.

Background

In many electrical systems, current measurement is used for control and protection purposes. Furthermore, current measurement can be used for measuring purposes such as for measuring electric power and/or electric energy. The electrical system can be for example a multiphase electrical system such as e.g. a three-phase electrical system. Applications of multiphase electrical systems range from household grid connections to photovoltaic systems. Examples of three-phase electrical systems fed by power electronics are pump and fan applications, industrial drives, drive trains of electrical vehicles, etc.

A straightforward approach to measure electric currents of an electrical system is to measure each of the electric currents with an individual current sensor. In this case, the number of the current sensors equals the number of the electric currents to be measured. In many cases, there is however a boundary condition related to the electric currents to be measured. In typical multiphase electrical systems e.g. three- phase electrical systems, the boundary condition is that the sum of all the electric currents is zero in a fault-free situation. In many cases, the boundary condition is wanted to be utilized to reduce the number of the current sensors and thereby to reduce costs and the number of components of a sensor system. For example, a typical sensor system used in conjunction with an M-phase electrical system comprises M - 1 current sensors each of which is configured to measure one of electric currents of the M-phase electrical system so that M - 1 electric currents are measured in total. In this case, the one of the electric currents which is not measured can be computed based on the measured electric currents and an assumption that the sum of all the electric currents is zero. This sensor system is however not free from challenges because one of the electric currents does not have any contribution on output signals of the current sensors. Therefore, a sensor system of the kind described above is typically not sufficiently capable of detecting for example overcurrent caused by a ground-fault in a phase that is not provided with a current sensor.

Summary

The following presents a simplified summary to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments.

In accordance with the invention, there is provided a new sensor system for measuring electric currents of an electrical system that can be for example a multiphase electrical system such as a three-phase electrical system.

A sensor system according to the invention comprises:

- one or more current sensors so that the number N of the one or more current sensors is the number M of the electric currents to be measured minus one, i.e. N = M - 1 , and each of the one or more current sensors is configured to measure a linear combination of two or more of the electric currents so that every one of the electric currents belongs to at least one linear combination measured with the one or more current sensors, and

- a computing circuitry configured to compute values of the electric currents based on the one or more linear combinations measured with the one or more current sensors and on information indicative of a value, e.g. zero, of a sum of all the electric currents.

As each current sensor is arranged to measure a linear combination of two or more of the electric currents instead of measuring one electric current only, every one of the electric currents can be arranged to have a contribution on an output signal of at least one current sensor even if the number of the one or more current sensors is less than the number of the electric currents to be measured. This improves the ability of the sensor system to detect for example over-current caused by a singlephase ground-fault in a multiphase electrical system regardless of a phase of the multiphase electrical system in which the single-phase ground-fault has taken place.

In accordance with the invention, there is also provided a new method for measuring electric currents.

A method according to the invention comprises:

- measuring one or more linear combinations each being a linear combination of two or more of the electric currents so that every one of the electric currents belongs to at least one measured linear combination and the number N of the one or more measured linear combinations is the number M of the electric currents to be measured minus one, i.e. N = M - 1 , and

- computing values of the electric currents based on the one or more measured linear combinations and on information indicative of a value, e.g. zero, of a sum of all the electric currents.

Various exemplifying and non-limiting embodiments are described in accompanied dependent claims.

Exemplifying and non-limiting embodiments both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and non-limiting embodiments when read in conjunction with the accompanying drawings.

The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of unrecited features.

The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality.

Brief description of the figures

Exemplifying and non-limiting embodiments and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which: figure 1 illustrates a sensor system according to an exemplifying and non-limiting embodiment, figure 2 illustrates a sensor system according to an exemplifying and non-limiting embodiment, figure 3 illustrates a sensor system according to an exemplifying and non-limiting embodiment, figures 4a and 4b illustrate a sensor system according to an exemplifying and nonlimiting embodiment, figure 5 illustrates schematically a sensor system according to an exemplifying and non-limiting embodiment, figure 6 illustrates schematically a sensor system according to an exemplifying and non-limiting embodiment, and figure 7 shows a flowchart of a method according to an exemplifying and non-limiting embodiment for measuring electric currents.

Description of the exemplifying and non-limiting embodiments

The specific examples provided in the description below should not be construed as limiting the scope and/or the applicability of the accompanied claims. Lists and groups of examples provided in the description are not exhaustive unless otherwise explicitly stated. Figure 1 illustrates a sensor system according to an exemplifying and non-limiting embodiment for measuring electric currents h , i2, and is of a three-phase electrical system. The sensor system comprises two current sensors 101 and 102 so that the current sensor 101 is configured to measure a linear combination of electric currents ii and i2 and the current sensor 102 is configured to measure a linear combination of electric currents i2 and is. In this exemplifying case, the linear combination of electric currents h and i2 is the sum h + i2 of these electric currents h and i2. Correspondingly, the linear combination of electric currents i2 and is is the sum i2 + is of these electric currents i2 and is. The sensor system comprises a computing circuitry 104 configured to compute values of the electric currents h, i2, and is based on the measured values of the sums h + i2 and i2 + is and on information indicative of a value of the sum ii + i2 + is of all the electric currents as:

11 = s - m2,

12 = r + m2 - s, is = s - mi, (1 ) where r is an output value of the current sensor 101 configured to measure the sum i 1 + i2 i .e . m 1 = ii + i2, m2 is an output value of the current sensor 102 configured to measure the sum i2 + is i.e. m2 = i2 + is, and s is the value of the sum ii + i2 + is of all the electric currents i.e. s = h + i2 + is. The above-presented equations 1 correspond to a situation in which the current sensors 101 and 102 give positive output values when directions of the electric currents are according to arrows presented in figure 1 . If, for example, the current sensor 101 gives a negative output value and the current sensor 101 gives a positive output value when directions of the electric currents are according to the arrows presented in figure 1 , the sign of r in the above-presented equations 1 is to be reversed.

In a sensor system according to an exemplifying and non-limiting embodiment, the computing circuitry 104 is configured to compute the values of the electric currents with a boundary condition that the sum of all the electric currents is zero, i.e. s = 0. It is however also possible that an electrical system comprises means which measure and/or estimate the sum of all electric currents. In this exemplifying case, the computing circuitry 104 can be provided with a signal-input for receiving from outside the sensor system a signal that expresses the sum of all the electric currents.

The computing circuitry 104 can be an analogue computing circuitry, a digital computing circuitry, or a combination thereof. In a case of an analogue implementation, the arithmetic operations corresponding to the above-presented equations 1 can be implemented with for example operational amplifiers. In a case of a digital implementation, the digital computing circuitry may comprise a programmable processor circuit provided with appropriate software, a dedicated hardware processor such as for example an application specific integrated circuit “ASIC”, and/or a configurable hardware processor such as for example a field programmable gate array “FPGA”. Furthermore, the digital computing circuitry may comprise one or more memory circuits each of which can be for example a Random- Access Memory “RAM” circuit.

In the exemplifying sensor system illustrated in figure 1 , each of the current sensors comprises a loop-shaped magnetic core that is configured to surround electric conductors which conduct the electric currents whose sum is to be measured with the current sensor under consideration. In figure 1 , the loop-shaped magnetic core of the current sensor 101 is denoted with a reference 105, the loop-shaped magnetic core of the current sensor 102 is denoted with a reference 106, and the electric conductors are denoted with references 107, 108, and 109. In this exemplifying case, the loop-shaped magnetic cores comprise airgaps 110 and 111 to linearize the operation of the current sensors 101 and 102. The current sensor 101 comprises a sensor coil 112 that surrounds the loop-shaped magnetic core 105 and produces a sensor signal in response to a situation in which electric currents h and i2 are alternating “AC” currents, or at least the sum h + i2 is alternating. Correspondingly, the current sensor 102 comprises a sensor coil 113 that surrounds the loop-shaped magnetic core 106 and produces a sensor signal in response to a situation in which electric currents i2 and is are alternating “AC” currents, or at least the sum i2 + is is alternating. Each loop-shaped magnetic core comprises material whose relative magnetic permeability p is greater than one. The material is advantageously ferromagnetic material. Each loop-shaped magnetic core may comprise e.g. a stack of electrically insulated steel sheets, ferrite, or soft magnetic composite such as e.g. Somaloy®.

Figure 2 illustrates a sensor system according to an exemplifying and non-limiting embodiment for measuring electric currents h , i2, and is of a three-phase electrical system. The sensor system comprises two current sensors 201 and 202 so that the current sensor 201 is configured to measure a linear combination of electric currents ii and i2 and the current sensor 202 is configured to measure a linear combination of electric currents i2 and is. In this exemplifying case, the linear combination of electric currents h and i2 is the sum h + i2 of these electric currents h and i2. Correspondingly, the linear combination of electric currents i2 and is is the sum i2 + is of these electric currents i2 and is. The sensor system comprises a computing circuitry 204 configured to compute values of the electric currents h, i2, and is based on the measured values of the sums h + i2 and i2 + is and on a value s, e.g. zero, of the sum of all the electric currents.

In the exemplifying sensor system illustrated in figure 2, each of the current sensors comprises a loop-shaped magnetic core that is configured to surround electric conductors which conduct the electric currents whose sum is to be measured with the current sensor under consideration. In figure 2, the loop-shaped magnetic core of the current sensor 201 is denoted with a reference 205, the loop-shaped magnetic core of the current sensor 202 is denoted with a reference 206, and the electric conductors are denoted with references 207, 208, and 209. The current sensor 201 comprises a Hall effect sensor 214 placed in a gap of the loop-shaped magnetic core 205 and configured to produce a sensor signal responsive to a magnetic field directed to the Hall effect sensor 214. Correspondingly, the current sensor 202 comprises a Hall effect sensor 215 placed in a gap of the loop-shaped magnetic core 206 and configured to produce a sensor signal responsive to a magnetic field directed to the Hall effect sensor 215. It is also possible that each current sensor of a sensor system according to an exemplifying and non-limiting embodiment comprises a Hall effect sensor and a compensation coil surrounding a loop-shaped magnetic core and the sensor system further comprises a controller configured drive an output signal of the Hall effect sensor to zero by controlling electric current of the compensation coil. In this exemplifying case, the measured sum of electric currents is directly proportional to the electric current of the compensation coil.

In the exemplifying sensor system illustrated in figure 2, an opening of the loopshaped magnetic core 205 contains support material 216 configured to mechanically support the electric conductors 207 and 208 to be at fixed positions with respect to the loop-shaped magnetic core element 205. Correspondingly, an opening of the loop-shaped magnetic core 206 contains support material 217 configured to mechanically support the electric conductors 208 and 209 to be at fixed positions with respect to the loop-shaped magnetic core element 206. The support material 216 and 217 can be for example cast plastic or resin. In the exemplifying sensor system illustrated in figure 2, the electric conductors 207-209 are advantageously parts of the sensor system and external electric conductors such as cables can be connected to the ends of the electric conductors 207-209. Instead, in the exemplifying sensor system illustrated in figure 1 , the electric conductors 107-109 can be for example cables that have been threaded through the openings of the loop-shaped magnetic cores 105 and 106 during installation of the sensor system shown in figure 1 . In the exemplifying sensor system illustrated in figure 2, the fixed placement of the electric conductors 207-209 with respect to the loop-shaped magnetic cores 205 and 206 makes it possible to determine the magnetic field with a higher accuracy and the installation variance is smaller than in a case in which e.g. cables are threaded through openings of loop loop-shaped magnetic cores during an installation phase.

Figure 3 illustrates a sensor system according to an exemplifying and non-limiting embodiment for measuring electric currents h , i2, and is of a three-phase electrical system. The sensor system comprises two current sensors 301 and 302 which are schematically presented with dashed lines in figure 3. The current sensor 301 is configured to measure a sum of electric currents h and i2, and the current sensor 302 is configured to measure a sum of electric currents i2 and is. The sensor system comprises a computing circuitry 304 which is schematically presented with a dashed line in figure 3. The computing circuitry 304 is configured to compute values of the electric currents ii, i2, and is based on the measured values of the sums h + i2 and i2 + is and on a value, e.g. zero, of the sum of all the electric currents. The exemplifying sensor system illustrated in figure 3 comprises a casing 318 that contains the current sensors 301 and 302 and supports electric conductors 307, 308, and 309 to be in fixed positions with respect to the current sensors 301 and 302. Like in conjunction with the exemplifying sensor system illustrated in figure 2, the fixed placement of the electric conductors 307-309 with respect to the current sensors 301 and 302 improves the measurement accuracy and reduces the installation variance.

Figure 4 illustrates a sensor system according to an exemplifying and non-limiting embodiment for measuring electric currents h , i2, and is of a three-phase electrical system. The sensor system comprises two current sensors 401 and 402. The current sensor 401 is configured to measure a sum of electric currents h and i2, and the current sensor 402 is configured to measure a sum of electric currents i2 and is. The sensor system comprises a computing circuitry 404 which is configured to compute values of the electric currents h , i2, and is based on the measured values of the sums h + i2 and i2 + is and on a value, e.g. zero, of the sum of all the electric currents.

In the exemplifying sensor system illustrated in figure 4, each of the current sensors comprises a loop-shaped magnetic core that is configured to surround electric conductors which conduct the electric currents whose sum is to be measured with the current sensor under consideration. In figure 4, the loop-shaped magnetic core of the current sensor 401 is denoted with a reference 405, the loop-shaped magnetic core of the current sensor 402 is denoted with a reference 406, and the electric conductors are denoted with references 407, 408, and 409. The current sensor 401 comprises a sensor coil 412 that surrounds the loop-shaped magnetic core 405 and produces a sensor signal in response to a situation in which electric currents ii and i2 are alternating “AC” currents, or at least the sum h + i2 is alternating. Correspondingly, the current sensor 402 comprises a sensor coil 413 that surrounds the loop-shaped magnetic core 406 and produces a sensor signal in response to a situation in which electric currents i2 and is are alternating “AC” currents, or at least the sum i2 + is is alternating. The exemplifying sensor system illustrated in figure 4 comprises a circuit board 419 that comprises the above-mentioned electric conductors 407-409. The loop-shaped magnetic core of each of the current sensors comprises a first part on a first side of the circuit board and a second part on second side of the circuit board. In figure 4, the first part of the loop-shaped magnetic core 405 is denoted with a reference 420 and the second part of the loop-shaped magnetic core 405 is denoted with a reference 421 .

Figure 5 illustrates schematically a sensor system according to an exemplifying and non-limiting embodiment for measuring electric currents h, i2, is, and i4 of a four- phase electrical system. The sensor system comprises three current sensors 501 , 502, and 503. The current sensor 501 is configured to measure a linear combination of electric currents h , i2, and is, the current sensor 502 is configured to measure a linear combination of electric currents i2, is, and i4, and the current sensor 503 is configured to measure a linear combination of electric currents h , is, and i4. In this exemplifying case, the linear combination of electric currents h, i2, and is is the sum ii + i2 + is of these electric currents. Correspondingly, the linear combination of electric currents i2, is, and i4 is the sum i ₂ + is + i4 of these electric currents i2, is, and i4, and the linear combination of electric currents h, is, and i4 is the sum ii + is + i4 of these electric currents h , is, and i4.

The sensor system comprises a computing circuitry which is configured to compute values of the electric currents h, i2, is, and i4 based on the measured values of the sums ii + i2 + is, 12 + is + i4, and ii + is + i4 and on a value, e.g. zero, of the sum of all the electric currents. The computing circuitry is not shown in figure 5.

The values of the electric currents h, i2, is, and i4 can be solved from the following set of equations:

11 + i2 + is = m-i,

12 + is + i4 = m2, ii + is + i4 = m3, ii + i2 + is + i4 = s, (2) where r is an output value of the current sensor 501 configured to measure the sum ii + i2 + is, m2 is an output value of the current sensor 502 configured to measure the sum i ₂ + is + i4, m3 is an output value of the current sensor 503 configured to measure the sum h + is + i4, and s is the value, e.g. zero, of the sum h + i ₂ + is + i4 of all the electric currents.

Figure 6 illustrates schematically a sensor system according to an exemplifying and non-limiting embodiment for measuring electric currents h and i ₂ . The sensor system comprises a current sensor 601 that is configured to measure a linear combination of electric currents h and i ₂ . In this exemplifying case, the linear combination of electric currents h and i ₂ is the difference h - i ₂ of these electric currents. The sensor system comprises a computing circuitry which is configured to compute values of the electric currents h and i ₂ based on the measured value of the difference h - i ₂ and on a value, e.g. zero, of the sum of the electric currents h and i ₂ . The computing circuitry is not shown in figure 6.

The values of the electric currents h and i ₂ can be solved from the following set of equations: ii - i ₂ = m, ii + i2 = s, (3) where m is an output value of the current sensor 601 configured to measure the difference h - i ₂ and s is the value, e.g. zero, of the sum h + i ₂ of the electric currents.

The computing circuitry is configured to compute the values of the electric currents ii and i ₂ as:

11 = (s + m)/2,

1 ₂ = (s - m)/2. (4)

An electrical system may comprise two or more subsystems each of which is a multiphase system such that a sum of electric currents in each subsystem is known or assumed to be a given value e.g. zero. For example, an electrical system may comprise two three-phase systems so that the sum of electric currents in each three- phase system is known or assumed to be a given value e.g. zero. In this exemplifying case, a sensor system for measuring electric currents may comprise two sensor subsystems so that each of the sensor subsystems is a sensor system according to an embodiment of the invention, for example such as illustrated in figures 1 , 2, 3, or 4a and 4b. In this exemplifying case, there are 2 x 2 current sensors for measuring 2 x 3 electric currents. In a general case in which there are n pieces of M-phase subsystems, the sensor system comprises n x (M - 1) = n x M - n current sensors for measuring n x M electric currents. Thus, in cases where there are many subsystems, i.e. n is significant, the number of the current sensors can be significantly smaller than the number of the electric currents to be measured. This is a corollary of the fact that there are many boundary conditions according to which the sum of electric currents in each subsystem is known or assumed to be a given value e.g. zero. It is to be noted that the subsystems do not need to be like each other, but they can differ from each other e.g. concerning the number of phases.

Figure 7 shows a flowchart of a method according to an exemplifying and nonlimiting embodiment for measuring electric currents. The method comprises the following actions:

- action 701 : measuring one or more linear combinations each being a linear combination of two or more of the electric currents so that every one of the electric currents belongs to at least one measured linear combination and the number N of the one or more measured linear combinations is the number M of the electric currents minus one, i.e. N = M - 1 , and

- action 702: computing values of the electric currents based on the one or more measured linear combinations and on information indicative of a value of a sum of all the electric currents.

In a method according to an exemplifying and non-limiting embodiment, the number N of the measured linear combinations is at least two and each of the measured linear combinations is a sum of at least two and at most N of the electric currents.

In a method according to an exemplifying and non-limiting embodiment, each of the measured linear combinations is a sum of exactly two of the electric currents.

In a method according to an exemplifying and non-limiting embodiment, the number M of the electric currents is three, the number N of the measured linear combinations is two, and the values of the electric currents are computed according to the following equations:

11 = s - m2,

12 = r + m2 - s,

13 = s - mi, (5) where h is the value of a first one of the electric currents, i2 is the value of a second one of the electric currents, is is the value of a third one of the electric currents, r the sum of the first and second ones of the electric currents, m2 is the sum of the second and third ones of the electric currents, and s is the value of the sum of the first, second, and third ones of the electric currents.

In a method according to an exemplifying and non-limiting embodiment, the values of the electric currents are computed with an assumption that the sum of all the electric currents is zero, i.e. s = 0.

In a method according to an exemplifying and non-limiting embodiment, each linear combination is measured with a current sensor that comprises a loop-shaped magnetic core that surrounds electric conductors which conduct the electric currents whose linear combination is measured with the current sensor under consideration. The loop-shaped magnetic core may comprise an airgap to linearize the measurement operation.

In a method according to an exemplifying and non-limiting embodiment, the above- mentioned current sensor comprises a sensor coil surrounding the loop-shaped magnetic core and configured to produce a sensor signal in response to a situation in which the electric currents whose linear combination is measured with the current sensor are alternating currents.

In a method according to an exemplifying and non-limiting embodiment, the above- mentioned current sensor comprises a Hall effect sensor that is placed in a gap of the loop-shaped magnetic core and produces a sensor signal responsive to a magnetic field directed to the Hall effect sensor.

In a method according to an exemplifying and non-limiting embodiment, an opening of the above-mentioned loop-shaped magnetic core contains support material configured to mechanically support the above-mentioned electric conductors to be at fixed positions with respect to the loop-shaped magnetic core element.

In a method according to an exemplifying and non-limiting embodiment, the current sensors for measuring the linear combinations are inside a casing configured to mechanically support the electric conductors to be in fixed positions with respect to the current sensors.

In a method according to an exemplifying and non-limiting embodiment, the electric conductors for conducting the electric currents are parts of a circuit board and the above-mentioned loop-shaped magnetic core comprises a first part on a first side of the circuit board and a second part on second side of the circuit board.

The specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims. List and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.