WO2020094866A1

WO2020094866A1 - Measuring device and method for measuring electrical variables in a three-phase alternating current network

Info

Publication number: WO2020094866A1
Application number: PCT/EP2019/080749
Authority: WO
Inventors: Florian BERGMANN
Original assignee: Phoenix Contact Gmbh & Co.Kg
Priority date: 2018-11-09
Filing date: 2019-11-08
Publication date: 2020-05-14
Also published as: DE102018128050A1

Abstract

For improved measuring of electrical variables in a three-phase alternating current network having three phase conductors (L1, L2, L3) and a neutral conductor (N), according to the invention, a measuring device (100, 101) is provided comprising a first current sensor (201) for measuring the current strength of a current flowing in a first (L1) of the three phase conductors, a second current sensor (202) for measuring the current strength of a current flowing in a second (L2) of the three phase conductors, a third current sensor (203) for measuring the current strength of a current flowing in the neutral conductor (N), and an evaluation unit (120, 121) connected to the current sensors (201, 202, 203), wherein the evaluation unit is designed to determine the current strength of a current flowing through the third phase conductor (L3) from the current sTrengths measured with the first, second and third current sensors.

Description

Measuring device and method for measuring electrical quantities in a three-phase AC network

description

The invention relates to a measuring device and a method for measuring electrical quantities, in particular for measuring the current of conductors through which current flows

Three-phase AC network and for determining quantities that can be derived from measured currents.

In a three-phase alternating current network, the current in the neutral conductor is canceled out when currents of the same current intensity, same phase shift and no harmonic component flow in all three outer conductors. If the current in the outer conductors is unequal, a current flows in the neutral conductor that compensates for the asymmetry and at most reaches the current of the most heavily loaded outer conductor. With different phase shifts of the currents in the outer conductors and / or when

However, harmonics, the current in the neutral conductor can also significantly exceed the current of the most heavily loaded outer conductor.

In known measuring devices, such as the measuring device 10 shown in FIG. 1, the current intensity of the current flowing in the outer conductors LI, L2 and L3 is measured by means of three current sensors 11, 12 and 13 and the current intensity of the current flowing in the neutral conductor from these measured values through totalization

determined. Only the respective one is used for the calculation RMS value and the phase of the phase currents

taken into account, it can happen, especially when harmonics occur, that a current strength of the neutral conductor current of I _N = 0 is calculated, although the current strength actually corresponds to the current strength of each of the

Phase conductor currents exceeds, ie I _N > I _Li , I _N > I _L2 and I _N > I _L3 . In extreme cases, this can lead to the current I _N destroying the neutral conductor.

The invention has for its object to show a way such as a measurement of electrical quantities, in particular a measurement of the current intensity of a through a conductor of a three-phase AC network

flowing current can be carried out in an improved and / or simplified manner.

This object is solved by the features of the independent claims. Advantageous embodiments are the subject of the dependent claims, wherein the

specified features and advantages can essentially apply to all independent claims.

A key concept of the invention is to be found in a three-phase AC network with three outer conductors and one neutral conductor, the current intensity of the currents flowing in these four conductors being determined with high accuracy and with the aid of only three current sensors, the neutral conductor current and two of the outer conductor currents being measured directly and the third phase current is determined from the measured values, in particular is calculated. A measuring device for measuring electrical quantities in a three-phase alternating current network with three outer conductors and a neutral conductor accordingly comprises a first current sensor for measuring the current strength of a current flowing in a first of the three outer conductors, a second current sensor for measuring the current strength of a current flowing in a second of the three outer conductors and a third current sensor for measuring the current intensity of a current flowing in the neutral conductor. Furthermore, the measuring device includes one with the

Evaluation unit connected to current sensors, which is designed to determine the current strength of a current flowing through the third outer conductor from the current strengths measured with the first, second and third current sensors.

In addition to the current measurement, a voltage measurement can advantageously also be provided, which enables the determination of values of further electrical quantities, in particular the determination of a power and / or energy, from the measured electrical quantities. For this purpose, the measuring device advantageously comprises

Measuring lines for measuring the voltage, which is present in each case on the three outer conductors and on the neutral conductor, the measuring lines being connected to the evaluation unit. The voltages are preferably measured relative to ground, the three-phase AC network

in particular in addition to the outer conductors and the neutral conductor also includes a ground conductor.

The evaluation unit preferably determines or calculates electrical quantities on the basis digital readings. Accordingly, the

Evaluation unit is advantageously designed to determine digital current and / or voltage measured values with a predetermined sampling rate.

The evaluation unit preferably comprises at least one microprocessor and a memory and can advantageously be part of a measuring device. Depending on the intended use, the measuring device can further comprise a user interface with an input unit and a display, or also a communication interface for communication with a higher-level monitoring device.

The measuring device can have its own power supply, for example in the form of a power supply unit or accumulator. It is also conceivable that the energy supply of the

Measuring device and in particular the evaluation unit by means of energy harvesting, the required

Energy is extracted from the magnetic field of the respective current-carrying conductor with the aid of at least one of the current sensors.

A method for measuring electrical quantities in a three-phase alternating current network with three outer conductors and one neutral conductor comprises measuring the current intensity of a current flowing in a first of the three outer conductors with a first current sensor, and measuring the

Current of a current flowing in a second of the three outer conductors with a second current sensor, measuring the current of one in the neutral conductor

flowing current with a third current sensor, and determining the current intensity through the third Outer conductor flowing current from the measured

Currents.

It is also advantageous that each of the three

Outer conductors and voltage applied to the neutral conductor, as well as advantageously from the measured

electrical quantities a value for a power and / or an energy determined. For further processing, digital electricity and / or

Voltage measured values determined with a predetermined sampling rate.

These and other features and advantages of the present invention also result from the

Embodiments, which below under

Reference to the accompanying drawings will be explained. It show

1 is a schematic representation of a measuring device known from the prior art,

Fig. 2 is a schematic representation of a first

preferred embodiment of a

measuring device according to the invention, and

Fig. 3 is a schematic representation of a second

preferred embodiment of a

inventive measuring _V orrichtung.

1 shows a measuring device 10 with three on it

connected current sensors 11, 12 and 13, by means of which the current strength of the current flowing in the outer conductors LI, L2 and L3 of a three-phase AC network 400 is measured. To the current of the im To determine the neutral current flowing, the measuring device can be designed to calculate this by summing the values measured for the outer conductors, where I _N = Ii + I2 + I3, and where I _{N is} the current intensity of the neutral conductor current and Ii, I ₂ and I3 denotes the current of the respective phase current.

If only the respective effective value and the phase of the phase currents are taken into account for the calculation, it can happen, especially when harmonics occur, that a current intensity for the neutral conductor current is calculated that is significantly below the actual current intensity of the neutral conductor current.

This problem can be eliminated by directly measuring the current flowing through the neutral conductor.

By measuring the neutral conductor current directly, the value for the current strength can also be higher

Accuracy can be determined if the

Neutral conductor current is much smaller than that

Line currents, which is the case in many applications.

At currents of the outer conductor currents in the

For example, the order of magnitude of 100 A and one

Current strength of the neutral conductor current in the order of magnitude of, for example, 5 A and a measurement accuracy of ± 1% of the respective measurement variable can

Outer conductor currents with an accuracy of ± 1 A and the neutral conductor current with an accuracy of ± 50 mA be measured. However, the current of the

Neutral conductor current from the measured values for the

Calculated outer conductor, the calculated current of the neutral conductor current is only an accuracy of ± 1 A due to the measurement accuracy of the

Calculation of inflowing quantities.

However, an additional current sensor is disadvantageously required for direct measurement of the neutral conductor current.

The invention advantageously provides only three

To use current sensors, whereby the neutral conductor current is measured directly, but one of the phase conductor currents is not measured. The non-measured phase current is calculated from the measured currents of the other two phase currents and

Neutral conductor current calculated.

In the example above, in which the currents of the outer conductor currents are in the order of 100 A and the amperages of the neutral conductor current in the order of 5 A and the measurement accuracy is ± 1% of the

the respective measured variable, results for the

calculated value of the outer conductor current a comparable accuracy as in a direct measurement, while the accuracy of the current strength of the neutral conductor current is significantly improved due to the direct measurement.

In addition, there is the advantage that, due to the direct measurement of the neutral conductor current, dangerously high Currents, for example, due to occurring

Harmonics are recorded correctly.

A preferred embodiment of a measuring device 100 according to the invention is exemplified in FIG. 2

shown. In the exemplary embodiment shown, the measuring device 100 comprises a measuring device 110 with a

Evaluation unit 120, and three current sensors 201, 202 and 203. The current sensor 201 is for measuring the

Amperage of the through the outer conductor LI des

Three-phase alternating current network 400 flowing current arranged. The current sensor 202 is for measuring the

Current of the through the outer conductor L2 of the

Three-phase alternating current network 400 flowing current arranged. The current sensor 203 is for measuring the

Current of the through the neutral conductor N des

Three-phase alternating current network 400 flowing current arranged. The evaluation unit 120 is designed to determine the current strength of the current flowing through the third outer conductor L3 from the current strengths measured with the current sensors 201, 202 and 203.

The measuring device 100 is thus designed to measure the neutral conductor current directly, but to dispense with the direct measurement of the current flowing through the outer conductor L3 and to calculate it from the measured current strengths. Instead of a direct measurement of the current flowing through the outer conductor L3, it would alternatively also be possible to dispense with a direct measurement of the current flowing through the outer conductor LI or L2.

It is essential that two phase currents and the

Neutral conductor current can be measured and the third Phase current is calculated based on the measured values.

The measurement and calculation of the values for the current intensity are explained in more detail below as examples.

On the basis of normative specifications, a summary measurement is typically carried out for a specified time window with a duration of in particular 200 ms, wherein a different duration could also be selected. The duration of

200 ms is used in particular because it is an integer number of periods for the usual alternating current frequencies of 60 Hz and 50 Hz

includes, namely 12 or 10.

When digitizing the analog measurement signal with a sampling rate of, for example, 500 kSPS (kilo

Samples per second) are 100,000 samples per current sensor after a measurement for a measurement duration of 200 ms, which are stored in a memory of the measuring device 110

2, and the memory is not shown in FIG. 2 and can be arranged, for example, in the evaluation unit 120. In the example shown, the evaluation unit 120 is advantageously designed to calculate an effective value for I _Li , ie for the current strength of the current flowing through the outer conductor LI, using the following formula:

in which

I _Li : RMS value of the current through the

Line conductor Li flowing current

k: index of the samples

i _{Li k} : kth sample of the current, ie a single one from an A / D converter

provided measured value

In an analogous manner, the evaluation unit 120 is designed to add an effective value for I _Li and I _N

calculate, with each channel (LI, L2, N) independently determining 100 k samples based on the signals provided by the current sensors 201, 202 and 203.

Furthermore, the evaluation unit 120 is particularly advantageously designed to in each case produce a value for from the measured sample values i _Li , i and i _N

to determine where

1L3 ⁼ IN ^_ iLi ^~~ lL2

To calculate the effective value of the current strength of the current flowing through the outer conductor L3, the

Evaluation unit 120 also designed to

Calculate the effective value using the following formula:

e of the by the

Outer conductor L of flowing current

k: index of the samples iLl k kth sample value of the current intensity of the current flowing through the outer conductor Li

lL2 k kth sample value of the current intensity of the current flowing through the outer conductor L2

iLN k kth sample value of the current intensity of the current flowing through the neutral conductor N.

Not only can the evaluation unit particularly advantageously determine effective values of the current strength of the currents flowing through the outer conductor and the neutral conductor, but also further electrical variables can be determined from the measured and calculated sample values. For this purpose, measuring lines are preferably used

Measurement of the electrical voltages applied to the outer conductors and the neutral conductor. A corresponding embodiment of a measuring device 101 is shown in FIG. 3. In the exemplary embodiment shown in FIG. 3, the measuring device 101 comprises a measuring device 111 with an evaluation unit 121, as well as three current sensors 201, 202 and 203. The current sensor 201 is arranged to measure the current intensity of the current flowing through the outer conductor LI of the three-phase alternating current network 400 Current sensor 202 is for measuring the

Current of the through the outer conductor L2 of the

Three-phase AC network 400 flowing current is arranged, and the current sensor 203 is for measuring the current intensity through the neutral conductor N des

Three-phase alternating current network 400 flowing current arranged. In addition, there are measurement lines 301, 302, 303, 304 and 305 connected to the evaluation unit 121 provided, which are each connected to the outer conductor LI, the outer conductor L2, the outer conductor L3, the neutral conductor N and the ground conductor PE, and which serve to measure the voltage applied to the conductors relative to the ground.

The current sensors 201, 202 and 203 shown in FIGS. 2 and 3 can each be designed, for example, as a so-called Rogowski coil and can be designed, for example, as a current probe. However, any other suitable design of a current sensor is also within the scope of the invention.

The evaluation unit 121 can advantageously be designed to determine one or more of the following values:

• total active power +/-

• Total reactive power vectorial +/- · Total apparent power vectorial +/-

• Total power factor vectorial +/-

• Active power phase 3 +/-

• reactive power phase 3 +/-

• Apparent power phase 3

Power factor phase 3 +/-

System stream

Total reactive power arithmetic +/- Total apparent power arithmetic +/- Total power factor arithmetic +/- phase angle UI3

Energy meter

positive total active energy

total positive reactive energy

Total apparent energy • negative total active energy

• total negative reactive energy

• current positive active energy

• current positive reactive energy

• current apparent energy

• current negative active energy

• current negative reactive energy

A decisive advantage of the measuring devices 100 and 101 over the prior art is the absolute gain in accuracy with regard to the determined current strength of the neutral conductor current.

Another example calculation is given below.

Given the currents I _L 3, ILI, IL2 = -100A and the current I _LN = ~ 1A. Furthermore, an accuracy of 0.2% of the respective measured value is assumed.

If one were to measure the current of the three phase currents and calculate the effective value of the current of the neutral current, this could be done using the following equation:

wobe

ILN · RMS value of the current through the

Neutral conductor N flowing current k: index of the samples iLl k kth sample value of the current intensity of the current flowing through the outer conductor Li

i _L 3 _k : kth sample value of the current intensity of the current flowing through the outer conductor L3

It must be assumed that the error is approx. 0.2% of 100 A and thus up to 0.2 A. This makes it 20% of 1 A.

However, if one uses the direct measurement of the

The neutral value, as is provided according to the invention, can be the effective value by means of the following

Equation can be determined:

in which

IL _N : RMS value of the current through the

Neutral conductor N flowing current k: index of the samples

ILN k · kth sample value of the current strength of the current flowing through the neutral conductor N In this case it can be assumed that the error is approximately 0.2% of 1 A and thus up to 2 mA. The reduction of the error from 200 mA to 2 mA corresponds to an improvement by a factor of 100. In the given example, the error in calculating I _L 3 using the method according to the invention differs using the equation given above

not from the error that results from a direct measurement and calculation using the equation

^, L3

would result. In both cases the error is approx. 200 mA and thus approx. 0.2 o of the measured value. In the following, the determination of the power and the energy is shown as an example for the determination of further electrical quantities by the evaluation unit 121.

In the following illustration it is assumed that the sampling takes place at a constant sampling rate, i.e. with constant time intervals between the individual samples. In this case, the

Inclusion of the individual time values in the calculation of the power or energy.

The evaluation unit is designed, for example, to determine power values using the following equations:

through the outer conductor Li, L ₂ or L flowing current

k: index of the samples

i Ll k r 1L2 k: kth sample value of the current strength of the through the outer conductor Li or

L flowing current

i LN k: kth sample value of the current intensity of the current flowing through the neutral conductor N.

^U Ll_k r ^U L2_k r ^U L3_k k-th sample value on the

External conductor Li, L ₂ , or L

applied voltage

P ¹ total r F '-' total · total power or total energy t (fLx_100fc) time of acquisition of the first or last sample value of the time window of 200 ms

The calculated values such as P total ^and E total refer normatively to a length of ~ 200ms. A

Energy meter function can be provided that the currently determined energy is added up with the previously determined energies.

Claims

1. Measuring device (100, 101) for measuring electrical

Sizes in a three-phase AC network (400) with three outer conductors (LI, L2, L3) and one neutral conductor (N), comprising

- A first current sensor (201) for measuring the

Current one in a first (LI) of the three

Outer conductor of flowing current,

- A second current sensor (202) for measuring the

Current one in a second (L2) of the three

Outer conductor of flowing current,

- A third current sensor (203) for measuring the

Amperage of a current flowing in the neutral conductor (N), and

- An evaluation unit (120, 121) connected to the current sensors (201, 202, 203), which is designed to determine the current strength of a current flowing through the third outer conductor (L3) from the current strengths measured with the first, second and third current sensors determine.

2. Measuring device according to claim 1, comprising

Measuring lines (301-305) for measuring the voltage which is applied to the three outer conductors (LI, L2, L3) and to the neutral conductor (N), the measuring lines (301-305) being connected to the evaluation unit (121).

3. Measuring device according to one of claims 1 or 2, wherein the evaluation unit (121) is designed to determine a value for a power and / or an energy from the measured electrical quantities.

4. Measuring device according to one of the preceding

Claims, wherein the evaluation unit is designed to determine digital current and / or voltage measured values with a predetermined sampling rate.

5. Method for measuring electrical quantities in a three-phase AC network (400) with three outer conductors (Li, L ₂ , L ₃₎ and a neutral conductor (N), with the

Steps:

Measuring the current intensity of a current flowing in a first (Li) of the three outer conductors with a first current sensor (201),

Measuring the current intensity of a current flowing in a second (L _{2) of} the three outer conductors with a second current sensor (202),

- Measuring the current strength of a current flowing in the neutral conductor (N) with a third current sensor (203), and

- Determining the current strength of a current flowing through the third outer conductor (L ₃₎ from the measured current strengths.

6. The method according to claim 5, further comprising each of the three outer conductors (Li, L ₂ , L ₃₎ and on the

Neutral conductor (N) applied voltage is measured.

7. The method according to any one of claims 5 or 6, wherein a value for a power and / or an energy is determined from the measured electrical quantities.

8. The method according to any one of claims 5 to

wherein digital current and / or voltage measured values are determined with a predetermined sampling rate.