RU2131128C1 - Portable digital meter of heavy direct currents - Google Patents

Abstract

FIELD: electric measurement technology. SUBSTANCE: invention is specifically meant to test stationary measurement systems for currents over 100 kA without their dismantling. Invention suggests that digital integrator based on converter of analog voltage to number of pulses should be used in meter to expand functional capabilities, to increase limits of measured currents and to reduce errors in measurement of currents. EFFECT: increased accuracy of integration, improved technical characteristics of meter. 3 dwg, 1 tbl

Description

 The invention relates to the field of electrical measurements, in particular to measurements of large constant currents without breaking the electric circuit, and can be used for periodic or occasional monitoring of the modes of electric circuits of large constant currents.

 The invention is aimed at solving the problem of providing periodic verification of stationary measuring current systems of more than 100 kA having an accuracy class of 0.2 or lower, without dismantling these systems.

 The authors are not aware of analogues that could be used to solve this problem.

 A patent is known for a portable current meter N 2006043 / Zykin F.A .; Diveev A.I.; Kazakov M.K .; Chistyakova T.O. (Bull. N 1, 01/15/94.), Which was supposed to be used for measuring direct currents up to 50 kA, and an attempt to use this invention for currents of 150 kA by the scientific and production concern Paramet (Ulyanovsk) is known.

 The fabricated sample was tested at the Ural Research Institute of Metrology and used to verify manufactured stationary measuring systems of constant currents at the Volgograd and Kamensk-Ural aluminum plants.

 Tests showed that the maximum relative error of the device did not exceed 0.07% (a certificate of metrological certification is attached). However, this analogue is not approved by such a class for the reasons indicated below.

 The closest in technical essence to the claimed invention is the specified analogue. The prototype and the claimed invention have the following similar essential features. In the prototype and in the inventive device uses a Rogowski belt on a solid basis and a reset button. The winding of the Rogowski belt gives information about the magnitude of the direct current in the form of EMF during the coverage of current-carrying buses with the Rogowski belt or its removal, the output signal of the belt is integrated, and the output voltage of the integrator, proportional to the measured current, is an informative signal.

 It should be noted that with a dense, uniform winding of the Rogowski belt and small cross-sectional dimensions compared to the current lead and the distance from the current lead to the belt, the mutual inductance M (t) will not depend on the length and configuration of the belt.

μ₀ - магнитная постоянная.This follows from the following reasoning. The thread linkage of a belt consists of flows permeating each turn. For the k ^th round of the magnetic flux determined by the expression
Ф _k = μ ₀ H _k S cosα, (1)
where H _k - intensity of the magnetic field on the k ^th coil plane;
S is the cross-sectional area of the coil;
α is the angle between the vectors

μ ₀ is the magnetic constant.

где по закону полного тока

(I - измеряемый ток), откуда

(3)
В выражении (2) l - длина пояса. Таким образом, M_max зависит только от сечения пояса и плотности его намотки.When the winding density W ₁ (the number of turns per unit length), the flux linkage of all turns of the belt covering the current lead is determined by the equation

where according to the law of total current

(I - measured current), whence

(3)
In expression (2), l is the length of the belt. Thus, M _max depends only on the section of the belt and the density of its winding.

 As studies and operating results have shown, the disadvantages of the prototype are a decrease in the measurement accuracy and the limit of the measured currents due to the following reasons.

 According to the principle of operation of the prototype in the integrator, which is part of the prototype, it is necessary to use a large capacitor (several microfarads) when measuring currents above 100 kA, which complicates the use of precision capacitors. An increase in the capacitance of the capacitor is associated with the need to increase the time constant of the integrator due to the increase in the measurement time due to the significant size of the current lead, and therefore the belt.

 In addition, in the prototype, errors occur due to zero drift of the operational amplifier, which is part of the integrator, which also has a significant effect when measuring currents above 100 kA for the above reasons.

 The aim of the invention is to expand the functionality in the direction of increasing the limit of the measured currents and increasing the accuracy of the measurement, which will allow verification of stationary measuring systems at workplaces without dismantling and transportation to special metrology laboratories, will reduce the complexity of verification of these measuring installations, which will improve the process .

 It should be noted that the tasks to be solved are very important for improving the metrological support system for measuring large constant currents, since the state of this system in our country is far from satisfactory, in particular, due to the lack of a network of special calibration laboratories, which significantly complicates the verification process.

 Considering also the peculiarity of high current circuits - the continuity of the power supply mode, which complicates the dismantling of measuring installations, one can understand the importance of developing portable precision high current meters.

 To achieve this goal, the claimed invention "Portable digital meter high constant currents" contains a Rogowski belt on a detachable non-ferromagnetic frame, self-opening reset button, a large-scale amplifier, digital integrator, decoder and digital indicator. One clamp of the Rogowski belt is connected to the zero bus, the second is connected to the input of a large-scale amplifier. The output of a large-scale amplifier is connected to the input of a digital integrator, the output of which through a decoder is connected to the input of a digital indicator.

 The totality of these common essential features develop additional features set forth in paragraph 2 of the claims, which clarify the structure of the digital integrator. It contains an analog integrator, an analog inverter, a clock, a frequency divider, a pulse counter, two comparators, seven electronic keys, two logical inverters.

 The output of a large-scale amplifier is connected to the first comparator, which controls the keys at the input of the counter and at the inverting input of the second comparator. The output of the large-scale amplifier is also connected to the input of the analog inverter and through the first key to the inverting input of the second comparator. The output of the first comparator is connected to the control input of the first key and to the first control input of the fifth key on the subtracting input of the pulse counter.

 The output of the first comparator through a logical inverter is connected to the control input of the second key in the circuit connecting the output of the analog inverter and the inverting input of the second comparator, and to the first control input of the fifth key on the summing input of the pulse counter. The output of the second comparator is connected to the second and first control inputs of the fifth and fourth keys, respectively, at the input of the pulse counter.

 The input of the analog integrator through the sixth key is connected to the positive terminal of a stable power source. The output of the clock generator through the third key, the control input of which is connected to the output of the frequency divider, is connected through the fourth and fifth cries to the counter input. The input of the frequency divider is connected to the output of the clock generator, and its output is connected to the control input of the sixth key and, through a logical inverter, to the control input of the seventh key in the feedback of the analog integrator. The positive output of the power source through a self-opening button is connected to the reset terminal of the pulse counter information.

 In relation to the prototype of the claimed invention has the following distinctive features. The winding of the Rogowski belt is connected to a large-scale amplifier, which is connected to the input of a digital integrator, replacing the analog integrator of the prototype. In this case, the exclusion of meter errors caused by the zero drift of the operational amplifier is achieved, as is the case in the prototype. This allows you to unlimitedly increase the measurement time, which is necessary when measuring currents above 100 kA.

The clock frequency affects the accuracy of the digital integrator, therefore, it largely determines the accuracy of the meter as a whole: the higher the frequency, the smaller the methodological error of integration. As the calculations showed, the methodological error at the frequencies of the clock generator f _tg = 100 kHz and the frequency divider f _dh = 200 Hz does not exceed 0.07% for the measurement duration t ₁ = 2 sec, and for f _tg = 500 kHz (the values of f _tg and t ₁ are the same) the error does not exceed 0.006%. Thus, the optimal choice of frequencies of the clock generator and the frequency divider methodological error of the proposed device can be reduced to very small values. In addition, by nature, this error is systematic and, if necessary, is eliminated when calibrating the device.

 According to the information available to the authors, the set of essential features characterizing the essence of the claimed invention is not known from the prior art, which allows us to conclude that the invention meets the criterion of "novelty."

 According to the authors, the essence of the claimed invention does not follow explicitly from the prior art for specialists, since it does not reveal the above effect on the obtained technical result — a new property of the object — a set of features that distinguish the claimed invention from the prototype, which allows us to conclude about it compliance with the criterion of "inventive step".

 The set of essential features characterizing the essence of the invention, in principle, can be repeatedly used when measuring large constant currents above 100 kA with high accuracy without breaking and disconnecting the measurement circuit, which leads to the achievement of the goal - expanding functionality in the direction of increasing the limit of measured currents and increasing accuracy measurement, which allows us to conclude that the invention meets the criterion of "industrial applicability".

The invention is illustrated by graphic materials which depict: in FIG. 1 is a block diagram of a device; in FIG. 2 - the frame of the Rogowski belt, covering a package of tires with a measured current; on phage. 3a - dependences of the output voltage of the scale amplifier u (t) and the output voltage of the analog integrator u $\genfrac{}{}{0ex}{}{\text{u}}{\text{out}}$ (t); in FIG. 3b - signals from the output of the frequency divider; in FIG. 3c - signals received at the input of a pulse counter from a clock generator. The time interval θ is shown on a larger scale for clarity.

 The claimed invention according to the first claim includes a Rogowski belt with a winding 1 (Fig. 1) on a detachable non-ferromagnetic frame 2, some of which are pivotally connected (Fig. 2). One clamp the winding of the belt is connected to the zero bus, the second clip is the output of the belt and is connected to the scale amplifier 3. The output of the scale amplifier 3 is connected to the input of the digital integrator 4, the output of which through the decoder 5 is connected to the digital indicator 6. Positive clamp (+) of the power supply through a self-extracting button 7 is connected to the clamp reset the digital integrator 4.

 According to the second claim in the inventive device, the inverting input of the first comparator 8 is connected to the input terminal of the digital integrator 4 with its grounded non-inverting input, and also, through the first key 9, the inverting input of the second comparator 10 and the input of the analog inverter 11. The latter is an operational amplifier with a grounded non-inverting input, and the impedances at the input and in the feedback of which are strictly identical. The output of the analog inverter 11 through the second key 12 is connected to the inverting input of the second comparator 10.

 The output of the clock generator 13 through the third key 14, the control input of which is connected to the output of the frequency divider 15, is connected through the fourth 16 and fifth 17 keys to the summing and subtracting inputs of the pulse counter 18. The output of the counter 18 is the output of the digital integrator 4. To the output of the clock generator 13 the input of the frequency divider 15 is also connected.

 The output of the frequency divider 15 is connected to the control input of the sixth key 19 and through the logical inverter 20 to the control input of the seventh key 21, which is in the feedback of the analog integrator 22.

 The output of the first comparator 8 is connected to the first control input of the fifth key 17 on the subtracting input of the pulse counter 18 and the control input of the first key 9. The output of the second comparator 10 is connected to the first control input of the fourth key 16 and to the second control input of the fifth key 17. Output of the first comparator 8 through a logical inverter 23 is connected to the control input of the second key 12 at the output of the analog inverter and the second control input of the fourth key 16 at the summing input of the pulse counter 18. Positive field a stable power supply 24 through the sixth key 19 is connected to the input of the analog integrator 22, and also through a self-opening button 7 - with the clip reset the information of the pulse counter 18. Note that in the latter case (to reset the information), you can use the usual (unstable) as a source ) source of power.

 In the process of measuring the inventive device operates as follows.

где I - измеряемый ток;
ψ - потокосцепление.When the frame 2 is closed, the Rogowski belt (Fig. 2) is brought up to the measurement object (DC buses). In this case, the initial value of the mutual inductance between the belt and the current lead M (O) = 0. By pressing the button 7 (Fig. 1), the possible initial information in the pulse counter 18 is reset. From this moment, the measurement process begins. When the Rogowski coil covers a package of current-carrying tires, the mutual inductance M (t) changes from M (O) = 0 to M (t ₁ ) = M _max , where t ₁ is the time of complete coverage of the tires with the Rogowski coil. EMF is induced in the winding of the belt:

where I is the measured current;
ψ - flux linkage.

заменяется алгебраической суммой

где интервал времени ΔT определяется периодом выходного сигнала делителя частоты 15,
K - коэффициент пропорциональности.The signal from belt 1 (Fig. 1) in the form of an EMF is fed to the input of a large-scale amplifier 3, the output voltage u (t) of which is supplied to the input of a digital integrator 4, in which the integration process

replaced by algebraic sum

where the time interval ΔT is determined by the period of the output signal of the frequency divider 15,
K is the coefficient of proportionality.

 It should be noted that at the beginning of the measurement at t = 0, the voltage u (t) = u (O) = 0. At the end of the measurement, also u (t) = 0. During the measurement, u (t) can change according to an arbitrary law, in including at some points may have negative values. It depends on the actions of the operator. However, the measurement process is slow and the output frequency of the divider 15 f = 1 / ΔT can be selected relatively low.

 In the analysis, a twofold change in the measurement frequency did not lead to changes in methodological errors.

 The integration process using node 4 (Fig. 1) is as follows.

The signal u (t) from the output of the scale amplifier 3 is fed to the inverting input of the first comparator 10 through the first switch 9, if the voltage u (t) is negative, or through the analog inverter 11 and the second switch 12, if the voltage u (t) is positive. From this moment, when the sixth key 19 is closed and the seventh key 21 is opened from the control signal from the frequency divider 15, the voltage U _О is supplied to the input of the analog integrator 22 from a stable power source 24. From the output of the integrator 22, the signal is supplied to the non-inverting input of the second comparator 10. From the output of the latter, signals are sent to turn on the fourth and fifth keys 16 and 17. Key 16 is turned on if the voltage of the large-scale amplifier is positive, and key 17 when it is negative. In these time intervals, the pulses from the clock are fed to the pulse counter 18 (respectively, to the summing input or subtracting).

In FIG. 3a shows the changes in time of voltage u (t) at the output of scale amplifier 3 and voltage u $\genfrac{}{}{0ex}{}{\text{u}}{\text{out}}$ (t) at the output of the analog integrator 22.

The integration process ends when u $\genfrac{}{}{0ex}{}{\text{u}}{\text{out}}$ (t) = u (t). At this moment, a signal is sent from the second comparator 10 to disable the keys 16 and 17.

 The interval τ (Fig. 3a), and therefore the number of pulses received by the counter, are directly proportional to the voltage u (t) (Fig. 3c).

By summing the signals for the entire measurement time t ₁ in the device, equation (6) is implemented. Next, the signal from the counter output through the decoder 5 is fed to the indicator 6. In the second half-period (Fig. 3b) of the voltage from the output of the frequency divider 15, the key 21 is closed, so the capacitor of the analog integrator 22 is discharged to zero voltage during this period of time.

 Tests of the prototype showed that when using this method of measuring direct current, high measurement accuracy can be achieved, and the errors are created mainly by the analog elements of the device.

 As noted, the value of the frequency of the clock generator (TG) 13 has a significant impact on errors. The table (see also the appendix) shows the relative errors for different frequencies.

 It should be noted that they are due to the discrete nature of the conversion of the analog voltage u (t) to the number of pulses. By nature, these errors are systematic and therefore, when calibrating the device, they can be eliminated. Errors, due to changes in the value and shape of the voltage u (t) with the right choice of frequencies of the generator 13 and the frequency divider 15, can also be reduced to very small values (units of hundredths of a percent).

 The inventive device can be used in the national economy as a metrological portable device for measuring currents above 100 kA in order to verify stationary measuring devices without dismantling them.

Compared to the prototype, the range extends from 50 kA to 250 kA and higher, and the measurement accuracy also increases, since the operating frequency of the analog integrator increases, therefore, it is possible to use more precision small capacitors in it. In addition, a significantly shorter integration period (τ ≪ t ₁ ) completely eliminates the error due to the zero drift of the integrator.

 Calculations and tests of the layout of the proposed device suggest that the error in measuring the current does not exceed the value of 0.05%.

 The inventive portable digital meter of high constant currents is of considerable interest to the national economy, as it will allow you to verify stationary measuring devices without stopping the process and simplify the verification process of these installations.

 The inventive device does not adversely affect the environment.

Claims (1)

 A portable digital high constant current meter containing Rogowski belt in the form of a winding located on a detachable non-ferromagnetic frame, the first clip of the Rogowski belt being connected to the zero bus, a self-opening reset button, an integrator, characterized in that a large-scale amplifier, decoder, digital indicator and a stable power source, and the integrator is designed as a digital integrator, which contains an analog integrator, a clock generator, a frequency divider, and a counter pulses, two comparators, seven electronic keys, two logical inverters, the second clamp of the Rogowski coil winding connected to the input of a large-scale amplifier, and its output connected to the input of the analog inverter, as well as to the inverting input of the first comparator and through the first electronic key to the inverting input the second comparator, the output of the analog inverter through the second electronic key is connected to the inverting input of the second comparator, the output of the clock generator is through the third electronic key, the control input of which connected to the output of the frequency divider, connected through the fourth and fifth electronic keys to the summing and subtracting inputs of the pulse counter, the frequency divider is also connected to the output of the clock generator, the output of which is connected to the control input of the sixth electronic key and through a logical inverter with the control input of the seventh electronic key in feedback of an analog integrator, the input of which through the sixth electronic key is connected to the positive output of a stable power source, to the output of the first compar ora attached first control input of the electronic key and the fifth control input of the first electronic key, the second comparator output is connected to a first control input of a fourth electronic switch and a second control input of the fifth electronic key; the output of the first comparator through a logical inverter is connected to the control input of the second electronic key and the second control input of the fourth electronic key on the summing input of the pulse counter, the positive output of the stabilized power supply through a self-opening button is connected to the reset terminal of the pulse counter information, the output of the latter through a decoder is connected to a digital indicator .

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