RU2515176C2 - Device for current measurement - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to electric measurement equipment and is designed to measure alternating currents of high level and to determine the moment of current transition over zero value in high-current circuits of industrial frequency networks. In the device for current measurement, comprising two coaxially arranged metal cylinders, connected at one end with the help of flanges, and at the other end having each their shunt, a high-frequency slot fixed on the flange of one of cylinders, with a coaxially arranged central electrode and at least one current tap located in the space between the inner and outer cylinders and connected by one end to the inner cylinder in the beginning of its working part, and by the other one - via a hole in the wall of the inner cylinder and an integrating RC-chain with a central electrode of the high-frequency slot, there is at least one additional resistor, connected between the outlet of the central electrode of the high-frequency slot and the body of the inner cylinder serially with the capacitor of the RC-chain, and values of lengths of the current tap and the working part of the inner cylinder are selected in accordance with the following ratio: $$\frac{l}{H}=0.5\pm \mathrm{0.3,}$$

where l - tap length; H - length of the working part of the inner cylinder. The current tap may be made in the form of a tube wit a longitudinal cut covering the inner cylinder. The capacitor of the RC-chain and the additional resistor may be installed in an electronic amplifying unit connected to the device with the help of a high-frequency cable. The RC-chain and the additional resistor may be installed in an electronic amplifying unit connected to the device with the help of a high-frequency cable.

EFFECT: higher accuracy of measurements due to reduction of unevenness of amplitude-frequency characteristic of a device, and also reduction of phase shift between voltage induced on a tap and current flowing in a device.

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Description

The invention relates to electrical engineering and is intended for measuring alternating currents of a high level and determining the moment of transition of a current through a zero value in high-current circuits of industrial frequency networks.

Known coaxial shunt containing two coaxially arranged cylinders, the outer of which is made in the form of current supply to the internal fixed on its bottom, which serves as calibrated resistance, the inner cylinder is made into four identical elements, two of which are made of material with a small, and the rest of material with high resistivity, interconnected and included in the bridge circuit, the ohmic resistance of the opposite and the inductance of all shoulders of which are equal to each other [1].

The disadvantage of this shunt is the dynamic error associated with the manifestation of the skin effect in the elements of the inner cylinder, as well as the dependence of the achieved effect (expanding the frequency range of measurement of pulsed currents) on the accuracy of manufacturing of the shunt elements.

A device for measuring current is known, comprising two metal coaxial cylinders that are connected at one end by a flange. At the opposite end, each of the cylinders has its own current supply. The inner cylinder is made of a material with high internal resistance. The voltage drop across the inner cylinder is brought to the oscilloscope using a solder located on the axis of the cylinders and connected to the central electrode of the RF connector mounted on the flange [2].

The disadvantage of this device is the dynamic error. Due to the skin effect in the inner cylinder, the transition characteristic has the form of an aperiodic curve, the rise time of which is proportional to the square of the wall thickness of the inner cylinder. This time, according to [3], is expressed as

where τ is the rise time of the transient response of the device;

µ is the magnetic permeability of the cylinder material;

d is the cylinder wall thickness;

ρ is the specific resistance of the cylinder material.

For a stainless steel cylinder, for example, 3 mm thick, which ensures thermal stability of the device when a current of 100 kA flows for 0.2 sec, the rise time is 3.6 · 10 ^-6 sec. At such a delay time, even if the electronic circuit finishes zero current, the current value at these times will be many times higher than the permissible level dictated by the requirements imposed on switching devices.

The closest analogue is a device for measuring current, containing two coaxial metal cylinders, a high-frequency connector and a current tap. Moreover, the cylinders are interconnected at one end using a flange, and at the other end they have their own current supply, one end of the tap is connected to the inner cylinder, and the other end to the central electrode of the RF connector located on the flange. The soldering is located in the space between the inner and outer cylinders and is connected through a hole in the side surface of the inner cylinder to the central electrode of the RF connector via an integrating RC circuit, which converts the voltage induced on the solder into a signal proportional to the measured current [4].

The disadvantage of this device is the unevenness of the amplitude-frequency characteristics of the device, as well as the presence of a significant phase shift between the voltage induced on the tap and the current flowing through the device.

The objective of the invention is to reduce the unevenness of the amplitude-frequency characteristics of the device, as well as reducing the phase shift between the voltage induced on the tap and the current flowing through the device.

The technical result is to reduce the response time of the device to a change in the measured current.

The problem is solved in that in a device for measuring current, containing two coaxially arranged metal cylinders connected at one end with flanges, and at the other end having their own current supply, a high-frequency connector, mounted on the flange of one of the cylinders, with a coaxially located central electrode and at least one current tap located in the space between the inner and outer cylinders and connected at one end to the inner cylinder at the beginning of its working part, etc. at least one additional resistor is inserted through the hole in the wall of the inner cylinder and the integrating RC circuit with the central electrode of the high-frequency connector, connected between the output of the central electrode of the high-frequency connector and the body of the inner cylinder in series with the capacitor of the RC chain, and the lengths soldering and working parts of the inner cylinder are selected in accordance with the ratio:

where l is the length of the tap;

H is the length of the working part of the inner cylinder.

The current tap can be made in the form of a tube with a longitudinal section covering the inner cylinder.

The capacitor of the RC chain and an additional resistor can be installed in an electronic amplifier block connected to the device using a high-frequency cable.

The RC circuit and an additional resistor can be installed in an electronic amplifier unit connected to the device using a high-frequency cable.

The introduction of an additional resistor in the device for measuring current, connected between the output of the central electrode of the high-frequency connector and the inner cylinder body in series with the capacitor of the RC circuit, can reduce the influence of the inductance of the measuring circuit on the measurement result, which reduces the response time of the device to a change in the measured current.

The choice of the tap length equal to half the length of the working part of the inner cylinder, allows to reduce the unevenness of the amplitude-frequency characteristics in the frequency range 10 ⁴ ÷ 10 ⁵ Hz. This is due to the fact that at l = 0.5 H, the stresses arising in the external circuit due to electric fields on the outer and inner surfaces of the inner cylinder act in antiphase, that is, one is ahead of the nominal value, and the other is behind it, with l = 0.5 H maximally compensate for each other's shortcomings and their sum differs little from the nominal value.

The location of the capacitor of the RC chain and the additional resistor, as well as the RC chain and the additional resistor in a separate unit provides more free access to the elements when they are configured.

Figure 1 shows a device for determining current.

Figure 2 shows a section aa of the device for determining the current with the implementation of the current soldering in the form of a tube with a longitudinal section.

Figure 3 shows a view of the amplitude-frequency characteristics of a device for determining current under various conditions.

Figure 4 shows the phase-frequency dependence of the output signal of the device for determining current.

The device for measuring current contains two coaxially arranged metal cylinders 1 and 2, connected at one end using flanges 3 and 4 (Fig. 1). Metal cylinders 1 and 2 at each other end have their own current supply 5 and 6. On the outer plane of the flange 4 of the inner cylinder 2, a high-frequency connector 7 is fixed, with the central electrode 8 coaxially located. At least one current tap 9 is located in the space between the inner 2 and external 1 cylinders and is connected at one end to the inner cylinder 2 at the top at the beginning of its working part, and to the other through an opening 10 in the wall of the inner cylinder 2 and an integrating RC circuit (resistor 11 and capacitor 12) with a central electric odom 8 of the high-frequency connector 7. At least one additional resistor 13 is inserted into the device, connected between the output of the central electrode 8 of the high-frequency connector 7 and the housing of the inner cylinder 2 in series with the capacitor of the RC chain 12, and the lengths of the current tap 9 - (l ) and the working part of the inner cylinder 2 - (H) are selected in accordance with the ratio:

where l is the length of the tap;

H is the length of the working part of the inner cylinder.

The current tap 9 is made in the form of a tube 14 with a longitudinal section 15, covering the inner cylinder 2 (figure 2).

To reduce the influence of an external magnetic field, the device contains two identical parallel-connected tap 9 and 16 located symmetrically relative to the axis of the inner cylinder 2.

All elements of the seals 9 and 16 are isolated from the cylinders 1 and 2 all the way from the place of their connection with the inner cylinder to the high-frequency connector 7. The inner cylinder 2, which is a calibrated resistance, is made sufficiently thick-walled of a well-conducting non-magnetic material, for example stainless steel.

In the actual design of the device on the inner plane of the flange 4 is a circular dielectric board 17 with three circular conductive paths. Between the first and second tracks, parallel-connected capacitors 12, 18 are formed, which form the capacitor of the integrating RC circuit, and between the second and third tracks connected to the body of the internal cylinder 2, parallel-connected resistors 13, 19 are formed, forming an additional resistor. Some terminals of the capacitors 12, 18 are connected to the central electrode 8 of the high-frequency connector 7, while others are connected to the terminals of the resistors 13, 19, the second terminals of which are connected to the housing of the inner cylinder 2.

An increase in the number of capacitors of the integrating circuit and additional resistors makes it possible to reduce the influence of their own inductance on the measurement results.

In one embodiment of the device, a board 17 with capacitors 12, 18 and additional resistors 13, 19 is installed, for example, in an electronic amplifier unit connected to the device using a high-frequency cable.

In another embodiment, in addition to the board 17 with capacitors 12, 18 and additional resistors 13, 19, an integrating resistor 11 is also transferred from the device to the electronic amplifier block.

These options provide easier access to the board 17 when setting it up.

To measure current, the device with current leads 5, 6 is included in the gap of one of the buses of the current circuit. The direction of current I in the device is shown in the drawing by arrows. The elementary theory explaining the operation of this device is reduced to the equation

On the left side, the EMF that appears at the tap location is recorded, on the right is the distribution of this EMF between different elements of the measuring circuit, including the current tap 9, connecting wires 20, 21, the resistor of the RC chain 11, the capacitor of the RC chain 12 and the additional resistor 13.

The values included in the right side indicate:

R is the resistance of the working part of the inner cylinder 2, taking into account the skin effect;

- коэффициент взаимоиндукции отпайки 9;

- the mutual induction coefficient of the tap 9;

µ is the relative magnetic permeability of the medium between cylinders 1 and 2;

l is the length of the tap 9;

h is the width of the gap between the outer part of the tap 9 and the inner cylinder 2;

a is the outer radius of the inner cylinder 2;

i is the current in the measuring circuit;

r ₁ is the resistance of the integrating resistor 11;

r ₂ is the resistance of the additional resistor 13 in the measuring circuit: the central electrode 8 of the high-frequency connector 7, the integrating capacitor 12, the contact of the capacitor 12 with the housing of the inner cylinder 2;

L is the inductance of the measuring circuit;

C is the capacity of the integrating capacitor 12;

q is the charge of the integrating capacitor 12.

, уравнение (1) может быть записано в виде:Given that

, equation (1) can be written as:

The optimal mode of operation of the device in terms of accuracy of reproduction of the current shape is the condition:

Performing it, equation (2) can be represented as:

,

.where indicated

,

.

The expressions F ₁ and F ₂ differ only in that r ₁ includes r ₀ - resistance of the working part of the inner cylinder 2 at constant current, and F ₂ - R - resistance of the working part of the inner cylinder 2 taking into account the skin effect.

To better visualize the properties of such a device for measuring current, we will analyze equation (4) in the frequency variant by introducing the circular frequency ω and assuming that all the variables vary in harmonic law, that is, are proportional to e- ^jωt .

In this case, we have

It follows that as ϖ · T >> 1 F ₁ → 0. When ϖ · T << 1 F ₂ → 0. But since the skin effect plays an insignificant role (R≈r ₀ ) at low frequencies, F ₂ ≈F ₁ here too. Therefore, subject to condition (3), at least at low and high frequencies, the voltage at the output of the high-frequency connector is proportional to the current, i.e.

. Здесь наблюдается отклонение выходного напряжения от значений I·r₀.This equality is violated in the vicinity of the frequency

. Here, the deviation of the output voltage from the values of I · r ₀ is observed.

It is impossible to determine the magnitude of these deviations within the framework of this analysis. For this, the electrodynamic problem was solved on the basis of Maxwell's equations. Relations have been obtained that make it possible to determine for a given frequency the distribution over the thickness of the inner cylinder of the current density and, therefore, the electric field strength. This allowed us to calculate the value of the output voltage depending on the frequency. Numerical calculations were performed on a computer using the Mathcad program.

The results of calculating the output voltage, taking into account the skin effect for a stainless steel cylinder with a diameter of 50 mm and a wall thickness of 3 mm, are presented in FIGS. 3 and 4. The calculation takes: M = 1.5 · 10 ^-9 H, r ₀ = 1.7 · 10 ^{- 4} ohms.

In the form of the amplitude-frequency characteristic shown in Fig. 3, curves 1 are obtained for the case when the length of the current soldering 9 is equal to the length of the entire working part of the inner cylinder 2 (l = H), curves 2 - for the case when l = 0.8 H, curves 3 - for the case l = 0.5 H, curves 4 - for the case l = 0.2 H, curves 5 - for the case l = 0.

On figa presents the calculation result without taking into account the resistor r ₂ , Fig.36 - taking into account this resistor, the resistance value of which is selected according to condition (3) with the accepted inductance calculation L = 1 · 10 ^-6 GN. It can be seen that for l = 0.5 H, the non-uniformity of the frequency response is minimal. It is reduced to 10 percent. The use of the resistor r ₂ extends the frequency band in the high frequency region.

The phase-frequency dependence of the output signal of the device shown in Fig. 4 also shows a similar effect: the smallest phase shift occurs at l = 0.5 H. This is especially pronounced at low frequencies. For example, at a frequency of 50 Hz, the phase (in radians) has the following values: 9 × 10 ^-3 - (1); -9 × 10 ^-3 - (5); 7 × 10 ^-7 - (3).

должно быть больше 0.5; наоборот, для запаздывания фазы, отношение

должно быть меньше 0.5.The variant l = 0.5 H is selected in the case when it is necessary to form the output signal that most accurately reproduces the shape of the current. If it is important to ensure phase advance, the ratio

must be greater than 0.5; conversely, for phase lag, the ratio

must be less than 0.5.

The condition l = 0.5 H is implemented in practice in a device whose wall thickness of the inner cylinder is made of non-magnetic stainless steel is 3 mm and the length of the working part is 10 cm. The device remains operational when a current of 100 kA flows for 0.2 s.

The choice of the tap length equal to half the length of the working part of the inner cylinder, allows to reduce the unevenness of the amplitude-frequency characteristics in the frequency range 10 ⁴ ÷ 10 ⁵ Hz.

The introduction of an additional resistor in the device for measuring current, connected between the output of the central electrode of the high-frequency connector and the inner cylinder body in series with the capacitor of the RC circuit, can reduce the influence of the inductance of the measuring circuit on the measurement result, which reduces the response time of the device to a change in the measured current.

The location of the capacitor of the RC chain and the additional resistor, as well as the RC chain and the additional resistor in a separate unit provides more free access to the elements when they are configured.

Information sources

1. USSR Copyright Certificate No. 813267, G01R 19/04, 1981.

2. Schwab A., Measurements at high voltage. Per. with him. M., "Energy", 1973, p.140.

3. Schwab A., Measurements at high voltage. Per. with him. M., "Energy", 1973, p.143.

4. RF patent No. 2216746, G01R 19/00, 2003.

Claims (4)

1. A device for measuring current, comprising two coaxially arranged metal cylinders connected at one end by means of flanges, and at the other end having their own current supply, a high-frequency connector, mounted on the flange of one of the cylinders, with a central electrode coaxially located and at least one current tap located in the space between the inner and outer cylinders and connected at one end to the inner cylinder at the beginning of its working part, and the other through a hole in the inner wall cylinder, and an integrating RC circuit with a central electrode of the high-frequency connector, characterized in that at least one additional resistor is inserted into the device, connected between the output of the central electrode of the high-frequency connector and the body of the inner cylinder in series with the capacitor of the RC chain, and the quantities the lengths of the current tap and the working part of the inner cylinder are selected in accordance with the ratio:

where l is the length of the tap;
H is the length of the working part of the inner cylinder. 2. The device according to claim 1, characterized in that the current tap is made in the form of a tube with a longitudinal section covering the inner cylinder. 3. The device according to claim 1, characterized in that the capacitor of the RC chain and an additional resistor are installed in an electronic amplifier unit connected to the device using a high-frequency cable. 4. The device according to claim 1, characterized in that the RC circuit and an additional resistor are installed in an electronic amplifier unit connected to the device using a high-frequency cable.

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