WO2004010154A1

WO2004010154A1 - A voltage sensor for monitoring ac current

Info

Publication number: WO2004010154A1
Application number: PCT/CN2003/000461
Authority: WO
Inventors: Youyuan Wang
Original assignee: Chint Group Corporation
Priority date: 2002-07-19
Filing date: 2003-06-16
Publication date: 2004-01-29
Also published as: CN2563582Y; AU2003242214A1; CN1455260A; CN1243246C

Abstract

This invention relates to a voltage sensor for monitoring AC current, which includes a conductor of primary circuit, secondary coil and iron core. The three parts are located by insulating fasteners. The current direction of the said primary circuit is perpendicular to the section of the said core, and the said core is open-type. The output voltage of the secondary inductance coil passes through bridge commutator and capacitive rectifier, and then exports DC voltage which is supplied for testing. In this invention, the voltage is directly used as the sampling signal, which has the advantage of high accuracy of measurements, great capability of anti-interference and wide measuring range, and also which makes the measurement separated from the primary circuit. Furthermore, the sensor can be disengaged from or closed to the primary circuit, which is convenient for the measurement of current at any time.

Description

TECHNICAL FIELD

The present invention relates to a sensor of a power system on-line current or a working current monitoring device of a power-consuming device, and in particular, it is a voltage sensor for AC current monitoring.

Background technique

At present, the monitoring of the working current of power systems or electrical equipment is mainly performed by current transformers. The structure of commonly used current transformers consists of a primary main circuit conductive plate, a secondary inductance coil and a toroidal core. , The large current of the main circuit is converted into a small current on the secondary coil for measurement. The traditional measurement method is to directly connect the ammeter in series with the secondary coil, but with the development of science and technology, digital arithmetic processing is gradually adopted. The digitally calculated sampling signal is a voltage signal. Therefore, the analog voltage signal on the secondary coil must be A / D converted first. Because the voltage signal of the secondary output of the current transformer is low, the amplified interference signal will also be amplified at the same time. However, a large error will occur, affecting the measurement accuracy, especially the poor anti-interference ability during the amplification process. At the same time, the current transformer with the above structure is also difficult to measure large currents, and it is impossible to measure the circuit from the main circuit.

According to the traditional electromagnetic theory, the current and the magnetic induction intensity function are multiple solutions, which can be seen from the hysteresis loop. Therefore, the induced voltage obtained by the secondary inductance coil is also multiple solutions and cannot be used as a measurement signal. When the induced voltage of the secondary inductance coil is used as the sample signal, the above problems are unavoidable.

However, the traditional electromagnetic theory ignores the role of eddy currents in the magnetic medium when analyzing the voltage of the secondary electromagnetic coil. The hysteresis loops obtained from it are also measured statically, so they cannot truly reflect the dynamic relationship between them. In fact, the induced voltage on the secondary electromagnetic coil includes the induced potential of the eddy current in the magnetic medium in the secondary inductance coil in addition to the part of the induced potential corresponding to the actual measured current, which is reflected on the hysteresis loop. The current value corresponding to the abscissa should be the superposition of the eddy current in the magnetic medium and the current in the primary circuit conductor. As can be seen from the more detailed analysis below, the relationship between the superimposed current and the magnetic induction intensity is linear, and This argument is confirmed by online dynamic measurements provided by modern technology.

Summary of the Invention

What the present invention is to solve is that due to the limitations of the existing current transformer and its sampling signal, there are a series of technical difficulties when using the current transformer to achieve analog-to-digital conversion (A / D), which limits the measurement accuracy. A voltage sensor for AC current monitoring is provided, which is based on the above analysis results, and converts AC current directly into a voltage signal, which can be directly converted into digital signals by A / D for digital operation. It can be directly converted into a sufficiently large voltage signal, so it will not be subject to radiated conduction interference, strong anti-interference ability, wide measurement range, and isolated measurement from the main circuit, and the sensor can be freely detached from or close to the main circuit. This provides convenient conditions for measuring current at any time. The technical solution used to solve the above problems is: A voltage sensor for AC current monitoring, which includes a primary main circuit conductor, a secondary coil, and an iron core. The three are positioned by insulating fasteners, and the main circuit current direction and the iron are The core cross section is perpendicular, the iron core is open, and the output voltage of the secondary inductor coil is bridge-rectified and capacitor-filtered to output a DC voltage for testing.

The hollow coil is used as the secondary coil, and the main circuit is the primary circuit. After the positions of the two are fixed, the magnetic flux generated by the primary circuit current and the flux linkage of the secondary coil Ψ _{21 are} proportional to L, and the proportionality factor is M _21. The mutual inductance coefficient is as follows:

When the primary main circuit current is AC, the induced potential U ₂ generated in the secondary coil satisfies the following formula:

U ₂ =-ά Ψ2ΐ = -M ₂₁ di! ⁽² -dt dt

Due to the hollow coil, the magnetic permeability of air can be regarded as a constant equal to the vacuum permeability, so M ₂₁ is a constant. This is the principle of measuring the current of the primary circuit with the hollow coil. However, because it is an air-core coil, the magnetic flux interlinked with the primary circuit is very weak, so the value of U ₂ is very small, and it is very difficult to directly measure U ₂ .

When the primary magnetic circuit and the secondary coil are laminated with a closed magnetic circuit using silicon steel sheets, the magnetic permeability of the iron core is a variable. The influence of the exciting current required by the iron core must be overcome to change the magnetic field Effect conductivity, it requires a very small impedance of the secondary circuit, then the secondary current coil ₁₂ and the secondary product of the number of turns N ₂ N _₂ I _2, N _₂ I ₂ when the exciting current is much larger than the core, then The core excitation current can be ignored, so the following formula holds:

NJ ^ N ₂ I ₂ (3)

^ Is the number of turns of the main circuit at a time,! ^ Is the primary circuit current after ignoring the core magnetizing current. Equation (3) is the working principle of the current transformer. Also, because the secondary coil gives a current signal and the secondary loop impedance must be very small, the voltage signal that can be taken out is also very small, which is difficult to meet the technical requirements for analog-to-digital conversion.

If the second ^ turn is wound on a section of iron core laminated with silicon steel sheet, the core length should be slightly larger than For the length of the secondary coil, a slot-shaped iron core with an equal cross-section can also be used. The secondary coil is wound on the core at the bottom of the slot. The secondary coil is wrapped and insulated. Keep the position unchanged, because the presence of the iron core increases the interlinking magnetic flux between the primary circuit and the secondary coil several hundred times, so that the induced potential generated by the primary circuit AC current in the secondary coil is hollow The coils increase hundreds of times. As long as the core magnetic permeability is constant and the position of the primary and secondary coils is constant, the mutual inductance M _{21 of the} primary and secondary coils will be constant. Formula (1) and formula (2) ) Still valid.

The substantial technical progress of the present invention is to find a method to ensure that the magnetic permeability of the iron core can be kept constant within a very large range of exciting current change. The primary loop current is used as the exciting current that generates the magnetic potential and is the measured current. I lmSin, which is substituted into formula (2) to get

= M ₂₁ ω Im Sin (ω ί- π / 2) (4) Formula (4) is the testing principle of the current-voltage sensor of the present invention. It is known from formula (4) that the phase angle difference between ¾ and the primary main circuit current is 90 Degrees (π / 2). The invention finds a new method for measuring AC current. This method has obvious advantages. First, the measurement device can be isolated from the current circuit under test, and the measurement device can be entered into the measurement position under the working state of the circuit under test. Measure without moving the loop and move the measuring device. The second is to directly convert the measured current into a high-voltage signal, which can be converted to digital by the A / D module after being divided by a resistor for direct display, calculation, program control or communication. And there is no need to consider the errors caused by interference and temperature effects. Third, the measurement range is wide, and the short circuit current value of the main circuit can be directly measured.

When the circuit under test has a very large short-circuit current of tens of thousands of amperes, U ₂ is bridge-rectified to the storage capacitor and allowed to convert the peak voltage into the storage capacitor's allowed voltage for a period of time. / D is converted into a digital signal. After the measured current exceeds several thousand amperes, the magnetic induction strength of the non-closed loop iron core will still saturate: However, because U _{2 is} ahead of L90 degrees (π / 2), U ₂ reaches the peak Um at I and the current crosses zero. Therefore, the Um value will not change the linear relationship due to the saturation of the core. Substituting _t = 0 into the formula (4), since sin (ω ί + π / 2) = 1 is:

Um = M ₂₁ ω Im (5) (5) Formula (5) shows that the peak value of the measured current of the primary circuit is proportional to the peak value of the induced voltage of the secondary warp loop, so the peak voltage value of the secondary coil is always the same as the measured current of the primary circuit. The peaks are linear and have nothing to do with whether the core is saturated or not. The formula (5) is the principle of measuring a very large short-circuit current, so the present invention provides a new method for measuring a very large current. That is the core of the non-closed magnetic circuit that uses the measured current as the secondary coil. The exciting current causes the secondary coil to generate an induced potential. The peak value of the secondary coil induced potential is proportional to the peak value of the measured current. The proportionality factor is the product of the angular frequency of the measured current and the mutual inductance of the measured current to the secondary coil. . In this way, the measured current signal is successfully converted into a voltage signal, and the relationship is linear.

In order to verify how large the range of the peak value of the measured current in formula (5) can ensure that the mutual inductance M ₂₁ is constant, it can be directly measured by experiments. For the convenience of the test, a primary coil is formed by winding N turns of wires on the outer layer of the secondary coil. The primary coil current is used as the exciting current instead of the main circuit current to be measured. I ^ NLn is connected in series with the primary coil with a small resistor 1 ?, and the voltage on the resistor Ur Then, I _in = Ur / R. Use a dual tracer to display I, η, and U ₂ on the fluorescent screen or send them to the printer for printing through the interface. When the U ₂ waveform is sinusoidal and no distortion occurs, explain M ₂₁ is constant. Gradually increasing Ln, the U ₂ waveform starts to appear distorted, and the current peak before the distortion point starts to appear. The M ₂₁ corresponding to Im is constant. Starting from the U ₂ distortion point, the M value of the core decreases toward saturation saturation, but from the waveform diagram, it can still be seen that Um and Im are still linear, and even Um has slightly increased. It is shown that the formula (5) holds from tens of amps to thousands of amps or tens of thousands of amps. The measurement principle of this newly invented current-voltage sensor is proved by experiments, the measurement range is very large, and the accuracy is good. Because the initial magnetic permeability of the iron core is small, the lower limit range of the measured current value that maintains the linear relationship of M ₂₁ decreases as it decreases. If the lower limit of the measured current value is still insufficient, the number of turns of the primary coil can be increased to further expand the lower limit.

The above experiment shows that the magnetic permeability of the soft magnetic material of the silicon steel sheet is a constant constant above the initial permeability of the exciting current and before reaching saturation. The above experiment is credible, and the current and voltage sensor of the present invention is also practical .

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described below with reference to the drawings and embodiments.

FIG. 1 is a schematic structural diagram of an embodiment when a large current is measured by the present invention.

FIG. 2 is a schematic structural diagram of an embodiment when a small current is measured by the present invention.

FIG. 3 is a schematic structural diagram of an embodiment when a small current is measured by the present invention.

FIG. 4 is a schematic structural diagram of a grooved iron core according to the present invention. detailed description

As shown in the figure, 1-secondary coil, 2—secondary coil frame, 3—iron core, 4—secondary main circuit conductive plate, 5—insulating fasteners, 6—bridge rectifier, 7—filter storage Capacitance, 8 once the main circuit wire. When the measured current of the main circuit is relatively large, the current and voltage sensor shown in Figure 1 is used.

—The secondary main loop, iron core and secondary inductance are fixed to ensure the same position and angle, so that the current direction of the main circuit is perpendicular to the cross section of the core. The primary circuit conductor uses a conductive plate, and the core uses a straight core. The output voltage of the secondary inductor is rectified to DC by the bridge, and the capacitor c is used to filter the energy storage output as a DC voltage. To ensure that the output DC capacitance is equal to the peak voltage of the secondary inductor, the load resistance is greater than the internal resistance of the secondary inductor

More than 2000 times.

When the measured current of the main circuit is small, the current and voltage sensor of the type shown in FIG. 2 is used, which is different from the embodiment of FIG. 1 in that the core uses a slot-shaped equal-section core. Referring to FIG. 4, the slotted iron core is butted on the bottom slot, so that it can be inserted into the coil frame. Each piece is not in the same position as the joint, and it can compact the iron core. -Referring to Figure 3, when the measured current of the primary circuit is small and the main circuit uses round wire 8, the primary circuit can be wrapped around the insulated secondary coil. The circuit is fixed on the secondary coil.

Claims

Claim

1. An AC current monitoring voltage sensor includes a primary main circuit conductor, a secondary coil, and an iron core. The three are positioned by insulating fasteners. The main circuit current direction is perpendicular to the cross section of the core. The iron core is open, and the output voltage of the secondary inductor coil is bridge-rectified and capacitor-filtered to output a DC voltage for testing.

2. The AC current monitoring voltage sensor according to claim 1, wherein the iron core is a straight type.

3. The voltage sensor for AC current monitoring according to claim 1, wherein the iron core is slot-shaped. '

4. The voltage sensor for AC current monitoring according to any one of claims 1 to 3, wherein the primary circuit conductor is a conductive plate.

5. The voltage sensor for AC current monitoring according to any one of claims 1 to 3, characterized in that the primary circuit conductor is a wire and is wound directly outside the insulated secondary coil.

6. The voltage sensor for AC current monitoring according to any one of claims 1 to 3, characterized in that the load resistance is more than 20 times greater than the internal resistance of the secondary inductance coil.

7. The voltage sensor for AC current monitoring according to claim 4, wherein the load resistance is more than 20 times greater than the internal resistance of the secondary inductance coil.

8. The voltage sensor for AC current monitoring according to claim 5, wherein the load resistance is more than 20 times greater than the internal resistance of the secondary inductance coil.