CN210376503U - Capacitive voltage divider low pressure measuring circuit - Google Patents
Capacitive voltage divider low pressure measuring circuit Download PDFInfo
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- CN210376503U CN210376503U CN201921236241.0U CN201921236241U CN210376503U CN 210376503 U CN210376503 U CN 210376503U CN 201921236241 U CN201921236241 U CN 201921236241U CN 210376503 U CN210376503 U CN 210376503U
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Abstract
The utility model discloses a capacitive voltage divider low pressure measuring circuit, it includes: the capacitive voltage divider is formed by connecting a high-voltage arm C1 and a low-voltage arm C2 in series, one end of an adjustable resistor R1 is connected with the intersection point of the C1 and the C2, the other end of the adjustable resistor R1 is connected with the cable core of the cable, and the cable sheath on the corresponding side is grounded; the cable core at the other side is connected with an adjustable resistor R2 and a resistor R3; the other end of the adjustable resistor R2 is connected with one end of an adjustable capacitor C3, one end of a switch K1 and one end of a switch K2, the other end of the switch K1 is connected with one end of a capacitor C4, and the other end of the switch K2 is connected with one end of a capacitor C5; the other end of the resistor R3 is connected with the adjustable resistor R4, and R4 is connected with the other ends of the adjustable capacitors C3, C4 and C5; the oscilloscope CRO is connected with the adjustable resistor R4 in parallel; the technical problems that a capacitive voltage divider low-voltage test circuit in the prior art may have large errors in voltage division ratio and amplitude are solved.
Description
Technical Field
The utility model belongs to the electric energy meter measurement field especially relates to a capacitive voltage divider low pressure measuring circuit.
Background
The overvoltage can endanger the safe operation of a power grid, so that the voltage waveform which is born on a power transmission line or power transformation equipment and exceeds the normal operation can endanger the insulation safety of the equipment and the line, and the production of the power grid is greatly threatened. Overvoltage of a power system is classified into two major categories, lightning overvoltage and internal overvoltage. The lightning overvoltage is related to meteorological conditions and is caused by external reasons of a power grid; the internal overvoltage is caused by the transmission or conversion of energy inside the power system, and is related to various factors such as the internal structure, parameters, operation state, power cut and transmission operation, and accident occurrence of the power system. External and internal overvoltages can in turn be divided into more than ten overvoltage types. The overvoltage has great difference in amplitude and waveform, and even if the overvoltage is internal overvoltage caused by different reasons, the overvoltage value, waveform, frequency and duration are not completely the same. This presents great difficulty in accurately acquiring the overvoltage waveform.
The voltage divider is the most commonly used instrument for measuring overvoltage, and is mainly divided into a resistor voltage divider, a capacitor voltage divider and a resistor-capacitor voltage divider. The resistor voltage divider is not suitable for measuring the operation surge voltage and the resonance overvoltage with longer duration due to resistance loss. Compared with a resistor voltage divider, the capacitive voltage divider has the advantage of no phase angle error, and the voltage expression of the distributed capacitive voltage divider is as follows:
u(t)≈U0(X/l)[1-C/6K]
in the formula, K is the body capacitance of the capacitive voltage divider, and C is the stray capacitance to ground of the capacitive voltage divider. It has been found that a capacitive voltage divider output voltage without the inductor being considered is frequency independent, i.e. contains only amplitude errors and no waveform errors. In practical engineering, the low-voltage arm of the capacitive voltage divider should adopt a low-inductance structure and a low-dielectric-loss capacitor. For safety and reliability, a long distance is often present between the acquisition unit (or oscilloscope) and the voltage divider, so that a coaxial cable is required for connection, the coaxial cable has certain wave impedance, and the wave impedance causes the waves to be refracted and reflected in the propagation process, so that the measurement result is interfered, and a resistor R with the same size as the wave impedance is required to be added at the head end1. Finally, an oscilloscope or an acquisition device is connected to the tail end of the cable, so that a low-voltage measurement loop of a capacitive voltage divider which is commonly used at present is formed as shown in figure 1, wherein in figure 1: c1、C2Is a voltage divider capacitance; r1Is a matching resistor; DL is a cable; the CRO is an oscilloscope.
Because the wave impedance of the cable needs to be considered when the cable transmits high-frequency signals, if the resistance of a device connected with the cable is not matched with the wave impedance, the phenomenon of wave refraction and reflection can occur. The cable only serves a connection function when propagating low frequency signals. Due to the characteristic difference of the cable under different frequency signals, the voltage division ratio of the measuring system under different frequencies can be different, thereby causing the distortion of the measuring waveform. The wave head part of the overvoltage has high frequency, the wave tail part of the overvoltage has low frequency, and the difference of the voltage division ratio of the wave head and the wave tail can be caused by measuring the surge overvoltage by adopting a capacitive voltage divider to measure the waveform distortion.
At the initial state (wave head):
at steady state (wave tail) there are:
k can be found in case of matching of the first segment0≠k∞Error of voltage division ratio of
For higher voltage divider C2Large, not too long cable, equivalent capacitance CcAnd the error is relatively small, and the error can be ignored under the common condition. If the capacitance of the voltage divider is smaller and the cable is longer, C is the timecAnd C2The comparison cannot be ignored, and the error is larger.
In addition, in practical engineering, the voltage divider is installed on site, and the acquisition unit is installed in the main control room, and therefore, has a long coaxial cable connection. However, because the structure and voltage grade of different transformer substations are greatly different, the lengths of cables for connecting the oscilloscope and the capacitive voltage divider are greatly different, so that whether C can be ignored or not cannot be determinedcThe influence of (c) is accurately determined, and the error caused by the increase of the cable length is also increased continuously. Meanwhile, one electrical device is often directly equivalent to a capacitor C in measurement1Then through a series capacitor C2The voltage division ratio is uncertain, and the amplitude of the data measured after one time of voltage division may have a large error.
In summary, the conventional voltage divider measurement loop may have large errors in both the voltage division ratio and the amplitude.
SUMMERY OF THE UTILITY MODEL
The utility model provides a technical problem: the low-voltage measuring circuit of the capacitive voltage divider is provided to solve the technical problems that the low-voltage measuring circuit of the capacitive voltage divider in the prior art possibly has larger errors in voltage division ratio and amplitude and the like.
The technical scheme of the utility model:
a capacitive divider low voltage measurement circuit comprising: the capacitive voltage divider is formed by connecting a high-voltage arm C1 and a low-voltage arm C2 in series, and is characterized in that: one end of the adjustable resistor R1 is connected with the intersection point of the C1 and the C2, the other end is connected with the cable core of the cable, and the cable sheath on the corresponding side is grounded; the cable core at the other side is connected with an adjustable resistor R2 and a resistor R3; the other end of the adjustable resistor R2 is connected with one end of an adjustable capacitor C3, one end of a switch K1 and one end of a switch K2, the other end of the switch K1 is connected with one end of a capacitor C4, and the other end of the switch K2 is connected with one end of a capacitor C5; the other end of the resistor R3 is connected with the adjustable resistor R4, and R4 is connected with the other ends of the adjustable capacitors C3, C4 and C5; the oscilloscope CRO is connected in parallel with the adjustable resistor R4.
The measuring method of the low-voltage measuring circuit of the capacitive voltage divider comprises the following steps:
step 1, the wave impedance of the cable is50 ohm and 75 ohm specifications, and adjusting the resistance R1And R2The resistance value enables the wave impedance of the head end and the wave impedance of the tail end of the cable to be matched; adding analog square wave signals, adjusting the waveforms of the head end and the tail end to be consistent, and eliminating interference of refracted waves and reflected waves;
step 2, adjusting the adjustable capacitor C3Adjusting to the minimum value, closing K1, and adding an adjusting capacitor C4Measuring the measured voltage U in the initial state1And a first partial pressure U2Calculating an initial voltage division ratio k0(ii) a Measure measured voltage U again after stabilization1And a first partial pressure U2Calculating the final voltage division ratio k∞；
Step 3, comparing k0And k∞When k is the size of0＜k∞When the input of the capacitor is too large, the switch K1 is opened, and the capacitor C is adjusted3To make the initial and final division ratios consistent; when k is0＞k∞When the input of the capacitor is insufficient, K1 is opened, and another switch K2 is closed to enable the capacitor C to be charged5Put into service, repeat step ② to measure voltage, then repeat step ③ to compare k again0And k∞Until the capacitance is adjusted to k0＝k∞；
And 4, measuring the overvoltage by using the adjusted measuring loop.
The primary voltage division U2Is calculated as:
in the initial state there are:
at steady state there are:
the utility model has the advantages that:
the utility model discloses in capacitive voltage divider low pressure test return circuit, change the single-ended matching commonly used into bi-polar matching method. By improving the original low-voltage arm measuring circuit, the variable resistor and the variable capacitor are added, and the initial voltage division ratio and the final voltage division ratio can be kept consistent under the conditions of different cable lengths and different voltage division ratios by selecting a proper capacitance value, so that the purpose of accurately measuring the overvoltage amplitude is achieved.
In addition, by adding the secondary voltage division circuit, the signal amplitude in transmission can be ensured to be large, and the signal-to-noise ratio in the signal transmission process is improved. And the amplitude of the signal entering the acquisition device can be reduced, and the requirements of different acquisition devices on input ranges can be met.
The utility model provides an improved over-voltage measuring circuit, which can ensure the voltage division ratio of a voltage divider to be consistent between the initial state and the final state and reduce the error on the amplitude, thereby ensuring the accuracy of the measuring result; the technical problems that a capacitive voltage divider low-voltage test circuit in the prior art may have large errors in voltage division ratio and amplitude are solved.
Drawings
Fig. 1 is a schematic diagram of a low-voltage testing circuit of a capacitive voltage divider according to the prior art of the present invention;
fig. 2 is a schematic structural diagram of the present invention.
Detailed Description
The capacitive voltage divider should have a low inductance structure and low dielectric loss, and for example, a monolithic mica capacitor may be used. In order to avoid the influence of the high-voltage test area on the electromagnetic field and the electrostatic field of the measuring instrument and also for safety reasons, the measuring instrument and the voltage divider need to be separated by a certain distance, which generally needs a distance of several meters to dozens of meters. The voltage divider is typically connected to the oscilloscope by a radio frequency coaxial cable. In order to reduce the coupling impedance value and improve the anti-interference performance, a coaxial cable with a double-layer shielding layer can be adopted.
Aiming at the situation that the length and the model of a cable are uncertain in the actual engineering, variable resistors R are connected to the first end and the last end of a low-voltage measuring loop1And R2The resistance value of the variable resistor is adjusted to be equal to the wave impedance of the cable, so that errors caused by wave refraction and reflection to measurement are avoided; considering the unknown condition of equivalent capacitance of certain electrical equipment in actual engineering, a secondary voltage division circuit is added in a measurement loop, voltage is further divided by a resistance voltage divider, the voltage amplitude is reduced at a measurement end, and the measuring range of the oscilloscope is ensured to meet the measurement requirement. In addition, the voltage amplitude should be large enough in the propagation path to ensure a large signal-to-noise ratio so as to reduce the influence of interference signals; finally, considering the influence of frequency on the cable capacitance and the problem of inconsistent initial and final voltage division ratios, an adjustable capacitor C is connected in series in an adjustable resistance branch circuit3And switchable capacitor C4、C5The consistency of the voltage division ratio is ensured by adjusting the capacitance.
After the above analysis, the resulting measurement circuit is shown in fig. 2.
The capacitive voltage divider is formed by connecting a high-voltage arm C1 and a low-voltage arm C2 in series, one end of an adjustable resistor R1 is connected with the intersection point of the C1 and the C2, the other end of the adjustable resistor R1 is connected with a cable core of a cable, and a cable sheath on the corresponding side is grounded. The cable core on the other side is connected with adjustable resistors R2 and R3, wherein the other end of R2 is connected with a capacitor C3, one end of a switch K1 and one end of a switch K2, the other end of a switch K1 is connected with one end of a capacitor C4, and the other end of the switch K2 is connected with one end of a capacitor C5. The other end of R3 is connected with adjustable resistor R4, and R4 is connected with the other ends of capacitors C3, C4 and C5, and is not grounded. The oscilloscope CRO is connected in parallel with the adjustable resistor R4.
Adjustable resistor R1And R2And the matching of wave impedances of different cables is realized.
In the circuit diagram shown in the upper diagram, U1For the voltage to be measured, U2And U3Respectively, after the first voltage division and the second voltage division.
For the circuit diagram described above, in the initial state there are:
at steady state there are:
to ensure the initial voltage division ratio is consistent with the final voltage division ratio, let C1+C2＝CC+C3. I.e. can be adjusted according to C3So as to achieve the purpose that the initial voltage division ratio and the final voltage division ratio are equal. Same as C4And C5Can also be used for regulation, except that C4、C5For coarse adjustment, whether the device is put into use or not can be selected by closing the switch. C3The method is used for fine adjustment, the measurement precision can be further improved, and Z is the wave impedance of the cable.
C5> C4> C3, and the maximum value of C3 is C5-C4. Through switch control, equivalent capacitance is changed in the ranges of C4-C4 + C3 and C5-C5 + C3. Theoretically, the minimum capacitance value is C4, and the maximum capacitance value is C3+ C4+ C5.
The operation steps in the actual engineering are as follows:
the cable model is uncertain, and the cable length is related to the actual distance between the field voltage divider and the acquisition unit.
① the wave impedance of the cable has two specifications of 50 ohm and 75 ohm, and the resistance R is adjusted1And R2And the resistance value enables the wave impedance of the head end and the tail end of the cable to be matched. Adding analog square wave signals, adjusting the waveforms of the head end and the tail end to be consistent, and eliminating interference of refracted waves and reflected waves;
② adjustable capacitor C3Returning to zero, putting a regulating capacitor C4Measuring U in the initial state1And U2Calculating an initial voltage division ratio k0(ii) a Is long enoughMeasure U again after meta-stability1And U2Calculating the final voltage division ratio k∞；
③ comparison k0And k∞The size of (2). When k is0＜k∞When the capacitance input is excessive, the switch K1 is opened, and the capacitance C is adjusted3To make the initial and final division ratios consistent; when k is0＞k∞When the capacitor input is insufficient, another switch K2 is closed to enable the capacitor C5Putting into service, keeping the capacitance small, closing both K1 and K2, putting C4 and C5, repeating step ② to measure voltage, repeating step ③ to compare K again0And k∞Until the capacitance is adjusted to k0＝k∞；
④ measure the overvoltage using the regulated measurement loop.
The voltage obtained on the oscilloscope is
Claims (1)
1. A capacitive divider low voltage measurement circuit comprising: the capacitive voltage divider is formed by connecting a high-voltage arm C1 and a low-voltage arm C2 in series, and is characterized in that: one end of the adjustable resistor R1 is connected with the intersection point of the C1 and the C2, the other end is connected with the cable core of the cable, and the cable sheath on the corresponding side is grounded; the cable core at the other side is connected with an adjustable resistor R2 and a resistor R3; the other end of the adjustable resistor R2 is connected with one end of an adjustable capacitor C3, one end of a switch K1 and one end of a switch K2, the other end of the switch K1 is connected with one end of a capacitor C4, and the other end of the switch K2 is connected with one end of a capacitor C5; the other end of the resistor R3 is connected with the adjustable resistor R4, and R4 is connected with the other ends of the adjustable capacitors C3, C4 and C5; the oscilloscope CRO is connected in parallel with the adjustable resistor R4.
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| CN110297121A (en) * | 2019-08-01 | 2019-10-01 | 贵州电网有限责任公司 | A kind of capacitive divider low pressure measurement circuit and measurement method |
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN110297121A (en) * | 2019-08-01 | 2019-10-01 | 贵州电网有限责任公司 | A kind of capacitive divider low pressure measurement circuit and measurement method |
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