CN106771510B - Method for improving low-range measurement precision of residual current - Google Patents
Method for improving low-range measurement precision of residual current Download PDFInfo
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- 238000006243 chemical reaction Methods 0.000 claims description 15
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Abstract
The invention relates to a method for improving the low-range measurement precision of residual current, which comprises the following steps of sampling the residual current of a distribution line through a residual current transformer; performing data processing on the sampled residual current to obtain a current signal which is same in frequency and phase with the residual current and is 50 mA; feeding the current signal of 50mA back to the input end of a residual current transformer; measuring the residual current in an output linear region of the residual current transformer; and subtracting 50mA from the measured value of the residual current to obtain a residual current measurement result meeting the high-precision requirement. According to the technical scheme, the residual current measurement error is reduced, the residual current monitoring data of the low-voltage distribution network are accurate and effective, the residual current is accurately measured, and a three-level protection system for the residual current of the distribution network can effectively cooperate with action and operate reliably.
Description
The technical field is as follows:
the invention relates to the field of high-precision measurement of residual current of a power distribution network, in particular to a method for improving low-range measurement precision of residual current.
Background art:
at present, the residual current measurement error of domestic residual current protectors is generally more than +/-10% (the measurement range is 30-1000mA), and the residual current measurement error is large, so that the data of a residual current monitoring system is distorted, a three-level protection system of the residual current of a rural power distribution network cannot be effectively matched to act, and the operation reliability is reduced.
The key to improve the measurement accuracy of the residual current protector is to improve the measurement accuracy of the low range.
The invention content is as follows:
the invention aims to provide a method for improving the low-range measurement accuracy of residual current, which reduces the measurement error of the residual current and enables the residual current monitoring data of a low-voltage distribution network to be accurate and effective.
In order to achieve the purpose, the invention adopts the following technical scheme: a method for improving the accuracy of low-range measurement of residual current comprises the following steps:
sampling residual current of a distribution line through a residual current transformer;
performing data processing on the sampled residual current to obtain a current signal which is same in frequency and phase with the residual current and is 50 mA;
feeding the current signal of 50mA back to the input end of a residual current transformer;
measuring the residual current in an output linear region of the residual current transformer;
and subtracting 50mA from the measured value of the residual current to obtain a residual current measurement result meeting the high-precision requirement.
Under the load resistance, the change curve of the output characteristic of the residual current transformer comprises three sections: a nonlinear region of the starting section, a linear region of the middle section and a saturation region of the end section; the output voltage of the residual current transformer is increased along with the increase of the residual current in the linear area.
Performing data processing on the residual current through a data processing circuit; the data processing circuit comprises a sampling amplifying circuit, a sine wave square wave conversion circuit, a phase discrimination low-pass filter circuit, a level conversion circuit, a function generator and a current amplifying circuit; the phase discrimination low-pass filter circuit, the level conversion circuit and the function generator form a phase-locked loop PLC circuit; and the sampled residual current sequentially passes through a sampling amplifying circuit and a sine wave square wave conversion circuit to obtain a square wave signal synchronous with the residual current of the distribution line, and the square wave signal is used as an input of the phase-locked loop PLC circuit.
The phase discrimination low-pass filter circuit comprises a phase discriminator and an RC low-pass filter arranged at the output end of the phase discriminator; the phase detector comprises a PDL; the PDL is made up of exclusive or gates, requiring the duty cycle of the input waveform to be 50%.
A square wave output pin of the function generator is connected with a pull-up resistor to a voltage V + so that the level of the square wave output by the function generator is within +/-12V; the output square wave is used as the other input of the phase-locked loop PDL circuit through the level conversion circuit; the sine wave pin of the function generator outputs a sine wave signal, and the sine wave signal passes through the current amplifying circuit and outputs 50mA of same-phase alternating current.
The function generator is an ICL8038 function generator; pins 4, 5 and 16 are respectively connected with resistors R13 and R14 and a capacitor C2; in order to enable the duty ratio of the sine wave and the square wave output by the ICL8038 function generator to be 50%, R13-R14 is taken; the output sine wave signal frequency is 50Hz, and when the loop is locked, the voltage at pin 8 is 9V, so that R13-R14-20K and C2-0.2 μ F are taken.
The resistance in the RC low-pass filter is 100 kilo-ohm, and the capacitance is 2.2 microfarads.
The PDL is CD4046 PDL.
The sine wave distortion degree of the ICL8038 function generator is reduced to 0.1% by adjusting a bridge circuit formed by a pin 1 and a pin 12 of the ICL8038 function generator.
Compared with the closest prior art, the technical scheme provided by the invention has the following excellent effects
1. According to the technical scheme, the residual current is accurately measured, so that a three-level protection system for the residual current of the power distribution network can effectively cooperate with action and operate reliably;
2. the technical scheme of the invention automatically tracks the frequency and the phase of the power grid voltage in real time, does not occupy the software and hardware resources of a processor, and greatly reduces the programming complexity;
3. the technical scheme of the invention can meet the required high-precision measurement result;
4. the technical scheme of the invention meets the requirement that the error is controlled within 3 percent in the measurement range of 20 mA-600 mA residual current.
Drawings
FIG. 1 is a schematic diagram of a measurement circuit according to an embodiment of the present invention;
FIG. 2 is a graph of the output characteristic of a transformer according to an embodiment of the present invention;
FIG. 3 is a functional block diagram of a measurement circuit according to an embodiment of the present invention;
FIG. 4 is a schematic diagram of a sampling amplifying circuit according to an embodiment of the present invention;
FIG. 5 is a schematic diagram of a sine wave/square wave conversion circuit according to an embodiment of the present invention;
fig. 6 is a schematic diagram of a phase detection low-pass filter circuit according to an embodiment of the present invention;
FIG. 7 is a schematic diagram of a level shift circuit according to an embodiment of the present invention;
FIG. 8 is a schematic diagram of a sine wave generation circuit according to an embodiment of the present invention;
fig. 9 is a schematic diagram of a current amplifying circuit according to an embodiment of the invention.
Detailed Description
The present invention will be described in further detail with reference to examples.
Example 1:
the invention provides a method for improving the low-range measurement accuracy of residual current, which is characterized in that as shown in fig. 1, a residual current transformer is used for sampling the residual current of a distribution line, and then a 50mA current signal which is in phase synchronization with the residual current of the distribution line is obtained through a series of circuit processing. The current is applied to the residual current transformer, the measurement starting point of the residual current is improved, the nonlinear region of the output characteristic of the residual current transformer is avoided, and the high-precision measurement result can be obtained by subtracting 50mA from the numerical value sampled and measured by the CPU of the protector. The method needs to use an alternating current signal synchronous with the residual current of the distribution line, namely a standard sinusoidal signal with the same frequency and phase as the residual current of the distribution line. The invention adopts a full hardware circuit to complete the function, namely, adopts a phase-locked loop and a sine function generator integrated circuit to automatically track the frequency and the phase of the voltage of the power grid in real time, does not occupy the software and hardware resources of a processor, and greatly reduces the programming complexity. The hardware circuit can be divided into a distribution line residual current sampling amplification part, a sine wave/square wave conversion part, a phase discrimination low-pass filtering part, a level conversion part, a sine wave generation part and a current amplification part in principle. The method comprises the steps of sampling and amplifying the residual current of the distribution line by using a residual current transformer, and obtaining a square wave signal synchronous with the residual current of the distribution line through a zero-crossing comparator to be used as one input of a Phase Discriminator (PD). The phase detector, the low pass filter and the function generator ICL8038 form a phase locked loop (PLC) circuit. When the loop is locked, the output sine wave is synchronized with the grid voltage, i.e. it is in phase with the same frequency, and there should be little delay in time. The schematic block diagram is shown in fig. 3.
(1) Sampling amplifying circuit
The millivolt level voltage signal of the residual current transformer is amplified, so that the amplified signal can be identified by a zero-crossing comparator, and the circuit is shown in fig. 4.
(2) Sine wave/square wave conversion circuit
The residual current transformer is utilized to sample the residual current of the distribution line, and then a square wave signal synchronous with the residual current of the distribution line is obtained through the zero-crossing comparator, and the circuit is shown in fig. 5.
(3) Phase discrimination low-pass filter circuit
The phase detector in this embodiment uses PDl of CD 4046. PDl are formed by exclusive or gates, requiring the duty cycle of the input waveform to be substantially 50%. The resistor R8 and the capacitor C1 form a low-pass filter, and the circuit is shown in FIG. 6.
(4) Level conversion circuit
Since the PDl output of the phase detector CD4046 is between-12V and +12V, the function generator ICL8038 requires that the input control voltage signal must be between 1/3V + and V +, i.e., between 4V and 12V. Level shifting is necessary so that the ICL8038 circuit can capture the input control signal well. The circuit is shown in fig. 7.
(5) Sine wave generating circuit
The function generator ICL8038 can generate a waveform such as a square wave, a sine wave, or a sawtooth wave with high accuracy by connecting a small number of external elements. In order to reduce the sine wave distortion degree, a bridge circuit is formed by a sine wave distortion degree adjusting pin 1 and a pin 12 of an ICL8038, so that the distortion degree is reduced to about 0.1%. Pin 2 outputs a sine wave. The square wave output pin 9 is connected with a pull-up resistor to V +, so that the output square wave level is within +/-12V. The output square wave is level-shifted as another input to PDl. Because the square wave and the sine wave output by the ICL8038 have a phase difference of 90 degrees, when the loop is locked, the requirement that the duty ratio of PDl input and output waveforms is 50 percent is exactly met, and the circuit is shown in FIG. 8.
(6) Current amplifying circuit
The function generator ICL8038 outputs a sine wave amplified output at pin 2, and can output 50mA of in-phase ac current, and the circuit is shown in fig. 9.
(7) Design of circuit key parameter
Low pass filter, fig. 6, parametric design of R8 and Cl: r8 and Cl have a great influence on the loop-capturing performance and the operational stability. If the time constant R8 × C1 is large, excessive delay is generated when the loop tracks the input frequency which changes rapidly; taking a small time constant R8 × C1 would cause the loop to track a rapidly changing input signal. Causing an abnormal change in the output frequency of the phase locked loop. The loop capture performance and the working stability are comprehensively considered. R8 ═ 100K Ω and C1 ═ 2.2 μ F were selected.
Parameters of ICL8038 external elements R13, R14 and C2 were selected: in order to make the duty ratio of the sine wave and the square wave output by the ICL8038 be 50%, R13 is taken as R14. The output sine wave signal frequency should be around 50Hz, and when the loop is locked, the pin 8 should be around 9V, so that C2 should be taken out to the maximum within the allowable range, and finally, R13 ═ R14 ═ 20K, and C2 ═ 0.2 μ F are taken.
Under a certain load resistance, the change curve of the error characteristic of the residual current transformer is shown in figure 2. The graph of fig. 2 is divided into three segments. It can be seen that the segment a indicates that the initial output value of the transformer is very low, the residual current is continuously increased, the output voltage is not proportional, and the transformer has a proportional output voltage value only after the residual current is increased to a certain value. The section b shows that the output voltage of the transformer increases along with the increase of the residual current, and the section can be regarded as the output of the transformer is linearly changed. The section c shows that when the output voltage of the mutual inductor reaches a certain value, the output voltage of the mutual inductor does not continuously increase along with the increase of the input current, but tends to be stable and even tends to decline, which indicates that the iron core of the mutual inductor reaches a saturated state. It can be seen that the b segment is a segment of the area that can be used for accurate measurements.
According to experimental research, the sampling error of the residual current transformer can be controlled within 3% when the sampling error is 70 mA-700 mA. In order to meet the measurement requirement of high precision (in the measurement range of 20mA to 600mA residual current, the error is controlled within 3%), a 50mA same-frequency and same-phase current is added to the residual current transformer, the sampling measurement range of the residual current transformer is moved from 20mA to 600mA to the linear range of 70mA to 650mA, and 50mA is subtracted from the numerical value sampled and measured by a CPU of the residual current protector, so that the high-precision measurement result meeting the requirement can be obtained.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting the same, and those skilled in the art should understand that although the above embodiments are referred to: modifications and equivalents may be made to the embodiments of the invention without departing from the spirit and scope of the invention, which is set forth in the claims below.
Claims (5)
1. A method for improving the low-range measurement accuracy of residual current is characterized in that: the method comprises the following steps:
sampling the residual current of the distribution line through a residual current transformer;
performing data processing on the sampled residual current to obtain a current signal which is same in frequency and phase with the residual current and is 50 mA;
feeding the current signal of 50mA back to the input end of a residual current transformer;
measuring the residual current in an output linear region of the residual current transformer;
subtracting 50mA from the measured value of the residual current to obtain a residual current measurement result meeting the high-precision requirement;
under the load resistance, the change curve of the output characteristic of the residual current transformer comprises three sections:
a non-linear region of the starting section, the linear region of the middle section and a saturation region of the end section; the output voltage of the residual current transformer is increased along with the increase of the residual current in the linear region;
performing data processing on the residual current through a data processing circuit; the data processing circuit comprises a sampling amplifying circuit, a sine wave square wave conversion circuit, a phase discrimination low-pass filter circuit, a level conversion circuit, a function generator and a current amplifying circuit; the phase discrimination low-pass filter circuit, the level conversion circuit and the function generator form a phase-locked loop PLC circuit; the sampled residual current sequentially passes through a sampling amplifying circuit and a sine wave square wave conversion circuit to obtain a square wave signal synchronous with the residual current of the distribution line, and the square wave signal is used as one input of the phase-locked loop PLC circuit;
the phase discrimination low-pass filter circuit comprises a phase discriminator and an RC low-pass filter arranged at the output end of the phase discriminator;
the phase detector comprises a PDL; the PDL is composed of exclusive-OR gates, and the duty ratio of an input waveform is required to be 50%;
a square wave output pin of the function generator is connected with a pull-up resistor to a voltage V + so that the level of the square wave output by the function generator is within +/-12V; the output square wave is used as the other input of the phase-locked loop PDL circuit through the level conversion circuit; the sine wave pin of the function generator outputs a sine wave signal, and the sine wave signal passes through the current amplifying circuit and outputs 50mA of same-phase alternating current.
2. The method for improving the accuracy of low-range measurement of the residual current as claimed in claim 1The method of (2), characterized by: the function generator is an ICL8038 function generator; pins 4, 5 and 16 are respectively connected with resistors R13 and R14 and a capacitor C2; in order to enable the duty ratio of the sine wave and the square wave output by the ICL8038 function generator to be 50%, R13-R14 is taken; the frequency of the sine wave signal output by the loop locking circuit is 50Hz, and the voltage at the pin 8 is 9V when the loop is locked, so that R13-R14-20K is taken
，C2＝0.2μF。
3. The method for improving the accuracy of measuring the low range of the residual current according to claim 2, wherein: the resistance in the RC low-pass filter is 100 kilo-ohm, and the capacitance is 2.2 microfarads.
4. The method for improving the accuracy of measuring the low range of the residual current according to claim 1, wherein: the PDL is CD4046 PDL.
5. The method for improving the accuracy of measuring the low range of the residual current according to claim 2, wherein: the sine wave distortion degree of the ICL8038 function generator is reduced to 0.1% by adjusting a bridge circuit formed by a pin 1 and a pin 12 of the ICL8038 function generator.
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2015
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| CN1145117A (en) * | 1994-12-28 | 1997-03-12 | 菲利浦电子有限公司 | DC and AC current sensor having a minor-loop orperated current transformer |
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