CN112034233B - High-precision alternating current testing device and method - Google Patents
High-precision alternating current testing device and method Download PDFInfo
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- CN112034233B CN112034233B CN202010897238.4A CN202010897238A CN112034233B CN 112034233 B CN112034233 B CN 112034233B CN 202010897238 A CN202010897238 A CN 202010897238A CN 112034233 B CN112034233 B CN 112034233B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R19/00—Arrangements for measuring currents or voltages or for indicating presence or sign thereof
- G01R19/0092—Arrangements for measuring currents or voltages or for indicating presence or sign thereof measuring current only
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R15/00—Details of measuring arrangements of the types provided for in groups G01R17/00 - G01R29/00, G01R33/00 - G01R33/26 or G01R35/00
- G01R15/14—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks
- G01R15/18—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using inductive devices, e.g. transformers
- G01R15/183—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using inductive devices, e.g. transformers using transformers with a magnetic core
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R35/00—Testing or calibrating of apparatus covered by the other groups of this subclass
- G01R35/04—Testing or calibrating of apparatus covered by the other groups of this subclass of instruments for measuring time integral of power or current
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Abstract
The disclosure provides a high-precision alternating current testing device and method, comprising: the I/V conversion circuit comprises a two-stage mutual inductor, a sampling circuit, a low-noise small-signal amplifier and a high-precision follower which are sequentially connected; the two-stage transformer converts the detected current into small current; the sampling circuit is used for converting the small current into voltage which can be measured by the analog-to-digital conversion circuit through sampling processing of different measuring ranges; the voltage is transmitted to an analog-digital conversion circuit for data acquisition after passing through a low-noise small-signal amplifier and a high-precision follower. According to the scheme, a two-stage mutual inductor measuring mode is adopted for current measurement in the verification process of the electric energy meter, and a special process temperature control circuit is used for guaranteeing a high-precision voltage reference, so that the current ratio difference and the angle difference measuring precision reach higher levels.
Description
Technical Field
The disclosure belongs to the technical field of current measurement, and particularly relates to a high-precision alternating current testing device and method.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
Common ways of ac current detection are resistor, hall element (LEM) and current transformer sampling. The resistor sampling is easy to realize, the circuit design is simple, the loss is large, the detection signal is easy to interfere, and the resistor sampling circuit is suitable for a low-power conversion circuit. Although the detection accuracy is high by using the Hall element, the cost and the volume are often unacceptable for a module power supply. The characteristic of a general current transformer is between a resistor and a hall element, and is one of the most used current detection methods.
First kind: resistance (shunt) method. The input and the output of the resistor (shunt) are not electrically isolated, the loss is large, and the detection signal is easy to interfere. In addition, when high frequency or large current is detected, the inductive character is provided, and the resistance (shunt) is connected to influence the waveform of the detected current, and the non-sinusoidal waveform cannot be truly transmitted.
Second kind: hall current sensor method. The characteristics are as follows: the output voltage of the Hall current sensor is in direct proportion to the current flowing through the primary side, the linearity is good, the dynamic performance is good, the response speed is high, and the precision is not high.
Third kind: a current transformer method. The insulation difficulty is high, especially more than 500 kV; the dynamic range is small, the CT can generate saturation phenomenon when the current is large, and the saturation can lead the secondary protection not to accurately identify the fault phenomenon. The CT open circuit can generate high voltage, and the personal safety and equipment safety of crisis are.
Therefore, when the AC electric energy meter is detected at the current stage, the problem of larger angle difference and ratio difference exists in current measurement.
Disclosure of Invention
In order to overcome the defects in the prior art, the present disclosure provides a high-precision alternating current testing device to improve the verification accuracy and efficiency of an alternating current electric energy meter.
To achieve the above object, one or more embodiments of the present disclosure provide the following technical solutions:
in a first aspect, a high-precision alternating current testing device is disclosed, comprising:
the I/V conversion circuit comprises a two-stage mutual inductor, a sampling circuit, a low-noise small-signal amplifier and a high-precision follower which are sequentially connected;
the two-stage transformer converts the detected current into small current;
the sampling circuit is used for converting the small current into voltage which can be measured by the analog-to-digital conversion circuit through sampling processing of different measuring ranges; the voltage is transmitted to an analog-digital conversion circuit for data acquisition after passing through a low-noise small-signal amplifier and a high-precision follower.
In a second aspect, a high-precision alternating current testing method is disclosed, comprising:
the detected current is converted into small current through a two-stage transformer and then is converted into voltage which can be measured by an ADC through sampling circuits with different measuring ranges;
the voltage is transmitted to the ADC for data acquisition after passing through the low-noise small-signal amplifier and the high-precision follower.
The one or more of the above technical solutions have the following beneficial effects:
according to the scheme, a two-stage mutual inductor measuring mode is adopted for current measurement in the verification process of the electric energy meter, and a temperature control circuit is used for guaranteeing a high-precision voltage reference, so that the current ratio difference and the angle difference measuring precision reach higher levels.
Compared with a common alternating current measurement method, the implementation method of the scheme improves the current measurement accuracy to 50ppm, wherein the ratio difference measurement error is reduced to 20ppm, and the angle difference measurement error is reduced to 0.001 degrees, so that the verification accuracy of the electric energy meter is greatly improved.
The technical scheme is well applied to the aspects of alternating current measurement, power measurement, electric energy measurement and the like through circuit tests and test calibration. The current measurement accuracy is improved to 50ppm.
Additional aspects of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Drawings
The accompanying drawings, which are included to provide a further understanding of the disclosure, illustrate and explain the exemplary embodiments of the disclosure and together with the description serve to explain the disclosure, and do not constitute an undue limitation on the disclosure.
FIG. 1 is a schematic block diagram of a circuit of an embodiment of the present disclosure;
FIG. 2 is a schematic diagram of a dual stage transformer according to an embodiment of the present disclosure;
FIG. 3 is a diagram of the internal structure of a voltage reference according to an embodiment of the present disclosure;
FIG. 4 is a circuit diagram of a temperature compensation portion of an embodiment of the present disclosure;
FIG. 5 is a circuit diagram of a temperature control portion of an embodiment of the present disclosure;
FIG. 6 is a schematic diagram of a low pass filter according to an embodiment of the present disclosure;
fig. 7 is a schematic diagram of an ADC according to an embodiment of the disclosure.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments in accordance with the present disclosure. As used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise, and furthermore, it is to be understood that the terms "comprises" and/or "comprising" when used in this specification are taken to specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof.
Embodiments of the present disclosure and features of embodiments may be combined with each other without conflict.
Referring to fig. 1, this embodiment discloses a high-precision ac current testing device, which includes:
the I/V conversion circuit comprises a two-stage mutual inductor, a sampling circuit, a low-noise small-signal amplifier and a high-precision follower which are sequentially connected;
the two-stage transformer converts the detected current into small current;
the sampling circuit is used for converting the small current into voltage which can be measured by the analog-to-digital conversion circuit through sampling processing of different measuring ranges; the voltage is transmitted to an analog-digital conversion circuit for data acquisition after passing through a low-noise small-signal amplifier and a high-precision follower.
And the sampling circuit is used for sampling the voltage converted by the transformer.
The circuit adopts a two-stage current transformer. The two-stage mutual inductor consists of an iron core I and an iron core II, and primary coils and secondary coils of the two mutual inductors are respectively connected in series. And a compensating coil N is wound on the second-stage transformer core B With a number of turns equal to the number of secondary windings N 2 As shown in FIG. 2, N B And N 2 The windings are connected in parallel.
The first-stage mutual inductor consists of an iron core I, a primary transformer and a secondary transformerSecondary coil N 1 And N 2 The composition is the same as that of a common transformer, and the error is formed by exciting ampere turns of an iron core I
And (3) determining:
the second-stage mutual inductor consists of an iron core II, primary coils, secondary coils and compensation coils N 1 、N 2 And N B Comprises exciting ampere-turns of
The magnetomotive force equilibrium equation is obtained by:
Z I the primary secondary load of the first-stage transformer is the passing current
Z II The primary secondary load of the second-stage transformer is a pass current of +.>
Z is the common secondary load of the two-stage mutual inductor, namely the secondary load of the two-stage mutual inductor, and the passing current is +.>
Due to N B ＝N 2 ：
One ampere turn of the two-stage transformer is
The secondary ampere-turns are->
The excitation ampere-turns are->
The error of the dual-stage transformer is thus the negative of the product of the errors of the first-stage and second-stage current transformers.
The first-stage current transformer is the same as the common current transformer, the magnetic density of the iron core is higher, and the error is of the order of 10 -3 . The primary current of the second-stage current transformer is the error current of the first stage, the ampere turns are about 3 orders of magnitude lower, the magnetic density of the iron core is extremely low, and the iron core belongs to the micro-magnetic density current transformer, and the nanocrystalline iron core with high magnetic permeability is selected and can reach 10 -3 On the order of magnitude of (2). Such that the error of the two-stage transformer is of the order of magnitude 10 -3 ×10 -3 ＝10 -6 。
In particular, when designing the reference voltage with high stability, the voltage reference source for the high-speed and high-precision ADC not only needs to meet the requirements of ADC precision and sampling rate, but also has lower temperature coefficient and higher power supply rejection ratio. The reference circuit is a circuit for providing reference for converting voltage from analog quantity to digital quantity, the circuit is critical to the whole circuit system, accurate and stable reference voltage is provided for other circuits in the system, and the current sources of most circuits in the system take the output of the reference circuit as reference. The system adopts the output voltage of the deeply buried zener diode as a reference standard, the zener diode has the advantages of low noise, good long-term stability and the like, and is a technical scheme of the optimal high-performance reference standard of the prior board-level circuit, but the output reference voltage of the zener diode is influenced by factors such as working current, temperature and the like, and the working current and the temperature must be controlled to obtain the optimal performance.
Reference internal structure as shown in fig. 3, Q1 is a temperature compensation transistor, and Q2 is a temperature sensing transistor.
Both parts take advantage of the PN junction temperature sensitivity of the transistor, whether heating control or temperature compensation control. The change in PN junction temperature produces a change in junction voltage drop that causes a change in base input current, and thus a voltage change occurs across the collector load resistor that drives the respective op-amp, which causes the respective load to transition in the desired direction. In addition, because the emitters of the two transistors are internally connected together and the emitter-base junctions are all positioned on the same silicon wafer, the voltage drop change trend of the junctions is similar at any working temperature.
The temperature compensation part circuit design is shown in fig. 4. The temperature of the reference voltage chip is controlled, and the output of the reference voltage chip is affected by the change of the temperature of the reference voltage chip, so that high precision and high speed cannot be ensured.
When the temperature changes, the base-emitter junction voltage drop of the temperature compensation tube also changes, and the change causes the base current to change, and further causes the collector current to correspondingly change, so that the collector load resistance is shown. The operational amplifier input voltage difference caused by the series of changes, and the working current input to the zener diode is adjusted through the output, so that the voltage on the sampling resistor changes, and the Ic of the temperature compensation transistor is kept at a certain value.
The temperature rise, the junction voltage drop of the transistor is reduced, the base current is increased, the collector potential is reduced, the output potential is reduced, the zener current is reduced, and the sampling resistance voltage drop is reduced. Otherwise, if the temperature decreases, the voltage drop across the final sampling resistor increases. In this process, the zener also changes due to the temperature, and the direction of change is opposite to the change of the junction voltage drop of the transistor, which forms a natural compensation relationship with the junction voltage drop of the transistor. Due to the negative feedback of the operational amplifier, the working current passing through the zener diode can change along with the change of temperature, which leads to the fact that the circuit is likely to be under-compensated when the temperature changes.
When the circuit is powered on, since the two input ends of the operational amplifier are low, if the operational amplifier is in negative offset, the Vout end can be clamped at the ground potential and can not be started, and therefore the output end of the operational amplifier is connected to Vout through an anti-reflection diode 1N4148, and the phenomenon can be prevented. In addition, a capacitor is connected to the collector of the temperature compensation transistor, which forms a simple low-pass filter with the collector load resistor, and the whole loop has high-pass filter characteristics.
The basic circuit of the temperature control part is as shown in fig. 5:
r3 is the collector load of the temperature sensing transistor, and R4 and R5 form a voltage divider for determining the operating bias of the temperature sensing transistor Q2.
When the ambient temperature changes, the voltage divider passes through enough current, so the change of the Q2 junction voltage drop only can change the base current and promote A1 to act, and the A1 increases heat or reduces heat by driving the triode heating resistor. If the temperature is increased at this time, the Q2 junction voltage drop decreases, the base current increases, the collector potential decreases, the A1 output decreases, and the heating resistance current decreases, otherwise the temperature increases by reverse change.
The voltage divider formed by R4 and R5 is connected across the reference voltage to give a specific voltage division point voltage according to a determined proportion. If the voltage division point voltage is unchanged, vbe2 of Q2 is also unchanged, and the heater is warmed or warmed by the change of the base current, thereby forcing the ambient temperature of the zener to remain at a temperature at which Vbe2 is equal to the voltage division point potential. Since the magnitude of Vbe2 is determined here in the form of a voltage divider, the temperature coefficient of the divided ratio has an important influence on the control characteristics.
The temperature control and the compensation are combined to control, so that the purpose of stabilizing the reference voltage is achieved. When the power is on in a cold state, the junction voltage drop Vbe1 of the temperature compensation transistor corresponds to the Vbe near the room temperature at the moment, and the operational amplifier A2 establishes a reference initial power supply. At this time, vbe2 of the temperature sensing transistor should be equal to Vbe1, but the proportional voltage given by the voltage dividers R4-R5 is lower, vbe2 is forced to be at a lower potential, so that the base current is smaller, the collector potential is higher, and the A1 outputs high potential full power energy to the heating resistor. The temperature rise gradually decreases Vbe2 to coincide with the potential given by voltage divider R4-R5, and the Q2 collector potential drop reduces the heater gain energy, thereby bringing the temperature close to and maintaining it at the desired temperature point. During this time, the reference voltage supplied to the voltage divider R4-R5 is kept substantially unchanged due to the self-regulation of the temperature compensation section, and the potential at the given voltage division point is also kept substantially unchanged, so that a temperature stabilization process is established.
Low noise amplifying circuit design: a low pass filter may be used for the sampled analog signal before it reaches the ADC to prevent out-of-band noise aliasing errors from occurring and to prevent the analog signal from generating superimposed high frequency noise. If the input signal noise exceeds one half of the converter sampling frequency, its noise amplitude remains unchanged, but the frequency changes when aliasing occurs in the signal. After digitizing the signal, the in-band noise can no longer be reduced by using a digital filter.
The conditioned signals are differential signals, so that the common-mode interference resistance is high, the signal to noise ratio is high, and the error of the mutual inductor small current measurement can be effectively compensated. The secondary output signal of the transformer is interfered by high-frequency noise and the like in transmission, the interference is suppressed by designing a low-pass filter, and the system adopts a second-order Butterworth low-pass filter for signal conditioning, as shown in fig. 6.
The analog front-end signal conditioning circuit must realize the highest possible signal-to-noise ratio, the design of low noise analog signals is crucial, the influence of space temperature and magnetic field disturbance is avoided from noise sources, an operational amplifier with extremely low noise and a gain resistor with low noise are selected, the influence of system signal background noise and common mode voltage is reduced by adopting the structure of an instrument amplifier, the sampling signal-to-noise ratio reaches 126db through the processing, and high-stability measurement is realized.
High-precision ADC circuit design: as shown in fig. 7, the system adopts 24-32bit ADC sampling technology, and has a maximum resolution of 32bit in low output rate mode, and in combination with digital filter technology, has a resolution of 24bit in ac sampling mode, and can select different digital filter configurations according to the measurement signal, thereby reducing the influence of noise on the measurement signal and realizing high-stability measurement. The conditioned signals are input into an ADC acquisition chip, the digitized values of the signals are obtained through high-precision acquisition conversion, and then the digitized values are transmitted to an FPGA through an isolation chip for algorithm processing, so that the accurate voltage and current measured values are finally obtained.
In another embodiment, a high-precision alternating current testing method is disclosed, comprising:
the detected current is converted into small current through a two-stage transformer and then is converted into voltage which can be measured by an ADC through sampling circuits with different measuring ranges;
the voltage is transmitted to the ADC for data acquisition after passing through the low-noise small-signal amplifier and the high-precision follower.
The method comprises the following specific steps: after the detected current I1 is input into the primary coil of the two-stage transformer, the small current I2 induced by the secondary coil of the transformer is converted into small voltage through the relay sampling circuit by selecting different measuring ranges, and the small voltage is conditioned into a differential signal through the low-noise amplifier and is transmitted to the high-speed ADC through the high-precision follower. The ADC converts the voltage analog quantity into digital quantity and transmits the digital quantity to the microprocessor for processing and interface display.
The foregoing description of the preferred embodiments of the present disclosure is provided only and not intended to limit the disclosure so that various modifications and changes may be made to the present disclosure by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present disclosure should be included in the protection scope of the present disclosure.
While the specific embodiments of the present disclosure have been described above with reference to the drawings, it should be understood that the present disclosure is not limited to the embodiments, and that various modifications and changes can be made by one skilled in the art without inventive effort on the basis of the technical solutions of the present disclosure while remaining within the scope of the present disclosure.
Claims (6)
1. A high-precision ac current testing device, comprising:
the I/V conversion circuit comprises a two-stage mutual inductor, a sampling circuit, a low-noise small-signal amplifier and a high-precision follower which are sequentially connected;
the two-stage transformer converts the detected current into small current;
the sampling circuit is used for converting the small current into voltage which can be measured by the analog-to-digital conversion circuit through sampling processing of different measuring ranges; the voltage is transmitted to an analog-to-digital conversion circuit for data acquisition after passing through a low-noise small-signal amplifier and a high-precision follower;
the sampling circuit provides accurate and stable reference voltage for other circuits, and comprises a temperature compensation transistor and a temperature sensing transistor which are connected with each other for temperature compensation control and heater control respectively;
the temperature compensation transistor is connected to the temperature compensation part circuit, when the temperature changes, the base-emitter junction voltage drop of the temperature compensation transistor changes simultaneously, so that the base current changes, and then the collector current changes correspondingly, and the collector load resistor R1 shows the voltage difference of the operational amplifier A2, and the operating current input to the zener diode D1 is adjusted through output, so that the voltage of the sampling resistor R2 changes, and the temperature compensation transistor is kept at a certain value;
when the ambient temperature changes, the current passing through the voltage divider is large enough, so that the change of the junction voltage drop of the temperature sensing transistor Q2 can only change the base current and promote the operational amplifier A1 to act, and the operational amplifier A1 increases heat or reduces heat by driving the triode heating resistor; if the temperature is increased at this time, the junction voltage drop of the temperature sensing transistor Q2 is reduced, the base current is increased, the collector potential is reduced, the output of the op-amp A1 is reduced, and the heating resistance current is reduced, otherwise, the temperature is increased by reverse change.
2. The high-precision alternating current testing device according to claim 1, wherein the two-stage transformer is a two-stage current transformer, the first-stage transformer consists of an iron core I, a primary coil and a secondary coil, the second-stage transformer consists of an iron core II, a primary coil, a secondary coil and a compensation coil, the primary coil and the secondary coil are respectively connected in series, and the compensation coil is connected in parallel with the secondary coil.
3. The apparatus of claim 1, wherein for the temperature compensation transistor and the temperature sensing transistor, the emitters of the two transistors are connected together internally and the emitter-base junction is located on the same silicon wafer.
4. The high-precision alternating current testing device according to claim 1, wherein the low-noise small-signal amplifier is used for carrying out signal conditioning on the sampled analog signal by adopting a second-order butterworth low-pass filter, and the conditioned signal is a differential signal.
5. The high-precision alternating current testing device according to claim 4, wherein the conditioned signal is input to an ADC acquisition chip, and a digitized value of the signal is obtained through high-precision acquisition conversion.
6. A high-precision alternating current testing method is characterized by comprising the following steps:
the detected current is converted into small current through a two-stage transformer and then is converted into voltage which can be measured by an ADC through sampling circuits with different measuring ranges;
the voltage is transmitted to an ADC to obtain a digital value of the signal after passing through a low-noise small-signal amplifier and a high-precision follower;
then the voltage and current measurement value is transmitted to the FPGA through the isolation chip for algorithm processing, and finally, the accurate voltage and current measurement value is obtained;
when the sampling circuits with different ranges are converted into voltages which can be measured by the ADC, the sampling circuits provide accurate and stable reference voltages for other circuits, and heater control and temperature compensation control are respectively carried out on the sampling circuits;
the temperature compensation transistor is connected to the temperature compensation part circuit, when the temperature changes, the base-emitter junction voltage drop of the temperature compensation transistor changes simultaneously, so that the base current changes, and then the collector current changes correspondingly, and the collector load resistor R1 shows the voltage difference of the operational amplifier A2, and the operating current input to the zener diode D1 is adjusted through output, so that the voltage of the sampling resistor R2 changes, and the temperature compensation transistor is kept at a certain value;
when the ambient temperature changes, the current passing through the voltage divider is large enough, so that the change of the junction voltage drop of the temperature sensing transistor Q2 can only change the base current and promote the operational amplifier A1 to act, and the operational amplifier A1 increases heat or reduces heat by driving the triode heating resistor; if the temperature is increased at this time, the junction voltage drop of the temperature sensing transistor Q2 is reduced, the base current is increased, the collector potential is reduced, the output of the op-amp A1 is reduced, and the heating resistance current is reduced, otherwise, the temperature is increased by reverse change.
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| CN1813402A (en) * | 2003-06-27 | 2006-08-02 | 希格半导体(美国)公司 | Integrated power amplifier circuit |
| CN101393256A (en) * | 2008-10-17 | 2009-03-25 | 广州市羊城科技实业有限公司 | Method for eliminating measurement error of transformer by active impedance vector electric voltage synthesis |
| CN202057784U (en) * | 2011-04-29 | 2011-11-30 | 广州市格宁电气有限公司 | Online calibrating and digital online monitoring system for electric energy meter |
| CN106557104A (en) * | 2017-01-06 | 2017-04-05 | 云南电网有限责任公司电力科学研究院 | A kind of high precision broad frequency wide-range current/voltage conversion equipment |
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