Current measurement method, device and circuit breaker

Abstract

This invention discloses a current measurement method. When the self-powered power supply is in operation, the measured values ​​of the primary current of each phase are corrected according to the following method: the sum of the measured values ​​of the primary current of each phase is taken as the total current measurement value I of the circuit under test. rms Based on the pre-calibrated correspondence between the measured total current and the actual total current under the condition of self-powered power supply supplying external power, the measured total current value I is determined. rms The corresponding actual value of total current I O Then, the number of digits is calculated using the following formula. i Corrected measured value I of the primary current of phase. Oi :I Oi =I O ×(I rmsi / I rms ), i =1,2,...,N, where N is the number of phases of the circuit to be measured, I rmsi For the first i The invention discloses a current measuring device and a circuit breaker. This invention provides current measurement results that meet the accuracy requirements of three-stage protection and grounding protection of a circuit breaker without increasing the size of the core material in the iron-core transformer.

Description

Technical Field

[0001] This invention relates to a current measurement method, belonging to the field of low-voltage electrical appliance technology. Background Technology

[0002] The primary function of a circuit breaker is current protection, typically featuring three-stage protection (long-delay, short-delay, and instantaneous) and grounding protection. With advancements in technology and user needs, circuit breakers are becoming increasingly smaller, requiring less internal space. Considering installation size and cost, current transformers with iron cores are generally used for current measurement.

[0003] Because most existing intelligent circuit breakers use self-powered operation, the core current transformer needs to simultaneously power the circuit breaker and sample current signals. This requires drawing power from the current transformer, which affects the current signal sampling function and causes signal distortion, with the degree of distortion varying under different operating conditions. Conversely, when the molded case circuit breaker uses an external power supply (such as when the circuit breaker has communication capabilities), the current signal distortion is significantly reduced because it does not need to draw power from the transformer.

[0004] Intelligent circuit breakers using iron-core current transformers face limitations in size and installation space. Even current transformers made from materials with good magnetic permeability cannot simultaneously meet the requirements for current detection and power supply. Without calibration, the current error exceeds 10% when the transformer is self-powered, failing to meet the accuracy requirements for circuit breaker current protection. Furthermore, the current distortion of iron-core current transformers varies under different current conditions and with the same current but different pole numbers. Traditional current curve fitting methods cannot account for the impact of varying current distortion levels.

[0005] To meet the accuracy requirements of circuit breaker current protection, the existing solution is to increase the size of the core material in the iron-core current transformer to improve its output characteristics. This solution requires more space and incurs higher costs, failing to meet the requirements for low-cost and miniaturized circuit breakers. Therefore, a more reasonable method is needed to correct the current signal of multi-phase current transformers to meet the accuracy requirements of the circuit breaker's three-stage protection and grounding protection. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a current measurement method that can correct the current measurement value according to different situations of whether the current transformer is self-powered or not. Thus, it is possible to obtain current measurement results that meet the accuracy requirements of three-stage protection and grounding protection of circuit breakers without increasing the size of the core material in the iron core transformer.

[0007] The present invention specifically adopts the following technical solutions to solve the above-mentioned technical problems: A current measurement method involves converting the primary current of each phase of the circuit under test into secondary current using a current transformer module, then converting the secondary current into a self-powered power supply, and simultaneously sampling and measuring the secondary current of each phase to obtain the measured value of the primary current of each phase of the circuit under test. When the self-powered power supply is in operation, the measured values ​​of the primary current of each phase of the circuit under test are corrected according to the following method: The sum of the measured values ​​of the primary current of each phase of the circuit under test is taken as the total current measurement value I of the circuit under test. rms Based on the pre-calibrated correspondence between the measured total current value and the actual total current value of the circuit under test when the self-powered power supply provides external power, the measured total current value I is determined. rms The corresponding actual value of total current I O Then, the first digit of the circuit under test is calculated using the following formula. i Corrected measured value I of the primary current of phase. Oi :I Oi =I O ×(I rmsi / I rms ), i =1,2,...,N, where N is the number of phases of the circuit to be measured, I rmsi For the circuit under test i The measured value of the primary current of the phase.

[0008] Furthermore, the system monitors in real time whether the self-powered power supply is supplying power. When the self-powered power supply is not supplying power to the outside, the measured values ​​of the primary current of each phase of the circuit to be measured are not corrected.

[0009] Preferably, the correspondence between the measured total current value and the actual total current value of the circuit under test when the self-powered power supply supplies power externally is pre-calibrated using the following method: Using a standard current source as the circuit under test, and with the self-powered power supply providing external power, a series of corresponding data were obtained between the actual total current value and the measured total current value of the circuit under test. Then, data fitting was performed on this series of corresponding data to obtain the correspondence between the measured total current value and the actual total current value of the circuit under test with the self-powered power supply providing external power.

[0010] More preferably, a polynomial fitting method is used to fit the data.

[0011] Based on the same inventive concept, the following technical solutions can also be obtained: A current measuring device, comprising: A set of current transformer modules is used to convert the primary current of each phase of the circuit to be measured into the secondary current. The current transformer power generation module is used to convert the secondary current of each phase into a self-powered power supply. The current sampling and measurement module is used to sample and measure the secondary current of each phase to obtain the measured value of the primary current of each phase of the circuit under test. The device also includes: A current correction module is used to correct the measured values ​​of the primary current of each phase of the circuit under test when the self-powered power supply is powered, according to the following method: The sum of the measured values ​​of the primary current of each phase of the circuit under test is taken as the total current measurement value I of the circuit under test. rms Based on the pre-calibrated correspondence between the measured total current value and the actual total current value of the circuit under test when the self-powered power supply provides external power, the measured total current value I is determined. rms The corresponding actual value of total current I O Then, the first digit of the circuit under test is calculated using the following formula. i Corrected measured value I of the primary current of phase. Oi :I Oi =I O ×(I rmsi / I rms ), i =1,2,...,N, where N is the number of phases of the circuit to be measured, I rmsi For the circuit under test i The measured value of the primary current of the phase.

[0012] Furthermore, the current measuring device also includes a power detection module for real-time detection of whether the self-powered power supply is supplying power; when the self-powered power supply is not supplying power externally, the measured values ​​of the primary current of each phase of the circuit to be measured are not corrected.

[0013] Preferably, the correspondence between the measured total current value and the actual total current value of the circuit under test when the self-powered power supply supplies power externally is pre-calibrated using the following method: Using a standard current source as the circuit under test, and with the self-powered power supply providing external power, a series of corresponding data were obtained between the actual total current value and the measured total current value of the circuit under test. Then, data fitting was performed on this series of corresponding data to obtain the correspondence between the measured total current value and the actual total current value of the circuit under test with the self-powered power supply providing external power.

[0014] More preferably, a polynomial fitting method is used to fit the data.

[0015] A circuit breaker includes a current measuring device, wherein the current measuring device is the current measuring device described in any of the above technical solutions.

[0016] Compared with the prior art, the technical solution of the present invention has the following beneficial effects: This invention performs corresponding software correction on the current measurement value according to different situations of whether the current transformer is self-powered or not. The current measurement accuracy can reach within 5%, which is sufficient to meet the accuracy requirements of the three-stage protection and grounding protection of the circuit breaker. Moreover, it does not require increasing the size of the iron core material in the iron core transformer, nor does it require adding new hardware circuits. It can be implemented based on the existing intelligent circuit breaker through software improvement, which makes it more adaptable and lower in cost. Attached Figure Description

[0017] Figure 1 This is a schematic block diagram of the current measuring device of the present invention; Figure 2 This is a circuit diagram of a specific implementation of the transformer circuit in a current transformer module. Figure 3 This is a circuit diagram of a specific implementation of a current sampling module; Figure 4 A circuit diagram of a specific implementation of a current transformer power generation module; Figure 5 A circuit diagram illustrating a specific implementation of an external power supply module; Figure 6 This is a circuit diagram of a specific implementation of a power detection module; Figure 7 This is a circuit diagram of a specific implementation of the current measurement module; Figure 8 This is a flowchart of the current correction process for the current correction module. Figure 9 This is an example of data fitting between the measured total current value and the actual total current value. Detailed Implementation

[0018] To address the shortcomings of existing circuit breaker current measurement technology, the present invention proposes to perform corresponding software corrections on the current measurement values ​​based on whether the current transformer operates under self-powered conditions, in order to meet the accuracy requirements of the circuit breaker's three-stage protection and grounding protection.

[0019] The present invention specifically adopts the following technical solutions to solve the above-mentioned technical problems: A current measurement method involves converting the primary current of each phase of the circuit under test into secondary current using a current transformer module, then converting the secondary current into a self-powered power supply, and simultaneously sampling and measuring the secondary current of each phase to obtain the measured value of the primary current of each phase of the circuit under test. When the self-powered power supply is in operation, the measured values ​​of the primary current of each phase of the circuit under test are corrected according to the following method: The sum of the measured values ​​of the primary current of each phase of the circuit under test is taken as the total current measurement value I of the circuit under test. rms Based on the pre-calibrated correspondence between the measured total current value and the actual total current value of the circuit under test when the self-powered power supply provides external power, the measured total current value I is determined. rms The corresponding actual value of total current I O Then, the first digit of the circuit under test is calculated using the following formula. i Corrected measured value I of the primary current of phase. Oi :I Oi =I O ×(I rmsi / I rms ), i =1,2,...,N, where N is the number of phases of the circuit to be measured, I rmsi For the circuit under test i The measured value of the primary current of the phase.

[0020] A current measuring device, comprising: A set of current transformer modules is used to convert the primary current of each phase of the circuit to be measured into the secondary current. The current transformer power generation module is used to convert the secondary current of each phase into a self-powered power supply. The current sampling and measurement module is used to sample and measure the secondary current of each phase to obtain the measured value of the primary current of each phase of the circuit under test. The device also includes: A current correction module is used to correct the measured values ​​of the primary current of each phase of the circuit under test when the self-powered power supply is powered, according to the following method: The sum of the measured values ​​of the primary current of each phase of the circuit under test is taken as the total current measurement value I of the circuit under test. rms Based on the pre-calibrated correspondence between the measured total current value and the actual total current value of the circuit under test when the self-powered power supply provides external power, the measured total current value I is determined. rms The corresponding actual value of total current I O Then, the first digit of the circuit under test is calculated using the following formula. i Corrected measured value I of the primary current of phase. Oi :I Oi =I O ×(I rmsi / I rms ), i =1,2,...,N, where N is the number of phases of the circuit to be measured, I rmsi For the circuit under test i The measured value of the primary current of the phase.

[0021] To facilitate public understanding, the technical solution of the present invention will be described in detail below through a specific embodiment and in conjunction with the accompanying drawings: like Figure 1 As shown, the current measuring device in this embodiment includes a current transformer module, a transformer power generation module, a power detection module, a current sampling and processing module, a current measurement module, a current correction module, and an external power supply module.

[0022] The current transformer module is used to convert the large primary current of each phase of the circuit under test into a small secondary current. It is configured with three or four sets of current transformers depending on the number of poles of the circuit breaker, and can be used for three-pole or four-pole circuit breakers. The ratio of the primary current to the secondary current is determined by the rated current of the circuit breaker; the larger the rated current, the larger the ratio. In this embodiment, the current transformer module includes a set of transformer circuits corresponding one-to-one with each phase of the circuit under test. Taking one transformer circuit as an example... Figure 2 As shown, the current transformer circuit includes a current transformer and a rectifier circuit D1. The primary current I1 is converted into a small secondary current signal by the current transformer, and then rectified into a DC voltage V by the rectifier circuit D1. O And the secondary side current signal I2.

[0023] The current sampling module samples the small current signal from the current transformer module, performs signal conditioning, and processes it into a voltage signal that the current measurement module can process; a specific implementation circuit is as follows: Figure 3 As shown, its basic structure is a current-to-voltage converter + inverting proportional amplifier circuit. The input current I2 flows through the sampling resistor R1. According to Ohm's law V1 = I2 × R1, the current signal is converted into a voltage signal V1, completing the conversion of the electrical signal type. R2 serves as a current limiting and transmission resistor. On the one hand, it limits the current flowing into the subsequent op-amp to avoid overcurrent damage to the op-amp input stage. On the other hand, it transmits the voltage signal V1 to the inverting input terminal of the op-amp. R4 is the balancing resistor at the non-inverting input of the op-amp, used to cancel the influence of the bias current at the inverting input and ensure the stability of the op-amp's DC operating point. Op-amp N1 and feedback resistor R3 form an inverting proportional amplifier with a gain A = -R3 / R2, which proportionally amplifies the input voltage signal. R5 is the output resistor, which can limit the op-amp output current to prevent short circuit damage to the op-amp from the subsequent load, and also serves as a load impedance match.

[0024] The current transformer power generation module is used to convert the small current on the secondary side of each phase into a self-powered power supply for the circuit breaker controller. The specific circuit of the current transformer power generation module in this embodiment is as follows: Figure 4As shown, the constant voltage control circuit detects the voltage Vcc through voltage divider resistors R6 and R7. When Vcc starts to rise to the design voltage, the comparator outputs a high level to drive the MOSFET. After the MOSFET turns on, the voltage Vo drops, and simultaneously the voltage Vcc drops. At this time, the output voltage of comparator N2, through feedback resistor R9, and Vcc, through the voltage divider resistor, act together on the input terminal IN+ of N2. When the input terminal IN+ voltage is lower than the input terminal IN- voltage, N2 outputs a low level to turn off the MOSFET. After the MOSFET turns off, the voltage Vo rises again, and the voltage Vcc also rises until the MOSFET turns on again, repeating the cycle to achieve constant voltage control. Vex is the output voltage of the external power supply. When the external power supply is connected, it is input to the input terminal IN+ of N2 through diode D3. When this voltage value is greater than +2.5V, the comparator outputs a high level to drive the MOSFET, and Vo is 0V, meaning the self-powered power supply does not supply power externally.

[0025] A specific implementation circuit of the external power supply module is as follows: Figure 5 As shown, when an external power supply Vex is connected, the external power supply module converts Vex into the operating voltage Vcc of the circuit breaker controller.

[0026] The power detection module is used to detect in real time whether the circuit breaker is powered by its own power supply or by an external power supply module, and sends the detection results to the current correction module. Figure 6 The diagram shows a specific implementation circuit of a power supply detection module. It uses a microcontroller to detect and compare the external power supply voltage Vex and the generated voltage Vo of the current transformer power generation module. When the external power supply voltage Vex is greater than the generated voltage Vo of the current transformer power generation module, it is determined that the power supply is from the external power supply; when the external power supply voltage Vex is less than the generated voltage Vo of the current transformer power generation module, it is determined that the power supply is from the current transformer.

[0027] The current measurement module is used to digitize the signal sampled and processed by the current sampling module to calculate the corresponding primary current measurement value. For example... Figure 7 As shown, taking a four-pole circuit breaker as an example, its function is to convert the analog voltage signal output by the current sampling module into a digital signal through the ADC module inside the microcontroller. The four digital signals corresponding to the four phases A, B, C, and N are AD1, AD2, AD3, and ADn, respectively. The instantaneous current value i is calculated based on the digital signals. AD1 i AD2 i AD3 i ADn The calculation formula is: Where K is the current coefficient, defined as the ratio of the rated current at the rated operating current to the sampled AD value; the current sampling signal is continuously sampled at an interval of 0.625ms (32 points sampled per 20ms cycle). After one full cycle of sampling, the effective value of the current is calculated using the following formula: The measured effective value of the four-channel current is I. rms1 I rms2 I rms3 I rmsN .

[0028] The current correction module corrects the primary current measurement value calculated by the current measurement module under different power supply conditions based on the detection results of the power supply detection module. When the circuit breaker controller is powered externally, the transformer output signal is in its optimal state and no signal correction is required. Therefore, the current correction module does not correct the primary current measurement value calculated by the current measurement module, but directly uses the primary current measurement value as the final measurement output. When the transformer power supply module is detected, in order to compensate for the current distortion of the current transformer, the current correction module corrects the current value calculated by the current measurement module. Specifically, the sum of the measured values ​​of the primary current of each phase of the circuit under test is used as the total current measurement value I of the circuit under test. rms Based on the pre-calibrated correspondence between the measured total current value and the actual total current value of the circuit under test when the self-powered power supply provides external power, the measured total current value I is determined. rms The corresponding actual value of total current I O Then, the first digit of the circuit under test is calculated using the following formula. i Corrected measured value I of the primary current of phase. Oi :I Oi =I O ×(I rmsi / I rms ), i =1,2,...,N, where N is the number of phases of the circuit to be measured, I rmsi For the circuit under test i The measured value of the primary current of the phase.

[0029] Taking a four-pole circuit breaker as an example, the current correction module is implemented through the pre-set software in the microcontroller, and its current correction process is as follows: Figure 8 As shown, within one cycle, the effective value of the four-phase current calculated by the current measurement module is I. rms1 I rms2 I rms3 I rmsN The current correction module calculates a unified total current measurement value based on the effective current measurements of each phase. The total current measurement value is the sum of the four-phase current measurements, I.rms :

[0030] The current correction module determines the total current measurement value I based on the pre-calibrated correspondence between the measured total current value and the actual total current value of the circuit under test when the self-powered power supply provides external power. rms The corresponding actual value of total current I O The correspondence between the measured total current and the actual total current of the circuit under test when the self-powered power supply provides external power is pre-calibrated using the following method: a standard current source is used as the circuit under test, and a series of corresponding data between different actual total current and measured total current values ​​of the circuit under test are obtained when the self-powered power supply provides external power; then, this series of corresponding data is fitted to obtain the correspondence between the measured total current and the actual total current of the circuit under test when the self-powered power supply provides external power.

[0031] Table 1 shows a series of corresponding data of different actual values ​​of total current and total current measurement values ​​of the circuit under test obtained by the current measuring device of a certain model 63A circuit breaker under self-powered conditions. The input current represents the actual value of total current input by the standard current source, and the output current represents the total current measurement value output by the current measuring module.

[0032] Table 1

[0033] By fitting the data in Table 1, the correspondence between the measured total current and the actual total current of the circuit under test when the self-powered power supply provides external power can be obtained. Existing methods such as polynomial fitting and spline fitting can be used for data fitting. Based on the characteristics of the data, this invention preferably uses a simple polynomial fitting method. Figure 9 The fitting curve obtained by the trinomial fitting method is displayed. The coefficients of the fitted trinomial are A = 0.0002439629, B = -0.0243890877, C = 1.6585556966, and D = -2.3989035230. This value is then stored in the current correction module.

[0034] Based on the total current measurement value I under self-powered conditions rms With the pre-fitted trinomial coefficients A, B, C, and D, the current correction module can calculate the corresponding actual total current value I. O :

[0035] Then, based on the measured effective value I of each phase current... rms1 I rms2 I rms3 IrmsN and the actual value of total current I O Calculate and output the corrected measured value I of the primary current of each phase. O1 I O2 I O3 I ON Used for circuit breaker current protection calculations:

[0036] The long-delay protection characteristics of the circuit breaker at its low end were tested using calibrated current measurements. The long-delay characteristics of this 63A circuit breaker are shown in Table 2 below. Table 2

[0037] The test results show that the operating characteristics meet the target requirements and satisfy the accuracy requirements of the circuit breaker's three-stage protection and grounding protection.

Claims

1. A current measurement method, comprising converting the primary current of each phase of the circuit under test into secondary current using a current transformer module, then converting the secondary current of each phase into a self-powered power supply, and simultaneously sampling and measuring the secondary current of each phase to obtain the measured value of the primary current of each phase of the circuit under test; characterized in that, When the self-powered power supply is in operation, the measured values ​​of the primary current of each phase of the circuit under test are corrected according to the following method: The sum of the measured values ​​of the primary current of each phase of the circuit under test is taken as the total current measurement value I of the circuit under test. rms Based on the pre-calibrated correspondence between the measured total current value and the actual total current value of the circuit under test when the self-powered power supply provides external power, the measured total current value I is determined. rms The corresponding actual value of total current I O Then, the first digit of the circuit under test is calculated using the following formula. i Corrected measured value I of the primary current of phase. Oi :I Oi =I O ×(I rmsi / I rms ), i =1,2,...,N, where N is the number of phases of the circuit to be measured, I rmsi For the circuit under test i The measured value of the primary current of the phase.

2. The current measurement method as described in claim 1, characterized in that, The system monitors in real time whether the self-powered power supply is supplying power. When the self-powered power supply is not supplying power to the outside, the measured values ​​of the primary current of each phase of the circuit to be measured are not corrected.

3. The current measurement method as described in claim 1, characterized in that, The correspondence between the measured total current value and the actual total current value of the circuit under test when the self-powered power supply provides external power is pre-calibrated using the following method: Using a standard current source as the circuit under test, and with the self-powered power supply providing external power, a series of corresponding data were obtained between the actual total current value and the measured total current value of the circuit under test. Then, data fitting was performed on this series of corresponding data to obtain the correspondence between the measured total current value and the actual total current value of the circuit under test with the self-powered power supply providing external power.

4. The current measurement method as described in claim 3, characterized in that, The data was fitted using a polynomial fitting method.

5. A current measuring device, comprising: A set of current transformer modules is used to convert the primary current of each phase of the circuit to be measured into the secondary current. The current transformer power generation module is used to convert the secondary current of each phase into a self-powered power supply. The current sampling and measurement module is used to sample and measure the secondary current of each phase to obtain the measured value of the primary current of each phase of the circuit under test. The device is characterized in that it further includes: A current correction module is used to correct the measured values ​​of the primary current of each phase of the circuit under test when the self-powered power supply is powered, according to the following method: The sum of the measured values ​​of the primary current of each phase of the circuit under test is taken as the total current measurement value I of the circuit under test. rms Based on the pre-calibrated correspondence between the measured total current value and the actual total current value of the circuit under test when the self-powered power supply provides external power, the measured total current value I is determined. rms The corresponding actual value of total current I O Then, the first digit of the circuit under test is calculated using the following formula. i Corrected measured value I of the primary current of phase. Oi :I Oi =I O ×(I rmsi / I rms ), i =1,2,...,N, where N is the number of phases of the circuit to be measured, I rmsi For the circuit under test i The measured value of the primary current of the phase.

6. The current measuring device as described in claim 5, characterized in that, It also includes a power detection module for real-time detection of whether the self-powered power supply is supplying power; when the self-powered power supply is not supplying power to the outside, the measured values ​​of the primary current of each phase of the circuit to be measured are not corrected.

7. The current measuring device as described in claim 5, characterized in that, The correspondence between the measured total current value and the actual total current value of the circuit under test when the self-powered power supply provides external power is pre-calibrated using the following method: Using a standard current source as the circuit under test, and with the self-powered power supply providing external power, a series of corresponding data were obtained between the actual total current value and the measured total current value of the circuit under test. Then, data fitting was performed on this series of corresponding data to obtain the correspondence between the measured total current value and the actual total current value of the circuit under test with the self-powered power supply providing external power.

8. The current measuring device as described in claim 7, characterized in that, The data was fitted using a polynomial fitting method.

9. A circuit breaker, comprising a current measuring device, characterized in that, The current measuring device is the current measuring device as described in any one of claims 5 to 8.

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