Method for increasing measuring range of direct-current electric energy meter

Abstract

The invention discloses a measuring range increasing method for a direct-current electric energy meter, and belongs to the technical field of heat dissipation of electrical measuring elements. According to the method, aiming at the problems of temperature rise, precision reduction and insufficient measuring range of the existing manganin shunt during large-current working, a heat dissipation device is designed by calculating the relationship between the maximum current under the allowable temperature rise and the heat dissipation requirement, so that the direct-current electric energy meter which is larger than the rated current of the manganin shunt can be measured, the precision is not lost during small current, and the measurement accuracy is improved. And the measurement precision and the measuring range are improved.

Description

Technical Field

[0001] This invention relates to the field of electrical measuring element technology, and in particular to a method for increasing the range of a DC energy meter. Background Technology

[0002] A manganin shunt is a DC current measuring element based on Ohm's law. It infers the current magnitude by measuring the millivolt-level voltage drop generated when current flows through a low-value manganin resistor. For example, a manganin shunt with a resistance R of 0.00025Ω will produce a voltage drop of 75mV when a current of 300A flows through it.

[0003] However, manganese-copper shunts generate heat during operation due to the Joule effect (P = I²R), causing their own temperature to rise. Although manganese-copper alloys have a low temperature coefficient of resistance (approximately 5 × 10⁻⁶), this can lead to a decrease in their temperature. -6 The temperature rise is approximately 100°C, but significant temperature rise can still cause resistance drift, affecting measurement accuracy. Moreover, it cannot support overload measurement. Using a manganese-copper shunt with a larger current rating will result in low accuracy at low currents, leading to insufficient measurement range.

[0004] Existing technologies attempt to improve the heat dissipation of manganese-copper shunts, such as adding simple heat dissipation fins. However, these methods have significant shortcomings: limited heat dissipation efficiency and failure to precisely match the thermal characteristics of the manganese-copper shunt.

[0005] Therefore, there is an urgent need for a systematic heat dissipation improvement method that can comprehensively solve the problem of improving the accuracy and expanding the range of manganese copper shunts under high current by means of thermal modeling, external heat dissipation, electromagnetic protection, and other aspects. Summary of the Invention

[0006] This invention aims to overcome the shortcomings of existing technologies and provide a method for extending the range of DC energy meters. It solves the problem of excessive temperature rise and decreased measurement accuracy caused by the self-heating effect of manganese copper shunt under high current. At the same time, while ensuring the original low current measurement accuracy, the range of high current is expanded, making the product more versatile.

[0007] To achieve the above objectives, the present invention employs a method for extending the range of a DC energy meter, comprising the following steps: Step S1: Obtain the minimum heat dissipation power Determine the rated parameters of the manganese-copper shunt, including the rated current I0 and the rated voltage drop U. rated Resistance R=U rated / I0, Maximum permissible operating temperature T max Calculate the heat generation power P of the manganese-copper shunt under the target operating current I1. heat = I1² × R, where I1 is the target operating current and R is the resistance of the manganese copper shunt. Under steady-state conditions, the heat generation power equals the heat dissipation power, therefore the minimum heat dissipation power is P.heat , Additionally, heat dissipation power = P0 × (T) max - T0), where P0 is the total thermal conductivity of the heat dissipation system (W / ℃), T max T0 is the maximum permissible operating temperature of the manganese-copper shunt, and T0 is the ambient temperature. From this, we can deduce that the total thermal conductivity of the heat dissipation device required to increase the current from I0 to I1 is P0 = I1. 2 ×R / (T) max -T0); Step S2: Heat dissipation device design Select a heat dissipation device with a heat dissipation power ≥ P heat The heat dissipation device is modularly composed of a thermoelectric semiconductor cooling chip and heat sink fins. The cold side of the thermoelectric semiconductor cooling chip is in close contact with the manganese-copper shunt, and its hot side is in close contact with the planar portion of the heat sink fins. By applying voltage to the thermoelectric semiconductor cooling chip, its cold side dissipates heat to the manganese-copper shunt, while its hot side dissipates heat through the heat sink fins, maintaining the temperature difference ΔT between the cold and hot sides of the thermoelectric semiconductor cooling chip within a predetermined range. Temperature sensors were installed on and near the manganese-copper shunt to measure the operating temperature T1 of the manganese-copper shunt and the actual ambient temperature T2. Step S3: Electro-cooling heat dissipation treatment When the current flowing through the manganese-copper shunt exceeds the cooling start-up threshold, the heat dissipation device is activated to cool the manganese-copper shunt. The voltage applied to the thermoelectric semiconductor cooling chip is dynamically adjusted in segments according to the current range of the manganese-copper shunt to achieve energy-saving management. Within a segment, the thermoelectric semiconductor cooling chip maintains a stable cooling power. The current segmentation method first divides the operating current of the manganese-copper shunt into N segments with small intervals, starting from the cooling start threshold and ending at the target current. When operating in each current segment, a different voltage is applied to the thermoelectric cooler. Based on the applicable voltage range of the thermoelectric cooler, the voltage changes from high to low in equal steps to ensure that when the manganese-copper shunt operates at its maximum value in that segment, the operating temperature T1 of the manganese-copper shunt is lower than the maximum value T. max The minimum applied voltage value of each segment current is obtained. Based on the measured applied voltage required for each of the N segments, the N voltage values ​​with the same value are merged and locally merged into segment M-1. Segment 1 indicates that the heat dissipation device is not activated, resulting in the final M segments, M≥2 and M<N, that is, the N segment currents correspond to M voltage values.

[0008] Preferably, in step S3, after the current of the manganese-copper shunt drops to the cooling start threshold, a voltage is still applied to the thermoelectric semiconductor cooling chip for a period of time, and the temperature of the manganese-copper shunt T1 and the ambient temperature T2 are monitored until T1≤T2 before the voltage is stopped.

[0009] Preferably, in step S3, the measurement accuracy error of the manganese-copper shunt caused by the magnetic field interference of the heat dissipation device is compensated. The cooling voltage applied to each segment current is stable. The measured values ​​of the manganese-copper shunt at the applied cooling voltage and actual ambient temperature of each segment are obtained in the laboratory and compared with the standard table of the calibration bench to obtain the compensation value under different applied voltages and ambient temperatures, and a compensation lookup table is obtained.

[0010] The beneficial effects of this invention are as follows.

[0011] Significantly improved heat dissipation efficiency: By guiding the design of the heat dissipation device through heat generation power and combining it with a modular heat dissipation device, the operating temperature of the manganese copper shunt can be significantly reduced.

[0012] High and stable measurement accuracy: Effective temperature rise control reduces drift of the manganese copper shunt caused by temperature changes.

[0013] The measurement range of the manganese copper shunt has been improved: while meeting the original accuracy requirements of the device, the measurement range can be significantly increased, ensuring both the accuracy of small current measurement and the accuracy of large current measurement. Attached Figure Description

[0014] Figure 1 This is the overall flowchart of the improvement method of the present invention.

[0015] Figure 2 This invention relates to a thermoelectric semiconductor refrigeration chip.

[0016] Figure 3 These are the heat dissipation fins of the present invention.

[0017] Figure 4 These are the voltage / current curves of the thermoelectric semiconductor cooling chip of this invention at different temperature differences. Detailed Implementation

[0018] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0019] Implementation Case: A Method for Extending the Range of a DC Energy Meter. This embodiment relates to extending the range of a DC energy meter equipped with a 300A / 75mV manganese copper shunt for use in DC charging piles.

[0020] Step S1: Obtain the minimum heat dissipation power Objective: To increase the current of the manganese copper shunt from the rated 300A to 450A.

[0021] Parameter: Rated voltage drop U of manganese-copper shunt rated=75mV, rated current I0=300A, ambient temperature T0=25℃, maximum allowable temperature T max =120℃.

[0022] Calculate: Resistance R = 0.075V / 300A = 0.00025Ω. Heat generation power P at the target current. heat = I1² × R = (450A) 2 ×0.00025Ω=50.625W.

[0023] The manganese copper section of the 300A / 75mV manganese copper shunt has a typical area of ​​35mm × 18.5mm, with an area S = 6.475cm². 2 .

[0024] Under steady-state conditions, the power of heat generation equals the power of heat dissipation.

[0025] Furthermore, based on the principle that heating power equals heat dissipation power, then P0 × (T) max - T0) = P heat Where P0 is the total thermal conductivity of the heat dissipation system (W / ℃), it can be deduced that the total thermal conductivity of the heat dissipation device required to increase the current from I0 to I1 is P0 = I1. 2 ×R / (T) max -T0).

[0026] Step S2: Heat dissipation device design The HT054130 thermoelectric semiconductor cooler is selected, with dimensions of 30mm × 20mm, a maximum cooling capacity of 61.8W, and a maximum temperature difference of 69℃. Because the height of the manganese-copper shunt can completely cover the thermoelectric semiconductor cooler, while its width is narrower, the maximum cooling capacity P1 in the area above the manganese-copper shunt is calculated as 61.8W × 18.5mm / 20mm = 57.165W, which is greater than the heating power P at the target current. heat =50.625W requirement, therefore the device meets the requirements.

[0027] After applying thermal grease to the cold side (21) of the thermoelectric semiconductor cooler, it is tightly attached to the manganese copper area of ​​the manganese copper shunt. After applying thermal grease to the flat side of the heat sink fins, it is tightly attached to the hot side (22) of the thermoelectric semiconductor cooler. When a voltage is applied to the thermoelectric semiconductor cooler, a cooling effect is generated, and a low temperature is formed on the cold side (21). A large temperature difference can be generated between the cold side (21) and the hot side (22). The cold side (21) is attached to the manganese copper shunt to dissipate heat from the manganese copper shunt. The heat sink fins dissipate the heat generated by the thermoelectric semiconductor cooler. In order to improve the heat dissipation effect, a small fan can be installed to blow on the heat sink fins to accelerate heat dissipation.

[0028] To ensure reliable measurement, a temperature sensor (23) is installed on the manganese copper shunt and an ambient temperature sensor (24) is installed near the manganese copper shunt to measure the operating temperature T1 of the manganese copper shunt and the actual ambient temperature T2.

[0029] With the above design, the measuring current of the manganese copper shunt can be increased from 300A to 450A under the original performance parameter requirements, thus expanding the measurement range and providing temperature monitoring function.

[0030] Step S3: Electro-cooling heat dissipation treatment When the current flowing through the manganese-copper shunt exceeds the cooling start threshold, the heat dissipation device is activated to power the thermoelectric semiconductor cooling chip and dissipate heat from the manganese-copper shunt.

[0031] The manganese-copper shunt originally maintained normal measurement accuracy at 300A, and in principle, 300A could be used as the cooling start-up threshold. However, after mounting the thermoelectric semiconductor cooler, it affected the heat dissipation of the manganese-copper shunt. Using 60% of the rated current, i.e., 180A, as the starting point and a target current of 450A as the ending point, the current was divided into segments of 10A each. Measurements from 180A to 450A were divided into N=27 segments with a 10A step size. Different voltages were applied to the thermoelectric semiconductor cooler during each current segment to ensure that the temperature of the manganese-copper shunt remained below the maximum value T during operation at that segment's maximum value. max When different voltages are applied to the thermoelectric cooler, the voltage is varied in steps from high to low according to the applicable voltage range of the thermoelectric cooler, so that the temperature of the manganese-copper shunt is lower than the maximum value T. max And the temperature difference ΔT of the thermoelectric semiconductor cooling chip is less than 85% of its maximum allowable temperature difference (corresponding to 58.65℃ for the device), so the minimum applied voltage value for each segment current is obtained, that is, N applied voltage values ​​are obtained. According to Figure 4 It can be seen that under the same temperature difference, the voltage and current are basically linear. Under the same voltage, the current increases synchronously with the temperature difference, but not nonlinearly. Therefore, according to the above method, the current and cooling power are different when the voltage is applied at different points, but the optimal value can be found.

[0032] The segmented approach is used because the power consumption of the cooling chip varies depending on the voltage applied. For example, applying 10V may consume 20W, while applying 9V only consumes 16W. Therefore, when conditions permit, 9V can be used instead of 10V, which is equivalent to achieving energy saving.

[0033] The recommended voltage range for the HT054130 thermoelectric semiconductor cooler is 30% to 80% of the highest voltage of 14.3V. Considering the optimal performance of the cooling control circuit in terms of cost, size, and power maintenance, seven integer voltages from 5V to 11V are selected, divided into M=8 segments. The first segment represents the cooling module not being activated, corresponding to the 0A~180A segment. Because the cooling capacity of the high current segment is higher than that of the low current segment, they can be grouped and merged. Based on the measured minimum voltages corresponding to N=27 segments (i.e., some of the 27 voltage values ​​are the same), adjacent cooling voltage segments are merged into segment M-1. Adding the 0A~180A segment without the cooling module activated, we obtain the final M segments. The N segment currents correspond to the M segment voltages. These measurements were obtained in the laboratory and stored in the energy meter for subsequent practical operation, allowing for the application of appropriate voltage values ​​based on the current values.

[0034] The electricity meter is internally designed with a DC power supply with a maximum power of 115W (greater than the product of the maximum operating voltage of the thermoelectric semiconductor cooler, 14.3V, and the current, 8A), which can output integer voltages from 5V to 11V to power the thermoelectric semiconductor cooler.

[0035] In actual DC charging pile applications, charging is intermittent. In some cases, the power is dynamically adjusted based on the power of the internal power module of the charging pile (e.g., 40kW). If charging is completed or the voltage is adjusted to below 180A, the manganese copper shunt may still be at a high temperature and cannot immediately stop heat dissipation. It is necessary to apply voltage to the thermoelectric semiconductor cooler for a period of time to maintain the applied cooling voltage value of the last monitoring. By monitoring the temperature T1 of the manganese copper shunt and the actual ambient temperature T2, the power is cut off to the thermoelectric semiconductor cooler until T1≤T2.

[0036] Because the heat dissipation device of this invention is active, it generates a magnetic field during operation, which affects the measurement accuracy of the manganese-copper shunt. To eliminate this, a compensation algorithm is employed. Since the voltage applied to each segment is stable, a segmented fixed compensation algorithm is used. First, the applied cooling voltage and actual ambient temperature of each segment are measured in the laboratory. By comparing this with the standard table on the calibration bench, a compensation lookup table for different voltages and actual ambient temperatures is obtained. In practical applications, the measured values ​​of the manganese-copper shunt are compensated by measuring the current, the operating voltage of the thermoelectric semiconductor cooling chip, and the ambient temperature to ensure its accuracy.

[0037] The MCU inside the electricity meter can collect the current of the manganese copper shunt, the temperature T1 of the manganese copper shunt, and the actual ambient temperature T2. The compensation lookup table is the data that is tested and fixed inside the electricity meter before production. The power supply current control and measurement value compensation of the thermoelectric semiconductor cooling chip mentioned above are all handled by the MCU.

[0038] Before actual operation, the present invention selects a heat dissipation device and sets the applied voltage and the data to be compensated according to the specific working current, so as to effectively achieve the heat dissipation and measurement range and accuracy goals in the actual working scenario.

[0039] The above implementation example uses a general-purpose thermoelectric semiconductor cooler, and the heat dissipation of the manganese-copper shunt only considers the manganese-copper part. In practice, there are many more ways to further improve the measurement range, including using thermoelectric semiconductor coolers with higher thermal conductivity, selecting thermoelectric semiconductor coolers that cover a larger area of ​​the manganese-copper shunt, or combining multiple thermoelectric semiconductor coolers, including customizing thermoelectric semiconductor coolers of the same size as the manganese-copper shunt, and adding fans to improve the heat dissipation effect.

[0040] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for increasing the range of a DC energy meter, characterized in that, Includes the following steps: Step S1: Obtain the minimum heat dissipation power Determine the rated parameters of the manganese-copper shunt, including the rated current I0 and the rated voltage drop U. rated Resistance R=U rated / I0, Maximum permissible operating temperature T max Calculate the heat generation power P of the manganese-copper shunt under the target operating current I1. heat = I1² × R, where I1 is the target operating current and R is the resistance of the manganese copper shunt. Under steady-state conditions, the heat generation power equals the heat dissipation power, therefore the minimum heat dissipation power is P. heat , Additionally, heat dissipation power = P0 × (T) max - T0), where P0 is the total thermal conductivity of the heat dissipation system (W / ℃), T max T0 is the maximum permissible operating temperature of the manganese-copper shunt, and T0 is the ambient temperature. From this, we can deduce that the total thermal conductivity of the heat dissipation device required to increase the current from I0 to I1 is P0 = I1. 2 ×R / (T) max -T0); Step S2: Heat dissipation device design Select a heat dissipation device with a heat dissipation power ≥ P heat The heat dissipation device is modularly composed of a thermoelectric semiconductor cooling chip and heat sink fins. The cold side of the thermoelectric semiconductor cooling chip is in close contact with the manganese-copper shunt, and its hot side is in close contact with the planar portion of the heat sink fins. By applying voltage to the thermoelectric semiconductor cooling chip, its cold side dissipates heat to the manganese-copper shunt, while its hot side dissipates heat through the heat sink fins, maintaining the temperature difference ΔT between the cold and hot sides of the thermoelectric semiconductor cooling chip within a predetermined range. Temperature sensors were installed on and near the manganese-copper shunt to measure the operating temperature T1 of the manganese-copper shunt and the actual ambient temperature T2. Step S3: Electro-cooling heat dissipation treatment When the current flowing through the manganese-copper shunt exceeds the cooling start-up threshold, the heat dissipation device is activated to cool the manganese-copper shunt. The voltage applied to the thermoelectric semiconductor cooling chip is dynamically adjusted in segments according to the current range of the manganese-copper shunt to achieve energy-saving management. Within a segment, the thermoelectric semiconductor cooling chip maintains a stable cooling power. The current segmentation method first divides the operating current of the manganese-copper shunt into N segments with small intervals, starting from the cooling start threshold and ending at the target current. When operating in each current segment, a different voltage is applied to the thermoelectric cooler. Based on the applicable voltage range of the thermoelectric cooler, the voltage changes from high to low in equal steps to ensure that when the manganese-copper shunt operates at its maximum value in that segment, the operating temperature T1 of the manganese-copper shunt is lower than the maximum value T. max The minimum applied voltage value of each segment current is obtained. Based on the measured applied voltage required for each of the N segments, the N voltage values ​​with the same value are merged and locally merged into segment M-1. Segment 1 indicates that the heat dissipation device is not activated, resulting in the final M segments, M≥2 and M<N, that is, the N segment currents correspond to M voltage values.

2. The method for increasing the range of a DC energy meter as described in claim 1, characterized in that: In step S3, after the current of the manganese copper shunt drops to the cooling start threshold, a voltage will still be applied to the thermoelectric semiconductor cooling chip for a period of time, and the temperature of the manganese copper shunt T1 and the ambient temperature T2 will be monitored until T1≤T2 before the voltage is stopped.

3. The method for increasing the range of a DC energy meter as described in claim 1, characterized in that: In step S3, the measurement accuracy error of the manganese-copper shunt caused by the magnetic field interference of the heat dissipation device is compensated. The cooling voltage applied to each segment current is stable. The applied voltage of each segment and the measured value of the manganese-copper shunt under the actual ambient temperature are obtained in the laboratory and compared with the standard table of the calibration bench to obtain the compensation value under different applied voltages and ambient temperatures, and a compensation lookup table is obtained.

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