With the deepening of economic system reform and driven by the market, digital energy meters have developed rapidly. China has now become the most dynamic market in the world's energy metering industry. With the increase in user power load, the requirements for power supply quality are getting higher and higher. The power supply department needs to understand the quality of the power grid and various power parameters of users, such as power, voltage, current, frequency, etc., so the application scope of three-phase energy meters has been expanded. The timeliness and accuracy of energy metering, electricity fee accounting and collection have become important issues for power users. At present, the meter reading interface of the energy meter is mainly 485 interface and infrared interface. These two methods gradually cannot meet the actual needs. For this reason, we have proposed a new meter reading solution - wireless meter reading. The realization of wireless meter reading is the first step towards distribution automation and helps to improve the level of power management in the power system.
Energy metering chip ADE7758
The three-phase multifunctional energy meter with accuracy of 0.5 and 0.5S can use ADI's ADE7758. ADE7758 has the following functions and features: it integrates 6 independent 16-bit Δ-ΣA/D converters, high-performance DSP, voltage reference and temperature sensor circuits, and the error is less than 0.1% within the dynamic range of 1000:1; it provides data such as active, reactive and apparent energy, voltage, current RMS and waveform sampling; it is compatible with three-phase three-wire/three-phase four-wire; power, phase and input offset can be digitally calibrated; under the condition of large changes in environmental conditions and long-term use, the ADC and DSP using patented technology can still ensure high accuracy; the DSP internally compensates for reactive energy; it provides independent active energy and reactive energy pulse output. These functional features greatly reduce the workload of MCU software development. The functional block diagram of the energy meter based on ADE7758 is shown in Figure 1.
Design method of three-phase electric energy metering
Three-phase energy meters are divided into transformer type and direct-entry type according to the conditions of use. For high-current users, an external primary transformer is used, and its current output is 5A according to the standard. The most common three-phase energy meter is the transformer type, and its transformer current specification is 1.5/6A.
1. Design conditions for three-phase energy metering
a. Voltage specification 220V/380V;
b. The current specification is 1.5/6A, that is, the basic meter is 1.5A and the maximum current is 6A.
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| Figure 1: Functional block diagram of an energy meter based on the ADE7758. |
2. Select ADE7758 as the main metering chip, based on the conditions initially determined by the design requirements
a. Three-phase four-wire mode;
b. Select the algebraic sum model of each phase electric energy;
c. Three-phase current input, maximum current is 6A, input signal is V=200mV, to ensure a certain current overload capacity;
d. When the phase voltage input is 220V, the input signal is V=200mV, which ensures the measurement linearity at 120%Ua (rated voltage).
3. Selection of three-phase current secondary transformer
a. 1.5(6)A/5mA, 20Ω load, accuracy 0.05 level. The current sampling circuit is shown in Figure 2. In the figure, R1 and R2 are sampling resistors of the current loop. Precision resistors with an error of 1% and a temperature coefficient of ±100ppm/℃ must be selected.
b. R3 and R4 are input resistors with a resistance of 1kΩ;
c. C1 and C2 are filter capacitors, and the sampling resistor value is selected as 5Ω. The current sampling design can basically meet the range of at least 6 times the rated current.
4. Design of three-phase voltage loop sampling circuit
According to the ADE7758 data sheet, the maximum input level of the analog signal is 500mV. Selecting an input amplitude of about 1/2 of the range can ensure real-time measurement accuracy and measurement accuracy when it is greater than 120% Ua. The voltage division adopts a resistor network method, and the sampling circuit is shown in Figure 3, Uap=180mV.
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| Figure 2: Current sampling circuit. |
5. Active pulse output calibration frequency (CF)
According to the principle of electric energy measurement, for a certain phase, the CF calculation formula is:
Among them, C is the meter pulse constant, Un is the rated phase voltage value (unit V), and Ib is the rated phase current value (unit A).
Take C = 3,200 imp/kWh, substitute Un = 220 V, Ib = 1.5 A into formula (1) to calculate: Fe = 0.293 Hz.
When the sampling of the voltage and current channel of one phase of ADE7758 reaches the maximum level of 500mV and the remaining two phases are zero, the output frequency is about 16kHz. In this way, when the current sampling voltage Uia=50mV and the voltage signal sampling voltage Uva=180mV, first determine the output frequency of ADE7758 without frequency division and verification:
After rounding 1,152/0.293, we get 3,931, and write 3,931 into the denominator of the frequency output division coefficient of ADE7758, and write 1 into the numerator. Then, by adjusting the gain, phase, and offset of each channel of ADE7758, we can get an accurate and stable frequency energy pulse.
Wireless RF communication module
For the wireless RF transceiver function, we use ADI's RF transceiver chip ADF7020. ADF7020 has a series of products. In this solution, we choose a dual-band transceiver chip that works in 431MHz to 478MHz and 862MHz to 956MHz, which meets the North American FCC and European ETSI-300-200 standards.
The chip has the following features: wide operating voltage range of 2.3V to 3.6V, suitable for different battery power supplies; standby current of only 1μA; programmable output power from -16dBm to +13dBm, with an adjustment step of 0.3dBm; built-in automatic frequency control compensation loop, which can compensate the crystal by ±25ppm in the frequency band of 862MHz to 956MHz, and 50±50ppm in the frequency band of 431MHz to 478MHz; the current is 19mA when receiving and 22mA when sending; it has a digital wireless signal receiving amplitude indication (RSSI) output. The design adopts a rate of 9,600bps, 2FSK modulation, a receiving sensitivity of -106dBm, an output power of 12dBm, and a communication distance of more than 500 meters.
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| Figure 3: Voltage division using resistor network, sampling circuit. |
Conclusion
The wireless chip selected for this solution has the characteristics of built-in automatic frequency control (AFC), high transmission rate, long distance, etc. The overall design is relatively simple, and takes into account the future direction of meter reading, which is more suitable for mountainous areas and countries and regions where wired transmission is inconvenient.
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变色卡
千斤顶
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