Keywords: PIC24FJ64 chip; intelligent photovoltaic junction box; AGF-M16 type; Hall sensor; design; application
0 Introduction With the continuous growth of the world ’s population and the continuous advancement of the urbanization process, fossil fuels are becoming depleted, and environmental pollution caused by burning fossil fuels is becoming more and more serious. As a result, solar energy, a clean energy source, has attracted much attention. Solar energy is a low-density planar energy source, which requires a large number of solar panel arrays to be combined in series and parallel to achieve the required power. In order to reduce the energy between the battery module and the inverter The connection line is convenient for future maintenance, and at the same time reduces the investment cost, it is necessary to configure the photovoltaic array lightning protection combiner box, DC cabinet, inverter on the DC side, and perform the primary and secondary convergence according to the section connection and the stepwise convergence.
This article introduces a smart photovoltaic AGF series bus collection device based on PIC24FJ64. This device uses Hall sensors to measure the current from the photovoltaic cell array in isolation. At the same time, it can measure its output voltage and monitor the lightning arrester and DC circuit breaker inside the bus box. Working state, equipped with temperature, wind speed, irradiation and other sensor input interfaces, with contact output, used to drive external actuators. The device communicates with the host computer via the RS485 bus and is used to receive computer instructions and upload detected photovoltaic cell arrays, combiner box components, and external environmental status. The device can monitor the output current of up to 16 photovoltaic cells, and can calculate the split-phase and combined-phase power according to the input voltage. The device is equipped with LED digital display and setting dial code, used to view data and set the device's communication address, data format, baud rate and other parameters.
1 Circuit design principle
PIC24FJ64 is an improved Harvard high-speed 16-bit low-power single-chip microcomputer of MicroChip company. The system performance can reach 16MIPS at 32M clock frequency, 17-bit x 17-bit single-cycle hardware multiplier, and the maximum can include 64k FlashROM, 8k system SRAM, 2 modules, 2 UART modules, allowing to redefine many external I / O port functions, increasing the flexibility of system design. The circuit block diagram is shown in Figure 1.
figure 1
1.1 Current input module The current input module is divided into two 8-channel modules, which can input up to 16 battery strings. The current sampling uses Hall sensors to achieve isolated measurement of photovoltaic current. The Hall effect refers to when there is a semiconductor sheet Assuming that from a three-dimensional perspective, a magnetic field of a certain intensity is applied in the Y-axis direction and a current is applied in the X-axis direction, then an electromotive force will be generated in the Z-axis direction of the semiconductor sheet, which is the Hall potential, As shown in Figure 2, the Hall sensor uses a single power supply 5V power supply mode, and the maximum input current at the input is -15A ~ + 15A corresponding to the output voltage of 0 ~ 5V. The output of the Hall sensor is connected to an analog switch of one of eight, and the 8 input channels are scanned one by one by the program control. The output of the analog switch passes through an RC filter circuit and then is connected to the A / D conversion chip ADS1110. ADS1110 is TI The company's 16-bit precision analog-to-digital converter has a wide operating power supply of 2.7 to 5.5V. The chip has a built-in 2.048V high-precision voltage reference with a temperature coefficient of 5ppm / ° C. The chip has a built-in PGA, which can amplify the input signal. The magnification is 1 , 2, 4, 8 times. The fastest speed of the chip using the bus interface can reach 400kHz (Figure 3).
figure 2
image 3
The current input module also has an input channel status indicator, which uses a two-color LED lamp to indicate the disconnection or normal working state of the battery string. The two-color LED driver uses two cascades of serial input and output shift registers 74HC595, of which U33 drives green LEDs Die, U36 drives the red die. (Figure 4)
Figure 4
1.2 Display part The display part circuit uses two parallel cascaded shift registers 74HC595, in which the output of U100 is used as the segment code of the digital tube, and the output of U101 is used as the bit selection signal of the digital tube. The dynamic display mode is used to light up one by one. The nixie tube, the bit selection signal of the nixie tube also serves as the key detection function. The CPU outputs the level on the "KEY" line. When a certain nixie tube is turned on, if the button connected to this nixie tube strobe line is pressed at this time When down, the "KEY" line will be pulled down, the CPU will detect a key event, and then through the key debounce processing to prevent multiple triggers or misjudgment. (Figure 5)
Figure 5
1.3 External digital input circuit The external digital input circuit has a local display LED lamp D28. The circuit mainly has U25, R65, Q3, R68 to form a constant current circuit. The digital input node and the primary side of the detection photocoupler are connected to In this constant current loop, the advantage of this switch detection circuit is that it can accept external contact input with a contact resistance of up to kΩ or optocoupler input with open-drain mode to avoid the oxidation of the input contact caused by environmental factors on site to cause sampling failure. To the digital input (Figure 6).
Image 6
1.4 DIP switch input circuit The DIP switch input circuit is used to set the device's communication address, communication baud rate, data format and other parameters. An eight-to-one analog switch U20 is used to dynamically perform each bit of the external dial code J12. Scanning detection, the control bus of the DIP switch input and the digital input is a common structure. The advantage of this method is to save the CPU's IO port line. The disadvantage is that the program processing is slightly more complicated, and the dynamic scanning method needs to be used to detect all the way (Figure 7).
Picture 7
1.5 External analog input circuit
The external analog input types are DC 0 ～ 20mA, DC 0 ～ 10V, PT100, 0 ～ 100mV, DC 0 ～ 1000V. These external signals are first conditioned by external partial voltage or shunt processing to voltage signals of the same range, and then input to An eight-for-one analog switch U44, the output of U44 is amplified by the op amp U45 to the acceptable voltage range of the A / D converter U46, the op amp U45 uses a 5V single-supply rail-to-rail op amp, and the A / D converter uses the ADS1110 . When the circuit works, it is controlled by the program to switch the external analog signal to the output for A / D conversion, and then the CPU reads the data (Figure 8).
Picture 8
1.6 Communication mode The communication mode adopts RS485 mode and uses high-speed optocouplers to achieve electrical isolation. Since the 485 interface chip U20 is a half-duplex structure, the characteristic of this circuit is that the sending and receiving of the 485 chip is controlled by the sending data to realize automatic data flow control , Omitting a flow control optocoupler, simplifying the hardware and software design. Reduced costs. (Figure 9)
Picture 9
1.7 Power supply The power supply module adopts PI company's TNY series switching power supply chip, the input range is AC / DC 80-270V, the power supply has 3 outputs, and provides power for some circuits such as photovoltaic cell voltage sampling, CPU, and communication.
2 The main technical indicators of the products are shown in Table 1
3 software design software process see figure 10
Picture 10
4 Installation dimensions The device is mainly composed of a core board and a bus bar. The bus bar is divided into a positive bus bar and a negative bus bar. The specific dimensions are shown in Figure 11.
Picture 11
5 Application case Take a 10MW photovoltaic power station as an example, a 16-channel combiner box needs to collect 16-channel current, panel voltage after the confluence, and surge arrester auxiliary contacts. The secondary plan is shown in Figure 12 (a), and the primary plan is shown in Figure 12 (b).
(a) (b)
Picture 12
6 Conclusion This product has been applied to the solar project in Tibet Sangri, the Xitieshan photovoltaic project in Qinghai, the rooftop solar project of a company in Nanjing, and the rooftop solar project of a company in Shanghai, which have produced good social and economic benefits.
About the Author:
Yu Jing, female, undergraduate, engineer of Wuhan Ankerui Electric Co., Ltd., the main research direction is intelligent power monitoring and power management system
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