Abstract : This paper introduces the design principle of AGF-M16, a multi-loop photovoltaic combiner acquisition device based on the 16-bit microcontroller PIC24FJ64 as the main control chip, applied to an intelligent photovoltaic combiner box, as well as the main technical specifications and applications of the product.
Keywords : PIC24FJ64 chip; intelligent photovoltaic combiner box; AGF-M16 type; Hall sensor; design; application
0 Introduction
With the continuous growth of the world's population and the ongoing urbanization process, fossil fuels are becoming increasingly depleted, and the environmental pollution caused by burning fossil fuels is becoming increasingly serious. This has brought solar energy, a clean energy source, into sharp focus. Solar energy is a low-density planar energy source that requires a large number of solar panel arrays connected in series and parallel to achieve the required power. To reduce the number of connecting wires between the battery modules and the inverter, facilitate future maintenance, and reduce investment costs, it is necessary to configure photovoltaic array lightning protection combiner boxes, DC cabinets, and inverters on the DC side, using a segmented connection and tiered current combining method for primary and secondary current combining.
This article introduces an intelligent photovoltaic AGF series combiner current acquisition device based on the PIC24FJ64. This device uses Hall effect sensors to isolate and measure the current from the photovoltaic array, and can simultaneously measure its output voltage. It monitors the operating status of the surge protectors and DC circuit breakers inside the combiner box. It is equipped with sensor input interfaces for temperature, wind speed, and irradiance, and has contact outputs for driving external actuators. The device communicates with a host computer via an RS485 bus to receive computer commands and upload detected photovoltaic array, combiner box components, and external environmental conditions. This device can monitor the output current of up to 16 photovoltaic arrays and can calculate the phase-by-phase and phase-by-phase power based on the input voltage. The device features an LED digital display and setting DIP switches for viewing data and setting parameters such as the communication address, data format, and baud rate.
1 Circuit Design Principles
The PIC24FJ64 is a high-speed, low-power 16-bit microcontroller from Microchip, featuring an improved Harvard architecture. It achieves a system performance of 16 MIPS at a 32MHz clock frequency, a 17-bit x 17-bit single-cycle hardware multiplier, up to 64KB of internal Flash ROM, 8KB of system SRAM, two modules, two UART modules, and allows for redefinition of many external I/O port functions, increasing system design flexibility. 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 series connections. The current sampling uses a Hall sensor to achieve isolated measurement of photovoltaic current. The Hall effect refers to the phenomenon that when a semiconductor wafer is viewed in three-dimensional space, and a magnetic field of a certain strength is applied in the Y-axis direction, and a current is passed through it in the X-axis direction, an electromotive force will be generated in the Z-axis direction of the semiconductor wafer. This electromotive force is the Hall potential, as shown in Figure 2. This Hall sensor adopts a single 5V power supply mode, and the output voltage is 0 to 5V when the maximum input current is -15A to +15A. The output of the Hall sensor is connected to an 8-to-1 analog switch, which scans each of the eight input channels sequentially under program control. The output of the analog switch passes through an RC filter circuit and is then connected to the ADS1110 A/D converter chip. The ADS1110 is a 16-bit precision analog-to-digital converter from TI, with a wide operating power supply range of 2.7–5.5V. The chip has a built-in 2.048V high-precision voltage reference with a temperature coefficient of 5ppm/°C and a built-in PGA, which can amplify the input signal by 1, 2, 4, or 8 times. The chip can reach a maximum speed of 400kHz using a bus interface (Figure 3).
Figure 2
Figure 3
The current input module also features an input channel status indicator, using dual-color LEDs to indicate whether the battery series connection is disconnected or operating normally. The dual-color LEDs are driven by two cascaded 74HC595 serial-in parallel-out shift registers, with U33 driving the green LED and U36 driving the red LED. (Figure 4)
Figure 4
1.2 Display Section
The display circuit uses two cascaded 74HC595 serial-in parallel-out shift registers. The output of U100 serves as the segment code for the digital tube, and the output of U101 serves as the digit selection signal. A dynamic display method is used, lighting up each digital tube sequentially. The digit selection signal also functions as a button detection function. The CPU outputs a level on the "KEY" line. When a digit of the digital tube is turned on, if a button connected to that digit's selection line is pressed, the "KEY" line will be pulled low, and the CPU will detect a button event. Debouncing is then performed to prevent multiple triggers or misjudgments. (Figure 5)
Figure 5
1.3 External Switch Input Circuit
The external digital input circuit has a local display LED D28. The circuit mainly consists of U25, R65, Q3, and R68 forming a constant current circuit. The digital input node and the primary side of the detection optocoupler are connected in series in this constant current circuit. The advantage of this digital detection circuit is that it can accept external contact input with contact resistance up to kΩ or optocoupler input with open drain, so as to avoid the situation where the input contact oxidation caused by environmental factors on site will result in the failure to sample the digital input (Figure 6).
Figure 6
1.4 DIP switch input circuit
The DIP switch input circuit is used to set parameters such as the communication address, communication baud rate, and data format of this device. An 8-to-1 analog switch U20 is used to dynamically scan and detect each bit of the external DIP switch J12. The control bus for the DIP switch input and the switch input is a shared structure. The advantage of this method is that it saves CPU I/O lines. The disadvantage is that the program processing is slightly more complicated, and it is necessary to use dynamic scanning to detect each bit one by one (Figure 7).
Figure 7
1.5 External Analog Input Circuit
External analog input types include DC 0-20mA, DC 0-10V, PT100, 0-100mV, and DC 0-1000V. These external signals are first processed by external voltage divider or current divider to condition them into voltage signals within the same range, and then input to an 8-to-1 analog switch U44. The output of U44 is amplified by operational amplifier U45 to the acceptable voltage range of A/D converter U46. Operational amplifier U45 is a 5V single-supply rail-to-rail operational amplifier, and the A/D converter is an ADS1110. During circuit operation, the program controls the switching of external analog signals to the output one by one for A/D conversion, and then the CPU reads the data (Figure 8).
Figure 8
1.6 Communication Methods
The communication method uses RS485, with high-speed optocouplers for electrical isolation. Since the RS485 interface chip U20 is a half-duplex chip, the characteristic of this circuit is that the transmission and reception of the RS485 chip are controlled by the data being transmitted, achieving automatic data flow control. This eliminates the need for a flow control optocoupler, simplifying the hardware and software design and reducing costs. (Figure 9)
Figure 9
1.7 Power Supply Section
The power module uses PI's TNY series switching power supply chip, with an input range of AC/DC 80-270V. The power supply has three outputs, which provide power to the photovoltaic cell voltage sampling, CPU, communication and other circuits.
2. The main technical specifications of the product are shown in Table 1.
3 Software Design
The software flow is shown in Figure 10.
Figure 10
4 Installation dimensions
This device mainly consists of a core plate and a busbar. The busbar is divided into a positive busbar and a negative busbar. The specific dimensions are shown in Figure 11.
Figure 11
5 Application Cases
Taking a 10MW photovoltaic power station as an example, a 16-channel combiner box needs to collect 16 currents, the voltage of the solar panels after combining, and the auxiliary contacts of the surge protector. The secondary scheme is shown in Figure 12(a), and the primary scheme is shown in Figure 12(b).
(a) (b)
Figure 12
6. Conclusion
This product has been applied in photovoltaic projects in Sangri, Tibet; Xitieshan, Qinghai; a rooftop solar energy project of a company in Nanjing; and a rooftop solar energy project of a company in Shanghai, generating good social and economic benefits.
About the author:
Cai Lei (1977-), male, research and development engineer, whose research field is the design and development of industrial control products.
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