Design and application of intelligent photovoltaic current collection device based on PIC24FJ64

0Introduction

With the continuous growth of the world's population and the continuous advancement of urbanization, fossil energy is becoming increasingly exhausted, and the environmental pollution caused by the burning of fossil fuels is becoming increasingly serious. As a result, solar energy, a clean energy, has attracted much attention. Solar energy is a low-density planar energy that requires a huge number of solar panel arrays to be connected in series and parallel to achieve the required power. In order to reduce the connection wires between the battery components and the inverter and facilitate future maintenance, while reducing investment costs, it is necessary to configure the photovoltaic array lightning protection junction box, DC cabinet, inverter on the DC side, and perform primary and secondary junctions in a segmented connection and step-by-step junction manner.

This article introduces an intelligent photovoltaic AGF series current collection device based on PIC24FJ64. This device uses Hall sensors to isolate and measure the current from the photovoltaic cell array. At the same time, it can measure its output voltage, monitor the working status of the lightning arrester and DC circuit breaker inside the junction box, and is equipped with temperature, wind speed, irradiation and other sensor input interfaces, with contact outputs for driving external actuators. This device communicates with the host computer through the RS485 bus to receive computer instructions and upload the detected photovoltaic cell array, junction box components and external environmental status. This device can monitor up to 16 photovoltaic cell group output currents, and can calculate the split-phase and combined-phase power according to the input voltage. This device has an LED digital display and a setting dial to view data and set the communication address, data format, baud rate and other parameters of this device.

1 Circuit Design Principles

PIC24FJ64 is a high-speed 16-bit low-power microcontroller with an improved Harvard architecture from MicroChip. The system performance can reach 16MIPS at a 32M clock frequency, with a 17-bit x 17-bit single-cycle hardware multiplier, a maximum of 64k FlashROM, 8k system SRAM, 2 modules, 2 UART modules, and allows many external I/O port functions to be redefined, increasing the flexibility of system design. See Figure 1 for the circuit block diagram

1.1 Current input module

The current input module is divided into two 8-way modules, which can input up to 16 battery series. The current sampling uses a Hall sensor 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 passed in the X-axis direction, then an electromotive force will be generated in the Z-axis direction of this semiconductor sheet. This electromotive force is the Hall potential, as shown in Figure 2. This Hall sensor uses a single power supply of 5V. When the maximum input current is -15A~+15A, the corresponding output voltage is 0~5V. The output of the Hall sensor is connected to an 8-choice analog switch, which is controlled by the program to scan the 8 input channels one by one. 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 a 16-bit precision analog-to-digital converter from TI, with a wide operating power supply of 2.7-5.5V. 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 by 1, 2, 4, or 8 times. The chip can reach a maximum speed of 400kHz using the bus interface (Figure 3).

The current input module also has an input channel status indicator, using a two-color LED light to indicate the disconnection or normal working status of the battery string. The two-color LED driver uses two 74HC595 serial input and parallel output shift registers in cascade, where U33 drives the green LED die and U36 drives the red die. (Figure 4)

1.2 Display part

The display part circuit uses two 74HC595 serial input and parallel output shift registers in cascade, where 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 the digital tubes one by one. The bit selection signal of the digital tube also serves as the key detection function. The CPU outputs the level on the "KEY" line. When a certain digital tube is turned on, if the key connected to the digital tube selection line is pressed at this time, the "KEY" line will be pulled low, and the CPU will detect a key event, and then the key debounce process is performed to prevent multiple triggers or misjudgments. (Figure 5)

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1.3 External switch input circuit

The external switch input circuit has an on-site display LED light D28. The circuit mainly includes U25, R65, Q3, and R68 to form a constant current circuit. The switch input node and the primary side of the detection optocoupler are connected in series to this constant current loop. The advantage of this switch detection circuit is that it can accept external contact inputs with a contact resistance of kΩ or optocoupler inputs with an open drain mode to avoid oxidation of the input contacts caused by environmental factors on site, resulting in the failure to sample the switch input (Figure 6).

1.4 DIP switch input circuit

The DIP switch input circuit is used to set the communication address, communication baud rate, data format and other parameters of the device. An eight-to-one analog switch U20 is used to dynamically scan and detect each bit of the external DIP switch J12. The control bus of the DIP switch input and the switch input is a common structure. The advantage of this method is that it saves the CPU's IO port line. The disadvantage is that the program processing is slightly complicated and requires the use of dynamic scanning to detect each bit one by one (Figure 7).

1.5 External analog input circuit

The external analog input types include DC 0～20mA, DC 0～10V, PT100, 0～100mV, DC 0～1000V. These external signals are first conditioned into voltage signals of the same range by external voltage division or current shunting, and then input into an eight-choice 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 power rail-to-rail op amp, and the A/D converter uses ADS1110 . When the circuit is working, the program controls the external analog signals to be switched to the output one by one for A/D conversion, and then the CPU reads the data (Figure 8).

1.6 Communication methods

The communication method adopts RS485 mode, and high-speed optical coupler is used 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, which realizes automatic data flow control, omits a flow control optical coupler, simplifies the software and hardware design, and reduces the cost. (Figure 9)[page]

1.7 Power supply part

The power supply module uses PI's TNY series switching power supply chip, with an input range of AC/DC 80-270V. The power supply has 3 outputs, which provide power to photovoltaic cell voltage sampling, CPU, communication and other circuits.

2 Main technical indicators of the product are shown in Table 1

3 Software Design

The software flow is shown in Figure 10

4 Installation Dimensions

This device is mainly composed of a core board and a busbar. The busbars are divided into positive busbars and negative busbars. The specific dimensions are shown in Figure 11.

5 Application Cases

Taking a 10MW photovoltaic power station as an example, a 16-way combiner box needs to collect 16 currents, the voltage of the solar panels after the combiner, and the auxiliary contacts of the surge arrester. The secondary solution is shown in Figure 12(a), and the primary solution is shown in Figure 12(b).

(a) (b)
Figure 12

6 Conclusion

This product has been applied in the Sangri photovoltaic project in Tibet, the Xiteshan photovoltaic project in Qinghai, the rooftop solar project of a company in Nanjing, and the rooftop solar project of a company in Shanghai, generating good social and economic benefits.