Photovoltaic power generation refers to the use of photovoltaic cells to convert solar energy directly into electricity. Photovoltaic power generation system in the energy conversion rate, equipment costs and practical applications have good prospects for development. Photovoltaic power generation has the characteristics of no pollution, no noise, short equipment construction period, simple maintenance, etc., and can be promoted and applied on a large scale. Photovoltaic power generation technology in Germany, the United States and other developed countries are more mature, the promotion of greater, such as Germany as early as 1990 that launched the “1000 solar roof program”; In 2002, the total amount of photovoltaic cell power generation in the United States has reached 112.9 MW, and Japan is nearly 254.5 MW, and an annual growth rate of 48.6%. China’s photovoltaic power generation technology started in 1970, after nearly 50 years of continuous efforts, the gap between domestic photovoltaic power generation technology and foreign countries is constantly narrowing. Photovoltaic power generation system can be divided into three modes according to the transmission mode: independent, grid-connected and distributed photovoltaic power generation, of which distributed photovoltaic power generation refers to the distribution and configuration of smaller photovoltaic power supply system at the user’s power supply site to meet the needs of specific users, with the advantages of decentralized layout and centralized management. Taking the distributed photovoltaic power generation control system as the research object, the control structure, hardware and software design are analyzed to improve the automation level of the control system, improve the operating efficiency of the system, and reduce the incidence of failure.
1. Control system structure
Distributed photovoltaic power generation control system structure block diagram as shown in Figure 1, according to the function can be divided into energy management, conversion/control management, load optimization management three parts. The energy management part is the photovoltaic cell array, which is the power source of the distributed photovoltaic system; Photovoltaic cells are processed by conversion/control management modules such as DC/AC converters, photovoltaic grid-connected devices, and control systems to provide continuous and reliable photovoltaic energy for the public power grid for load use. The conversion/control management part is the core of the distributed photovoltaic system, which can realize the automatic switching of AC and DC of photovoltaic power. The load optimization management part can realize the storage, intelligent management and energy flow control of photovoltaic energy, and can provide the optimal control strategy for users or loads.
2. Hardware Design
The core hardware of the distributed photovoltaic power generation control system is a PLC controller, and the selected model is Mitsubishi FX5U-64MR. The input signal of the photovoltaic power generation control system is mainly divided into two categories: command signal and position/protection signal, command signal has emergency stop, reset, automatic/manual, etc. Position/protection signals include direction knob, pendulum limit signal, etc. The control system can control the swing rod moving along the east-west, west-east and vertical directions; The knob can be used to control the rotation of the photovoltaic module in four directions: east, west, south and north to obtain the maximum solar energy. Distributed photovoltaic PLC controller I/O input address allocation table details are shown in Table 1I0. 0 ~ I0.7 and I1.0 ~ I1.3. The input signal of the PLC controller also includes an inclination sensor to measure the inclination of the photovoltaic module, which is an analog current signal; Temperature sensor for detecting the temperature of photovoltaic modules; Light intensity sensor, used to detect the light intensity of sunlight. According to the collected temperature and light intensity data, the control system controls the automatic movement or rotation of the pendulum rod and photovoltaic module, and uses the inclination sensor to measure the inclination Angle of the pendulum rod and photovoltaic module in real time. The output signal of the photovoltaic power generation control system is mainly composed of relays and contactors, such as relays that control light color change, relays that control buzzer switches, relays that control DC/AC electrical signals, and contactors that control photovoltaic power generation systems off the grid and connected to the grid. The selected model of the temperature sensor is CYB-20S-kW, the sensor uses the thermal resistance PT1000 as the thermal sensor, can detect the temperature range of -50 ~ 260℃, the detection accuracy is! 0.2% F.S, the excitation voltage is 12 ~ 24VDC, the shape and size of the temperature sensor meet the installation size requirements, the detection accuracy meets the system requirements. Inclination sensor selected for the model ZCT290L-LHS-17, the sensor is a dual-axis structure, can measure the X axis and Y axis of the two directions of the inclination, the measurement range is! 90°, can output -10 ~ 10V voltage signal or 4 ~ 20mA current signal. The selected light intensity sensor model is ZCT182L-LHZ-1, and the detection accuracy of the sensor is! 0.3% F.S, power supply is 12 ~ 24VDC. In addition, the monitoring platform of the distributed photovoltaic power generation control system is realized by force control configuration software, and the PLC controller and force control configuration software use RS485 communication mode for data transmission and mapping.
3. Software Design
3.1 PLC Design
The distributed photovoltaic power generation control system has two control modes: manual and automatic. In manual mode, the PV module starts to move after rotating/pressing the knob; After loosening the knob, the photovoltaic module stops moving immediately; In automatic mode, after pressing the knob in the specified direction, the photovoltaic module starts to move according to the set speed until it reaches the limit position, and then moves in reverse and repeats. Distributed photovoltaic power generation control system is designed to east, west, south, north four directions of the knob can control the photovoltaic module to rotate in the specified direction according to the requirements, manual mode, press the knob, the photovoltaic module begins to rotate according to the requirements, when the knob is released (or reach the limit position), stop rotating; In automatic mode, the photovoltaic module can simulate the sun continuously running in the specified direction until it reaches the limit position in the direction, and after the stop time of T seconds, it runs in the reverse direction. The distributed PV power generation control system software process first detects that the system is powered on and begins the system self-test process. If a fault occurs during the self-check, the fault information is pushed to the monitoring platform. The system can resume after the fault is rectified. In the software design, the system self-recovery/reset function is added to ensure that the system can run continuously in an emergency.
3.2 Monitoring platform design
Distributed photovoltaic power generation control system monitoring platform design block diagram as shown in Figure 3, by the monitoring interface, data interface and auxiliary interface of three parts. The PLC controller transmits all data of the system operation to the force control configuration software through RS485 communication mode and maps to the controller address. The monitoring platform is designed by user login interface, login in the form of user name + password; It can view and query the historical data of the system operation in real time, as well as the curve change trend of a certain parameter, and print the system data report. The monitoring interface is mainly composed of four sub-interfaces of operation management, inverter and load monitoring, alarm and parameter setting, which can view the input and output data such as horizontal/vertical limit button, direction button, light, temperature/illumination/inclination sensor value in real time. The data interface is composed of four sub-interfaces: curve display, historical data query, report and printing, which is convenient for technical and staff to view the historical information of each parameter and print management. The auxiliary interface consists of two sub-interfaces, user management and password modification, which can add, delete, modify user information, and set user rights.
4. Concluding Remarks
The designed distributed photovoltaic power generation control system has been tested and coordinated by the system, and has been used on site to improve the automation level and operation efficiency of the control system. Through the monitoring platform, the operating status and fault information of the power generation control system can be monitored in real time to achieve the design goal.