Introduction

In practical applications, photovoltaic usually refers to the conversion of solar energy to electrical energy, that is, solar photovoltaic. Its implementation is mainly through the use of solar panels made of semiconductor materials such as silicon, using light to generate direct current.

Photovoltaic grid-connected power generation system refers to the direct current generated by solar modules, which is converted into alternating current that meets the requirements of the mains grid by the grid-connected inverter and then directly connected to the public grid. The photovoltaic grid-connected power generation system forms a parallel connection with the power grid to supply power to the load. When the power of the photovoltaic grid-connected power generation system is insufficient, the grid supplies power. The inverter refers to a component that converts DC to AC.

However, after the photovoltaic power generation system is integrated into the grid, it is easy to generate harmonics and three-phase current imbalance, and the randomness of the output power is likely to cause grid voltage fluctuation and flicker. According to the requirements of the domestic Golden Sun Project for grid-connected photovoltaic systems, the photovoltaic system on the user side must be spontaneous and self-use. So the anti-backflow device came into being.

Brief introduction of anti-backflow device 

    The principle of the anti-backflow controller is to control or cut off the output of the grid-connected inverter by monitoring the input power on the grid side, so that the photovoltaic grid-connected power generation system will not feed the grid.

     By turning on or off the output of several inverters, the output power of the power grid is stabilized within a certain range. When a reverse current is detected on the grid side, the controller will immediately open the gate of the inverter and merge into the grid to cut off the output of all inverters.

     The anti-backflow controller mainly includes: main controller, operating host computer, measurement and control instrument, and inverter. The main controller is PLC, which reads the grid power Pg detected by the measurement and control instrument through RS485 communication; and communicates with the slave station where the inverter is located through RS485 to control the switching of each inverter. The upper and lower limits of grid power Pu and Pd can be set through the touch screen.

    When Pg> Pu, turn on the inverter;

     When Pg <Pd, disconnect the inverter.

Anti-backflow device system plan 

      1 、System requirements

    The system consists of touch screen, master station PLC and 4 slave stations PLC. Among them, in addition to the MODBUS communication connected to the touch screen, the master station PLC also needs MODBUS communication to read the power, voltage, current and other data of the measurement and control instrument. At the same time, according to the data read from the measurement and control instrument, the inverter of the four slave stations is controlled through the MODBUS communication Swit ch on and off. The last slave station PLC uses MODBUS to communicate with the monitoring terminal for data and status.

     2 、The configuration of the McGemett system

    Controller: Megmeet PLC: MC200-2012BRA*5

    Special expansion module: MC200-RS485*2

    Touch screen: Megmeet MZ600-TT07SK

      3 、Key points of system solution

     Megmeet MC200 series PLC main module comes with two communication ports, and one communication port can be expanded. Among them, PORT0 port is used as a slave station to connect to the touch screen; PORT1 port is used as a master station to connect 4 PLC slave stations; PORT2 port is extended as a master station to connect two measurement and control instruments; , As a slave station to transmit relevant data to the monitoring center.

     In programming, the master PLC needs to determine the on-off status of each inverter in each slave station in real time, so that it can control whether each inverter is turned on or off according to the actual power requirements of the grid returned by the measurement and control instrument. At the same time, the status (X, Y) of each inverter must be converted into D element before it can be transmitted through communication.

Summary

In this device, there are more than thirty inverters in the slave station, and the number of inverters in each slave station is also different. In order to simplify the procedure and reduce the scanning time, the indexing method is used to check the on-off status of each inverter in real time.