Solar powerplant control using a UniPi Axon controller

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Thanks to their flexibility and software openness the UniPi programmable logic controllers are used in a number of projects for controlling solar powerplants. The Schönwald PV powerplant, built by the Solar Global Service a.s. company can serve as a perfect example. 

The Solar Global a.s. group is one of the leading producers of renewable-sourced energy in the Czech Republic. Subsidiaries of the group focus on a broad spectrum of services related to renewable energy sources and photovoltaics, ranging from the construction and maintenance of solar powerplants to investments in renewable sources to energy distribution and battery systems. The most significant part of the group is the Solar Global Service a.s.company, which realized several hundreds of solar powerplants and is currently directly servicing 62 installations with the total power output of over 100 MWp.

Its most recent project is a solar powerplant in Schönwald, Germany finished in December 2018. The powerplant, located in the area of a recultivated landfill, consists of polycrystalline modules placed on concrete holders. A total of 26 power inverters is connected to a master communication module. A distributor can regulate the powerplant's output by setting the effective power to one of four power levels (100%, 60%, 30%, 0%), while the reactive power can be regulated in 0,95kap - 1 - 0,95ind range. In total, there are 15 possible commands with all their respective combinations, which are routed as a 24V voltage to digital output terminals and then to the terminals of communication module's digital input.

The communication module features only a limited number of digital inputs, with a method for combining the switching of both effective and reactive power being also needed. Last but not least, the project also required a special "data tunnel" for the distributor, as the distributor cannot read or write data on the communication module via the Modbus TPC protocol due to safety reasons. Due to above-mentioned requirements the Solar Global Service a.s. company decided to install the  UniPi Axon M205 programmable logic controller. This particular model was deemed the most suitable due to its high number of digital inputs and the potential of the Mervis software platform.


Schematic of the installation

The controller, using an FBD programme containing all available inverter commands and their combinations, reads data from the data module using the Modbus TCP protocol and sends the data into the Solar Global Service a.s. surveillance centre. Relay outputs of the controller are then connected to the inverter data module inputs. The controller also features an onboard Modbus server that allows the local energy vendor to monitor the power production in real-time and to shut down or restart the powerplant if needed. The Modbus server allows the vendor to access only a limited portion of data specified by the vendor's requirements. 

Solar Global Service a.s. values the Axon controller for its ability to control the whole powerplant without the need for multiple controllers. As for future plans the company plans to use the Axon's potential for other projects, such as a smaller installation utilizing the UniPi Axon S115 controller, a model featuring four RS485 serial lines, to collect data from three different devices and relay them to a surveillance centre.

Next step

Photo documentation


Aerial shot of the installation

Axon M205 PLC in the distribution box

Installation of the control system

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