Photovoltaic systems have to play their part in ensuring high-level grid stability and supply reliability. Here, Ethernet-based networking ensures the failsafe transmission of diagnostics data and control commands between the various installed inverters, transformer stations, grid connection points and monitoring systems – whether via cable-based or wireless communication.
As a renewable energy, photovoltaics (PV) is making a considerable, sustainable contribution to meeting the globally increasing demand for energy. Planning, constructing and managing large PV systems, however, demands a huge amount of expertise and experience.
Zebotec, based in Konstanz, Germany, has established itself in this field over the last 15 years to become one of the world’s leading independent system integrators for controlling photovoltaic power stations. Zebotec is a part of the BayWa r.e. Group, which brought together various companies from within the field of renewable energies in 2009.
The Munich company’s field of activities includes the design, construction and marketing of PV power stations within the solar project management sector, among other things. Within these projects, Zebotec’s responsibilities include the systems for monitoring and control technology, among others. The range of services also includes the construction of efficient Ethernet networks that are used for networking system sections and for data exchange.
VLANs prevent unwanted communication
The Ethernet networks installed in the photovoltaic systems are used to transmit the diagnostics data recorded in the inverters, the weather stations, the temperature sensors mounted in the transformer stations, and the energy measuring devices. Forwarding the control data for grid feed-in, in particular, places high demands on the failsafe performance of this communication, because if the receiver does not receive the control values reliably, this may lead to the system feeding into the power grid without control, which would in turn put the grid’s stability at risk.
Certain special considerations have to be taken into account when networking the individual transformer stations in particular. A good example of this is the 45 MW ground-mounted system in Oosterwolde de Boer, the Netherlands, realised by Zebotec and the Dutch subsidiary of BayWa r.e., GroenLeven.
Firstly, there are large distances to be bridged between the stations in this system, and secondly, the Ethernet cables have been laid in cable ducts with very little clearance to the AC and DC cables in the system. Due to this proximity, electromagnetic interference (EMI) may arise when using classic twisted-pair copper cables and, in the worst case, can result in loss of data.
To prevent this, the copper cables would have to be equipped with special shielding or laid separately. Due to both the length of the cables and the possibility of EMI influences, Zebotec therefore decided to use fibreglass cables that, because of their immunity against EMI, proved to be an installation-friendly and error-tolerant solution.
Zebotec also installed managed switches from Phoenix Contact at the central grid connection point to further increase the stability of the network. The Ethernet transmission of the transformer stations connected in several lines comes together there. In this topology, each line is configured as a separate virtual network (VLAN). This prevents the various system sections from being able to inadvertently exchange data between each other. In turn, this prevents unnecessary data streams and improves communication efficiency.
Zebotec also sets up redundant network structures, especially in large systems, and usually in the form of a ring topology via RSTP (rapid spanning tree protocol) to achieve an even higher level of failsafe performance. This concept means that data transmission between all system sections is ensured, even in the event of a connection failure in one fibreglass line. Zebotec also uses Phoenix Contact’s FL Switchn 2200 Series managed switches here to ensure the necessary redundancy functionality.
Floating transformer stations connected via WLAN
Zebotec is also confronted with another characteristic that brings with it further requirements on the networking due to a new type of PV system. Alongside large free-standing systems, the company has recently started working together with BayWa r.e. on photovoltaic power stations on bodies of water that are not intended for tourism or ecological management purposes.
Transforming unused lakes into highly efficient photovoltaic systems brings with it considerable advantages. For example, they contribute to reducing CO2 emissions, generate high yields due to the water cooling effect, and prevent conflicts in terms of the previous necessity of taking up land. One example is the floating 27 MW system anchored to the lake bed of the Bomhofsplas gravel quarry close to the city of Zwolle in The Netherlands.
Because of the positioning on the lake, cabling the transformer stations and grid connection point would have been complex and expensive. Zebotec therefore decided upon wireless Ethernet networking: The WLAN clients installed in each transformer station establish connections with WLAN access points mounted in stations close to the shore. The transformer stations, which are themselves far from the shore, are coupled via repeaters to an access point to ensure consistent communication.
Here, the high-level reliability and robustness of the WLAN components used were a key factor. Zebotec therefore decided in favour of using the FL WLAN 5110 devices from Phoenix Contact. These industrial-grade devices can be configured as WLAN clients, as repeaters or as access points. This provided Zebotec, as the system integrator, with the flexibility to first construct the control cabinets for all transformer stations to be identical, and then to configure the final WLAN network topology once installed in the respective system.
Two omnidirectional antennas are installed on each transformer station for exchanging data between the FL WLAN 5110 devices. Due to the greater distances involved, Zebotec used directional wireless antennas for connecting the access points to an FL WLAN 5110 installed at the central grid connection point on land. All of the antenna cables are protected with surge protection equipment from Phoenix Contact to protect the photovoltaic system.
Networking concept to be adopted in future projects
To date, the Konstanz-based system integrator has had excellent experiences with both the managed and unmanaged switches, and the WLAN components from Phoenix Contact. “This was one of the reasons why we decided in favour of the infrastructure components, because the devices are industrial-grade and therefore robust enough to satisfy our high demands on high-level availability,” explains Werner Neff, CEO of Zebotec. Therefore, the company will also install the networking concepts described in future photovoltaic power stations.
As an aside, this also applies to the field of control technology. Zebotec has used AXC 3050 and ILC 191 Series controllers, as well as bus couplers, from Phoenix Contact’s Inline product family for processing the diagnostics and control data within its PV systems for years. As a Phoenix Contact solution partner in the field of renewable energies, Zebotec is also one of the first companies in the world to use the new, open PLCnext Technology control platform.
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