[PUBLICATION – EU PVSEC18] Spatial representation of low-voltage network hosting capacity for photovoltaic roof-top installations using an open-source tool
On the occasion of the European Photovoltaic Solar Energy Conference and Exhibition (EU PVSEC), Hespul has published a study on « Spatial representation of low-voltage network hosting capacity for photovoltaic roof-top installations using an open-source tool ». For this work, two areas of investigation have been chosen in the Auvergne-Rhône-Alpes Region (France) including Grenoble-Alpes Metropole within the European Commission funded FP7-City-Zen project focusing on innovation in the transition process. A look back at this study with Nicolas Lebert, smart grid project manager at Hespul.
What is the purpose of this tool?
Solar cadastres are extensively used in France today to assess the photovoltaic potential on territories. A result of those cadastres is that there remains a huge potential on rooftops that is not exploited yet. Those rooftops are mainly connected to the low-voltage network, where the connection costs per kWp are the highest, and can be prohibitive. This applies particularly to rural areas where a large part of the potential cannot be connected because the network is too meshed and there is not necessarily much consumption.
All these issues have led to the creation of a network cadastre tool that aims to model photovoltaic systems integration into the low-voltage network. The results are useful to both the project holders and the territorial communities.
The tool provides indeed information about the distance between buildings and their nearest substation, distance calculated as the real network distance. From this, the project holder can evaluate if the quotation of the grid connection by the distribution system operator (DSO) will be economically prohibitive or not.
Aggregated to substations and municipalities, the tool also give information about the areas where connections of photovoltaic systems are more constrained. This is a useful information for the municipalities int their discussion with the DSO about network reinforcement.
The results published are based on two territories: Valence-Romans and Grenoble-Alps Metropole.
Any unexpected results?
Not necessarily because we didn’t have any specific expectations, but we had some interesting results. In urban areas, there is a more meshed network and therefore a greater connection potential. Network constraints will be more likely to be found in rural areas. We have a fraction of the photovoltaic potential that can be connected to the low voltage network estimated between 15 and 30% depending on the type of community; those results were similar in Grenoble and Valence Romans.
15% is a percentage that may seem low but applied to the whole photovoltaic potential it’s huge! So if 15% of the photovoltaic potential on rooftop is connected to the grid it will be very positive yes.
What is the next step?
We have identified levers that make the capacity of the grid increase: in particular, undersizing the inverters at 70% of the system peak power only causes a 1% production loss but increases the connection potential by 25%. A modification of the voltage setting of the transformers may also help to eliminate part of the congestions in the LV network.
These technical levers are available to local authorities and, in this case, to the Metropole of Grenoble, which can use them when negotiating concession renewal contracts with the grid operator.
Within the framework of City-zen this tool has been improved, especially in the calculation times to make it more efficient, and we are still working on it. We had for example simplified constraints in the first version and we are currently trying to add real tension calculation in order to have something more realistic.
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Abstract – Just as solar cadastres have become the norm to assess the roof-top PV potential in the last years, assessing the hosting capacity of the low-voltage network for electrical production is on the path to becoming a necessity for an efficient solar production action plan. The model developed here is a hybrid tool composed of a physical model computing results based on a real infrastructure and a statistical model using empirical rules based on frequently encountered cases. The tool, built entirely on PostgreSQL/PostGIS, can be easily launched by non-experts on any set of data and on any computer, and has a low computation time. Results show that the major limit in connecting high shares of PV in existing grids is above all the low summer power consumption at noon. Key messages can be drawn from results: 1) hosting capacity of the existing infrastructure on the two territories studied is important but must be utilized with care by avoiding buildings located far from a substation, 2) on the long run, a deep reflexion on grid settings and planning is compulsory in order to achieve ambitious shares of renewable electricity at low marginal cost.