Study of the impact of a low Short Circuit Ratio (SCR) on the interconnection of a hybrid power plant with the grid

Within the framework of designing a hybrid power plant (solar panels + batteries), Capsim carried out a dynamic study focusing on the interconnection of the hybrid power plant with the grid in order to address an issue related to the low short-circuit power of the upstream network at the point of delivery. This low level of short-circuit power results in a low Short Circuit Ratio (SCR), which can lead to significant instabilities during transient events on the network (inverter synchronisation, short circuits, etc.).

SCR definition:
The short-circuit ratio is defined as follows: SCR = Scc / Sinverters

With:

• the short-circuit power at the point of delivery;
• the rated power of the inverters within the facility.

Usual recommendations suggest having a SCR greater than 3. However, with the architecture originally considered for this study, the SCR was initially below 0.5.

The first step involved numerically modelling the upstream network as well as the PV and BESS inverters using a dedicated simulation software.

This modelling was carried out in EMT in order to obtain a more detailed representation of the system and to perform an in-depth analysis. The complete model was validated through a short-circuit calculation to confirm the value of the short-circuit power S_cc at the delivery substation.

Once the modelling was completed, several simulations—such as inverter synchronisation events and short circuits—were performed, demonstrating that certain modifications made the system capable of remaining stable during these events.

However, after discussions with the client and the grid operator, the decision was made to modify the electrical architecture in order to significantly reduce the total installed inverter capacity. This improved the SCR and therefore the stability of the system, while maintaining the same power capacity.

With these modifications on the considered hybrid power plant—multiplying the SCR by approximately four—the dynamic results for the same simulations improved. The change in architecture made it possible to reduce the number of inverters required while enhancing the plant’s performance when facing unforeseen events.

Thanks to the dynamic study carried out to assess the impact of the low short-circuit ratio at the point of delivery, Capsim was able to identify a more suitable hybrid power plant architecture that meets the power requirements mentioned in the technical specifications while maintaining system stability