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Task 3.2 deals with the technical specifications of the 3 demonstrators foreseen in OSMOSE WP3:
1. an utility-scale battery energy storage system (BESS) at EPFL (Lausanne, Switzerland),
2. a small scale BESS also located at EPFL campus,
3. an hybrid energy storage system (HESS) connected to the RTE grid (RingoLab in France).
One of the main goals of these demonstrators is to assess the required conditions to implement the grid forming controls proposed in MIGRATE project considering different infrastructures and operational environments. For this purposes, the two demonstrators considered at EPFL are based on existing facilities, while RingoLab will be specifically built for the project using off-the-shelf equipment.
In the former cases, first investigations were focus on the possibility of upgrading an existing control.
In the latter case, efforts were devoted to the definition of the HESS technical specifications and to the development of its control in close collaboration between RTE and Ingeteam.
This report first presents the technical description of the EPFL demos and secondly the final specification of RingoLab. In both cases, the characteristics of the connection grids are included:
1. The utility-scale BESS consists in a 720 kVA/560 kWh Lithium-Titanate (LTO), equipped with a 720 kVA 4-quadrant converter, and connected to a 20 kV radial feeder of the EPFL campus medium voltage (MV) grid via a 630 kVA 3-phase transformer. This grid has an aggregated peak power consumption of about 300 kW and is equipped with 95 kWp of PV rooftop installations which make it a suitable test bed prone to highly variable voltage and current profiles.
2. The small scale BESS is a LTO battery with 25 kWh capacity, equivalent to one string of the utility-scale BESS, connected to a low voltage (400V) experimental microgrid through a 25 kVA
4-quadrant converter.
3. For RingoLab, we have specified a 1 MVA fully containerized solution based on a six lithiumion battery racks (for 0.5 MW - 60min in total), six supercapacitors racks (1MW-10s), a 1 MVA low voltage inverter and a 0.6/20 kV transformer. It will be installed in the south of France and connected to the secondary of a 20 MVA 63/20 kV transformer serving an industrial load.
The choice of the selected substation (Castelet) was mainly driven by the availability of a 20kV connection point, which is not common in RTE network, while low short circuit ratio and load variability were desirable features. A transient fault recorder was installed at the 20 kV bus bar in February 2019. The analysis of a first measurement campaign let us think that the grid forming robustness will be challenged in operation as significant load changes and single-phase short circuits were recorded.
Regarding the modifications to the control of EPFL existing facilities, low level control layers were found to be closed in the utility-scale converter which prevent us from implementing the grid forming control as proposed in MIGRATE. To tackle this shortcoming, a solution to achieve grid-forming performances in this demonstrator through an outer loop is under investigation. The small-scale BESS offers full access to the converter control, so MIGRATE grid forming control can be implemented.
For RingoLab, Ingeteam has plugged the MIGRATE grid forming control to an electromagnetic model of the demonstrator specifically developed for the project. System performances were validated on MIGRATE test cases. However, some challenges remain regarding the behaviour of the grid forming algorithm and the current limitation strategy under unbalanced conditions such as asymmetrical faults. Possible solutions are under study. This report details control updates required for industrial
implementation purposes and simulation results. In particular, detailed hardware limitations were implemented in the model at early stage of control design to accurately represent transient behaviour.
Finally, different DC control strategies for power and energy sharing between the different DC components were described and simulated. The final choice will be made later on in the project.
This report also addresses some challenges related to the characterization of the robustness and effectiveness of the grid forming function. Indeed, grid forming was defined in MIGRATE to provide smoothing services to the frequency and the voltage amplitude. Specific DC controls will have the same effect on the battery output in a HESS setting. Here, we specify measurable key performance indicators taking into account the specific settings of each demonstrators.
These metrics include the well known RoCoF (Rate of Change of the Frequency) and the frequency nadir, but also the derivative of the active power and voltage, as well as the ratio between the battery and supecapacitor output variations. Therefore, details about the monitoring infrastructure and communication protocols are also provided at this stage of the project. Both EPFL grids are highly instrumented with PMUs. The data is collected in a dedicated server and Graphana open source tool will be used for analysis. RingoLab must be integrated to the RTE industrial telecommunication infrastructure, whose underlying constraints are taken into account while ensuring required monitoring and control capabilities of the demonstrator.
Finally, although multiservice control algorithms were discussed in the scope of Task 3.1, an insight on the compatibility of grid forming smoothing services with more classical ones, such as primary frequency control (PFC), is provided within the framework of the DC side energy management strategies. For instance, traditional PFC using storage systems may be challenging to achieve in practice as their state of charge may drift with the average system frequency. Mitigation measures must be
put into place. For this purpose, a modified version of the IEEE 39 bus system has been modelled by EPFL in a real time simulation platform in order provide a reliable benchmark with long term testing capabilities. In addition, it will offer the possibility to compare grid forming solution with their grid
following counterparts, for different scenarios (topology, renewable penetration, outage, etc.) and multiservices settings.
In conclusion, at the current stage of the project the objective are reached. Portability of the MIGRATE grid forming solution to the existing EPFL demonstrator and Ingeteam off-the-shelf equipment has been, to a sufficient extend, confirmed. RingoLab control, based on MIGRATE solution, is under development in collaboration between RTE and Ingeteam. Associated services and metrics to assess performances have been agreed and the required monitoring infrastructure is now deployed.
