Market

BESS, battery energy storage

Storage plants connected at MV or HV. We cover grid connection studies, the point-of-connection substation, protection and inverter behaviour, from the connection offer to commissioning.

A storage plant alternates between export and import, sometimes within the same minute. We therefore carry out all studies in all four quadrants, charging and discharging, absorbing and supplying reactive power.

In 2026 the CRE approved ‘template’ (flexible) connection offers, under which injection or withdrawal is prohibited during certain time windows and seasons (CRE, deliberation 2026-32). They allow an earlier connection, provided that the operation of the installation within these limits is verified in detail.

Points of attention

Whether charging or discharging, the internal transformers and cables absorb reactive power. The P/Q capability actually available at the point of delivery is therefore smaller than that of the inverters, and it varies with the direction of power flow.

Requirements for grid-forming inverters are being introduced, and storage is the first concerned. Manufacturer models must be checked before being used as evidence.

Some inverters limit their fault current to a value barely above rated current. With these low short-circuit current sources, conventional overcurrent protection is no longer always sufficient.

On the DC side, arc flash on the battery side, coordination of DC fuses and circuit-breakers, and isolation all have to be addressed.

The evidence depends on the network operator. At MV with Enedis, it covers the NF C 13-100 point-of-connection substation and the interface protection. At HV with RTE, it addresses RTE's connection requirements. We cover both cases.

01What we do

Studies

Load flow and P/Q when charging and discharging
Short-circuit with inverter-based sources
EMT of grid-following and grid-forming inverters
Harmonics and resonances
Protection plan, earthing, battery-side arc flash

Services

Support to the project owner and review of the integrator's documents
Testing of the power plant controller and control loops
Relay testing at the point-of-connection substation

02Examples

Fig. 01Storage charging and discharging. At the point of delivery, the P/Q range is shifted towards absorption and reduced by the internal network losses, differently depending on the direction of power flow.Worked example

Fig. 02Voltage dip to 0.3 pu for 150 ms. The inverter injects reactive current and reduces its active current, then recovers.Simplified model

Extract from a 13.8 kV MV single-line diagram of a protection plan — Fig. 03Extract from the single-line diagram of a 13.8 kV MV protection plan.Anonymised deliverable

03Our offer

Usual content of an offer for this type of assignment. The exact scope, deliverables and price are set in the quotation.

Package 1Analysis of the connection offer
Export and import capacities, limitations of the offer, consequences for sizing.
Package 2Substation and internal network
Transformers, switchgear, cables, earthing, protection plan.
Package 3Inverter behaviour
P/Q when charging and discharging, voltage dips, EMT, harmonics.
Package 4Review of the integrator's work
Review of the single-line diagram and notes, deviations ranked blocking, major, minor.

04How we work

01Connection offer
Export and import capacities, constraints of the offer, schematic diagram.
02Substation and internal network
Transformers, switchgear, cables, earthing, protection.
03Inverter behaviour
Four-quadrant P/Q, voltage dips, EMT, harmonics.
04Testing and commissioning
Power plant controller, protection, control loops, acceptance.

05References

Storage developer

HV / MV substation for a battery storage plant

Transformers, switchgear, cables, protection and earthing. Preparation of tender documents and follow-up through to construction.

39 MW storage plant

Audit of a 33 kV single-line diagram

Review of the integrator's diagram and list of corrections to request, for the project owner.

Storage plant connected at 225 kV

EMT study method in PowerFactory

Grid-following and grid-forming inverter models, study cases defined according to CIGRE publications.

All references

06Frequently asked questions

Do you work on ‘template’ connection offers?

Yes. We check how the storage plant operates within the limitations of the offer, and their consequences for sizing.

Do you review the integrator's documents?

Yes, in particular the single-line diagram, calculation notes and protection plan. Deviations are ranked blocking, major or minor.

MV or HV?

Both. We work on storage plants connected at 20 or 33 kV as well as at 225 kV.