Two 400 kV supergrid transformers energized at Hams Hall 350 MW battery site

The energisation of two 400 kV, 220 MVA supergrid transformers at Hams Hall marks a pivotal milestone for large transmission-connected storage schemes. Connecting directly into the National Grid network, the high-voltage plant provides the primary electrical link for the 350 MW, 1.22 GWh battery energy storage system. Completed on September 30, 2026, the energisation establishes the high-voltage connection well before full commercial operation commences.

Large battery facilities require dedicated high-voltage substation capacity capable of handling continuous, bi-directional load transfers. Unlike conventional generation collector substations that manage unidirectional power delivery, battery substations experience frequent power reversals during charging and discharging cycles. For transmission-scale installations of 350 MW, managing these power swings places demanding mechanical and thermal stress on transformer windings, tap changers and high-voltage bushings. Utilizing twin 220 MVA units provides vital operational redundancy, allowing partial site dispatch even when an individual unit requires maintenance or takes an unexpected trip.

Energising supergrid transformers ahead of system-wide commissioning permits transmission operators and developers to perform essential no-load testing and auxiliary power verification. Establishing the transmission interface early also allows engineering teams to validate protection schemes, metering systems and supervisory controls before introducing full inverter power onto the network. This phased strategy mitigates harmonic interactions and unexpected resonance between high-voltage switchgear, stepping transformers and distributed inverter banks.

What this means for asset owners in the UK and Ireland: Substation operators and battery developers across the UK must prepare for extended procurement lead times when specifying bespoke 400 kV supergrid plant. Securing high-voltage transformer slots and scheduling transmission connection outages requires coordination several years in advance. Operators must also plan comprehensive post-energisation diagnostic testing during initial soaking periods to capture incipient bushing and dielectric insulation risks before full cycling begins.