Distributed restart is one approach being explored to deliver black start capability in a network with increasing levels of renewable generation.

If distributed restart is to be delivered using grid forming BESS, the practical question becomes how to energise large network assets from relatively small sources.

The scale challenge

In one project, an 11.6 MVA grid forming battery energy storage system was used to energise a 90 MVA transformer and associated 132 kV network.

The concept is straightforward. The behaviour during switching is less so.

When attempted without control of the switching process, transformer energisation resulted in a high inrush current, leading to voltage instability and disconnection of the source. This highlights the constraint that energisation places on smaller systems.

Understanding transformer energisation

This behaviour is linked to the residual flux within the transformer and the point on the voltage waveform at which it is energised.

If energisation occurs at an unfavourable point on the waveform, the core can be driven into saturation, resulting in a large transient current and a corresponding voltage depression.

Controlling the switching process

Managing this requires control of the switching process itself.

Point-on-Wave control allows the circuit breaker to close at a defined point on the voltage waveform. By taking into account the residual flux within the transformer and the system voltage at the time of switching, it is possible to select a closing point that avoids saturation.

The effect is a significant reduction in inrush current at the point of energisation.

From instability to control

When applied in this case, the reduction in inrush current allowed the battery energy storage system to maintain stable operation and successfully energise the transformer and network.

What had previously resulted in instability became a controlled and repeatable operation.

Implications for distributed restart and black start

This enables a more practical approach to restoration, where larger assets can be brought online without exceeding the capability of the source.

It also reflects a broader point. As approaches to black start evolve, it is not only new technologies that matter, but how they are applied in the context of real network behaviour.

The full project case study here explores this application in more detail, including the observed behaviour during energisation and the impact of controlled switching on system stability.