St Marys Substation 110/22 kV Transformer Replacement

Three aspects of this project required more than standard engineering, and all three were solved in the design office before any outage was called.

The first was the staged SEF (Sensitive Earth Fault) check scheme. The existing and replacement transformers had different CT secondary ratings, which meant the existing SEF check scheme was not directly compatible with the new equipment during the intermediate stages of the commissioning sequence. Megavar identified this incompatibility during the design phase and developed a staged SEF check scheme that kept protection functional and compliant through every step of the staged commissioning, rather than finding the problem at a relay during an outage window with a clock ticking.

The second was the interlocking scheme for the 22 kV station service transformer, which needed to be supplied from either 22 kV Bus A via feeder B252 or 22 kV Bus B via feeder H252, but never both at once. Megavar designed and proposed a full interlocking arrangement to TasNetworks, which was then reviewed, modified and implemented in accordance with TasNetworks’ operational requirements. The final scheme satisfies the operational need, the safety requirement, and the relevant TasNetworks standards.

The third was a straightforward operational decision: the protection relay setting files were developed in the Megavar office from the approved setting reports, rather than being generated on site during each outage. The as-installed setting files were downloaded from the relays before the works began, and the new setting files were prepared, reviewed and verified against the scheme drawings ahead of mobilisation. That meant site outage time was spent verifying behaviour and energising equipment, not writing settings. It is the kind of sequencing decision that is easy to get wrong and quietly expensive when it goes wrong.

On site, the staged sequence was executed against the outage plan, with the two transformers, the station service transformer, the ring main unit and the 22 kV feeder protection all brought through integration, testing and commissioning in the planned windows. Protection relay commissioning stepped through the full installed suite: GE T60 transformer management, Siemens 7UT613 transformer differential, MicroTapp MT101 tap changer control, Areva P143 and P122 feeder and bus protection, and the RMS 2C138 element. The 22 kV cable work was proved with HV Diagnostics VLF testing alongside Omicron MPD 600 partial discharge measurement, and the SCADA integration was verified point-to-point through to the network control room.