Sheffield Substation 25 MVA Transformer Post-Fault Testing and HV Withstand

Sheffield Substation is a TasNetworks 110/22 kV substation in north-west Tasmania. Its T5 transformer is a 110/22 kV, 25 MVA unit built by Wilson Transformer Company and installed in 2018. Megavar carried out its site acceptance tests in December 2018.

In early 2024, after a lightning strike, T5 began producing gas. TasNetworks needed to know whether the transformer could safely stay in service or was at risk of failure. TasNetworks engaged Megavar directly for two stages of work: a full set of post-fault diagnostic tests in February 2024, then an applied voltage (HV withstand) test with Megavar’s 300 kV resonant test system in May 2024.

Because Megavar had tested the same transformer at commissioning, the post-fault results could be compared against Megavar’s own 2018 baseline, not just the factory test sheet.

• 1 x 110/22 kV 25 MVA power transformer T5 (Wilson Transformer Company, 2018, YNyn0, ONAF)
• 23-position on-load tap changer and drive motor
• 4 x ABB GSA123-OA 110 kV bushings (three phases and neutral)

The question was not simply whether T5 would pass a set of tests. It was whether TasNetworks could keep a gassing transformer in service with confidence. Megavar split the work into two steps so the second could be judged on the results of the first.

The February diagnostic tests looked for mechanical and insulation damage without stressing the insulation. Short-circuit impedance matched the nameplate to within 0.02 percentage points on taps 1, 8 and 23. Ratio errors were 0.06 per cent or less on all 23 taps. The dielectric response put moisture in the main insulation at 0.8 per cent. The SFRA traces followed the same pattern as the 2018 traces, with minor deviations in the core region and around internal connections; Megavar noted these could come from residual magnetism or the energised 110 kV yard around the transformer, and recommended the manufacturer review the traces. The dissipation factor of the 22 kV winding to tank had risen from 0.29 per cent in 2018 to 0.49 per cent.

With no sign of winding movement, the remaining question was whether the insulation would still withstand a voltage well above operating stress. That is what an applied voltage test proves. Megavar set the levels at 80 per cent of the factory test voltages, the guide AS/NZS 60076.3 gives for insulation that has been in service, so the test would prove the insulation without over-stressing a transformer TasNetworks wanted to keep.

A resonant test system was used because it reaches the 184 kV needed on the 110 kV winding from a small site supply, and because resonance collapses immediately if the insulation breaks down, which limits damage to the transformer under test.