Vacuum-breaker switching can produce steep-front transients depending on current chopping, restrikes, cable length and surge impedance, transformer terminal capacitance and protective devices. The transformer cannot be assessed in isolation. A switching study should include breaker characteristics, cable/GIS connection, surge arresters or RC protection where applicable and the transformer high-frequency terminal model.
Current chopping creates an energy-transfer problem
If an inductive current is interrupted before its natural zero, stored magnetic energy can transfer into the connected capacitances and produce an overvoltage. A very simplified energy balance gives a voltage increment proportional to Ichop√(L/C), illustrating sensitivity to current and network capacitance. It is not a complete switching-surge prediction: existing voltage, damping, arc behavior and repeated reignitions change the waveform. Obtain the breaker's applicable switching characteristics rather than assigning every vacuum breaker the same chopping value.
The cable and transformer form one transient circuit
Cable length, surge impedance and propagation time influence the waveform reaching the winding terminals. The transformer input is frequency dependent, so a power-frequency leakage reactance alone may not represent the relevant steep-front stress. Model the actual connection and any adjacent equipment, including the protective-device lead paths. The same transformer switched through a different cable arrangement can face a different terminal waveform even when the nominal system voltage and breaker rating remain unchanged.
Verify the mitigation in the assembled network
Surge arresters, RC networks or other engineered measures address different aspects of the transient and must be coordinated with their own ratings. An arrester can limit terminal voltage yet leave a steep-front waveform requiring further assessment of internal winding stress. The study should compare candidate measures under credible breaker and network variations, then document the approved arrangement and component duties. This is a design task for the equipment and system specialists, not a generic field retrofit instruction.
Technical reference framework
[1] CIGRE Technical Brochure 568 — Transformer energization in power systems: A study guide.
[2] IEC 60076-8:1997 — Application guide.
[3] IEC 60071-2:2023 — Insulation co-ordination, Application guidelines.