Temporary overvoltage (TOV) lasts much longer than an impulse and can be driven by ground faults, load rejection, resonance or system control. Transformer and arrester selection therefore needs TOV magnitude-duration cases, grounding method and system operating states. It is incorrect to substitute a lightning-impulse withstand value for a TOV capability assessment.
A high impulse rating does not establish TOV capability
Temporary overvoltage is generally assessed as a power-frequency or relatively slow voltage excursion over its duration, not as the peak of a standard lightning impulse. A withstand value for a short impulse therefore cannot be substituted into a voltage-time assessment for a sustained condition. State whether each value is RMS or peak and whether it is phase-to-earth or phase-to-phase. Mixing those bases can create an apparent insulation margin that does not exist.
Retain the mechanism and clearing condition
Ground faults with neutral displacement, load rejection and voltage-control behavior can produce different TOV envelopes. The duration may depend on primary protection, backup clearing or control recovery. Use the relevant operating and grounding states, including credible failures, rather than one unexplained multiplier. Frequency should also be retained where it changes, because increased V/Hz can create an overexcitation duty in addition to the dielectric voltage exposure.
Check transformer and arrester limits separately
The transformer dielectric and overexcitation assessments are not the same as the arrester's thermal TOV capability. Compare each device with the relevant manufacturer-supported characteristic, including prior energy duty where applicable for the arrester. A device can satisfy impulse coordination yet fail a long-duration TOV requirement. The report should identify the governing amplitude-duration cases and the assumptions that keep them within the selected equipment's capability.
Technical reference framework
[1] IEC 60071-1:2019 — Insulation co-ordination, Definitions, principles and rules.
[2] IEC 60071-2:2023 — Insulation co-ordination, Application guidelines.
[3] IEC 60076-3:2013 + AMD1:2018 — Insulation levels, dielectric tests and external clearances.