Susceptibility of Distribution Transformers to Low-Voltage Side Lightning Surge Failure

The susceptibility of common designs of single-phase overhead distribution transformers to failure caused by lightning current surges entering the transformer via the normally unprotected low-voltage terminals has been investigated. Evidence from examination of transformers that have failed in service, transient analysis measurements of scaled surges applied to the high-and low-voltage windings, and a full-scale current surge impulse test, indicate that the shell-type designs ut'ilizing a non-interlaced 120/ 240-volt rating are most susceptible to failure. Shell-type and core-type designs with interlaced low-voltage windings are least susceptible. Susceptibility is inversely related to kVA rating, or more specifically, to the volts per turn employed by the designer. With the increasing trend to lower core loss transformers, reducing the volts per turn will increase this hazard on noninterlaced windings.