Very Fast Transient Overvoltages on power transformers

by G Semiano, WEG Equipamentos Electricos SA


Published in: Transformers and Substations Handbook: 2014

Chapter 2: Design and installation of a substation - page 30

Enquiries: Zest WEG Group. Tel. 011 723 6000 or email


Read the full article on Very Fast Transient Overvoltages on power transformers in PDF format.


Of all design elements in a power transformer, insulation is one of the most important. Its function is the dielectrically guarantee no discharge occurrence during the field operation of the equipment. The electric field distribution that may occur in the insulation system of power transformers connected to a Gas Insulated Switchgear (GIS) system is not considered in the standards (IEEE, IEC).

Standardised verifications only refer to the normal linkage used to connect the transformer to the power systems and do not have the special conditions for application or operation with a Gas Insulated Switchgear system. In verifying the transformer insulation, during the final tests, the standards are a guide to executing tests of applied voltage, induced voltage, lightning impulse and switching impulse, depending on the equipment characteristics.

Each test has its own purpose and all dielectric tests complement each other. While the tests of applied voltage and induced voltage are requirements for industrial frequency (low frequency), the tests of lightning impulse and switching impulse are required for high frequency and apply High Voltage (HV) gradients to the windings. On the induced voltage test, the voltage gradient distributes equally along the winding; while on the impulse test, the voltage initially distributes in one function of the winding capacitances and applied waveform characteristics in the frequency domain.


Take note

  • Transformers connected to a Gas Insulated Switchgear system are exposed to Very Fast Transient Overvoltages (VFTO).
  • Identification of the transient characteristics that may occur in the Gas Insulated Switchgear (GIS) is important for the optimal design of the transformer.
  • Transient characteristics must be taken into account in the design of the electrical insulation as well as in the internal configuration of each transformer.