Column: Transformer lifecycle
Short-circuit withstand capability of power transformers – Part I
This article discusses verification of the ability of a transformer to survive short circuit. Fault current leads to large axial and radial forces on windings, which have to be managed by proper structural design. Test experience, showing a failure rate of 20-25% of well-prepared transformers, suggests that the highest degree of reliability with respect to short-circuit withstand verification is ensured through full-scale testing in accordance with the international standards. The prime failure mode is winding deformation, but many other deficiencies, which would not be discovered in a design review, have also been observed. Since 2016, KEMA Laboratories is equipped to short-circuit test transformers up to the 800 kV class.
Keywords: transformer, short circuit, testing, certification, reliability
The power transformer is the most vital substation component since its unavailability creates a major problem, given the high costs and the long time involved in repair or replacement.
Power transformers are especially sensitive to short-circuit events, as will be made clear in the following. The effects of short-circuit currents in transmission and distribution networks for electric energy are tremendous, both for the equipment and for the stability of the networks. Since short circuits are not rare events (as a rule of thumb, one short circuit per 100 km overhead line per year), short-circuit withstand capability is regarded as one of the main characteristics of the equipment installed. The capability to withstand a short circuit is recognized as a major and an essential requirement of power transformers. Failure to withstand it results in damage to the internal (and even external) parts, and can lead, in short or longer term, to loss of service.
Short-circuit current leads to electro-dynamic forces on the windings that cause mechanical stresses in the radial as well as axial direction. They result from the interaction of the current with the leakage magnetic field between the windings, in radial and axial direction.
Radial and axial forces may have the following effect on the winding:
- The radial forces (see Fig.1) tend to compress the turns in the inner (normally the lower voltage) winding and to expand the turns in the outer winding (higher voltage). When the mechanical design of the supports is not adequate, radial stresses may lead to forced buckling of the inner winding, which is frequently observed.