The effects of short-circuit currents in transmission and distribution networks for electrical energy are tremendous, both on the equipment and on the stability of the networks. Since short-circuits occur quite often, the short-circuit withstand capacity must be one of the main characteristics of the equipment installed .

Design

Transformers, like series reactors, have the quality to limit the short-circuit currents to values predominantly determined by the transformer's impedance. In this way, the design of a power transformer with respect to short-circuit current withstand capacity is directed towards limitation of the current values proper as well as towards control of the forces and stresses exerted by the same short-circuit currents inside the transformer.

With an increase of the short-circuit power throughout the years, the severest short-circuit currents will appear when the transformer has aged. These short-circuit currents must be withstood without impairing the transformer. Also, the short-circuit withstand capacity is defined as the ability to withstand several full asymmetrical short-circuit currents in each phase and in each representative tap position without impairing the transformer's suitability for normal service conditions. 

Experience with short circuits in service as well as with short-circuit tests in high-power laboratories force users and manufacturers to pay serious attention to the short-circuit withstand characteristics of power transformers.

The final part of the distribution system at medium voltage are the distribution transformers. Due to the low impedance voltage, this type of transformer will not substantially limit the short-circuit current in the case of a fault on its secondary side. It is therefore common practice that distribution transformers are type-tested to their ultimate short-circuit conditions. Distribution transformers may be oil-filled (IEC 60076) or dry-type (IEC 60726). KEMA can perform complete type tests, which means doing all relevant tests in the High Power Lab and the High Voltage Lab. Regularly, a manufacturer offers a complete range of five distribution transformers, which can be very efficiently certified in one test performance.

Power transformers exist in the categories: distribution transformers; smaller than 2500 kVA (cat. 1), between 2500 kVA and 100 MVA (cat. 2) and larger than 100 MVA (cat 3). There are three-phase and single-phase transformers, the latter in special applications for railways as well (and in frequencies of 25 of 16 2/3 Hz). Oil-filled transformers at KEMA can be tested according to IEC 60076 or ANSI C.57-12.

KEMA offers the service to support all oil preparation and expertise on transports. KEMA is committed to optimizing preparation time before the test shift. Depending on bushing, oil treatment etc., preparation time generally ranges from one day to less than a week.