Summary
The short circuit withstand capability is an important requirement especially for large power transformers operating in high voltage transmission systems. Operating experience shows that over their expected long-term life, power transformers are exposed to several short circuit events. The consequences of these stressful events can be, potentially, catastrophic, leading to the scrapping of the unit and, also, to fire occurrence. According to IEC standard, the short circuit withstand capability can be evaluated using three methods: by check against the manufacturer’s design rules for short-circuit strength, comparison with a similar reference transformer which has passed successfully the test or by the execution of the test. Terna’s technical specification does not allow a comparison with the design rules of the manufacturer therefore the most effective way to proceed is to test, in a laboratory, the prototype. Within the certification process of a 400 MVA autotransformer, this paper describes Terna’s requirement and how the test has been developed in the Rondissone Laboratory.
Read more Read lessThe unit tested is a 400 MVA 400/155 kV KDAF auto-transformer, 5-limbs core with a short circuit impedance of 13,26 % in the principal tapping.
For standard three-limb auto-transformer, Terna’s technical specification requires the singlephase test arrangements that simulates a three-phase test, the so-called single-phase equivalent
(IEC 60076-5 clause 4.2.5.4). In case of a five-limbs core, the equivalent single-phase scheme may result in uncontrolled dielectric stress due to the neutral terminal voltage oscillations. In order to avoid this useless stress Terna decided to carry out, for the mentioned unit, the test by reproducing the short circuit that occurs most frequently in operation, that is single phase to ground.
It is important, when using this method, in a core type design, to pay attention also to core magnetization. The best way to proceed, in this regard, is to have a three-phase voltages supply, so that the magnetization of the main limbs is imposed by the grid. Another point of attention is the ground connection that must be arranged for the expected short circuit current.
Additional informations
| Publication type | Session Materials |
|---|---|
| Reference | A2_11053_2026 |
| Publication year | |
| Publisher | CIGRE |
| Country | Italy |
| Study committees | |
| File size | 867 KB |
| Price for non member | 30 € |
| Price for member | 30 € |
Authors
SACCO Simone - TERNA, Italy
Keywords
