Short-circuit current, at a sufficient level, causes a major force, the Lorentz force, between conductors especially when they are close. Under the influence of these forces, for example, conductors of faulted overhead lines smash together violently: the “kissing conductor” phenomenon. A less romantic date is the one between power transformers and short-circuit current. Short-circuit current running through the transformer windings causing tremendous mechanical stresses on the turns and on the winding as a whole. When the device is not designed robust enough, mechanical deformation can easily occur, having a negative impact on the transformer’s lifetime.
Short-circuit withstand capability is far from obvious for transformers: 20 – 25% does not pass the tests, in spite of the advanced multi-physics calculation tools that designers have at hand these days to calculate the forces.
It is generally considered that short-circuit testing is the only reliable means to verify that a transformer can survive a short-circuit without damage. This was understood already in the early years of KEMA Laboratories, but only in the mid-eighties testing of large power transformers became a routine practice. At first sight, the test seems straightforward, since only high current needs to pass through the test object, but there is more: power and switches.
To deal with transformers in the several hundreds of MVA range, very large power is needed from the laboratory generators. Power transformers are designed for certain utility projects, and they need to be put in service immediately after testing. This means great care has to be taken to limit the number of tests to the minimum required in the standard, as well as to limit damage when an unforeseen malfunction inside the transformer might occur. Our answer to both is “switches”. From the beginning on, it was understood that highly reliable, custom made switches to start and end tests are key to a secure performance. “Making switches” switch in the short-circuit current at a precisely synchronized moment, to generate the exact wave shape as required. “Master breakers” need to switch off the current, extremely fast when needed, in order to limit damage to test objects. When not, the damage may get out of control and a root cause analysis of the reason of damage can never be traced back.
Having “big power” and “fast switches”, KEMA was ready to enter this market.
From the beginning on it was realized that very large transformers could better not be hoisted or rolled into the laboratory, so a test site was created at a jetty close to the Rhine river. Power is fed into the transformer that remains on board of the barge that it delivered, so testing can be done safely and efficiently. Around 2010, the request for testing of large transformers became so large, that a second jetty was opened in Nijmegen, 20 km from Arnhem. Here, transformers are prepared for the final test, a few hours downstream in the big laboratory. This would be the last stretch of a long journey, only to experience a few short-circuits lasting no more than a quarter of a second each.
The latest improvement is sheer power. The laboratory extension from four to six generators plus another four power test transformers greatly increases the reach of transformers to be tested. Having around 13,000 - 15,000 MVA of power and 550 kV of direct voltage available, short-circuit testing of transformers in the 800 kV class has become reality. In fact, very soon after commissioning of the first stage of the laboratory extension, already three 800 kV class power transformers have been short-circuit tested. Also, the additional power is addressed: the old performance record of 440 MVA has now been shifted to 600 MVA.