This means that utilities have to get their power transformers comprehensively tested and certified before installing them in their networks. In order to help ensure this IEC and ANSI standards have specified the various type and routine tests to be done, based on the collective experience of experts working in power networks, manufacturing and testing around the world.
Testing & Certification
Before a transformer can be certified completely it has to undergo a set of tests including the shortcircuit tests successfully.
Short-circuit testing of transformers requires a large test facility such as that of KEMA Laboratories, Arnhem, the Netherlands. Before commencing the short-circuit tests, routine tests have to be performed on the transformer. KEMA Laboratories can perform these routine tests on transformers rated up to 20 MVA beyond which these tests are performed at the manufacturer’s works.
In KEMA High-Power Laboratory the short-circuit impedance of the transformer is measured after the installation is made for short-circuit testing Between the short-circuit test cycles, it is measured again to ascertain that there has been no substantial change. At the end of the shortcircuit tests all routine tests are to be repeated successfully. The mandatory order for these checks in accordance with STL Guide is first to perform the routine tests after short-circuit tests. Thereafter the transformer shall be untanked for inspection of the active parts. Visual inspection is necessary, because deformations and displacements in supporting structures, clamping systems, insulation materials external connections from coils cannot be detected by the reactance measurements only.
If the short-circuit routine tests and visual inspection are performed at the manufacturer’s works, then those tests shall be witnessed by a KEMA Laboratories inspector for issuing a Certificate/Report of Performance in accordance with the standards.
Expansion of the KEMA High-Power Laboratory Arnhem, the Netherlands
The present capacity of the KEMA High-Power Laboratory Arnhem (8400 MVA, 16.67 to 60 Hz) though to very large is inadequate for large power transformers of rating 800 kV. For this purpose construction is on-going to expand the KEMA High-Power Laboratory with two more short-circuit generators and associated equipment. After the expansion, the maximum available power will be 13000 MVA in the test cells and 800 kV power transformers can be short-circuit tested. Many of the mock-up tests can be replaced by full scale tests. This expansion of the laboratory also makes testing of 1200 kV switchgear up to 120 kA feasible.
The fast master circuit-breakers of the laboratory makes it possible to interrupt the shortcircuit current within one half-cycle if some irregularities are detected in the performance of the transformers (half-cycle protection). Our experience shows that this fast protection reduce drastically the damage due to a fault in the transformer, and enables us to investigate the root cause.
When it is fully ready for testing, the transformer is transported by the river to the laboratory jetty in about 4 hours. On an average the whole process for preparations, testing and disassembly before shipping back takes about 15 days.
Since several transformers can be prepared simultaneously while tests are performed on one transformer the capacity for testing large power transformers has increased considerably. For this reason the KEMA High-Power Laboratory is very flexible with test dates for transformer testing but proper planning by the manufacturer is still essential to get their transformer ready in time.
Failure rate during short-circuit testing In the chart below, the failure rate is shown from the past 18 years. Commonly, the reason of not passing short-circuit tests is because the variation in reactance is larger than specified in the standards. This variation is mostly confirmed by visual inspection. On the other hand, in the cases that the variation is within the tolerances set by the standards, it is KEMA Laboratories’ observation that visual inspection still leads to rejection of a certificate. Therefore visual inspection is necessary because some deformations cannot be detected by the reactance measurements only. The “average” failure rate throughout the years is approx. 25-30%.