High-power test technology has been a main driver behind the development of substation equipment. A large number of test circuits and test methods have been developed since 1911 when AEG built the world’s first high-power laboratory in Kassel, Germany, with a short-circuit power of 150 MVA. High-power laboratories basically need to verify two capabilities: every component of the power grid that is conducting current needs to be able to withstand short-circuit current and, in addition, circuit breakers need to interrupt it, a far more challenging task.
Short-circuit current can basically be generated in two ways: drawing power for the power system or generating power (for a short duration) with stand-alone short-circuit generators.
KEMA’s first high-power laboratory, starting from 1938 already chose for generators, in order to operate independently from the grid and to adjust power frequency and test voltage flexibly. Starting with 500 MVA in 1938, by 1974 it evolved to 8400 MVA. Because of the possibility of synthetic testing, this power was for decades sufficient to test even the largest circuit breakers, and with a maximum current of 400 kA there was never a “no go” to any short-circuit withstand capability request.
Around the millennium change, the tendency towards small-scale local generation took on a stronger course. To test and research power-electronic based equipment for renewables a flexible power grid laboratory (foto beschikbaar) was realized in 2008. At the same time in many countries large scale transmission at 800 kV and above emerged rapidly. Supergrids evolved, interconnecting gigantic hydro plants, mega-cities, provinces and even countries.
It was in this context, that plans were developed to further increase KEMA Laboratories’ capacity. A new UHV synthetic installation was commissioned in 2012, followed by a HVDC cable system test laboratory in 2013.
But the big chunk was added following a decision made soon after KEMA was acquired by DNV in 2012. An investment of 70 million euro was made to add two generators (to the existing four) and four short-circuit transformers (to the existing six).
The aim was twofold. The first one is to increase the laboratory capacity in terms of available test space to cater for more customers by adopting parallel testing and so cutting down the waiting lists. The second one was increasing the power and voltage available for short-circuit testing of very large power transformers, a booming market and HVDC circuit breakers.
Then, the search for a short-circuit generator manufacturer started, not easy in a world focused to standard products. A strong requirement from control and maintenance was the embedding of the new generators in the existing installation, which raised the preference of having modernized versions of the existing Oerlikon generators. But the original company and engineers disappeared, and Alstom Italy was selected to make a re-design based on old drawings keeping the key electrical characteristics of the 1974 generators. The first one arrived April 2015. Power electronic drives were installed, instead of the rotating converters of the existing generators. This choice had an important drawback: the super excitation power, needed to keep the voltage constant at short circuit, was naturally provided by the rotating mass of the converter. But without any rotating mass in the drive chain of the new generators, another solution was needed for super excitation. This was found in the application of supercapacitors, that, when stacked in huge numbers and switched properly, can now provide the last kick of few tens of megawatts of excitation at the critical instant.
Supergrid power transformers are in the 800 kV class, and for testing these, direct power is needed with full voltage and short-circuit current. The direct available voltage was increased from 245 kV to 550 kV. Therefore, four new test transformers have been added. Royal Smit from the Netherlands made a dedicated design, which was tested in KEMA’s installation. Since these transformers need to be super resistant to short circuit, a rigorous test program was passed through. The market reacted positively: in the first year of operation of the extended laboratory already several 800 kV transformers were short-circuit tested, the power supplied by ten transformers and six generators.