Switching systems in high-voltage networks have to meet increasingly exacting performance specifications. This is because the demand for electricity is rising and networks now require new capabilities to ensure that power can be distributed as efficiently, safely, and reliably as possible. As a rule, the circuit breakers used to control the flow of current in overhead transmission lines and power supply systems are filled with the gas sulfur hexafluoride (SF6). This serves to insulate the contacts and extinguish the arcs that are produced when the contacts are disconnected. Sulfur hexafluoride is well suited for this purpose because its dielectric strength is almost three times that of air. This describes the maximum field strength that an insulating material can withstand without breaking down, i.e. without conducting a current. This type of circuit breaker has been in use for decades and is inexpensive to manufacture. However, it requires relatively frequent replacement, since the contacts wear out quickly and can even be destroyed by strong currents. Furthermore, SF6 is the most potent greenhouse gas known to scientists. Great care must therefore be exercised to ensure that none escapes, not even in small amounts, during manufacture and maintenance of SF6 circuit breakers.

In vacuum circuit breakers, the contacts are separated in a vacuum. The vacuum arc arising between the contacts quenches very quickly, and in the absence of a gas between the contacts there is also no gas discharge. The contacts don’t wear out, so this type of circuit breaker is ideal for frequent switching and will continue to operate for several decades with almost no maintenance requirements. Given such advantages, production of vacuum circuit breakers has now become so advanced that they have ousted rival technologies in the medium-voltage range. Renz has been involved in this success story from the very beginning. Born in Berlin, he studied mechanical engineering and then physics at the TU Berlin, before joining Siemens to work in power distribution. That was 29 years ago, a period in which Renz has remained loyal to not only his home city but also his field of work. “When I started, the advantages of vacuum interrupters had just been recognized, and at the Plasma Physics Research Laboratory of Siemens Corporate Technology we began working on ways to exploit this technology for Siemens,” he explains. “My main job back then was to act as a go-between between research and product development and to make sure that they both understood each other.”

Over the years, Renz continued to advance the vacuum interrupter. In perfecting the contacts, vacuum tubes, and protective covering, he ensured that vacuum circuit breakers could be manufactured inexpensively enough for the medium-voltage range, despite the need for exacting production conditions such as vacuum technology and a clean-room environment. “Unfortunately, these methods don’t work for the high-voltage range,” Renz explains. Power is initially distributed to the regions by means of high-voltage networks. In order to be able to withstand overvoltage or lightning stroke voltage, the circuit breakers need to have a high voltage endurance. The required load capability of the vacuum circuit breakers increases not linearly but rather quadratically with relation to voltage, which means, for example, that in order to double the voltage endurance, the distance between the contacts must be increased by a factor of four. This, however, makes the circuit breakers too large.

Renz therefore focused on redesigning the vacuum tubes in such a way that voltage endurance could be increased while retaining their compact dimensions. “And we only managed that because Siemens has so much experience in this field,” he says. Of crucial importance here was the development of new contacts and, above all, research into multiple ceramic structures used to form the insulator of the vacuum tubes. “By increasing the number of ceramic insulators and thereby decreasing their length, we were able to improve the voltage endurance,” Renz explains. In combination with the components and production methods used for medium-voltage vacuum tubes, this new design will enable inexpensive production of vacuum circuit breaker in the high-voltage range.

Over the decades, Renz has continued to find his work in the field of vacuum tubes highly challenging and stimulating. “Everybody in this field worldwide is working flat out to develop new applications,” he says. His contribution has been impressive, with 60 invention applications and 146 granted individual patents registered in 53 IPR families to his name. After 15 years of heading development, Renz has been serving all the relevant departments at Siemens for some time now as an expert consultant on vacuum tubes. In addition, he travels from his beloved Berlin relatively often, representing Siemens at international conferences and committee meetings. In his free time, Renz cultivates his passion for music, playing guitar to the hits of blues legends, in a band together with his grown-up sons.