Dr. Tevfik Sezi - Preventing Leakage from Power Lines
Power grids have become safer and safer in recent years through the use of new technologies. But one problem that remains unsolved is the occurrence of "high-impedance ground faults." These are short circuits that occur during a hurricane or earthquake, for example, when utility poles fall over but the power lines remain intact. The lines lying on the ground still carry high-voltage current and are a great danger for people and animals.
Dr. Tevfik Sezi, 55, is the Principal Key Expert for Smart Grids at Siemens in Nuremberg, Germany. He has developed a new method that can safely detect short circuits of this kind. The system reliably measures very small earth currents and then ensures that the power is switched off. "I developed the idea when I was doing research into ways to avoid blackouts," says the engineer, who began working for Siemens after receiving his doctorate at the Technical University of Berlin in 1985. These issues are related, because when undetected faults are followed by the occurrence of another fault in the power grid, they can trigger a chain reaction that can end with the total collapse of the power grid.
When there is a high-impedance fault, the current that flows off along the ground is much too small to be detected by the protective relays at either end of the line. The protective relays are special electronic measuring units that constantly measure the state of a line and switch it off when there is a short circuit. Between a protective relay and the high-voltage line there is a current transformer that makes the measurement possible in the first place. At present, however, these have an imprecision of between three and five percent. In practice, this means that the line can only be switched off when the measuring system detects a relatively large differential current, because the imprecise measurement requires this safety margin.
With the new method created by Sezi, the absolute precision of the measurement is no longer the crucial factor. In his technique, the measuring instruments at both ends of the line measure the same current at precisely the same time. The measuring error is thus equally large at both ends and is irrelevant when it comes to detecting any possible differences in the current. The implementation of this idea is far from easy, however. After all, power lines may be up to several hundred kilometers long, and the protective relays with their associated current transformers cannot be removed and compared with one another.
To perform the comparison, the protective relays on both ends of the line are therefore equipped with high-precision satellite receivers that receive a GPS signal (Global Positioning System) and its time information. In this way, each protective relay associates a measurement with a time stamp. Then they are connected via a telephone line. During the calibration, both protective relays begin making current measurements at precisely established times. The measured values are saved. In this way, a series of compensation values are determined for a correctly operating line during a learning phase lasting several weeks. On the basis of these values, it is then possible to measure practically any "leakage current," no matter how small, in the event of an actual fault (short circuit). Once the calibration process is completed, the operating personnel then switch the protective system to "live" operation.
Any tiny deviations that are then measured can be ascribed with certainty to a leak resulting from a high-impedance ground fault. "In the U.S., the power companies are already liable for electrical accidents resulting from fallen power poles or similar occurrences," says Sezi. There is talk of introducing liability claims of this sort in Europe too. The new measurement technique is currently still in the development phase. Sezi is convinced that power companies will be keenly interested in the new protective relays.
When he attends a meeting or conference where he hears about a problem of the power companies, Sezi immediately begins thinking about solutions. As a result, he has 24 patents and 17 inventions. Sezi has always remained faithful to Siemens. One reason, among others, is that the company offered him the opportunity to work for three years in the U.S. A native of Turkey, he attended the Austrian Sankt Georg Gymnasium secondary school in Istanbul, where he gained the ideal qualifications for working in a global company: at the time of his final exams, he already spoke not only his mother tongue but also fluent German and English. Sezi has now settled in Erlangen with his family and works in Nuremberg in the Energy Automation business unit of the Distribution Division in the Energy Sector.