C. Meyer

Solid-state circuit breakers and current limiters for medium-voltage systems having distributed power systems

State-of-the-art mechanical circuit breakers in medium-voltage systems allow a safe handling of short-circuits if the short circuit power of the grid is limited. Using delayed turn-off times, the circuit breakers can be coordinated with lower level protection gear. Hence, a high availability of the grid can be guaranteed. However, during a short-circuit a significant voltage sag can be noticed locally in the medium-voltage grid. Sensitive loads such as computers will fail even if the voltage returns within a few seconds. A semiconductor circuit breaker, however, is able to switch fast enough to keep voltage disturbance within acceptable limits. The optimization and selection of power electronic switch topologies is critical. In this paper, different semiconductors are briefly compared considering the requirements of a solid-state switch integrated into a 20-kV medium-voltage grid. Based on these semiconductor characteristics, various switch topologies are developed, which are compared under technical and economical aspects. It is shown that solid-state circuit breakers offer significant advantages when compared to present solutions and can be used in todays medium-voltage power systems.

Control and Design of DC-Grids for Offshore Wind Farms

Recently, the interest in offshore wind farms has been increased significantly. Besides the huge amount of space available, they have the advantage of an increased and more constant wind speed, leading to a higher and more constant production of power. Although, a lot of mechanical problems for constructing these farms have been solved during the past decade, the electrification is still a major issue in research. For large wind farms situated more than 60 km from the shore, an HVDC connection is favorable. Consequently, a pure dc system could be an interesting and cost-effective solution for offshore wind farms. In this paper, a control structure for such a dc grid is presented and analyzed. Furthermore, different configurations are compared and the developed simulation models are presented.

Optimized Control Strategy for a Medium-Voltage DVR—Theoretical Investigations and Experimental Results

Most power quality problems in distribution systems are related to voltage sags. Therefore, different solutions have been examined to compensate these sags to avoid production losses at sensitive loads. Dynamic voltage restorers (DVRs) have been proposed to provide higher power quality. Currently, a system wide integration of DVRs is hampered because of their high cost, in particular, due to the expensive DC-link energy storage devices. The cost of these DC-link capacitors remains high because the DVR requires a minimum DC-link voltage to be able to operate and to compensate a sag. As a result, only a small fraction of the energy stored in the DC-link capacitor is used, which makes it impractical for DVRs to compensate relatively long voltage sags. Present control strategies are only able to minimize the distortions at the load or to allow a better utilization of the storage system by minimizing the needed voltage amplitude. To avoid this drawback, an optimized control strategy is presented in this paper, which is able to reduce the needed injection voltage of the DVR and concurrently to mitigate the transient distortions at the load side. In the following paper, a brief introduction of the basic DVR principle will be given. Next, three standard control strategies will be compared and an optimized control strategy is developed in this paper. Finally, experimental results using a medium-voltage 10-kV DVR setup will be shown to verify and prove the functionality of the presented control strategy in both symmetrical and asymmetrical voltage sag conditions.

Circuit breaker concepts for future high-power DC-applications

The development of advanced transmission and distribution technologies is steadily gaining interest. Especially the large number of wind farms leads to a demand for new and innovative solutions. Considering the interconnection of offshore wind farms, new technologies must be investigated. One promising solution for an interconnection is a DC distribution system which is also discussed for new (onshore) medium-voltage distribution systems. Beside the advantages of DC distribution (low losses, no reactive power), DC has major disadvantages concerning control and switching actions. Since present circuit breakers are not able to switch large DC currents, new solutions must be found. After a brief introduction of the fundamental principles for switching DC currents, different concepts of DC circuit breakers are presented and compared in this paper. Furthermore, this analysis is accomplished for different voltage levels and all solutions are compared under technical and economical aspects.

Solid-state circuit breaker based on active thyristor topologies

Solid-state circuit breakers (SCBs), based on modern high-power semiconductors, offer considerable advantages when compared to mechanical circuit breakers with respect to speed and life. For instance, voltage quality of a power grid can be improved during a short circuit because the fault current is reduced. The interval over which voltage distortions occur, caused by a three-phase short circuit, can be limited to a few 100/spl mu/s. In contrast to this, present circuit breaker technology requires at least 100ms to clear a fault. Major drawbacks of all SCB solutions presented so far are material costs and on-state losses. Primarily because of these disadvantages, SCB systems have not been widely used yet. Consequently, new solutions should be developed which are more cost effective and offer reduced on-state losses. This paper presents a conceptual approach based on active thyristor circuits. It is proven that these topologies can lead to a wider integration of SCBs in existing grids due to the economical advantages of silicon-controlled rectifiers compared to turn-off semiconductors. After a brief introduction and a presentation of the fundamentals, different topologies are described and assessed in this paper.

Power Electronics for Future Utility Applications

Medium-voltage converters, originally developed for industrial drives (e.g. in steel and paper mills), have nowadays entered utility applications. For several years, the growing need for power quality in distribution systems in conjunction with the large scale integration of renewable energy sources has boosted the demand for new technologies. Together with communication systems, power electronics are the key enabling technology to meet these challenges. This paper addresses several utility applications for power electronics, some of which are in use already today though market penetration is still low. In the field of high power inverters, conventional 3-level hard-switching converters as presently used e.g. in wind turbines are presented alongside with soft-switching converters and their possible applications such as STATCOMs or mini-turbines. The possibility of using DC instead of AC transmission and distribution systems will be discussed. DC systems have already been used for several decades to transmit bulk power. New opportunities for the use of modern VSC-HVDC will be shown, such as DC distribution systems. For this novel application, new technologies are needed, such as multi-megawatt DC-DC converters and DC circuit breakers; possible concepts for these technologies will be addressed. Cycloconverters, a rather conventional technology, have been applied recently in innovative ways to increase the efficiencies of very high-power pumped- hydro storage systems. Due to the fact that the development of new converter systems is always strongly related to the available device technology, future high-power devices will finally be discussed.

LCC analysis of different resonant circuits and solid-state circuit breakers for medium-voltage grids

The increasing usage of distributed power generation, leads to an increased short-circuit power in medium voltage grids. As a consequence, new concepts for limiting short-circuit currents are required. One possible solution is based on resonant circuits. These circuits are able to limit short-circuit currents fast, at preset levels. Four different circuits will be presented and compared with regard to technical aspects. In order to integrate new technologies into present power systems, costs are a decisive factor. In this paper, a complete Life Cycle Cost (LCC) analysis is carried out. It will be shown that, the design of resonant circuits offers some degree of freedom, which gives the opportunity to design circuits at minimal costs. The optimized solution strongly depends on the economical life time, which is assumed to be 10 or 20 years in present power systems. Finally, these costs are compared with solid-state circuit breakers, which are able to turn-off a short-circuit within a few 100 /spl mu/s.

Stability Analysis of High-Power DC Grids

During the last several years, increasing effort has been spent to advance offshore wind power. So far, only small test fields have been realized, and the electrification of offshore wind farms still remains a research topic. Currently, offshore wind farms are equipped with three-phase ac grids, some of them will be connected to the transmission grid via a high-voltage direct-current (dc) link. Consequently, a dc system encompassing the whole wind farm might constitute an alternative to a conventional ac grid. In this paper, the transient stability of such a dc grid is analyzed. Based on traveling wave theory, critical frequencies and cable lengths are calculated analytically. Furthermore, a large dc grid for an offshore wind farm is simulated. The goal is to provide guidelines that support stable operation of an offshore dc grid.

Optimized Control Strategy for a Medium-Voltage DVR

Most power quality problems in distribution systems are related to voltage sags. Therefore, different solutions have been examined to compensate these sags to avoid production losses at sensitive loads. Dynamic voltage restorers (DVRs) have been proposed to provide higher power quality. Currently, a system wide integration of DVRs is hampered because of their high cost, in particular, due to the expensive DC-link energy storage devices. The cost of these DC-link capacitors remains high because the DVR requires a minimum DC-link voltage to be able to operate and to compensate a sag. As a result, only a small fraction of the energy stored in the DC-link capacitor is used, which makes it impractical for DVRs to compensate relatively long voltage sags. Present control strategies are only able to minimize the distortions at the load or to allow a better utilization of the storage system by minimizing the needed voltage amplitude. To avoid this drawback, an optimized control strategy is presented in this paper, which is able to reduce the needed injection voltage of the DVR and concurrently to mitigate the transient distortions at the load side. In the following paper, a brief introduction of the basic DVR principle will be given. Next, three standard control strategies will be compared and an optimized control strategy is developed in this paper. Finally, experimental results using a medium-voltage 10-kV DVR setup will be shown to verify and prove the functionality of the presented control strategy in both symmetrical and asymmetrical voltage sag conditions.

Design of a Three-Phase Series Resonant Converter for Offshore DC Grids

Offshore wind farms for the generation of electrical power is an ongoing discussion for several years. A major challenge today is to reduce the cost for the electrification of these large scale wind farms. A possible solution is to use a DC collector grid instead of conventional AC systems. However, to realize this, high-power DC/DC converters are needed to transform the voltages within the grid. The Series Resonant Converter, which will be discussed in this paper offers a possible solution that is efficient and cost effective. First, the fundamentals on three-phase SRC systems are discussed, before the soft-switching characteristics of high-power semiconductors are discussed and measurement results are presented. Next, a modified topology of the SRC is presented allowing an asymmetric bi-directional power flow which is essential for the usage in offshore wind farms. Finally, the total efficiency of converter will be determined tor different frequencies.