Sebastian Nielebock

A modular bi-directional hybrid circuit breaker for medium and high voltage DC networks

The development of dc transmission and distribution technologies are gaining traction. With renewable energy being fed to the grid constantly increasing, there is growing interest in development of dc grids. A major hindrance for their development is attributed to the lack of protection during short-circuit fault. Hybrid circuit breakers, a combination of mechanical and semi-conductor devices are understood to be the most effective method to protect dc grids against such faults. Several of such hybrid breakers are published and patented but still a practical high voltage dc breaker has not been developed. In this paper a novel modular hybrid circuit breaker is presented. This paper tries to address limitations present in existing breakers and a scalable model which can be used for both medium and high voltage dc networks is proposed.

Voltage source operation of parallel PV/wind inverters-stabilized hybrid power plants in “diesel off-mode”- an experimental verification

Microgrids are emerging as an important component of large-scale smart grids. It is a very effective way of integrating various distributed generation resources into the AC power system. Microgrids or off grid hybrid power plants utilize renewable generation to offset fuel cost in remote areas. Maximizing the economic benefit requires maximizing the utilization of the renewable generation and therefore conventional generation to be reduced whenever possible. However conventional generators typically require a certain minimum power set point, and therefore consume fuel, even if there is enough renewable energy. Consequently it is desirable to switch off the conventional generation completely (“diesel-off mode”). During this mode storage systems or synchronous condensers may be used for grid stabilization tasks, which are normally performed by the generators. However only in rare cases the high cost for such extra installations are financially reasonable. This paper presents a novel renewable inverter control method to enable using existing renewable inverters for grid stabilization in diesel-off, and also in diesel-parallel mode. The approach enables parallel connected inverters to run in voltage source mode, compensate load fluctuations and run in parallel to conventional generators. The concept is derived for photovoltaic inverters and can be applied to wind inverters even more easily. By using short-term renewable forecasts the amount of spinning reserve needed to compensate unforeseen renewable fluctuations can be minimized. The control algorithm is presented and its stability is discussed and demonstrated in a laboratory experiment.

Modular parallel interleaved converter for high current application

Parallel interleaved converter belongs to the family of modular multilevel converter. This converter has dual nature; it is a multilevel converter and also a parallel converter. The advantages of both multilevel converters and the parallel converters are available in this family of converters. The specialty of this converter is that, multilevel voltage can be generated by using only one DC voltage level. Capacitor voltage balancing is not necessary in this type of multilevel converter. The interleaving switching method generates circulating current within the converter modules. The magnitude of the circulating current depends on the switching frequency, module inductance, dc link voltage and the numbers of parallel modules. It is also observed that, the circulating current of a parallel interleaved converter is constant for a switching frequency, module inductance and the dc link voltage. This means the converter has better efficiency with high output current. This converter can be very suitable for low voltage and high current applications.

Control of M2C direct converter for AC to AC conversion with wide frequency range

Modular Multi-level converters are the most discussed topic in the field of HVDC for AC to DC conversions. In this paper it is shown that modular multi-level converters with series connected H-bridges have the capability to convert 3-phase AC to single phase AC. Additionally M2C with full bridge modules has many degrees of freedoms in terms of frequency and voltage levels. In addition, it does not need any passive component for the AC to AC conversion like in case of resonant converters. The output single phase AC can have a wide range of frequency, independent of the 3-phase grid frequency. This ability of M2C makes it suitable for varieties of application. For example in railway electrification, the conversion from single phase AC to 3 phase AC to run the three phase motors attached to the wheels. It can be used in solid state grid application for the high frequency isolation without using isolated DC/DC converters. The control challenges of this type of the Modular Multilevel Converter are presented in this paper.