Infoscience

Thesis

Medium Voltage DC-DC Converter for Power Collection Links: A Multistage Stacked Boost Architecture

Nowadays electrical energy generation is an important topic discussed in the society. Since technologies like wind or solar energy generation are most of the time placed far away from the city centres one point to focus on is the transmission of the energy opening opportunities for DC-distribution within the energy distribution. Focussing on solar plants as application it became obvious that to achieve high power levels a large surface area is used. Covering such a large area, where the energy is generated, means that the power must be collected over long distances, which leads to a low efficiency if the transmission is done under low voltage conditions. A better efficiency could be expected by increasing the voltage to a medium voltage level using a DC-DC converter. The introduced converter is based on the current diverter topology, which is used to equalize the voltage between different cells. To use this topology as a step-up converter a boost stage was added as input stage forming the Multistage Stacked Boost Architecture. To ensure that this topology would be a possible solution for the before mentioned challenges calculations were done to verify a satisfying efficiency of the converter. In addition an Energetic Macroscopic Representation was done and a control was designed. This control uses two cascaded controllers in each stage with feedback of the load current to generate the PWM. Since that control relies on the measurement of nine state variables, it was considered important to introduce a second control. The first step was to introduce a State-Space control, which is based on the description of system with n first-order differential equations. Then the research continued into a control strategy using an observer. The observer is basically a copy of the state space system; it has the same input and almost the same differential equations. An extra term compares the actual measured output to the estimated output; this will cause the estimated states to approach the values of the actual states. To verify simulation results and the working of the proposed controls a low voltage prototype was built. The measurements that were conducted all included normal steady-state operation as well as test to verify the correct functioning during perturbations such as variations of the load. All the measurement showed that the controls feasible. In addition, there were only very small resonant phenomena that are of no consequence for the correct functioning of the converter. To conclude, the work that has been done for this thesis included considering applications for medium voltage DC-DC converters. In addition, the problems of the approach of having huge solar plants use a low voltage distribution inside were shown. A converter that could be used to elevate said voltage to a medium voltage level was introduced - The Multistage Stacked Boost Architecture. This topology was then investigated in detail and calculations for the efficiency of the topology were conducted. Three different dedicated controls were investigated. A control based on the feed forwarding of the output current, a state-space control and an observer control. All control schemes were first tested with simulations and then verified on a low voltage prototype. The measurement results obtained with the different controls proved satisfactory, showing that the Multistage Stacked Boost Architecture is a reliable and interesting topology.

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