Comparison of Different Methods to Measure Submodule Capacitor Voltages of Modular Multilevel Converters for HVDC Applications
The Modular Multilevel Converter (M2C) is proven to be a key converter technology which is suitable for various high-voltage high-power applications. It offers several advantages over the conventional Voltage Source Converters(VSC) and multilevel converters.This Thesis deals with the measurement system for an M2C. Different methods to measure submodule capacitor voltages are analyzed, implemented on circuit boards and verified experimentally. The aim is to define the best approach to measure submodule capacitor voltages from reliability, speed,accuracy and simplicity point of view. Initially, a detailed study on the operation of a M2C is given in order to define the importance of having a fast and accurate measurement system of the submodule capacitor voltages. Secondly, a study of different methods to measure capacitor voltages is carried out. First the configuration of an ADC (Analog-to-Digital Converter) is presented and afterwards an alternative method based on voltage-to-frequency conversion is presented. Then, a research of the electronic components which are suitable to fulfill the demands of such measurement systems and which are available on the market has been carried out. In particular, two different families of components are examined; VCOs (Voltage Controlled Oscillators) and VFCs(Voltage-to-Frequency Converters). As next step, the description of the digital interface between the submodule and the Field Programmable Gate Array (FPGA) is given. The FPGA receives information about the submodule capacitor voltages. This process is programmed using VHDL (VHSIC-very-high-speed-integrated-circuit Hardware Description Language). Finally, a hardware implementation of the measurement systems is performed, in order to verify the effectiveness of the proposed methods.
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