A Novel Cascaded Multilevel Converter Topology Based On Three-phase Cells
Name: RENNER SARTÓRIO CAMARGO
Publication date: 24/08/2021
Advisor:
Name |
Role |
|---|---|
| DOMINGOS SÁVIO LYRIO SIMONETTI | Co-advisor * |
| LUCAS FRIZERA ENCARNAÇÃO | Advisor * |
Examining board:
| Name |
Role |
|---|---|
| DOMINGOS SÁVIO LYRIO SIMONETTI | Co advisor * |
| LUCAS FRIZERA ENCARNAÇÃO | Advisor * |
| WALBERMARK MARQUES DOS SANTOS | External Examiner * |
| WEDER TÓTOLA NUNES | External Examiner * |
Summary: Due to the structural characteristics of modern electrical grids, composed mainly of an energy matrix that is very diverse and sparse, and by electrical loads that demand high power levels, maintaining electrical energy within the quality standards defined by norms becomes increasingly essential. Based on these requirements, the use of equipment based on power electronics to guarantee the perfect functioning of electrical grids has grown sharply, encouraging the study and development of equipment based on these technologies. Combined with these characteristics, the increase in the number of high-powered equipment in industrial parks has resulted in great demands for electrical energy, making it sometimes necessary to increase voltage levels in specific installations. The increment in the electrical current directly affects the installation cost due to an increase in the gauge of the required conductors. However, this solution has as a technological barrier the physical semiconductor switches limit usually used to drive these loads connected directly in power grids with higher voltage levels. In some cases, converters with multiple voltage levels, known in the literature as multilevel converters, are chosen to solve these technological difficulties. These devices are essential for reducing electric currents and improving the current and voltage total harmonic distortion. In this way, the set of available solutions considerably expands since several types of multilevel converters with different topologies are better suited to a specific application. Multilevel converters based on H bridge cells known in the literature as CHB (Cascaded H bridge Converter) are the most outstanding among this category. This converter has several structural advantages, such as modularity and moderate growth of its components as the number of voltage levels increases, presenting a more modest and attractive construction cost. Due to its characteristics and the fact that it is very present in the literature, CHB is considered an excellent option for several types of equipment in power electronics at medium voltage applications. However, the natural switching of the multilevel CHB converter in specific configurations, such as a back-to-back connection (CHB-B2B), presents several short-circuit states, making its performance unfeasible or limiting. This issue may require additional stages of isolation, increasing its implementation cost and reducing its competitiveness. Under these circumstances, this work proposes a new multilevel converter topology based on H bridge cells, without isolation stages, with three-phase characteristics and superiority in some metrics compared to a CHB of the same specifications. It also has a lower number of components, lower construction cost, and similar performance.
This proposal mainly differs from the classic CHB. It presents a three-phase structure, with sharing continuous voltage link capacitors (DC-link) between the phases, instead of one DClink per module per phase, present in the CHB, which gives it monophasic characteristics. This novel proposed topology, named SDC-CHB (Cascaded H Bridge Converter with Single DC-link), also presents several short-circuit states as well as CHB. However, the converter
operation has defined switching states, being a characteristic of power electronics problems. Thus, the semiconductor switches operating mode in this new topology also presents wellmapped and well-known short-circuit stages, becoming an attractive branch for model-based predictive control (MPC) application. In summary, the MPC of the converter to be controlled predicts the state of the target variable and adopts the best switching set necessary to produce the result closest to the desired one. This control acting differently from conventional switching strategies and enabling the inhibition of the short-circuit stages inherent in the SDC-CHB topology. This work is also dedicated to the mathematic study and the mapping of the short circuit states of the new topology proposed in a power electronics application such as STATCOM and
comparing its performance with this device using a CHB converter with similar characteristics. This topology was subjected to simulations in Simulink Matlab® software for data analysis and later implemented on a hardware-in-the-loop (HIL) real-time platform from the manufacturer OPAL-RT, model 5700, to prove its applicability and to validate the proposal. To analyze the efficiency of the converter, measurements of losses by conductivity and switching were carried out using the PLECS® Plexim software, WHERE the energy consumption of the converter in different modes of operation can be observed. Finally, future work and improvements to be developed for this topology are proposed to improve knowledge and expand its applications in power electronics devices.
Keywords: Multilevel Converters; Cascaded H Bridge Converter (CHB); Cascaded H Bridge Converter with Single DC-link (SDC-CHB); Model-based Predictive Control (MPC); Simulink Matlab; Hardware-in-the-Loop (HIL); OPAL-RT.
