A Rapid DC Component Elimination Strategy for Alternative Current in Frequency Converter

The principle achievement of this contribution is to develop and to describe a rapid DC component elimination strategy for AC currents in the frequency converter, simply by selecting transient period. To avoid DC component on the line current, it’s known that the transition of active and/or reactive power should not be rapid. If the transition time is relatively long compared to the fundamental period, the current may be considered as symmetrical and the DC component can be neglected. In comparison to the usually slow transient that characterizes a DC component free current transient; in this paper much faster transient also without DC component is achieved, simply by choosing a well-defined transition period. The present method is verified for a simple linear circuit. It can also be used, for example, with a three or more level inverter including, but not limited to, a Neutral Point Clamped. Simulation and verification results for different operating points and transitions between them highlight the capabilities of the proposed control strategy. These include the ability to operate with unity power factor and better current quality without continuous component.

Convertisseur de fréquence indirect à rapport de tension fixe

Aziza Benaboud

Many electric power generators use gas turbines as power sources. Typically they are connected to the turbines through a mechanical gearbox in order to adapt their synchronous speed to the optimal rotation speed of the turbine, which is very often much higher than the synchronous speed. However, due to direct network connection, the generator speed cannot be variable : it is imposed by the network and constant. To overcome this problem, we propose to replace the mechanical gearbox by a flexible electronic solution which offers a high efficiency. Using this approach, the turbine is directly connected to the synchronous generator, which is connected to the grid through an indirect static frequency converter with an intermediary DC circuit. However, this type of converter is not common in this application because of very high switching losses due to the high frequency of the PWM technique normally used for its control. In this dissertation, a new control strategy is proposed for the three level Neutral Point Clamped converter, characterized by its high efficiency due to the use of square-wave operation mode. The main advantage of this mode is the quasi absence of switching losses. In this mode, only the frequency can be varied between the input and the output voltage, but their magnitudes are not freely controllable. A voltage magnitude adaptation can be done by the generator's excitation. The produced active and reactive power can be controlled by the generator excitation as well as both the angle shift between the generator and rectifier voltages and between the inverter and network voltages. The capacitive intermediary circuit brings the advantage of decoupling of harmonics between the generator and the network currents. A control method is also proposed to resolve some problems incurred by using square wave operation mode, in particular to reduce the harmonics distortion of the output inverter voltage and current. As second important contribution, this thesis proposes a new fast-ramping DC-component elimination strategy for AC currents. In comparison to the usually slow transient that characterizes a DC component free current transient, we achieve that much faster transients also without DC-component, simply by choosing a well defined transition period. Simulation and experimental results for different operating points and transitions between them highlight the capabilities of the proposed control strategy. These include the ability to operate with unity power factor and better current quality without continuous component and less harmonics.

EPFL2007

Gas Turbine: Optimization of Energy Production and High Efficiency by Using Power Electronics

Aziza Benaboud, Alfred Rufer

The use of gas turbines in the power generation industry has been increasing. Their demand represents more than 50% of the world market of thermal power plants. Electric power generators using gas turbines as power sources are connected to the turbines through a mechanical gearbox, in order to adapt their synchronous speed to the optimal rotation speed of the turbine, which is very often much higher than the synchronous speed. However, due to direct network connection, the generator speed cannot be variable: it is imposed by the network and constant. To overcome this problem, we propose to replace the mechanical gearbox by a flexible electronic solution which offers a high efficiency. Using this approach, the turbine is directly connected to the synchronous generator, which is connected to the grid through an indirect static frequency converter. However, this type of converter is not common in this application because of very high switching losses due to the high frequency of the PWM technique used for its control. In this paper, a new control strategy is proposed for the multi-level converter, characterized by its high efficiency due to the use of square-wave modulation. The main advantage of this mode is the quasi absence of switching losses. In this mode, only the frequency can be varied between the input and the output voltage, but their magnitudes are not freely controllable. A voltage magnitude adaptation can be done by the generator's excitation. The produced active and reactive power can be controlled by the generator excitation as well as both the angle shift between the generator and rectifier voltages and between the inverter and network voltages. Simulation and experimental results for different operating points highlight the capabilities of the proposed control strategy. These include the ability to operate with unity power factor and better current quality. (C) 2016 The Authors. Published by Elsevier Ltd.

Elsevier Science Bv2016

Nouveaux mécanismes de commutation pour des structures dédiées aux convertisseurs de puissance de haute efficacité et interrupteurs du futur

Daniel Siemaszko

Nowadays, the scarcity of cheap energy sources requires a particular effort in optimizing the performance of all conversions. The electric power vector is particulary expected to grow in importance, in conjunction with the development of renewable energy sources. Research in the field of power electronics considers several aspects, including conversion topologies, command strategy as well as the structure and performance of power switches. This work focusses on the power component and the study of dedicated power converting structures. One goal is the use of active switches to replace elements with spontaneous behaviors presenting dissipative phenomena, in order to attempt to reduce switching and conduction losses. Indeed, several works introduce bidirectional monolithic switches adopting new structures, and predict high efficiency components in the near future. On the other hand, the emergence of components without spontaneous properties leads to a redefinition of switching mechanisms in power converters in order to enable the replacement of diodes by active devices. One aspect of this work is the investigation of a close control system including the detection of zero-crossings based on the measurement of both voltage and current in bidirectional switches. A voltage-current representation for the exploitation of experimental results is provided in order to allow for comparison with theoretical behaviors. Several material and internal structure types of components are compared in terms of switching performances. The concept of automatic switching was recently introduced to describe mechanisms based on the detection of non-compliant situations in the switching cell, namely the short-circuit of a voltage source and the opening of a current source. The proposal of applying a new control strategy allows to compensate for the absence of spontaneous extinction. Where sources of a classic switching cell are in a situation of energy exchange or isolation, a third state appears in which source values (converter input/output currents and voltages) are subject to fast changes. The use of passive elements becomes compulsory in order to allow a limitation in the theoretical infinite variations of source values. An intelligent close control system allowing a bidirectional component to switch its state on the basis of measurements of both current and voltage was designed and implemented. An automatic diode with synthesized spontaneous switching functions was studied and experimented for the validation of the proposed self-switching mechanisms model including passive circuits. The use of bidirectional switches allows the extension of conventional converter topologies to new modes of operation. This work presents the modification of a current converter topology traditionally implemented with thyristors, to allow for compensation of reactive power. The spontaneous extinction of thyristors is replaced by a controlled extinction which becomes independent of the polarity of source values. However, the forced opening of the input line inductors current path requires to be absorbed by an input filter whose study is proposed in this work.

EPFL2009