Investigation and control of a hybrid asymmetric multi-level inverter for medium-voltage applications

Power-electronic inverters are becoming popular for various industrial drives applications. In recent years also high-power and medium-voltage drive applications have been installed. However, the existing solutions suffer from some important drawbacks. Hybrid asymmetric multi-level inverters promise significant improvements for medium-voltage applications. This dissertation investigates such a hybrid inverter. To simplify the topology, some inverter parts are deprived of their feeding from the net and can only supply reactive power. The non-supplied intermediate-circuit capacitor voltages are inherently unstable and require a suitable control method for converter operation, preferably without influence on the load. Apart from normal operation, also converter start-up is an issue to consider, for which it is desirable to limit additional equipment. In this dissertation, we investigate the behaviour of this new inverter, and develop methods to obtain its reliable operation for the considered applications. These methods include modulation, voltage stabilization and start-up. We establish suitable models for their foundation. The principle achievement of this work is the development of a control method to stabilize a multitude of capacitor voltages which have no equilibrium state. Power balancing is performed by varying the common-mode output voltage, using a non-linear model-predictive controller. This method, which is new to power electronics, is applied to our hybrid asymmetric nine-level inverter driving an asynchronous motor. Computer simulations and measurements on an experimental drive system demonstrate stable behaviour in steady-state and during transients over the whole operating range. The obtained results prove the possible implementation of such a complex control algorithm for fast real-time operation. As second important accomplishment, this thesis proposes a start-up method that charges the non-supplied intermediate-circuit capacitors in parallel with the supplied ones, without additional equipment. Measurements show its successful application in the investigated drive system.

Symmetrical multilevel converters with two quadrant DC-DC feeding

In the technology sector of power electronics and control, the multilevel converter technology is still a rather new research area, but the application possibilities in the field of power drives and energy will demand more solutions with this promising technology. In the future, more converter systems will be realized with the multilevel topology. Up to now, multilevel converters have only been used in very particular applications, mainly due to the high costs and complexity of the multilevel converter system. The high costs are due to the fact that the latest technology on semiconductors, magnetic material for inductor and transformer cores and control system technology had to be used. But nowadays new developments in the fields of power semiconductors such as the IGBT, IGCT and perhaps in the future SiC switches as well as improvements of the performance of magnetic cores used in medium frequency transformers will favor the multilevel converters for many other application fields. It can be noted that the industrial trend is moving away from heavy and bulky passive components towards power converter systems using more and more semiconductor elements controlled by powerful processor systems integrating intelligent multi-task control algorithms. The presented work is a contribution to the large field of multilevel converters. It shows a certain kind of multilevel converter in a single phase and a three-phase configuration, called the series-connected four-quadrant converters (SCFQ). The two specialties of the presented converter type are a) that all the multilevel converter steps are fed by an identical DC voltage and b) that every multilevel converter step is realized with an individual AC-DC converter or four-quadrant converter. This type of multilevel converter is called multilevel converter with symmetrical feeding. In this work, a general theoretical development has been done for the use of this multilevel converter type. A special type of DC-DC converter is presented, in order to feed the individual four-quadrant converters of the multilevel converter with a constant DC voltage. All the developments and methods used are based on mathematical expressions. Various simulations using the latest software simulation tools are accomplished and are used to study different cases. The feasibility of the developments is underlined with a series of experimental results with all types of the used converters, which have been realized in the framework of this thesis. The main application for the multilevel converter presented in this work is the frontend power converter in locomotives. Instead of using a heavy low-frequency transformer to reduce the high-voltage from the catenary to a supportable voltage for the semiconductors, a multilevel converter concept is used. The multilevel converter is directly coupled to the catenary. There are many advantages compared to the existing solutions. In the same context, a novel solution of a multilevel converter has been developed for a locomotive usable on different power lines. The converter allows not only the operation on the high AC voltage power line (15kV), but also can be coupled to a medium-voltage DC power line (3kV). Three different configuration types of the locomotive converter have been developed and tested in a complex simulation environment. Besides the locomotive application, there are many more interesting applications for the symmetrical multilevel converter, e.g. in the fields of energy transmission (FACTS, static VAR compensators, electronic high-voltage transformers, etc.) and industrial drives. But certainly in the future with the availability of cheap semiconductors adapted to the needs of the multilevel converter, even more applications in lower power fields will be realized.

EPFL2000

Control of a Hybrid Asymmetric Multilevel Inverter for Competitive Medium-Voltage Industrial Drives

Alfred Rufer, Martin Veenstra

In symmetric multilevel inverters, there is a tradeoff between the output quality and the reliability and efficiency of the converter. New asymmetric and hybrid solutions, using different voltages and devices in various parts of the inverter, promise significant improvements for medium-voltage industrial drives. This paper investigates such a hybrid asymmetric nine-level inverter. It consists of a three-phase three-level integrated gate-commutated thyristor inverter (main inverter), with a two-level insulated-gate bipolar transistor H-bridge (subinverter) in series with each phase. To keep the power part simple and the efficiency high, the subinverters have no feeding from the net and can only supply reactive power. This is a very interesting solution in terms of power quality, efficiency, reliability, and cost. But the nonsupplied intermediate-circuit capacitors form an unstable system. This paper proposes a control method to stabilize their voltages. Power balancing is guaranteed by varying the common-mode voltage, using an online nonlinear model-predictive controller. The controller predicts the system evolution as a function of the control inputs. A cost function of system and control quantities is iteratively minimized in real time, to find the optimal control to apply to the system. Simulations and measurements demonstrate stable behavior in steady state and during transients. Precharging of the nonsupplied capacitors is also an issue to consider. This paper proposes a startup method that charges them in parallel with the supplied ones, without any additional equipment. Measurements show its successful application in the proposed drive system.

2005

Symmetrical multilevel converters with two quadrant DC-DC feeding

EPFL2000