Stephan Kenzelmann

Vector Control of Single-Phase Voltage-Source Converters Based on Fictive-Axis Emulation

This paper presents an alternative way for the current regulation of single-phase voltage-source dc-ac converters in direct-quadrature (dq) synchronous reference frames. In a dq reference frame, ac (time varying) quantities appear as dc (time invariant) ones, allowing the controller to be designed the same as dc-dc converters, presenting infinite control gain at the steady-state operating point to achieve zero steady-state error. The common approach is to create a set of imaginary quantities orthogonal to those of the real single-phase system so as to obtain dc quantities by means of a stationary-frame to rotating-frame transformation. The orthogonal imaginary quantities in common approaches are obtained by phase shifting the real components by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. In the proposed approach of this paper, the orthogonal quantities are generated by an imaginary system called fictive axis, which runs concurrently with the real one. The proposed approach, which is referred to as fictive-axis emulation, effectively improves the poor dynamics of the conventional approaches while not adding excessive complexity to the controller structure.

Multivariable-PI-Based

This paper presents a linear direct-quadrature current control strategy for voltage source converters (VSCs) in a rotating reference frame (RRF). The described method is based on multivariable-proportional-integral (PI) regulators and provides fast dynamics and a zero steady-state error. Contrary to the well-known conventional PI-based control strategies in RRFs, the presented method provides practically decoupled axes with a superior disturbance rejection capability. Moreover, its implementation is relatively simple and does not impose excessive structural complexity compared to its conventional PI-based competitors. The method is applicable to both single- and three-phase systems and also to anisotropic three-phase systems, e.g., synchronous motors with different direct and quadrature impedances driven by VSCs. Implementing a three-phase test system, the performance of the presented method is experimentally evaluated.

dq

In the future, new aspects from decentralized generation using different dc voltage levels are expected to influence the general concept of power exchange. Converter are needed to adapt the voltage between low voltage (LV), medium voltage (MV), and high voltage (HV). In this paper, a galvanic isolated bidirectional dc/dc converter based on the modular multilevel converter is studied, with a large potential for secure and flexible dc power flow control. The use of medium frequency transformation allows savings in copper and iron. A fundamental frequency modulation method is introduced for the presented converter, that enables variable stepup or step-down between primary and secondary dc voltage in discrete steps. The balancing mechanism of the internal power storage components is explained and verified by simulation and experiment.

Current Control of Voltage Source Converters With Superior Axis Decoupling Capability

This paper presents an alternative way for controlling the current of single-phase voltage source dc-ac inverters in a dq synchronous reference frame. In a dq reference frame, ac (time varying) quantities appear as dc (time invariant) quantities allowing the controller to be designed as for dc-dc converters presenting infinite control gain at the steady-state operating point to achieve zero steady state error. The common approach is to create a set of imaginary quantities orthogonal to those of the real single-phase system so as to obtain dc quantities by means of a stationary to rotating frame transformation. The orthogonal imaginary quantities in common approaches are obtained by phase shifting the real components by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. In the proposed approach of this paper, the orthogonal quantities are generated by an imaginary system called Fictive Axis, which runs concurrently with the real one. The proposed approach, which is referred to as Fictive Axis Emulation (FAE), effectively improves the poor dynamics of the conventional approaches while does not add excessive complexity to the controller structure.

Isolated DC/DC Structure Based on Modular Multilevel Converter

This paper presents a bidirectional modular DC/DC converter. It consists of two Modular Multilevel Converters (MMC) linked by a transformer to achieve galvanic isolation. This topology serves as “DC transformer” interfacing low and medium voltage DC grids. By the means of the “front-to-front” connection, the converter is able to handle short circuits without additional protection. A higher operating frequency makes it possible to decrease the size of the transformer, the capacitors and the branch inductances. Two fundamental frequency modulation methods are presented with the aim to reduce the switching frequencies without penalizing the performance of the transformer. The multilevel modulation yields a conventional staircase voltage waveform, whereas the two-level modulation, which is inspired by the conventional double active bridge converter, results in a two-level voltage waveform. A case study of a wind power converter will serve as point of comparison between the two methods. The semiconductor losses are evaluated and the efficiencies for different transformer designs are discussed.

Vector control of single-phase voltage source converters based on Fictive Axis Emulation

The paper introduces an advanced multilevel-based power converter structure for flexible power management of future electricity networks. The structure is then analysed so that appropriate control approaches are proposed in order to overcome technical challenges in power flow control. The validity of the study is finally verified by simulations. The results show that the proposed system control works satisfactorily and thus fulfills expected functionality for the system.

Modular DC/DC converter: Comparison of modulation methods

Summary This paper presents the concept of a multi-cellular power converter structure targeted towards use in future electricity networks. Control of such a converter is challenging because of the distributed energy storage intrinsic to the concept. If the energy flow through the converter is not carefully controlled poor performance and even shutdown of the converter may result. This paper details the operation of such a converter and presents an example of a control scheme which can achieve the desired performance. Validations of the converter concept and the control principles are provided by experimental results at low voltage using a two port prototype.