Showing papers by "Hans Ertl published in 2015"

Effects of unbalanced mains voltage conditions on three-phase hybrid rectifiers employing third harmonic injection

Abstract: Multitudinous industry applications require single-phase or three-phase rectification circuits. Hybrid rectifiers offer fairly flexible fields of applications (compared to purely implemented passive or active rectifiers) due to their optional active converter topology. The low harmonic input stage, however, is in general exposed to unbalanced AC-side mains voltage conditions. Depending on location and/or facility, input voltage variations can be 3% or higher (short-term). Imbalance in mains input voltages on hybrid rectifiers which are employing the third harmonic injection principle are therefore analysed in detail. It is shown that unbalanced AC-side input voltages not only directly influence THD of input currents and power factor of the system, but also increase current stress of active switches and passive components. Basic considerations are discussed from a mathematical point of view and are finally confirmed by simulation and experimental results of a 10 kW/10 kHz laboratory prototype (passive three-phase diode bridge rectifier equipped with ‘Flying’ Converter Cell (FCC) active current injection cell).

Analysis of a three-phase flying converter cell rectifier operating in light/no-load condition

Abstract: The “Flying” Converter Cell (FCC) rectifier allows the extension of an existing passive diode bridge rectifier to a low-harmonic unity power factor input stage by adding a combination of additional converter topologies to the DC-side of the passive circuit. In general, light-load condition of such an active rectifier, however, may lead to undesired effects as, e.g., impaired total harmonic distortion of input currents (THDi). In this paper the operation of the active rectifier circuit under no-load or light-load conditions and corresponding effects are therefore analyzed in detail also considering the control of the two FCC DC voltages. Three different operating modes are proposed and different parameters and characteristics of these modes are discussed accompanied by simulation results. It is shown that the inductance value of the coupled three-phase current injection choke directly influences the operating behavior and design guidelines for this coupled inductor are therefore derived. This work further comprises a closer look at current control and voltage balancing of FCC DC voltages during light- and no-load condition and resulting optimization issues evoked due to appropriate control of dedicate control structures. Discussed side effects are finally verified by experimental results taken from a laboratory prototype of 10kW output power and 10 kHz switching frequency.

Topology Survey of DC-Side-Enhanced Passive Rectifier Circuits for Low-Harmonic Input Currents and Improved Power Factor

Abstract: Passive diode bridge (B6) rectifiers are widely used in industry for three-phase AC-to-DC conversion due to simplicity of circuit and design, high efficiency as well as robustness and low cost. For applications which require high input current quality (low THDi and high power factor) active three-phase rectifiers have to be used which in general are dedicated systems fully replacing the passive rectifier stage. For specific applications (e.g., AC drives) now a concept is attractive which opens the opportunity that an existing B6 rectifier optionally can be upgraded to high quality mains currents if required. This paper provides a concise topology survey and proposes several new circuits regarding such DC-side located add-on options for standard B6 rectifiers. All proposed topologies (introduced and also new circuits) are based on the third harmonic injection principle. The most attractive concepts are analyzed in detail concerning injection inductor design, DC- link capacitor design and voltage/current stress of the required switching semiconductor devices. Measurements of a laboratory prototype finally illustrate good stationary and dynamic performance of the implemented system.

A passive three-phase rectifier enhanced by a DC-side high switching frequency add-on SiC-converter stage for unity power factor applications

Abstract: Recent developments showed that existing passive three-phase rectifier circuits with DC-side located smoothing inductor can be extended to a low harmonic input stage by adding an additional converter stage on the DC-side of the passive rectifier system. In this paper an implementation of this converter stage using reduced number of semiconductors (only two half-bridges) is proposed. The system shows input current distortions which can be reduced considerably to values below 1% if switching frequencies of 72kHz or above are used. A proper design of the injection chokes is thereto required and several operating modes and optimization possibilities are discussed. Design guidelines are verified by simulation results and a laboratory prototype based on a 10kW/72kHz rectifier system using SiC-MOSFETs.