In this paper, three generalized configurations of three-phase pulsewidth modulated (PWM) rectifiers are presented. They are composed of cascaded cells, each formed by a three-phase bridge of switching devices and a three-phase transformer to cascade the cells and isolate the load from the power source. One configuration is bidirectional, whereas the other two are unidirectional alternatives that replace power switches for diodes in some legs. The turns ratios of transformers from each cell are designed in order to provide the required gain and number of levels in the inverted multilevel voltage, enhancing power quality while attending requirements for proper operation in each case. Configurations with two and three cells are compared by means of simulations between themselves and with three conventional three-phase PWM rectifiers: the two-level insulated gate bipolar transistor bridge and the multilevel neutral point clamped (NPC) bridge and NPC H-bridge. Many degrees of asymmetry between the transformers in each cell, determined by the turns ratios, are considered to evaluate their impact on the topologies with relation to performance and cost. It is shown that proposed multilevel rectifiers present lower harmonic distortion, semiconductor losses, and switching stress, and some options with intermediate asymmetry are considered as good alternatives to the most asymmetrical cases. Experimental results are also provided for configurations with two cells, to demonstrate their lower harmonic distortion and power losses compared to the conventional isolated two-level PWM rectifier.

Asymmetrical Cascaded Three-Phase AC–DC Converters With Injection Transformers

In this paper, three configurations of three-phase ac-dc cascaded converters are presented. They are referred as basic configurations, and are composed by two three-phase bridges, where the second bridge is cascaded to the first by means of a three-phase injection transformer. Basic configurations have one bidirectional option (composed only by IGBT switches), and two unidirectional ones (composed by IGBT and diodes). Their generalizations, which are composed by an indefinite number of cascaded three-phase bridges, are also proposed. Each additional cascaded bridge is a three-phase IGBT bridge and requires one additional three-phase injection transformer. Transformers turns ratios are calculated in order to maximize the number of levels produced in the ac side of each rectifier, enhancing power quality. Basic configurations are compared by means of simulations between themselves and with the conventional three-phase IGBT bridge, to show that proposed multilevel rectifiers present better performance in terms of ac power quality drew from the ac source, with lower semiconductor losses. Experimental results are also exposed for the basic configurations to validate them and show their feasibility.

Asymmetrical cascaded three-phase AC-DC converters with injection transformers

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).

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

A passive three-phase rectifier circuit equipped with a “Flying Converter Cell” (FCC) active current shaping network, based on the third harmonic injection concept, has been introduced recently. This additional/optional circuit allows the upgrade of a standard B6 diode rectifier to a low harmonic input stage with unity power factor. In order to guarantee properly shaped sinusoidal mains input currents, dedicated current and voltage controllers of the active circuitry must be designed carefully. As the active current injection unit consists of an assembly of three individual converter stages, the design procedure of both current and superimposed voltage controllers results in fairly high complexity. It is, therefore, going to be discussed in detail in this work. Based on the analysis of the rectifier system, an appropriate control concept is developed which is implemented using a digital signal processor. It is furthermore shown that the midpoint voltage of the FCC can advantageously be used for balancing of the dc capacitor stage of the cell without deteriorating total harmonic distortion of mains input currents. Measurement results taken from a 10 kW/10 kHz laboratory prototype finally confirm promising characteristics of the total hybrid system.

Control Concepts for Hybrid Rectifiers Utilizing a Flying Converter Cell Active Current Injection Unit

In order to reduce input current distortions and improve power factor of a passive three-phase diode bridge rectifier, an active add-on-option circuitry (”Flying Converter Cell”) has been introduced recently. A novel voltage balancing concept for such an active three-phase rectifier which is based on the third harmonic injection principle using a Flying Converter Cell (FCC) is proposed in this paper. It is shown that the midpoint voltage of the FCC can advantageously be used for balancing of the DC capacitor voltages without disturbing the input currents. The proposed balancing concept can simply be added to the existing controller structure consisting of current controllers and a voltage controller. Using the proposed active balancing any losses of alternative passive balancing concepts (resistor networks) can be avoided which maximizes the system’s efficiency. The proposed concept is described in a simple mathematical manner, limitations of the concept are derived and basic operation is verified by simulation results. Measurement results taken from a laboratory prototype with an output power of 10 kW finally demonstrate the good balancing characteristic of the proposed concept.

/pdf/dc-voltage-balancing-of-flying-converter-cell-active-8eijd1aa37.pdf

DC Voltage Balancing of Flying Converter Cell Active Rectifier

Many applications require three-phase rectifier circuits with high input current quality (THD i less than 5%) and a power factor above 0.95 which cannot be achieved by application of simple and robust passive rectifier bridges. Active rectifier circuits are able to fulfill these requirements but existing passive rectifier systems can typically not be extended to these topologies. In this work a novel family of three-phase rectifier systems using a “Flying” Converter Cell is proposed which allows the extension of an existing passive rectifier to a low-harmonic rectifier circuit. The operating principle is discussed, design guidelines based on the calculated current stress of the rectifier circuit are derived and possible operating modes are addressed. The proper operation of the proposed topology is finally demonstrated by simulation of a 170kW rectifier.

Active three-phase rectifier system using a “Flying” Converter Cell

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Active three-phase rectifier system using a “Flying” Converter Cell