Cascaded Multicell Converter Research Articles

Coupled Inductor Based Regenerative Cascaded Multicell Converter for Drives With Multilevel Voltage Operation at Both Input and Output Sides

Cascaded multicell converter (CMC) is proved to be an effective solution for medium voltage drives. Three-phase diode bridge rectifiers are normally used in this converter, which do not have regeneration capability. Pulsewidth-modulated (PWM) rectifier-based power cells enable the drives with regeneration capability. However, due to their two-level voltage operation at the input side, an inductor is required for each cell to filter out the switching frequency components from the input current. In this paper, a new configuration with single-phase PWM rectifier-based power cell is proposed to achieve multilevel voltage operation at both the input and output sides of the converter. Therefore, the devices at the rectifier side are also operated at reduced switching frequency without using any filter inductor for each cell. In the proposed converter, simple modular transformers are used instead of a single large and complex transformer that is used in the conventional CMC. Further, input phase currents of the converter are controlled in the synchronously rotating d–q frame, which requires only two current sensors at the input side instead of individual current sensors for each cell. Experimental validation of the proposed converter configuration is carried out on a 4.5-kVA prototype, and the results are presented.

Modular Transformer-Based Regenerative-Cascaded Multicell Converter for Drives With Multilevel Voltage Operation at Both Input and Output Sides

Cascaded multicell converter (CMC) is proved to be an effective solution for medium voltage drives. Three-phase diode bridge rectifiers are normally used in this converter, which do not have regeneration capability. Pulsewidth-modulated (PWM) rectifier-based power cells enable the drives with regeneration capability. However, due to their two-level voltage operation at the input side, an inductor is required for each cell to filter out the switching frequency components from the input current. In this paper, a new configuration with single-phase PWM rectifier-based power cell is proposed to achieve multilevel voltage operation at both the input and output sides of the converter. Therefore, the devices at the rectifier side are also operated at reduced switching frequency without using any filter inductor for each cell. In the proposed converter, simple modular transformers are used instead of a single large and complex transformer that is used in the conventional CMC. Further, input phase currents of the converter are controlled in the synchronously rotating d-q frame, which requires only two current sensors at the input side instead of individual current sensors for each cell. Experimental validation of the proposed converter configuration is carried out on a 4.5-kVA prototype, and the results are presented.

A New Breed of Optimized Symmetrical and Asymmetrical Cascaded Multilevel Power Converters

Multilevel voltage source power converters are the state-of-the-art and key elements for medium-voltage (MV) high-power applications. The cascaded multicell (CM) topologies reach higher output voltage and power levels, and also retain higher reliability due to their modular and fault-tolerant features. This paper initially proposes an optimized topology for symmetrical CM (SCM) multilevel converters. The superiority of the proposed SCM, as compared with the conventional CM converter structure, is that the number of required high-frequency power switches is reduced. Next, a new topology of an asymmetrical CM (ACM) converter, which is formed based on the proposed optimized modules of an SCM converter, is suggested. The advantage of the proposed new ACM converter in comparison with the traditional ACM topology is that the variety of the dc links with different voltage ratings reduces, which makes the proposed topology more modular. The simulation results and the experimental measurements taken from the laboratory prototypes are presented for the proposed converters in order to validate the effectiveness and the advantages of these converters as well as their control strategy.

Control Scheme for an Induction Motor Fed by a Cascade Multicell Converter Under Internal Fault

Cascade multicell (CM) converters are characterized by their high modularity, allowing their power to be easily increased. This also allows any faulty module (cell) to be isolated, so the load can be fed by the remaining operative cells. This leads to operation with an unbalanced number of cells. The use of modified voltage references to avoid the unbalanced operation in steady state has been proposed in the literature (denoted as fundamental phase-shifted compensation). This paper presents the transient operation from normal to faulty operation when such voltages are used and an induction motor is driven by the converter. In the first place, a simple way to include fundamental phase-shifted compensation (FPSC) in closed-loop operation, specifically with a field-oriented control (FOC) scheme, is presented. Then, the use of two supervisor controllers is proposed to modify the FOC speed and flux references. These new references avoid hard torque pulsation while automatically determining the maximum power available from the converter and applying it to the load. Experimental results obtained with the proposed technique in a low-power CM converter are shown for two different cases, constant torque load and fan curve load, showing a significant improvement on the signal behavior.

Extended Operation of Cascade Multicell Converters Under Fault Condition

Multilevel converters are an interesting alternative for high power drives, due to their good quality output signals. Despite their advantages, the large number of components required increases the fault probability. Among the multilevel topologies, the cascade multicell converter presents advantages when operating under internal fault conditions, due to its high modularity. Previous works proposed to compensate the unbalanced operation due to a fault by changing the canonical fundamental output phase shift to precalculated angles, depending on the fault condition. This solution assumes that, if the maximum output phase voltage on each leg is used, the maximum line-to-line voltage will be at a maximum as well. This paper shows how this assumption is not always valid and presents the optimum angles and modulation indexes that must be used in order to obtain the maximum balanced load voltages.

Fault Detection on Multicell Converter Based on Output Voltage Frequency Analysis

Multilevel converters use a large amount of semiconductors, allowing the reconfigurate of the converter to work even on internal fault condition. This paper presents a method to detect faulty cells in a cascaded multicell converter requiring just one voltage measurement per output phase. The method is based on high-frequency harmonic analysis, using a dynamic prediction of their behavior, avoiding erroneous detection on transients while keeping the precision under real fault events. Once the faulty cell is detected, it can be bypassed allowing the converter to keep working according to previously reported techniques. Experimental results confirm accurate and fast fault detection, with a good rejection to normal operation transients.

Cascaded Multilevel Inverter With Regeneration Capability and Reduced Number of Switches

Multilevel converters are a very interesting alternative for medium and high power drives. One of the more flexible topologies of this type is the cascaded multicell converter. This paper proposes the use of a single-phase reduced cell suitable for cascaded multilevel converters. This cell uses a reduced single-phase active rectifier at the input and an H-bridge inverter at the output side. This topology presents a very good performance, effectively controlling the waveform of the input current and of the output voltage and allowing operation in the motoring and regenerative mode. The results presented in this paper confirm that this medium voltage inverter effectively eliminates low frequency input current harmonics at the primary side of the transformer and operates without problems in regenerative mode.