Modulation for Cascaded Multilevel Converters in PV Applications with High Input Power Imbalance

Cascaded multilevel inverters, such as the cascaded H-Bridge (CHB) converter, are an attractive solution for multi-string photovoltaic (PV) systems, because they enable direct connection to the medium voltage grid and maximum power point tracking of multiple strings. As a challenge of the topology, the operation with high power imbalance in the strings is constrained by the over-modulation. This limitation is analyzed for sinusoidal modulation and the impact on the maximum power imbalance is demonstrated. For increasing the operating range with maximum power tracking in the strings, a discontinuous modulation with extended maximum power imbalance and reduced losses is proposed. The method is analyzed in terms of maximum power imbalance, efficiency and power quality. In addition, the method is validated on an experimental test bench

Thermally-Compensated Modulation Strategies for Modular Power Converters

Power electronics converters are employed in various fields to take the advantages. Especially, a modular structure is getting attention, which exhibits a constitution of multiple basic cells rated for a lower power. Nevertheless, the reliability issue has been intensively argued due to the higher number of power semiconductor devices and capacitors. It has been revealed that the thermal stresses are highly responsible for the major failures of power semiconductor devices. The active thermal control methods have been considered, since they can be simply adopted by a software. However, the existing methods intend to increase the losses for compensating the temperature swing, which implies a limited lifetime improvement and a decreased efficiency. Considering the modular converter consisting of modular inverter and DC/DC converter, two active thermal control methods are proposed: multi-frequency and discontinuous modulations, which aims at controlling a loading power of each module. The DC/DC converters connected in parallel can have a different loading according to their remaining useful lifetime. Remarkably, the discontinuous modulation allows to improve the reliability of both DC/DC converters and inverter. Electrolytic capacitors are degraded due to the thermal stresses, which are determined with ESR and capacitor current. Therefore, the major task is to reduce the capacitor current. The optimal DC-link design approach has been introduced in literature. However, a clear relation between current frequency and thermal stresses has not been investigated. The power converter for deriving the relation is proposed, which injects a designated AC current and DC bias voltage to capacitor under test. The multi-level converter is most suitable with allowing lower THD and lower rated power devices. Also, the proposed control scheme simultaneously regulates two parameters. Finally, the correlation between the current frequency and the thermal stresses is experimentally derived

Improved harmonic performance of cascaded H-Bridge converters with thermal control

Among multilevel converter topologies, the cascaded H-bridge converter (CHB) is one suitable solution for multiple applications such as flexible ac transmission systems and motor drives. CHB presents a natural high modularity because it is formed by the serial connection of H-bridges. This paper deals with a CHB where the cells do not have the same aging because the maintenance during the years of operation forces to replace some damaged cells of the converter with new or repaired ones. A method based on clamping one power cell can be used to reduce the power losses of that cell reducing its temperature and increasing its lifetime. However, clamping one cell of the CHB introduces high harmonic distortion around twice the carrier frequency due to the CHB unbalanced operation when a conventional phase-shifted PWM is applied. A deep harmonic distortion analysis of the CHB output voltage with thermal control based on clamping one cell is presented. Using this analysis, the harmonic distortion at twice the carrier frequency is eliminated applying a non-conventional phase-shifted PWM where the angles between the carriers of consecutive power cells are modified. Experimental results show how the thermal control applying the clamping of a power cell is achieved whilst the harmonic distortion around twice the carrier frequency is eliminated

Sampling-time harmonic control for cascaded H-bridge converters with thermal control

Cascaded H-bridge converter (CHB) is a multilevel topology that is a well-suited solution for multiple applications such as flexible ac transmission systems or motor drives. This paper is focused on a CHB where the cells present an aging mismatch. This can be caused by the maintenance operation which forces the replacement of some damaged cells of the converter with new or repaired ones. In this paper, a new improved approach of the active thermal control (ATC) of the CHB using discontinuous pulsewidth modulation (PWM) (D-PWM) is presented. The D-PWM technique is used to reduce the power losses of one cell reducing its average temperature in order to increase its remaining lifetime. However, the combination of D-PWM with traditional phase-shifted PWM (PS-PWM) introduces high harmonic distortion in the output voltage of the CHB converter at twice the carrier frequency. A detailed harmonic distortion analysis of the CHB output voltage when the D-PWM based ATC is active is presented. From this analysis, a modification of the traditional PS-PWM is derived to eliminate the harmonic distortion at twice the carrier frequency. Experimental results show how the ATC using D-PWM is achieved whereas the harmonic distortion around twice the carrier frequency is eliminated. © 1982-2012 IEEE