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The modular multilevel cascade converter (MMCC) family has been attracting attention from power electronics and power system engineers because it has been showing considerable promise as one of the next-generation high-power high-voltage/medium-voltage power converters. This paper provides an intensive discussion on power-flow analysis of three MMCC members with star configuration, focusing on dc-mean-voltage balance/regulation and ac-voltage mitigation control of all the floating dc capacitors. Most existing analysis methods are based on single-phase circuits, which make it difficult to derive capacitor voltage regulation/mitigation methods relying only on circulating currents and zero-sequence voltage. The power-flow analysis and regulation/mitigation methods developed in this paper are characterized by applying the αβ0 coordinate transformation(s). The validity of the power analysis and the regulation/mitigation method is confirmed by simulation using the PSCAD/EMTDC software package.

This paper presents a physics-based IGBT model and a junction temperature analysis of parallel-connected IGBTs under PWM operating conditions using a physics-based IGBT model. The authors developed a physics-based IGBT model in which the excess carrier distribution within a drift region is represented using a one-dimentional ambipolar diffusion equation. The physics-based IGBT model makes it possible to predict losses and switching waveforms for converter applications. IGBTs are connected in parallel for medium- or large-capacity converters. In these applications, a transient current imbalance might occur owing to a difference in the wiring inductance or device characteristics between the IGBTs. An experiment shows that a difference in the wiring inductance between the two IGBTs causes a transient current imbalance, and the result is in excellent agreement with the result of a simulation using the physics-based IGBT model. The two parallel-connected IGBTs in this study correspond to a power module for 3.7kW motor drives, and the junction temperatures of both IGBTs are simulated by electro-thermal simulation under the following two conditions: a difference in the wiring inductance and a difference in the device characteristics. The temperature difference between the two IGBTs is approximately 4-7℃ under the applied conditions: a wiring inductance mismatch (20nH) and a threshold voltage mismatch (0.5V). The validity of the physics-based IGBT model is verified, and the model is found to be very useful when designing power converters.

This paper presents a modular multilevel cascade converter based on triple-star bridge-cells (MMCC-TSBC) with a focus on its decoupled control. The TSBC is one of the direct ac-to-ac power converters having the capability of bidirectional power flow with three-phase sinusoidal input (supply-side) and output (motor-side) currents. Therefore, it is suitable for a medium-voltage motor drive requiring regenerative braking. This paper has an intensive discussion on control strategy for the TSBC, which includes current control, capacitor-voltage-balancing control, and fluctuation-mitigating control for low-speed operation. This control strategy succeeds in achieving decoupling among supply (input) currents, motor (output) currents, and circulating currents, using the αβ0 coordinate transformation. The validity and effectiveness of the control strategy are confirmed by theoretical analysis and computer simulation.