Analysis and Investigation of Internal AC Frequency to Minimize AC Current Magnitude and Reactive Power Circulation in Chain-Link Modular Multilevel Direct DC–DC Converters

Abstract

Chain-link modular multilevel direct dc-dc converters (CLMMCs) have attracted much interest recently in for dc power systems because they achieve higher device utilization, lower power losses and they are physically more compact than the alternative front-to-front modular multilevel dcac-dc converters (FFMMCs). The CLMMCs rely on circulating an internal ac current to manage energy balance of the sub-module (SM) stacks but this current inevitably contributes to extra current stresses for circuit components and can also lead to excess reactive power circulation within the converter. This paper presents a circuit analysis that there exists a frequency value for the internal ac components that may minimize the current stresses and avoid excessive reactive power circulation. For illustration, the circuit analysis is applied to one of the base formats, the buckboost CLMMC as an example. The key relationship between the CLMMC and the standard dc-ac modular multilevel converter is explored and established. The equivalent circuits for their dc and ac components with consideration of SM capacitance and SM voltage ripples are created and analyzed in detail, and a full derivation is provided for the specific ac frequency. From this example, this analytical method is extended and applied to other base formats of CLMMCs. The theoretical analysis and results are verified by a set of full-scale simulation examples and down-scaled experiments on a laboratory prototype.

Flexible Interlinking Converter With Enhanced FRT Capability for On-Board DC Microgrids

“…However, these converters suffer from well-known disadvantages regarding the magnetically coupled circuit (i.e., incorporation of a high-frequencythough relatively bulkytransformer which moderates power density and raises saturation issues) and the complexity of the control scheme to achieve soft/zero switching operation. On the other hand, a non-isolated multilevel converter topology [16], [17], such as the MMC, is an alternative attractive solution for high-power conversion in modern DC MGs [18], [19], [20]. The merits of this converter over conventional ones are the inherent redundancy, the current-fed control that facilitates self-balancing of the capacitors, the low device ratings, the very low output harmonic distortion, the scalability and the possibility of common dc-bus configuration for multidrive applications; on the contrary, the practical bottlenecks that may prohibit the use of the MMC for higher voltage and power levels (as reported in [18]) are: the voltage stresses across the flying capacitors that may lead to a bulky capacitor bank, the operational limitations due to the complex charging pattern and the hard switching characteristics (that limit the increase of the switching frequency in practice).…”

Modular Multilevel Converter Impedance Computation Based on Periodic Small-Signal Analysis and Vector Fitting

“…1 is mostly formulated in the abc-frame but some of its sub-systems (mainly controllers, as it happens in a real design) are formulated in the dq0-frame. The resulting model is non-linear and also potentially affected by the negative sequence (consider for instance the presence of possible unbalances in the AC grid) [34], [35]. Therefore, since the model is formulated in a mixture of the abc-and dq0-frames, a periodic steady-state solution is obtained.…”