Abstract
Modular multilevel converters (MMCs) have enabled new and demanding applications for power electronics in the high voltage/high power range. In these applications, very fast and tight control of all variables of the MMC is of prime importance. This article presents a new multivariable control (MVC) concept, which enables independent and extremely fast control of all essential variables. For each variable, a tolerance band can be specified, which is strictly met even under unexpected severe transients and disturbances. The minimized dead times achieved by the control are more than one order of magnitude smaller than the state of the art. The MVC has been implemented based on FPGA-hardware and digital signal processing of the measured values. Experimental results of the control performance and the robustness against parameter variation are presented, using a down scaled MMC with 96 submodules. These results include the analysis of steady-state operation, dynamic performance, and fault scenarios.
Highlights
M ODULAR multilevel converters (MMC) are key components in many existing and emerging high power system applications [1], [2], such as wind or photovoltaic energy conversion systems [3], high voltage direct current transmission systems [4], electric ships [5] and multiterminal dc grids [6], [7]. In all these applications – in particular in possible multiterminal dc grids – MMCs will increasingly have the task to control and to electronically protect the associated ac and dc grids and themselves even during fast transients and unexpected severe fault conditions
This model predictive control (MPC) variant achieves a direct ac current control, the other control variables are still included in the form of contributions to the cost function – after all, the central role of a cost function is what makes up any MPC algorithm
For the strongly increased dynamic challenges tackled by multivariable control (MVC), these MPC algorithms are not appropriate since all what has been stated above about modulation schemes and about MPC applies to them
Summary
M ODULAR multilevel converters (MMC) are key components in many existing and emerging high power system applications [1], [2], such as wind or photovoltaic energy conversion systems [3], high voltage direct current transmission systems [4], electric ships [5] and multiterminal dc grids [6], [7] In all these applications – in particular in possible multiterminal dc grids – MMCs will increasingly have the task to control and to electronically protect the associated ac and dc grids and themselves even during fast transients and unexpected severe fault conditions. In order to enable a fast and tight control of the internal arm energies, the circulating currents (cc) and the common-mode (cm) voltage should be controlled with high bandwidth and minimized delay time.
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