MMC Standby Charging via Hybrid Cell Subset Segmentation
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Solution Overview
Problem
Modular Multilevel Converters (MMC) experience significant no-load losses and voltage deviations in standby and charging modes due to circulating currents, which are costly and inefficient, especially in applications like Statcoms and rail interties.
Innovation Solution
Implement a hybrid operating mode where converter cells are divided into subsets operating in 'pulse block' and 'bypass' modes, with alternating cell assignments to manage DC voltage deviations without circulating currents, reducing no-load losses and charging voltage requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If circulating currents are used to balance cell capacitor DC voltages in standby mode, then voltage balance is maintained, but no-load losses increase significantly
Solution Approach 1:
The converter branch is segmented into multiple converter cells, and the cells are further divided into subsets (first subset and second subset). This segmentation allows selective control of individual cells, enabling the patent to apply different operating modes to different subsets, thereby balancing voltages without requiring circulating currents through the entire branch.
Solution Approach 2:
Different subsets of converter cells are assigned different operating modes based on their local voltage conditions. The first subset operates in pulse-blocked mode when its cells need charging, while the second subset operates in bypass mode. This local differentiation allows voltage balancing to occur only where needed, eliminating the need for system-wide circulating currents.
2Stability of the object's composition
If all semiconductor switches actively switch to enable current flow for voltage balancing, then voltage balance is achieved, but operational complexity and losses increase
Solution Approach 1:
The converter cells are divided into distinct subsets that can be independently controlled. This segmentation simplifies the control strategy by allowing the controller to manage only the necessary subsets in pulse-blocked mode while keeping other subsets in simpler bypass mode, reducing overall operational complexity.
Solution Approach 2:
The controller periodically monitors cell capacitor DC voltages and alternates between operating modes for different subsets. This periodic action creates a simplified cyclic control pattern that achieves voltage balance without requiring complex continuous control of all switches.
3Speed
If full nominal charging voltage is applied to all cells simultaneously, then charging speed is maximized, but inrush current increases significantly
Solution Approach 1:
The charging process is segmented by dividing cells into subsets that are charged sequentially rather than simultaneously. The first subset is charged in pulse-blocked mode while the second subset operates in bypass mode, preventing the simultaneous inrush current that would occur if all cells were charged at once.
Solution Approach 2:
The charging process uses periodic alternation between operating modes for different subsets. The controller periodically switches subsets between pulse-blocked and bypass modes, creating a staged charging sequence that maintains high charging speed while limiting inrush current through time-based separation.
Data Source
AI summary
The present invention proposes a hybrid converter branch operating mode for a Modular Multilevel power Converter MMC with MMC cells in distinct subsets operating according to a “pulse blocked” cell operation mode with DC cell voltage increase or according to a “bypass” cell operation mode without DC cell voltage increase. Repeated cell subset assignment and corresponding alternation of cell operating mode allows to reduce or at least manage a mean deviation of the cell capacitor DC voltages of the converter cells. The invention also reduces no-load losses of the MMC in standby mode and a charging voltage in an MMC charging mode.