Buck-Boost Converter Cell for MMC Capacitance Reduction
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Solution Overview
Problem
Conventional Modular Multilevel Converter (MMC) cells require excessive cell capacitance due to unused energy storage capacity, leading to bulky and inefficient designs, especially when handling DC voltage ripple in applications like HVDC and STATCOM.
Innovation Solution
Implementing Buck-Boost (BB) operation in converter cells, which allows for efficient use of energy storage capacity by switching between primary and secondary energy storage via semiconductor switches and an inductor, reducing cell capacitance and semiconductor rating while enabling compact designs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional HB or FB cells are used with linear modulation, then the cell voltage ripple is limited to 10% peak-to-peak to ensure linear modulation for all operating points, but this results in 90% of the cell energy storage energy being unused, making the cell unnecessarily bulky
Solution Approach 1:
The patent implements dynamic switching between buck and boost modes based on real-time operating conditions. The converter operates in buck mode during normal linear modulation and switches to boost mode when voltage ripple exceeds thresholds, dynamically adapting the energy storage utilization to actual needs rather than designing for worst-case static conditions
Solution Approach 2:
The patent changes the operating parameters by allowing the cell voltage to exceed the traditional 10% ripple limit through boost mode operation. The control system adjusts the switching duty cycle and mode based on voltage ripple magnitude, enabling parameter changes that optimize energy storage utilization while maintaining reliability
2Volume of stationary object
If the cell capacitance is reduced to utilize more energy storage capacity, then the footprint and capacity of energy storage are substantially reduced, but this requires Buck-Boost operation which increases device complexity
Solution Approach 1:
The patent makes the converter cell multi-functional by integrating both buck and boost capabilities in a single cell design. The same converter topology performs both voltage reduction (buck) and voltage increase (boost) operations by switching between different modes, eliminating the need for separate buck and boost converters and reducing overall system complexity despite the enhanced functionality
3Quantity of substance
If 90% of cell capacitor energy is used for handling DC voltage ripple, then cell capacitor reduction up to 80% can be achieved, but this requires advanced control strategies to manage the increased voltage ripple
Solution Approach 1:
The patent implements feedback control by continuously monitoring the cell voltage ripple and comparing it against predefined thresholds. When the voltage ripple exceeds the threshold, the control system activates boost mode to reduce the ripple. This closed-loop feedback mechanism manages the increased voltage ripple automatically without requiring complex predictive control algorithms
Solution Approach 2:
The converter cell autonomously manages its own voltage ripple through self-regulating buck-boost operation. The control system uses simple threshold-based decisions rather than complex centralized control, allowing each cell to independently adjust its operation based on its own voltage conditions, reducing the overall control complexity
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces cell capacitance by up to 80%, making the converter cells more compact and cost-effective, suitable for high-voltage applications, and eliminates the need for central controllers, enhancing harmonic stability and reducing the footprint of energy storage.
Implementation Method 1
when both the first semiconductor switch and the second semiconductor switch are switched to non-conducting, a current is allowed to flow from the charged inductor to the primary energy storage, charging the primary energy storage
Implementation Method 2
when power is flowing into the cell and the first semiconductor switch is switched to conducting and the second semiconductor switch is switched to non-conducting
Data Source
Figure 1~2
Figure 3a~3c
AI summary
The present disclosure relates to a converter cell (4) for an MMC. The cell comprises a primary energy storage (Cm), an inductor (Lf), and a secondary energy storage (Cf); and first and second converter valves (Τ1, T2). The secondary energy storage (Cf) is connected in series with the first converter valve (T1), and together with said first converter valve in parallel with the inductor (Lf), and the primary energy storage (Cm) is connected in series with the second converter valve (T2), and together with said second converter valve (T2) in parallel with the inductor (Lf).