Switching-Cycle Capacitor Voltage Control for Modular Multi-Level Converters
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Solution Overview
Problem
Modular multi-level converters require large and high-value capacitors to buffer power fluctuations, especially at low frequencies, which limits power density and makes it challenging to achieve high torque during AC motor starting.
Innovation Solution
The implementation of switching-cycle capacitor voltage control (SCCVC) decouples capacitor ripple from output line frequency, allowing capacitor current control at the module switching frequency, and utilizes additional switching states to charge capacitors to an offset voltage, reducing capacitor size and value requirements while maintaining ripple specifications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If capacitors are used to buffer power fluctuations at line frequency in normal MMC operation, then power delivery requirements are met, but capacitor size and value become very large especially at low frequencies
Solution Approach 1:
The patent changes the operating frequency parameter from line frequency to switching frequency for capacitor current control. By controlling capacitor current at the higher switching frequency rather than line frequency, the capacitor energy ripple is significantly reduced, allowing much smaller capacitor values to achieve the same ripple specification.
Solution Approach 2:
The patent segments the capacitor current control from the output line frequency control. By decoupling these two functions and controlling capacitor current independently at switching frequency, the capacitor size requirement is reduced while maintaining power delivery stability at variable line frequencies.
2Reliability
If capacitor voltage ripple is reduced by injecting high frequency circulating current, then capacitor energy ripple decreases, but stress on switches increases and converter must be de-rated
Solution Approach 1:
The patent introduces an intermediary control mechanism that uses switching-frequency capacitor current control as an intermediate step between power delivery requirements and capacitor ripple reduction. This intermediary approach achieves ripple reduction without requiring high-frequency circulating current injection that would stress the switches.
3Adaptability or versatility
If output line frequency is reduced to enable AC motor starting from zero Hz, then motor starting capability is improved, but capacitor voltage ripple becomes infinite
Solution Approach 1:
The patent makes the capacitor current control frequency dynamic and independent from the line frequency. By controlling capacitor current at switching frequency regardless of line frequency, the system can operate at zero Hz for motor starting without capacitor voltage ripple becoming infinite, as the capacitor ripple frequency remains tied to switching frequency rather than line frequency.
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
SCCVC effectively reduces capacitor size and value, increases power density, and enables high-torque AC motor starting, independent of line frequency, including operation at zero Hz, by balancing capacitor voltages within each switching cycle.
Implementation Method 1
the capacitors in the respective modules must buffer power fluctuations at line frequency and second order harmonic of the line frequency
Implementation Method 2
two further switching states interleaved alternately between the main switching states during which the capacitor is either bypassed or connected to a voltage to charge the capacitor to an offset voltage value
Data Source
AI summary
In a modular multi-level power converter, additional switching states are interleaved between main switching states that control output voltage or waveform. The additional switching states provide current from a DC-link to charge capacitors in respective modules or cells to an offset voltage from which the capacitor voltages are controlled toward a reference voltage during each switching cycle rather than being allowed to build up over a period of an output waveform of variable line frequency, possibly including zero frequency. Since the switching cycle is much shorter than the duration of a line frequency cycle and the capacitor voltages are balanced during each switching cycle, output voltage ripple can be limited as desired with a capacitor of much smaller value and size than would otherwise be required.