PWM Cell Switching Control for Modular Multilevel Converter Loss Reduction
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Solution Overview
Problem
Multilevel converters experience significant switching losses due to unnecessary switching around peak voltage levels, which is not effectively addressed by existing control methods like dead-band pulse width modulation (DBPWM), leading to increased frequency and complexity in hardware implementation.
Innovation Solution
The method involves regulating the modulation index above 0.8 using tap-changer control, providing identical but time-delayed carrier waves for each phase leg, and stopping cell switching when waveform values are within a selected interval around peak values, allowing switching only when outside this interval, thereby reducing unnecessary switching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If dead-band pulse width modulation (DBPWM) is used to limit switching at peak values, then switching losses are reduced, but unnecessary switching occurs at areas with steepest slopes leading to increased device complexity
Solution Approach 1:
The patent modifies the carrier wave parameters by adjusting its shape in specific regions. The carrier wave is designed with modified slopes in certain intervals to prevent unnecessary switching events while maintaining effective switching control in other regions, thereby reducing switching losses without complicating the hardware implementation
2Reliability
If conventional DBPWM is applied in cell based converter, then switching at peak values is limited, but a lot of additional switching occurs leading to increased switching frequency
Solution Approach 1:
The patent applies different carrier wave characteristics to different time intervals. In intervals where peak values occur, the carrier wave is modified to prevent unnecessary switching. In other intervals, the carrier wave maintains its standard characteristics to ensure proper switching control. This localized approach reduces overall switching frequency while maintaining reliable switching control where needed
Data Source
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AI summary
The invention concerns the control of cells in a phase leg of a multilevel converter through regulating, using tap-changer control, the modulation index of the converter to above 0.80, providing a group of carrier waves (CAR) for the phase leg, where these carrier waves are identical but displaced in time from each other with a time delay, providing a voltage reference (Vref) for the phase leg being separate from voltage references provided for other phase legs, stop switching of the cells when waveform values of a power transmission converter waveform are detected to be in a selected interval (IV) around a peak value (PV1, PV2) of the power transmission converter waveform (Vref) and control switching of the cells based on comparisons of the respective carrier waves with the voltage reference (Vref) when the waveform values of the power transmission converter waveform are detected to be outside the selected interval (IV).