MMC Cell Bypass Control for Arm-Current Harmonic Suppression
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Solution Overview
Problem
The existing modular multilevel converter (MMC) systems do not effectively suppress harmonic components in the output voltage after a converter cell fails, leading to unbalanced capacitor voltages and potential system shutdown due to energy concentration of harmonic components in remaining cells.
Innovation Solution
A power conversion device with a control system that bypasses a failed converter cell and adjusts the normal cells within the arm to suppress the harmonic component in the arm current, using a control device to manage the ratio of harmonic components and maintain balanced capacitor voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a converter cell is bypassed after failure, then the system continues operating, but harmonic components in arm current increase causing capacitor voltage deviation
Solution Approach 1:
The control device detects harmonic components in the arm current after converter cell failure and dynamically adjusts the PWM reference phase shift of normal converter cells to suppress these harmonics, creating a closed-loop feedback control system that maintains balanced capacitor voltages
Solution Approach 2:
The invention changes the PWM reference phase shift parameter of normal converter cells dynamically after failure detection, adjusting it to compensate for the unequal phase relationships caused by the bypassed failed cell, thereby suppressing harmonic components in arm current
2Object-generated harmful factors
If PWM reference phase is equally shifted for each converter cell, then harmonic components are canceled, but this equality breaks down when a cell fails and output voltage becomes zero
Solution Approach 1:
The invention introduces asymmetric phase shift adjustments among normal converter cells after failure, where each normal cell receives a differentiated phase shift command based on its position and status, breaking the original symmetric phase relationship to compensate for the failed cell's absence and restore harmonic cancellation
Data Source
AI summary
A power converter includes an arm in which a plurality of converter cells are connected in series, each of the converter cells including at least two switching elements, a power storage element, and a pair of output terminals. A control device controls the power converter. The converter cell includes a switch to have the converter cell bypassed. When the control device senses failure of the converter cell, it has the failed converter cell within the arm bypassed and controls a normal converter cell within the arm to suppress a ratio of a harmonic component in an arm current that increases due to the failure.