MMC Cell Bypass Circuit for Lower Conduction Loss

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Solution Overview

Problem

Modular multilevel converters (MMC) face increased conduction loss due to the number of switching elements used, which hinders efficient power conversion.

Innovation Solution

A power converter design that includes a modular multilevel converter with cells connected in series, each equipped with a pair of switching elements and a parallel capacitor, along with a bypass circuit that reroutes current through the bypass circuit when the cell outputs zero voltage, reducing conduction loss by minimizing the number of switching elements involved in current flow.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Stress or pressure

If the number of cells in cascade connection is increased to increase withstand voltage, then the voltage handling capability is improved, but the conduction loss increases due to more switching elements

Engineering Contradiction:

Improvewithstand voltageVSAvoidconduction loss

Core Design Contradiction:

Stress or pressureVSLoss of energy

Solution Approach 1:

The power converter is divided into multiple cells connected in series, where each cell contains switching elements and a bypass circuit. This segmentation allows the system to handle high voltage while providing individual bypass paths that can be activated to reduce conduction loss in specific cells, thus resolving the contradiction between voltage handling capability and energy loss.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention dynamically changes the operational parameters of each cell by controlling the bypass circuits. When a cell is operating at zero voltage output, the bypass circuit is activated to change the current path, effectively reducing the conduction loss without affecting the overall voltage handling capability of the series-connected cell structure.

Inventive Principle:

Principle #35Parameter changes

2Reliability

If bypass circuit is provided for each cell to continue operation during cell failure, then the reliability is improved, but the device complexity increases

Engineering Contradiction:

Improveoperation continuity during cell failureVSAvoidnumber of bypass circuits

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The bypass circuit is designed to serve multiple functions: it acts as a failure bypass path when a cell malfunctions, and it also serves as a loss-reduction path during normal zero-voltage operation. This multi-functionality reduces the need for separate dedicated components, thereby improving reliability without proportionally increasing device complexity.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

Each cell is equipped with its own bypass circuit that can be independently controlled, allowing the cell to self-manage its operational state. When failure or zero-voltage condition is detected, the bypass circuit automatically activates to maintain system operation, reducing the need for complex external control mechanisms.

Inventive Principle:

Principle #25Self-service

Data Source

PatentEP3373435B1Power conversion device

Publication Date: 2023.10.18 MITSUBISHI ELECTRIC CORP

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AI summary

Since a power converter (100) including a modular multilevel converter uses a large number of cells (10) each combining a plurality of switching elements and a DC capacitor, there is a problem of conduction loss due to the switching elements. The conduction loss is reduced by connecting a bypass circuit (20) between terminals of each of the cells (10), controlling to open and close the switching element, and controlling to short-circuit the bypass circuit (20) connected to the cell (10) controlled to output zero voltage.