MMC Cell DC Chopper Control for Safe Capacitor Discharge

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Solution Overview

Problem

Modular Multilevel Converters (MMC) in wind turbine generators face issues with circulating currents, large capacitor voltage ripples, and inefficient heat dissipation, leading to high power losses and increased costs due to the use of large braking resistors and special IGBTs, while passive discharge methods incur constant power loss and hardware faults complicate safe de-energization.

Innovation Solution

Integrate DC choppers within each MMC cell, utilizing controllable switches and dump-load resistors to efficiently discharge capacitors and manage energy dumping, independent of hardware failures, reducing the need for large chopper dump-load resistors and enabling modular replacement of components.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If passive discharge is used with bleed resistors connected across capacitors, then capacitors can be de-energized during maintenance, but constant power loss occurs and additional resistive components are required

Engineering Contradiction:

Improvesafe de-energizationVSAvoidconstant power loss

Core Design Contradiction:

ReliabilityVSLoss of energy

Solution Approach 1:

The MMC cells perform their own discharge function through integrated DC choppers and dump-load resistors, eliminating the need for separate passive bleed resistors. The control system activates discharge only when needed (during faults or maintenance), rather than maintaining constant discharge capability through always-connected resistors.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The discharge system transitions from a static passive configuration (always-connected bleed resistors) to a dynamic active configuration where DC choppers are switched on-demand. The control system dynamically activates discharge paths only when voltage thresholds are exceeded or during maintenance modes, optimizing energy efficiency.

Inventive Principle:

Principle #15Dynamics

2Reliability

If large braking resistors are used to dissipate excess power during grid faults, then over-voltage protection is achieved, but system cost and complexity increase

Engineering Contradiction:

Improveover-voltage protectionVSAvoidsystem complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The discharge function is segmented from the traditional centralized DC-link chopper architecture and distributed to individual MMC cells. Each cell has its own integrated DC chopper and dump-load resistor, allowing localized energy dissipation and eliminating the need for a single large braking resistor at the DC link.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The DC chopper circuit is merged directly into each MMC cell structure, combining the converter function and energy dissipation function in a single integrated module. This eliminates separate braking resistor components and simplifies the overall system architecture.

Inventive Principle:

Principle #5Merging (Combining)

3Stress or pressure

If press-pack IGBTs are used for high voltage applications, then adequate voltage handling is achieved, but system cost increases

Engineering Contradiction:

Improvevoltage handling capabilityVSAvoidsystem cost

Core Design Contradiction:

Stress or pressureVSEase of manufacture

Solution Approach 1:

The high voltage stress is segmented and distributed across multiple series-connected MMC cells rather than requiring a single high-voltage device. Each cell operates at a lower voltage level, allowing the use of more economical IGBT modules instead of expensive press-pack IGBTs.

Inventive Principle:

Principle #1Segmentation

4Loss of energy

If MMC topology is used for efficient power conversion, then converter efficiency is improved, but circulating currents and capacitor voltage ripples increase

Engineering Contradiction:

Improveconverter efficiencyVSAvoidcirculating currents

Core Design Contradiction:

Loss of energyVSObject-generated harmful factors

Solution Approach 1:

The harmful circulating currents and voltage ripples are extracted and redirected through the integrated DC chopper circuits within each MMC cell. The dump-load resistors provide a controlled path for these unwanted currents, converting them into manageable heat dissipation rather than allowing them to circulate and cause losses.

Inventive Principle:

Principle #2Taking out (Extraction)

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

The integrated DC choppers provide efficient energy dumping and capacitor discharge, reducing system costs and complexity, while ensuring safe and space-conscious operation under various conditions, including hardware faults and over-voltage protection.

Implementation Method 1

a switch arranged in series with the resistor, the switch being arranged to selectively switch the resistor into a parallel connection to the energy storage device

Methodology Applied

Scientific EffectJoule heating: Joule Heating

Data Source

PatentEP4014308B1Intelligent discharge control for modular multilevel converter

Publication Date: 2025.10.01 VESTAS WIND SYSTEMS AS

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AI summary

This disclosure proposes procedures and systems for discharging system capacitors and de-energizing power transmission systems having Modular Multilevel Converter (MMC) topologies by intelligent control of MMC cell components including configuration of bypass and insert switches using integrated DC choppers to effectively de-energize MMC cell capacitors and/or DC-link capacitors under operating conditions such as after a normal stop, for protection against over-voltages, dumping turbine energy, and under certain hardware fault conditions.