MMC Circulating Current Injection for Power Density

Solution Overview

Problem

Modular multilevel converters (MMC) face challenges in minimizing capacitor size and cost due to the need to buffer large power fluctuations, leading to low power density and high costs, especially in high-voltage applications where conventional methods require large capacitances and are inefficient at low phase current frequencies.

Innovation Solution

The proposed solution involves injecting a high-frequency common-mode signal into the circulating current control loop of the MMC controller, allowing for reduced capacitance and increased switching frequency, using semiconductor devices like SiC MOSFETs, which enables faster switching and higher power density without the need for large capacitors.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional MMC control methods are used to buffer power fluctuations, then capacitor voltage balance is maintained, but capacitor size increases and power density decreases

Engineering Contradiction:

Improvecapacitor voltage balanceVSAvoidpower density

Core Design Contradiction:

ReliabilityVSProductivity

Solution Approach 1:

The patent applies dynamics by making the capacitor charging/discharging behavior adaptive through high-frequency switching control. The switching submodules dynamically adjust their operation between charging and discharging states at high frequency (e.g., 10 kHz), allowing the system to respond flexibly to power fluctuations without requiring oversized capacitors. This dynamic control enables capacitors to be sized for average power rather than peak fluctuations.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent implements periodic action through high-frequency switching of the power transistors in the switching submodules. By switching at high frequency (e.g., 10 kHz), the system creates rapid periodic charging and discharging cycles of the capacitors, which distributes the power buffering function over many small cycles rather than requiring large capacitors to handle entire fluctuation periods. This periodic high-frequency switching is the core mechanism that reduces capacitor size while maintaining voltage balance.

Inventive Principle:

Principle #19Periodic action

2Reliability

If large capacitances are used to buffer power fluctuations, then capacitor voltage balance is maintained, but device volume and cost increase

Engineering Contradiction:

Improvecapacitor voltage balanceVSAvoidcapacitor volume

Core Design Contradiction:

ReliabilityVSVolume of moving object

Solution Approach 1:

The patent applies dynamics by making the capacitor charging/discharging behavior adaptive through high-frequency switching control. The switching submodules dynamically adjust their operation between charging and discharging states at high frequency (e.g., 10 kHz), allowing the system to respond flexibly to power fluctuations without requiring oversized capacitors. This dynamic control enables capacitors to be sized for average power rather than peak fluctuations.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent implements periodic action through high-frequency switching of the power transistors in the switching submodules. By switching at high frequency (e.g., 10 kHz), the system creates rapid periodic charging and discharging cycles of the capacitors, which distributes the power buffering function over many small cycles rather than requiring large capacitors to handle entire fluctuation periods. This periodic high-frequency switching is the core mechanism that reduces capacitor size while maintaining voltage balance.

Inventive Principle:

Principle #19Periodic action

3Productivity

If switching frequency is increased to improve power density, then response speed improves, but semiconductor device losses increase

Engineering Contradiction:

Improvepower densityVSAvoidsemiconductor device losses

Core Design Contradiction:

ProductivityVSLoss of energy

Solution Approach 1:

The patent applies parameter changes by transitioning to wide bandgap semiconductor devices (SiC MOSFETs or GaN HEMTs) that fundamentally change the electrical parameters of the switching devices. These materials enable higher switching frequencies (e.g., 10 kHz) with significantly reduced conduction and switching losses compared to conventional silicon devices. The parameter change in semiconductor material physics allows the system to achieve high-frequency operation without prohibitive energy losses.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS10153712B2Circulating current injection control

Publication Date: 2018.12.11 VIRGINIA TECH INTELLECTUAL PROPERTIES INC

AI summary

In one example, a power converter includes a modular multilevel converter (MMC) electrically coupled between a first power system and a second power system. The MMC includes an arrangement of switching submodules, and the switching submodules include an arrangement of switching power transistors and capacitors. The MMC also includes a controller configured to inject a common mode frequency signal into a circulating current control loop. The circulating current control loop is relied upon to reduce at least one low frequency component in power used for charging the capacitors in the switching submodules. By injecting the common mode frequency signal into the circulating current control loop, the switching submodules can be switched at higher frequencies, the capacitances of the capacitors in the MMC can be reduced, and the power density of the MMC can be increased.