DC Chopper Integrated MMC Cell for Heat Dissipation
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Solution Overview
Problem
Modular Multilevel Converter (MMC) topologies in wind turbine generators face issues with circulating currents, large current ratings, capacitor voltage ripples, balancing capacitor voltages, and high switching losses, leading to increased heat dissipation and costs, with existing heat removal systems being costly and inefficient, and requiring passive discharge methods that incur constant power loss during maintenance.
Innovation Solution
Integrating DC choppers into MMC cells, with each cell having at least two main transistors connected in series between voltage rails and a controllable DC chopper, allowing for improved heat distribution and elimination of large dump resistors and bleed resistors, enabling safer and more efficient energy and heat removal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a braking chopper is connected on the DC-link to dissipate excess power, then excess power can be dissipated into a braking resistor, but the braking resistor becomes quite large and the system requires high voltage ratings
Solution Approach 1:
The patent divides the single large braking chopper into multiple smaller choppers distributed across different MMC cells. Each cell has its own chopper that can dissipate power locally, eliminating the need for one large centralized braking resistor. This segmentation allows power dissipation to be distributed throughout the system rather than concentrated in a single component.
Solution Approach 2:
The patent transitions from a centralized DC-link chopper architecture to a distributed cellular architecture where choppers are embedded within individual MMC cells. This dimensional change from centralized to distributed layout allows each chopper to handle only a fraction of the total power, reducing the size requirements for each individual resistor while maintaining overall power dissipation capability.
2Reliability
If passive discharge is used with bleed resistors during maintenance, then capacitors can be de-energized, but constant power loss occurs
Solution Approach 1:
The patent replaces static passive bleed resistors with dynamic controllable choppers that can be activated or deactivated based on system state. During normal operation, choppers remain inactive. During maintenance or fault conditions, they are dynamically activated to provide active discharge paths, eliminating continuous power loss while ensuring capacitor de-energization when needed.
Solution Approach 2:
The integrated choppers within MMC cells can be controlled to discharge cell capacitors during maintenance procedures. The system uses its own embedded chopper components rather than external passive resistors, allowing the MMC to service itself by routing discharge current through available chopper-resistor combinations within the modular structure.
3Adaptability or versatility
If MMC topology is used for scalability, then converter design becomes simpler, but circulating currents and capacitor voltage ripples increase
Solution Approach 1:
The patent applies local quality by giving each MMC cell its own dedicated chopper and resistor combination, allowing local power dissipation and voltage regulation. This local control capability enables each cell to independently manage its voltage and dissipate circulating currents locally, improving overall voltage stability while maintaining the scalable modular architecture.
4Temperature
If integrated DC choppers are implemented in MMC cells, then heat distribution improves and large dump resistors are eliminated, but device complexity increases
Solution Approach 1:
The patent merges the chopper function with the existing MMC cell structure by integrating chopper switches and resistors directly into the cell modules. This consolidation eliminates the need for separate external braking resistor assemblies and DC-link chopper circuits, reducing overall system complexity despite adding components within each cell. The merged design allows heat to be distributed across multiple locations rather than concentrated in a single external resistor.
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
This solution maintains modularity, reduces the need for additional resistive components, and enhances scaling and chopping ability, providing improved heat distribution and energy dissipation while eliminating the need for large chopper dump resistors and cell bleed resistors, thus reducing power losses and costs.
Implementation Method 1
The power dissipated by the braking resistor is determined by a chopper switch. Because a potentially large amount of power may need to be dissipated, the braking resistor may be quite large
Data Source
AI summary
This disclosure proposes a topology that integrates a DC chopper into the Modular Multilevel Converter (MMC) cells of a power converter. The integrated DC chopper may include chopper resistors that may also be advantageously integrated into a heat sink for a power module including at least the power transistors of the MMC cell. Embodiments herein also describe a method for using an MMC cell's IGBTs and chopper resistors for providing a safe discharge of both cell capacitors and DC-link capacitors in different operating conditions.