MMC Pre-Charging via Cell Capacitor Equalization
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Solution Overview
Problem
Modular Multilevel Converters (MMCs) in wind turbine generators face challenges in efficiently pre-charging capacitors, leading to potential power surges and system instability due to the reliance on additional charging components and transformers, which increases operational complexity and points of failure.
Innovation Solution
A novel control schema that uses auxiliary power from the wind turbine generator to pre-charge DC link capacitors, omitting the need for additional charging components by tuning the switching rates of bypass and insert switches to equalize charges in cell capacitors, thereby simplifying the charging process and reducing system complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional pre-charging components (transformers, diode bridges, current limiter resistors) are used to pre-charge DC link capacitors, then the capacitors can be charged to predefined voltage levels, but the system complexity increases and the number of potential failure points increases
Solution Approach 1:
The patent extracts and eliminates the traditional pre-charging components (transformers, diode bridges, current limiter resistors) from the system by using the MMC cell capacitors themselves to perform the pre-charging function. The cell capacitors are charged through bypass switches and then discharge through insert switches to pre-charge the DC link capacitors, removing the need for separate pre-charging equipment and reducing system complexity while maintaining reliability.
2Ease of operation
If additional charging components are added to pre-charge capacitors, then voltage equalization can be achieved, but the operational footprint and deployment footprint increase
Solution Approach 1:
The patent applies multi-functionality by making the MMC cell capacitors serve dual purposes: their primary function of voltage level generation and their secondary function of pre-charging the DC link capacitors. The same cell capacitors that create the multilevel voltage structure are also used to charge the DC link, eliminating the need for separate dedicated pre-charging components and reducing the operational footprint.
3Reliability
If traditional pre-charging schema with multiple components is used, then capacitors can be charged to predefined levels, but the number of points of failure increases requiring more maintenance
Solution Approach 1:
The patent removes multiple potential failure points by eliminating transformers, diode bridges, and current limiter resistors from the pre-charging path. By using only the bypass switches and insert switches that are already part of the MMC structure, along with the cell capacitors, the system reduces the number of components that could fail, thereby improving reliability and reducing maintenance complexity.
4Device complexity
If bypass switches and insert switches are tuned to equalize charges, then pre-charging can be achieved without additional components, but the control complexity increases
Solution Approach 1:
The patent employs feedback control by monitoring the voltage across the DC link capacitors and the charged state of cell capacitors during the pre-charging process. The controller adjusts the switching of bypass and insert switches based on real-time voltage measurements, ensuring that cell capacitors are charged to appropriate levels and that DC link capacitors reach the target voltage, thereby managing control complexity through intelligent regulation.
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 approach reduces operational and deployment footprints, enhances system reliability, and eliminates the need for additional charging components, leading to fewer points of failure and simplified maintenance while ensuring stable voltage equalization across capacitors.
Implementation Method 1
tuning a rate at which individual bypass switches switch to equalize charges in the plurality of cell capacitors as the cell capacitors charge; and tuning a rate at which individual insert switches switch to equalize charges in the plurality of cell capacitors as the cell capacitors discharge
Data Source
AI summary
According to embodiments described herein a Modular Multilevel Converter (MMC) is pre-charged by: driving a bypass current from an auxiliary power source through a plurality of bypass switches included in a corresponding plurality of cells; in response to a summed voltage across a plurality of cell capacitors included in the plurality of cells satisfying a drive threshold, driving an insert current through a plurality of insert switches included in the plurality of cells; and in response to a voltage across a Direct Current (DC) link capacitor satisfying a pre-charge threshold when driving the insert current, opening a circuit breaker connecting the auxiliary power source with the plurality of cells and connecting a generator with external power line rails between which the DC link capacitor is connected.