MMC Capacitor Pre-Charging With Harmonic Filter Synchronization
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Solution Overview
Problem
Modular Multilevel Converters (MMCs) in wind turbine generators face challenges in synchronizing and pre-charging capacitors, leading to inrush currents and potential damage to harmonic filters and other system components during power grid connections.
Innovation Solution
A method involving a controller-driven schema that initializes contactors to connect the power grid to a harmonic filter and MMC, pre-charges cell capacitors in an un-driven stage, and then boosts the charge level in a driven stage by progressively reducing the number of capacitors bypassed, using pre-charge resistors and forced bypass modes to synchronize the harmonic filter with the grid voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If direct connection is made between power grid and MMC without pre-charging, then connection speed is improved, but inrush currents damage harmonic filters and system components
Solution Approach 1:
The patent applies preliminary action by implementing a pre-charging stage before the main connection operation. The MMC capacitors are pre-charged through a pre-charging contactor and resistor network to reach a voltage threshold (e.g., 300V) before the transmission circuit breaker closes. This preliminary charging action prevents inrush currents during subsequent grid connection while maintaining relatively fast connection speed through automated control.
Solution Approach 2:
The patent uses pre-charging resistors as intermediary components between the power grid and MMC capacitors. These resistors limit the charging current during the pre-charging phase, acting as a mediator that protects the system from inrush currents. The resistors are temporarily engaged through pre-charging contactors and then disconnected once capacitors are sufficiently charged, enabling safe main connection.
2Reliability
If pre-charging resistors are used to limit inrush currents, then component protection is improved, but connection time increases due to additional pre-charging stage
Solution Approach 1:
The pre-charging stage performs preliminary action by charging capacitors to a threshold voltage before main connection. This preliminary charging protects components from inrush currents while minimizing time loss through automated threshold detection and rapid transition to main connection upon threshold achievement.
Solution Approach 2:
The patent dynamically changes system parameters during the connection process. The pre-charging resistors are engaged only when needed (when capacitor voltage is below threshold), and the transmission circuit breaker is closed automatically once the voltage threshold is reached. This parameter change approach balances protection needs with connection speed requirements.
3Device complexity
If all cell capacitors are charged simultaneously, then charging simplicity is improved, but voltage distribution uniformity deteriorates due to capacitor tolerance variations
Solution Approach 1:
The patent segments the capacitor charging process into multiple phases: un-driven stage where capacitors charge naturally through diodes, and driven stage where specific capacitors are selectively charged by bypassing others. This segmentation allows progressive voltage equalization across all cell capacitors, achieving uniform voltage distribution (e.g., within 5% tolerance) while managing complexity through structured control stages.
Solution Approach 2:
The patent implements dynamic charging control where the charging path for each capacitor is adjusted based on real-time voltage measurements. During the driven stage, the controller dynamically selects which capacitors to charge and which to bypass, adapting the charging strategy to achieve uniform voltage distribution across all capacitors despite manufacturing tolerances and aging effects.
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 prevents inrush currents, extends the lifespan of filter components, and ensures smooth power transmission by gradually charging capacitors to the operational voltage, thereby enhancing the reliability and efficiency of the power conversion and transmission system.
Implementation Method 1
closing a pre-charging contactor disposed on a second path, wherein the pre-charging contactor is disposed between the power grid and the converter system, wherein the second path includes a set of pre-charge resistors
Data Source
AI summary
Synchronizing a grid side harmonic filter and pre-charging cell capacitors in modular multilevel converters by: opening a transmission circuit breaker disposed on a first path between a power grid and a converter system; closing a pre-charging contactor disposed on a second path between the power grid and the converter system that includes a set of pre-charge resistors; connecting the power grid to the converter system and the harmonic filter over the second path; selectively charging cell capacitors in the converter system until a charge threshold is reached, wherein a smaller subset of the cell capacitors is charged at a given time than in an earlier time, and each cell capacitor is charged to a higher cell voltage than in the earlier time; and closing the transmission circuit breaker and connecting a generator to the converter system via a generator circuit breaker while leaving the pre-charging contactor closed.