Offshore Wind MMC Frequency Conversion
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Solution Overview
Problem
Existing offshore wind farm systems require large and heavy equipment for low-frequency AC transmission, which is inefficient and costly due to the need for additional frequency conversion at onshore grid connection points.
Innovation Solution
A system utilizing a modular multilevel converter (MMC) to convert variable-frequency electricity from wind turbines to a fixed low frequency, reducing equipment size and weight by using a gearbox to increase rotational speed and an AC-to-AC converter with a shared DC link and cascaded power electronic switching circuits, allowing for efficient electricity collection and transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If low-frequency AC transmission is used in offshore wind farms, then the economic distance of HVAC connections can be extended, but large and heavy equipment is required
Solution Approach 1:
The patent segments the frequency conversion process into multiple stages using a cascaded H-bridge converter structure. Instead of using a single large low-frequency generator, the system divides the conversion into several smaller converter modules that operate at higher frequencies, thereby reducing the size and weight of individual components while achieving the overall frequency conversion goal.
Solution Approach 2:
The patent replaces the traditional mechanical low-frequency generator system with an electrical frequency conversion system using power electronics. The cascaded H-bridge converter uses electrical circuits to perform frequency conversion that would otherwise require large mechanical generators, significantly reducing equipment weight and size while maintaining the ability to transmit power over extended distances.
2Speed
If traditional low-frequency generators are used, then electricity can be generated at low frequency, but the equipment size and weight become unacceptably large
Solution Approach 1:
The patent replaces the mechanical low-frequency generator with an electrical frequency conversion system. The wind turbine generator operates at its optimal higher rotational speed to produce high-frequency electricity, and the cascaded H-bridge converter electronically transforms this to the required low frequency for transmission, eliminating the need for heavy mechanical gear systems or large low-speed generators.
Solution Approach 2:
The patent changes the operating parameters of the system by allowing the generator to operate at higher frequencies than traditional low-frequency generators. The power electronic converter then adjusts the frequency parameter as needed, enabling the generator to be smaller and lighter while still achieving the required low-frequency output for offshore transmission.
3Adaptability or versatility
If frequency conversion equipment is added at onshore grid connection point, then LFAC transmission can be implemented, but device complexity increases
Solution Approach 1:
The patent performs frequency conversion in advance at the offshore wind farm location rather than at the onshore grid connection point. The cascaded H-bridge converter transforms the generator's high-frequency output to the required low frequency before transmission begins, so that the transmitted power already has the correct frequency for direct connection to the onshore grid, eliminating the need for additional frequency conversion equipment at the destination.
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 enables the collection and transmission of electricity at a lower frequency than the utility grid, reducing the size and weight of equipment needed and extending the economic distance of HVAC connections, while maintaining efficient power transmission.
Implementation Method 1
a gearbox configured to mechanically change a variable first rotational speed of a wind turbine into a higher variable second rotational speed
Implementation Method 2
a generator configured to be driven at the variable second rotational speed by an output of the gearbox and to thereby generate electricity at a correspondingly variable first frequency
Implementation Method 3
an AC-to-AC converter which comprises an MMC and which is configured to convert the electricity from the generator into electricity output from the AC-to-AC converter at a fixed low frequency
Data Source
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AI summary
According to one aspect of the teachings herein, a system for obtaining electricity from wind turbines provides advantageous operation with respect to offshore wind turbines where the size and weight of electricity generation and collection equipment are key considerations. The contemplated system includes an apparatus that is configured for collecting wind-generated electricity at a fixed low frequency and at a desired collection voltage, based on the advantageous configuration and use of a modular multilevel converter or MMC.