Variable Speed Wind Turbine Grid Interface via Power Converter
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Solution Overview
Problem
Renewable-energy turbines with variable voltage and frequency output pose a challenge for matching with nominally fixed voltage and frequency power grids, requiring efficient conversion methods to maintain optimal energy capture and transmission.
Innovation Solution
A power transmission system comprising renewable-energy turbines connected in parallel to a power converter, with a controller that adjusts stator electrical quantities based on turbine operating speed and fluid flow speed to maintain maximum efficiency, utilizing induction generators and semiconductor power switching devices for bi-directional power flow and grid fault ride-through capabilities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the speed of the renewable-energy turbine is increased to capture more energy, then the power output increases, but the stator voltage and frequency become unsuitable for connection to the fixed voltage and frequency power network
Solution Approach 1:
A power converter is introduced as an intermediary device between the variable-speed renewable-energy turbine and the fixed-frequency power network. The power converter includes a first active rectifier/inverter connected to the turbine and a second active rectifier/inverter connected to the power network, with a DC link between them. This intermediary converts the variable voltage and frequency output from the turbine into fixed voltage and frequency suitable for grid connection, resolving the incompatibility while allowing the turbine to operate at optimal variable speeds for maximum power capture.
2Adaptability or versatility
If a power converter is used to match variable turbine output to fixed grid requirements, then grid compatibility is achieved, but system complexity increases
Solution Approach 1:
The power converter is designed to perform multiple functions within a single integrated system. It simultaneously converts variable voltage to fixed voltage, variable frequency to fixed frequency, provides bidirectional power flow capability, enables grid fault ride-through, and supports both rectifier and inverter operations. This multi-functionality reduces the need for separate dedicated devices for each function, thereby managing system complexity while achieving comprehensive grid compatibility.
3Productivity
If the turbine operates at variable speeds to optimize energy capture, then efficiency is improved, but the equipment placed on the seabed or underwater must handle variable electrical conditions
Solution Approach 1:
The complex power conversion and control equipment is extracted from the underwater environment and placed onshore. Only the turbine assembly and induction generator remain underwater, connected to the onshore power converter via simple transmission cables. The onshore power converter handles all the complex variable-to-fixed conversion, bidirectional power flow control, and grid synchronization functions, significantly simplifying the underwater equipment while maintaining optimal energy capture efficiency through variable-speed turbine operation.
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
The system ensures renewable-energy turbines operate at peak efficiency, minimizing seabed equipment and allowing for stable power transmission to the grid, even during high flow conditions, without the need for pitch control or power shedding, while maintaining rated power generation.
Implementation Method 1
The ac frequency that is developed at the stator terminals of the generator (the 'stator voltage') is directly proportional to the speed of rotation of the rotor
Implementation Method 2
utilizing induction generators and semiconductor power switching devices for bi-directional power flow
Data Source
AI summary
A power transmission system may include a plurality of renewable-energy devices such as wind turbines or subsea turbines. The devices are connected together in parallel to a subsea cable that carries an ac transmission voltage. Each device includes a turbine assembly that is rotated by wind or water current flows, and a variable speed ac induction generator. A power converter is connected to the subsea cable and is used to interface the generators to a supply network or power grid. The power transmission system is operated such that an indicated operating speed of one or more of the devices is used to control the power converter (e.g. the PWM strategy that is used to open and close the power semiconductor devices) to achieve desired stator electrical quantities at each generator.