Offshore Wind Farm Frequency Control via Diode Rectifier HVDC
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Solution Overview
Problem
The challenge of replacing the offshore VSC HVDC terminal with a high-voltage diode rectifier in offshore wind farms requires robust control methods to manage active power transfer and maintain grid frequency and voltage magnitude, especially in large-scale systems with hundreds of wind turbines, without relying on communication links or GPS signals.
Innovation Solution
A method and controller for controlling wind turbines via a DC link, using a reference quantity based on the difference between sensed and setpoint grid frequencies to regulate the energy conversion device, allowing indirect control of wind turbines without synchronizing signals, and implementing droop control algorithms to maintain setpoint grid frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of stationary object
If a high-voltage diode rectifier is used to replace the offshore VSC HVDC terminal, then cost, space and weight are reduced, but control complexity increases and communication links are required
Solution Approach 1:
The patent introduces an intermediary control mechanism where the diode rectifier's firing angle is controlled based on DC-link voltage feedback, and the VSC operates in voltage control mode rather than direct power control. This intermediary approach allows the simplified diode rectifier to work effectively without requiring complex communication protocols between offshore and onshore systems.
2Stability of the object's composition
If all wind turbines operate in grid-forming mode to maintain offshore grid frequency, then frequency stability is improved, but control complexity and interdependence increase
Solution Approach 1:
The patent segments the control functions between different components: wind turbines operate in simple grid-following mode, the diode rectifier provides frequency-dependent power modulation through firing angle control, and the onshore VSC handles voltage regulation. This segmentation allows frequency stability to be achieved without requiring all turbines to operate in complex grid-forming mode.
Solution Approach 2:
The offshore AC grid frequency directly modulates the diode rectifier's firing angle through natural frequency-to-voltage conversion, creating a self-regulating mechanism. The system uses the frequency deviation itself as the control signal, eliminating the need for external frequency measurement and complex control algorithms at each turbine.
3Measurement precision
If GPS signals are used for synchronizing wind turbines, then frequency control precision is improved, but system complexity and communication requirements increase
Solution Approach 1:
The system uses the offshore AC grid frequency itself as the reference signal, which is naturally available at each turbine. By using local frequency measurement rather than external GPS synchronization, the system achieves adequate precision without requiring complex communication infrastructure or GPS receivers at each turbine.
Data Source
AI summary
A method of controlling a wind farm including a plurality of wind turbines installed in an AC subgrid, which is connected via a high-voltage rectifier and a DC line to an energy conversion device, the method including: controlling a power infeed of each wind turbine to maintain a setpoint grid frequency of the subgrid; sensing a grid frequency in the subgrid; determining a reference quantity based on a difference of the sensed grid frequency and the setpoint grid frequency; and controlling the energy conversion device in accordance with the determined reference quantity.