Doubly-Fed Induction Machine Power Control for Unbalanced Grids
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Solution Overview
Problem
Existing control methods for doubly-fed induction machines (DFIG) in wind turbines are not optimized to handle unbalanced grid voltages, leading to torque pulsations and increased mechanical wear, and lack dynamic performance for fault-ride-through operations.
Innovation Solution
A direct power control method using active and reactive power in positive and negative sequence systems as independent state variables, with a state controller generating manipulated values for state feedback in the stator frame coordinate system, eliminating the need for additional control loops and allowing separate feedback for different components, enhancing dynamic performance and enabling fault-ride-through operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If common control methods are used for DFIG, then the control structure is simple, but the method cannot handle unbalanced grid voltages effectively, causing torque pulsations and mechanical wear
Solution Approach 1:
The control method segments the power control into positive sequence components and negative sequence components. The active and reactive power are decomposed into Ppos, Qpos (positive sequence) and Pneg, Qneg (negative sequence), allowing independent control of each component. This segmentation enables the controller to separately manage balanced and unbalanced grid conditions, eliminating torque pulsations caused by unbalanced voltages.
2Reliability
If additional control loops are added to handle unbalanced voltages, then the ability to handle unbalanced grid voltages improves, but the device complexity increases
Solution Approach 1:
The control method merges the positive sequence control and negative sequence control into a unified direct power control framework. Both sequence components are controlled simultaneously through a single controller that processes the decomposed power components (Ppos, Qpos, Pneg, Qneg) and generates appropriate rotor voltage commands. This merging achieves fault-ride-through capability without requiring separate additional control loops, thus maintaining simplicity while improving reliability.
3Speed
If traditional vector control is used, then the control structure is straightforward, but the dynamic performance during fault conditions is insufficient
Solution Approach 1:
The control method changes the control parameters from traditional flux-oriented or stator-voltage-oriented variables to direct active and reactive power components in the stator frame coordinate system. By using Ppos, Qpos, Pneg, and Qneg as control variables, the system achieves faster dynamic response during faults. The direct power control approach allows immediate adjustment of power output in response to grid disturbances, enabling effective fault-ride-through operations with improved control dynamics.
Data Source
Figure 1
AI summary
The invention relates to a method to control power output of a doubly-fed induction machine to a grid including the steps of measuring grid voltage and grid current in a three phase coordinate system, transforming grid voltage and grid current into a stator frame coordinate system, decomposing the grid voltage and grid current in the stator frame coordinate system in a positive sequence system and in a negative sequence system, calculating active and reactive power in the positive and negative sequence system, and controlling active and reactive power in the positive and negative sequence system. The object to provide a method to control power output of a doubly-fed induction machine which provides good dynamics and is able to allow fault-ride-through operations when unbalanced grid voltages occurs is solved in that active and reactive power in the positive and negative sequence system are used as independent state variables in a state controller, whereas the state controller generates manipulated values in the positive and negative sequence system separately which are subjected as manipulated state vectors to a state feedback in a stator frame coordinate system without further control loops before the manipulated vector resulting from state feedback is used to set the rotor voltage.