DFIM Speed Control for Integer-Shifted Slot Harmonics
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Solution Overview
Problem
Doubly-fed induction machines (DFIM) in wind turbines generate harmonic and inter-harmonic distortions due to rotor slots, which degrade power quality and require costly mitigation techniques like active cancellation devices to meet grid regulatory requirements.
Innovation Solution
Operate the DFIM at a rated rotational speed where the harmonic order is shifted by an integer number, eliminating inter-harmonic distortions and allowing harmonic distortions to be mitigated with standard methods, thereby reducing the need for additional filtering equipment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If standard harmonic cancellation techniques (slot adjustment, pitch shortening, slot skewing) are applied, then harmonic distortions are reduced, but device complexity and manufacturing costs increase
Solution Approach 1:
The patent changes the operating parameter (rotational speed) of the DFIM to shift harmonic orders. By operating at a specific rated rotational speed where the slip frequency causes harmonic orders to shift by integer values, inter-harmonic distortions are eliminated. This parameter-based solution avoids structural modifications while achieving distortion reduction.
2Object-generated harmful factors
If tuned filters or active inter-harmonic cancellation devices are employed, then inter-harmonic distortions are reduced, but device complexity and operational costs increase
Solution Approach 1:
The patent converts the harmful effect of slot harmonics into a beneficial outcome by exploiting the relationship between rotational speed and harmonic order shifting. By carefully selecting the rated rotational speed, the harmonic distortions that would normally cause inter-harmonics are transformed into integer-order harmonics, which are easier to manage and do not require additional cancellation equipment.
Solution Approach 2:
The patent applies preliminary action by pre-selecting the rated rotational speed during the design phase to ensure that at this specific operating point, the harmonic orders shift by integer values. This preventive measure eliminates inter-harmonic generation before it occurs, avoiding the need for reactive filtering or cancellation devices.
3Object-generated harmful factors
If additional filters or active cancellation systems are installed, then power quality improves, but manufacturing costs and operational expenses increase
Solution Approach 1:
The patent uses parameter change (operating speed selection) as a cost-effective solution to improve power quality. By setting the rated rotational speed such that harmonic shifts are integer values, the system achieves better power quality without requiring expensive additional hardware, thereby maintaining ease of manufacture and reducing both capital and operational costs.
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 effectively reduces inter-harmonic distortions, allowing compliance with grid emission limits at minimal cost by shifting harmonic orders to integer values, thus minimizing the requirement for costly active cancellation devices.
Implementation Method 1
the rated rotational speed of the rotor is set to a value selected such that a shift of the harmonic order of one or more of the slot harmonic distortions at the rated rotational speed of the rotor is an integer number
Data Source
AI summary
A method of controlling the operation of a doubly fed induction machine is provided. The DFIM includes a stator electrically coupled to a power grid and a rotor rotating with a rotational speed. The DFIM has a synchronous rotational speed of the rotor and a rated rotational speed of the rotor. Rotation of the rotor at the synchronous rotational speed generates one or more slot harmonic distortions having a harmonic order. The method comprises operating the DFIM at the rated rotational speed of the rotor, wherein the rated rotational speed of the rotor is set to a value selected such that a shift of the harmonic order of one or more of the slot harmonic distortions at the rated rotational speed of the rotor is an integer number or is within a predefined limit of an integer number.