DC Link Voltage Control for Higher-Order Harmonic Damping
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Solution Overview
Problem
Gearless wind energy conversion systems with permanent magnet synchronous generators experience low frequency oscillations in DC link voltages due to back electromotive force (BEMF) harmonics, which can violate grid standards and cause stability issues, especially when these oscillations coincide with resonance frequencies of the grid impedance.
Innovation Solution
A control method for a converter system that involves determining AC-side currents and voltage references to control AC-to-DC converters, thereby damping higher order harmonics in the average DC link voltage. This is achieved by using a DC link ripple indicator to calculate a converter reference correction, which is then applied to the voltage references to eliminate specific higher order harmonics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If passive filters are added to damp resonance and harmonics, then grid standard compliance and stability are improved, but system cost and device complexity increase
Solution Approach 1:
The patent replaces mechanical/passive filtering components with an active control system that uses power electronic converters and control algorithms to damp harmonics and resonance. The converter system actively compensates for BEMF harmonics through controlled switching and current injection, eliminating the need for passive LC filters while meeting grid standards.
Solution Approach 2:
The patent changes the operating parameters of the converter system, specifically the switching frequency and modulation index, to avoid resonance frequencies of the grid impedance. By dynamically adjusting these parameters, the system prevents harmonic amplification without requiring additional filtering components.
2Stability of the object's composition
If passive filters are added to damp resonance, then stability is improved, but system cost increases
Solution Approach 1:
The patent replaces expensive passive filter components with control algorithms implemented in the converter system. The active damping is achieved through software-based control that injects counter-phase currents to cancel resonant oscillations, eliminating the need for costly passive filter hardware.
Solution Approach 2:
The converter system provides its own stability function by using its existing power electronic components and control infrastructure to actively damp harmonics and resonance. The system serves its own filtering and stabilization needs without requiring external passive filter additions.
3Device complexity
If higher order harmonics are not damped, then device complexity is reduced, but grid standard compliance deteriorates
Solution Approach 1:
The patent segments the control strategy into distinct components: one part handles fundamental frequency conversion while another part specifically targets higher order harmonic damping. This segmentation allows the system to address grid compliance requirements without overwhelming complexity, as each component has a focused function.
Solution Approach 2:
The patent applies partial action by selectively damping only the problematic higher order harmonics that cause grid standard violations, rather than attempting to filter all frequency components. This targeted approach achieves compliance with minimal additional control complexity.
Data Source
AI summary
A converter system includes at least two branches, where each branch includes an AC-to-DC converter and a DC link cascade connected with each other. A method includes: determining at least two AC-side currents, each of which is input into one of the AC-to-DC converters; determining from the AC-side currents, a voltage reference for each of the AC-to-DC converters; determining a DC link ripple indicator; determining a converter reference correction from the DC link ripple indicator, such that a higher order harmonic in the average DC link voltage is damped; determining corrected voltage references for the AC-to-DC converter by adding the converter reference correction to the voltage references of the AC-to-DC converters or by adding the converter reference correction to an average current reference for the AC-to-DC converters; and controlling the AC-to-DC converters with the respective corrected voltage references.