Converter Filter Cell Control for Low-Speed Flux Error Reduction
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Solution Overview
Problem
Existing control schemes for medium voltage electrical converters, such as the 3L(A)NPC+AF topology, face challenges in optimizing hardware design, ensuring reliable operation, and reducing computational demand, particularly at low speed and stand-still operations, while maintaining high power quality and minimizing total harmonic distortion and losses.
Innovation Solution
A method for controlling electrical converters that involves determining main and cell pulse patterns using pulse width modulation, with a pattern determination part that computes pulse patterns online for the next modulation period and a model predictive control part that modifies switching instants to minimize flux errors, while ensuring cell voltage balancing and reducing computational burden.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If optimized pulse patterns are computed offline to cover medium speed to high speed operation, then power quality is improved, but the control scheme cannot cover low speed and stand-still operation
Solution Approach 1:
The control scheme dynamically adapts by switching between two different control modes: model predictive control for low speed and stand-still operation, and optimized pulse patterns for medium to high speed operation. This dynamic adaptation ensures reliable control across the entire operating range without relying on a single static control strategy
Solution Approach 2:
The operating range is segmented into two distinct regions: low speed/stand-still region controlled by model predictive control, and medium/high speed region controlled by optimized pulse patterns. This segmentation allows each control method to be optimized for its specific operating range, improving overall system reliability
2Measurement precision
If model predictive control is applied to modify pulse patterns in real-time, then flux trajectory accuracy is improved, but computational demand increases
Solution Approach 1:
Model predictive control is applied partially - only for low speed and stand-still operation where it provides the most benefit for flux trajectory accuracy. At medium to high speeds, the computationally lighter optimized pulse patterns are used, thus avoiding excessive computational demand while maintaining sufficient performance
Solution Approach 2:
The control parameters and strategy are changed based on operating conditions: model predictive control with its real-time optimization is activated only when needed for low speed operation, while optimized pulse patterns handle medium to high speed operation. This parameter change reduces overall computational complexity while maintaining flux trajectory accuracy where required
3Reliability
If filter cells are directly controlled with floating topology, then cell voltage balancing is improved, but control complexity increases
Solution Approach 1:
The filter cells utilize their own capacitor voltages to contribute to the output voltage, with the model predictive control automatically adjusting switching instants to maintain cell voltage balancing. The control system serves itself by using the existing filter cell structure and voltages without requiring additional balancing circuitry or complex external control mechanisms
Data Source
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AI summary
An electrical converter (10) comprises a main stage (12) adapted for converting a DC voltage into an intermediate voltage comprising at least two voltage levels and a filter cell stage (14) with a filter cell (30) for each phase of the intermediate voltage, each filter cell (30) being adapted for adding or subtracting a cell voltage of the filter cell (30) to the intermediate voltage. A method for controlling an electrical converter (10) comprises a pattern determination part (44) comprising the steps of: determining a main pulse pattern (60) for the main stage (12) with pulse width modulation, wherein the main pulse pattern (60) is determined from a voltage reference signal (Vref,abc) for the output voltage and wherein the main pulse pattern (60) comprises switching instants for the main stage (12) over a next modulation period of the main stage (12); and determining a cell pulse pattern (70) for the filter cell stage (14) with pulse width modulation, wherein the cell pulse pattern (70) is determined from a difference of the voltage reference signal (Vref,abc) and a main stage voltage signal (62) determined from the main pulse pattern (60) and wherein the cell pulse pattern (70) comprises switching instants for the filter cells (30) over the next modulation period. The method comprises further a model predictive control part (46) comprising the following steps, which are performed several times during the next modulation period: modifying the main pulse pattern (60) and the cell pulse pattern (70) by moving at least one transition time of a switching instant, such that a flux error determined from a difference between an estimated flux of the electrical converter and a reference flux trajectory (76) is minimized; and applying at least a next switching instant from the main pulse pattern (60) and the cell pulse pattern (70) to the electrical converter (10).