Converter Filter Cell Control With OPP-Based Voltage Balancing
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing control schemes for 3L(A)NPC+AF converters face challenges in balancing active filter cell voltages across a wide range of operating points, leading to oscillations and inefficiencies, particularly at low ratios of filter cell voltage to main stage half DC-link voltage, and require complex computational demands.
Innovation Solution
A method involving offline-computed optimized pulse patterns (OPPs) is used to determine and adjust switching instants, combined with model predictive control, to balance filter cell voltages and maintain flux trajectory, reducing computational load and harmonic distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If common mode injection balancing scheme is used, then filter cell voltage balancing is achieved, but oscillations occur in certain operating points
Solution Approach 1:
The patent implements dynamic selection of balancing schemes based on operating conditions. The control system automatically switches between common mode injection and differential mode injection methods depending on the operating point, preventing oscillations while maintaining effective voltage balancing across all conditions
Solution Approach 2:
The patent changes the control parameters by introducing differential mode injection as an alternative to common mode injection. By adjusting the injection mode parameter based on operating conditions, the system eliminates oscillations while maintaining balancing effectiveness
2Measurement precision
If optimized pulse patterns are modified online, then flux trajectory tracking is improved, but computational demand increases
Solution Approach 1:
The patent pre-calculates and stores optimized pulse patterns in lookup tables before operation. During real-time control, the system retrieves pre-computed patterns and applies minimal online modifications, significantly reducing computational demand while maintaining flux trajectory accuracy
Solution Approach 2:
The patent uses pre-computed optimized pulse patterns as templates that are stored in lookup tables. The control system copies these pre-optimized patterns and applies them with minimal adjustments, avoiding the need for complex real-time optimization calculations
3Ease of manufacture
If filter cell voltage ratio to DC-link voltage is low, then converter cost is reduced, but balancing control becomes difficult
Solution Approach 1:
The patent introduces an intermediary balancing control mechanism that actively manages filter cell voltages. By using dedicated balancing circuits and control algorithms, the system maintains effective voltage balancing even when the filter cell voltage ratio to DC-link voltage is low, enabling cost-effective designs without sacrificing controllability
Data Source
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 main stage (12), 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: selecting and reading an offline-computed optimized pulse pattern (OPPbase) from a database, wherein the selected optimized pulse pattern (OPPbase) comprises switching instants for the main stage (12) and the filter cells (30) over a next computation window (αwin) of a predetermined width (αw); adjusting the optimized pulse pattern (OPPbase) by moving its switching instants such that average output voltages generated in the filter cells (30) are shifted towards an average output voltage reference (Ubal,abc) of the filter cells (30), which is determined from measurements in the filter cells (30), and compensating these adjustments by corresponding modifications of switching instants of the main stage (12). The method comprises further a model predictive control part (46) comprising the following steps, which are performed several times during the next computation window (αwin): modifying the adjusted optimized pulse patterns (66) by moving at least one transition time of a switching instant, such that a flux error determined from a difference between an estimated flux (74) of the electrical converter (10) and a reference flux trajectory (Ψref) is minimized; and applying at least a next switching instant from the modified adjusted pulse pattern to the electrical converter (10).