Neutral-Point Voltage Balance in Three-Level Converters
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Solution Overview
Problem
Existing methods for common-mode voltage reduction in neutral-point-clamped three-level converters are inadequate for full power factor ranges, leading to increased electromagnetic interference and damage to motors due to uncontrolled common-mode currents and voltages.
Innovation Solution
A neutral-point voltage balance control method using large, medium, and zero vector modulation to synthesize a reference voltage vector, determining duty cycles for each vector, and updating switching sequences to reduce common-mode voltage across all power factors, incorporating a PI controller and sector-based analysis to manage neutral-point potential.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional space vector modulation is used in NPC three-level converters, then the converter can operate at unity or high power factor, but common-mode voltage remains high causing electromagnetic interference and motor damage
Solution Approach 1:
The patent applies dynamics by making the small vector selection adaptive to real-time operating conditions. The control method dynamically selects whether to use small vectors based on the instantaneous power factor and neutral-point potential status, transitioning between different control modes (PF1, PF2, PF3) as operating conditions change. This dynamic adaptation enables effective common-mode voltage reduction across the full power factor range from 0.8 leading to 0.8 lagging, rather than being limited to unity or high power factor operation only.
Solution Approach 2:
The patent changes the parameter of vector selection criteria based on operating conditions. By monitoring the power factor and neutral-point potential, the system adjusts which vectors (large, medium, small, or zero) are selected for synthesis. The duty cycle of small vectors is specifically adjusted according to power factor conditions, and the selection of small vectors is modified based on sector and current direction, enabling the system to maintain low common-mode voltage across varying power factor conditions.
2Object-affected harmful factors
If existing common-mode voltage reduction methods are applied, then electromagnetic interference is reduced at unity power factor, but the method fails to maintain effectiveness across full power factor range
Solution Approach 1:
The control method dynamically adapts to different power factor conditions by implementing three distinct control modes (PF1, PF2, PF3) that are activated based on real-time power factor measurement. The system transitions between these modes as operating conditions change, ensuring continuous effectiveness of common-mode voltage reduction across the full operating range from 0.8 leading to 0.8 lagging power factor, rather than being effective only at unity power factor.
Solution Approach 2:
The patent modifies the control parameters (vector selection and duty cycles) based on power factor conditions. The duty cycle of small vectors is adjusted according to power factor, and the selection of small vectors is modified based on sector and current direction. This parameter adaptation enables the system to maintain low common-mode voltage and electromagnetic interference across varying power factor conditions.
3Stability of the object's composition
If neutral-point potential control is not implemented, then the converter operates simply, but low-frequency oscillation of neutral-point potential occurs causing dc unbalance
Solution Approach 1:
The patent implements feedback control by monitoring the neutral-point potential and using this information to adjust the selection and duty cycle of small vectors. The control method calculates the neutral-point potential based on the voltage difference between two dc-link capacitors and uses this feedback to determine when to activate small vectors for potential balance. This feedback mechanism eliminates low-frequency oscillation and prevents dc unbalance without requiring complex additional hardware.
Solution Approach 2:
The control method uses the existing converter components and control structure to achieve neutral-point potential balance. By utilizing the same vector modulation framework already in place for voltage synthesis, the system self-regulates the neutral-point potential through intelligent vector selection and duty cycle adjustment, avoiding the need for separate complex control systems or additional balancing circuits.
Data Source
AI summary
A neutral-point voltage balance control method and system for a three-level converter in a full power factor range. The method includes: using a large, medium, and zero vector modulation method to synthesize a reference voltage vector, and duty cycles of a large vector, a medium vector, and a zero vector; obtaining a voltage difference between two dc-link capacitors of signal acquisition, and the voltage difference as a neutral-point potential of the three-level converter; according to a value relationship between the neutral-point potential of the three-level converter and a specified threshold, selecting a small vector and calculating a duty cycle of the small vector; and updating a duty cycle of each basic vector, and obtaining a switching sequence for controlling a power switch of a three-phase bridge arm. An amplitude of a common-mode voltage of an NPC three-level converter is equal to one sixth of a dc-link voltage.