Three-Level PFC Voltage Control Without Midpoint Balance
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Solution Overview
Problem
The existing three-level three-phase active PFC circuit control schemes result in significant unbalance of the input current waveform and increased current harmonics due to midpoint balance control of half-bus voltages, leading to instability and power imbalance.
Innovation Solution
A voltage control method that adjusts the half-bus voltages based on their relationship with overvoltage protection values and the required load voltage, compensating the output duty ratio of switches to ensure the first half-bus voltage meets the load demand and stays within protection limits, thereby eliminating the need for midpoint balance control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If midpoint balance control is used to make upper and lower half-bus voltages equal, then voltage balance is improved, but input current waveform unbalance and current harmonics increase
Solution Approach 1:
The patent applies asymmetry by allowing the upper and lower half-bus voltages to have different reference values instead of forcing them to be equal. The control method sets different target voltages for the two half-bus based on their respective overvoltage protection values and load requirements, eliminating the need for midpoint balance control while reducing current harmonics and waveform unbalance.
Solution Approach 2:
The patent changes the voltage reference parameters dynamically by comparing actual half-bus voltages with overvoltage protection values and load requirements. The control method adjusts the reference voltages for upper and lower half-bus independently based on real-time conditions, allowing optimal voltage levels that prevent overvoltage while meeting load demands, thereby reducing current harmonics.
2Shape
If midpoint balance control is implemented, then voltage symmetry is improved, but input current waveform quality deteriorates
Solution Approach 1:
The patent deliberately introduces asymmetry in voltage control by allowing different half-bus voltages to operate at different levels based on their specific requirements. This asymmetric control approach eliminates the harmful waveform unbalance caused by forced symmetry, improving input current waveform quality while maintaining adequate voltage levels for reliable operation.
Solution Approach 2:
The patent discards the conventional midpoint balance control approach that forces voltage symmetry, replacing it with a more effective method that prioritizes waveform quality. By eliminating the restrictive symmetry requirement, the system achieves better input current waveform characteristics and reduced harmonics.
3Stability of the object's composition
If half-bus voltages are controlled to be equal, then voltage balance is improved, but load voltage requirements may not be met
Solution Approach 1:
The patent applies local quality by allowing each half-bus voltage to have different control characteristics suited to its specific load requirements. The control method independently adjusts the reference voltage for each half-bus based on local conditions such as overvoltage protection values and load demands, enabling each half-bus to optimally serve its connected loads while maintaining overall system stability.
Solution Approach 2:
The patent implements dynamic control by continuously comparing actual half-bus voltages with overvoltage protection values and load requirements, and adjusting reference voltages in real-time. This dynamic adaptation allows the system to meet varying load voltage requirements while preventing overvoltage conditions, enhancing both stability and adaptability.
Data Source
AI summary
A voltage control method includes obtaining two half-bus voltages at a direct current side of a three-level converter, overvoltage protection values of the two half-bus voltages, and a load voltage required by a load. One of the two half-bus voltages is configured to supply voltage to the load. The method further includes determining a voltage control strategy based on a relationship among the two half-bus voltages, the overvoltage protection values, and the load voltage. The voltage control strategy compensates for an output duty ratio of a switch of the three-level converter, such that the one of the two half-bus voltages is greater than or equal to the load voltage and each of the two half-bus voltages is smaller than or equal to a corresponding one of the overvoltage protection values. The method also includes performing voltage control on the two half-bus voltages based on the voltage control strategy.