Three-Level Converter Neutral Point Balancing via Switch Control
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Solution Overview
Problem
Three-level neutral-point-clamped converters require complex and costly LCL filters and separate regulating circuits to balance the neutral DC pole, increasing complexity and expense.
Innovation Solution
Control the AC poles of the converter to only connect with the positive and negative DC poles, eliminating the need for a separate regulating circuit and using simple balancing resistors, and replacing the LCL filter with an L filter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If PWM control method is used to control the ANPC converter, then the desired average voltage can be provided to the AC output, but an LCL filter and complex regulating circuit are required to balance the neutral DC pole potential
Solution Approach 1:
The patent extracts and eliminates the complex LCL filter and neutral point potential regulating circuit from the system by adopting a different control strategy that inherently balances the neutral point without requiring these additional components
Solution Approach 2:
Instead of using PWM control with complex filtering and regulation, the patent inverts the approach by using selective switch control where switches are turned on for complete half-cycles or thirds of the AC period, achieving power transfer with simpler L-filter and inherent neutral point balancing
2Power
If the ANPC converter provides a current path to the neutral DC pole, then power transfer is enabled, but the potential of the neutral DC pole may shift requiring separate regulating circuit
Solution Approach 1:
The control method makes the system self-balancing by selectively controlling which switching branches connect to the neutral point during different half-cycles, enabling the neutral point potential to self-regulate without external regulation circuits
Solution Approach 2:
The patent applies periodic switching patterns where switches are controlled for complete half-cycles or thirds of the AC period, creating inherent balancing action that prevents neutral point potential shifts during normal operation
Data Source
Figure 1~2
Figure 3~7
Figure 4a~4b
AI summary
A three-level converter and a method for controlling a three-level converter, wherein the third (S31, S32, S33), the fourth (S41, S42, S43) and the fifth (S51, S52, S53) controllable semiconductor switch of a switching branch having, out of all the switching branches, the most positive voltage in its alternating current pole (AC1, AC2, AC3) is controlled to be non-conductive for the whole period of time when the switching branch in question has the most positive voltage in its alternating current pole, and the first (S11, S12, S13), the second (S21, S22, S23) and the sixth (S61, S62, S63) controllable semiconductor switch of a switching branch having, out of all the switching branches, the most negative voltage in its alternating current pole is controlled to be non-conductive for the whole period of time when the switching branch in question has the most negative voltage in its alternating current pole.