Multilevel Converter Switching Control for Neutral-Point Voltage Swing
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Solution Overview
Problem
Existing 3- or 5-level electrical converters face significant challenges in managing the peak-value swing of neutral-point voltage (VNP) due to unequal loading of partial DC-link capacitors, leading to excessive voltage swings, harmonic distortions, and potential semiconductor damage, especially at high modulation indices.
Innovation Solution
A method and control unit that determine and avoid forbidden states in the state-sequence of the electrical converter, minimizing the difference between the sum of positive and negative partial DC-link voltages by using a state-sequence free from these states, and adjusting dwell-times to reduce VNP swing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If modulation index is increased to boost power density, then converter power density is improved, but peak-value swing of neutral-point voltage becomes excessive
Solution Approach 1:
The control method proactively identifies and avoids forbidden states before they can cause excessive neutral-point voltage swing. By determining the state-sequence in advance and selecting only permitted states, the system prevents voltage swing problems before they occur, enabling high modulation index operation without exceeding voltage swing limits.
Solution Approach 2:
The modulation method incorporates feedback by continuously monitoring the neutral-point voltage and adjusting the state-sequence selection accordingly. The control system uses the actual voltage swing information to adaptively choose from permitted states, ensuring the swing remains within acceptable limits while maintaining high power density operation.
2Ease of operation
If conventional modulation schemes are used, then implementation simplicity is maintained, but ability to reduce voltage swing becomes limited at high modulation indices
Solution Approach 1:
The method segments the state-space into permitted and forbidden states, allowing selective use of only those states that maintain voltage swing within limits. This segmentation enables the control system to manage complexity by dividing the problem into identifiable state categories while maintaining reliable voltage swing control at high modulation indices.
3Volume of moving object
If minimal DC-link capacitance is used to boost power density, then converter size is reduced, but peak-value of voltage swing becomes significant
Solution Approach 1:
By determining and avoiding forbidden states in advance, the method compensates for the reduced capacitance buffering capability. The proactive state selection prevents excessive voltage swing that would otherwise require larger capacitance, enabling compact converter design without sacrificing voltage stability.
4Adaptability or versatility
If forbidden states are not avoided, then state-sequence flexibility is maintained, but semiconductor damage may occur due to excessive voltage swing
Solution Approach 1:
The method converts the constraint of limited permitted states into a benefit by systematically identifying and avoiding forbidden states. This approach transforms what could be seen as a limitation into a protective mechanism that ensures semiconductor safety while maintaining adequate state-sequence flexibility through intelligent selection from permitted states.
Data Source
Figure 1~2
Figure 3~4
Figure 5
AI summary
A method for controlling an electrical converter (10) is described. The electrical converter (10) comprises a DC terminal (12) coupled to a DC device (26), an AC terminal (14) coupled to an AC device (28), and a DC-link (15) having a neutral interconnection point (NP) providing a neutral point voltage (VNP), at least one positive interconnection point (Px) coupled to a positive potential terminal of the DC device (26), and at least one negative interconnection point (Nx) coupled to a negative potential terminal of the DC device (26). The electrical converter (10) is configured for converting a DC-link voltage (VD) supplied by the DC device (26) via the DC terminal (12) into a three-phase AC voltage (vAC) supplied to the AC device (28) via the AC terminal (14) or vice versa, wherein at least a first positive partial DC-link voltage (VD,px) results between the positive interconnection point (Px) and the neutral interconnection point (NP) and at least a first negative partial DC-link voltage (VD,nx) results between the neutral interconnection point (NP) and the negative interconnection point (Nx). The method comprises the steps of: determining reference voltages (va,ref, vb,ref, vc,ref) for the three phases (a, b, c) of the three-phase AC voltage (vAC); determining a state-sequence, wherein the state-sequence is a sequence of states of the three phases (a, b, c) to be applied one after the other at the AC terminal (14) such that the reference voltages (va,ref, vb,ref, vc,ref) are in average generated at the corresponding phases (a, b, c) of the AC terminal (14), wherein the state-sequence is free from any predetermined forbidden states; determining an absolute value of the difference between a sum over all the positive partial DC-link voltages (VD,px) and a sum over all the negative partial DC-link voltages (VD,nx); determining for each state of the determined state-sequence a corresponding dwell-time (di) such that the absolute value of the difference is minimized during one switching cycle of the electrical converter (10); generating a switching signal (SWS) for the electrical converter (10) depending on the determined state-sequence such that each state of the determined state-sequence is applied at the AC terminal (14) for the determined corresponding dwell-time (di); and applying at least a next switching instant to the electrical converter (10) by supplying the switching signal (SWS) to the electrical converter (10).