Parallel NPC 3-Level Inverter Without DC Midpoint Interconnection
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Solution Overview
Problem
Existing 3-level inverters with parallel converters require DC midpoint interconnection, which is undesirable due to increased complexity, cost, and the need for over-current protection, as conventional control methods unbalance neighboring midpoints when the DC midpoints are disconnected.
Innovation Solution
A parallel 3-level inverter design without DC midpoint interconnection, where each converter's midpoint is disconnected, and a method involving determining average DC bus half voltages and modulation indices to achieve equal upper and lower DC bus half voltages, enabling zero-sequence control for global balancing without physical midpoint connections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If DC midpoint interconnection is implemented for parallel 3-level inverters, then control and balancing of DC midpoint can be achieved, but system complexity and cabling requirements increase
Solution Approach 1:
The patent extracts the DC midpoint interconnection from the parallel inverter system, allowing each converter to operate with its own isolated DC midpoint. This eliminates the need for complex interconnecting cables and connectors while maintaining reliable control through alternative balancing methods that monitor and adjust each converter's midpoint voltage independently based on AC output voltages.
2Reliability
If DC midpoint interconnection is implemented, then DC midpoint voltage balancing can be maintained, but additional connectors and cables are required between converters
Solution Approach 1:
The patent removes the physical DC midpoint interconnection between converters, eliminating the need for additional connectors and cables. Each converter's DC midpoint is kept isolated, and voltage balancing is achieved through control methods that adjust PWM switching based on AC output voltage measurements, thereby simplifying the physical installation and manufacturing process.
3Ease of operation
If conventional control methods are used with disconnected DC midpoints, then each converter can operate independently, but neighboring midpoints become unbalanced
Solution Approach 1:
The patent implements a feedback control mechanism where each converter monitors its own DC midpoint voltage and adjusts its PWM switching strategy based on measurements of AC output voltages. This closed-loop control enables independent converter operation while maintaining DC midpoint voltage balance, as each converter self-regulates based on its own operating conditions and feedback from the AC side.
Solution Approach 2:
The patent changes the control parameters by using AC output voltage measurements to infer and control DC midpoint voltages, rather than directly measuring or interconnecting DC midpoints. This parameter transformation allows independent converter operation while maintaining balancing, as the control system adjusts switching duty cycles based on AC voltage feedback to keep DC midpoints balanced without physical interconnection.
Data Source
AI summary
Embodiments of the present disclosure relate to parallel 3-level inverter without midpoint connection. The parallel 3-level inverter includes a plurality of parallel converters coupled in parallel between a common DC bus and a common AC output, wherein each of the plurality of parallel converters comprises a midpoint and the midpoints of the plurality of parallel converters are disconnected from each other.