Multilevel Inverter Balancing Circuit for Low-Ripple DC Link Control
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Solution Overview
Problem
Multi-level inverters face challenges with high cost and voltage unbalances, especially under high reactive loads, due to increased complexity and current ripples in existing balancing methods, which affect the stability and efficiency of power converter systems.
Innovation Solution
A power converter system with a multi-level inverter and a balancing circuit that includes a plurality of switches, a flying capacitor, and an inductor, controlled by a cascaded control loop comprising proportional-integral controllers to maintain balanced DC link capacitor voltages and flying capacitor voltage, reducing current ripples and operational complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If multi-level inverter topology is used, then harmonic content and switching losses are reduced, but voltage unbalance in DC link capacitors increases
Solution Approach 1:
A balancing circuit is introduced as an intermediary component between the DC link capacitors and the inverter stages. This balancing circuit includes switching elements and capacitors that actively transfer charge between unbalanced capacitors, mediating the voltage difference and restoring balance without affecting the main power conversion function.
Solution Approach 2:
The patent implements a control system that continuously monitors the voltages across DC link capacitors and adjusts the switching states of the balancing circuit accordingly. This feedback mechanism detects voltage unbalance and triggers corrective switching actions to maintain equal capacitor voltages throughout operation.
2Stability of the object's composition
If additional hardware is added to balance DC voltages, then voltage unbalance is reduced, but device complexity increases
Solution Approach 1:
The balancing function is segmented into modular switching cells that can be integrated into the existing inverter structure. Each balancing cell consists of minimal components (switches and capacitors) that can be independently controlled, allowing the balancing function to be added without proportionally increasing overall system complexity.
Solution Approach 2:
The balancing circuit components are designed to serve multiple functions: they participate in both the main power conversion operation and the voltage balancing operation. The same switching elements and capacitors used for power inversion also contribute to voltage equalization, eliminating the need for entirely separate dedicated balancing hardware.
3Stability of the object's composition
If known balancer topology is used, then voltage balancing is achieved, but current ripples increase
Solution Approach 1:
The balancing circuit employs dynamic switching strategies where the switching frequency and duty cycles are continuously adjusted based on the instantaneous voltage differences between capacitors. This dynamic operation allows the circuit to achieve voltage balancing while minimizing current ripples by adapting to changing operating conditions rather than using fixed switching patterns.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides a robust, reliable, and cost-effective power converter system with stable current output, reduced oscillations, and efficient operation by optimizing switching states and voltage control, leading to improved performance and reduced harmonic distortion.
Implementation Method 1
an inductor (L) connected between said DC link capacitor mid point and a mid point (7) between the first pair of switches (T1, T2) and second pair of switches (T3, T4)
Implementation Method 2
a flying capacitor (C3) connected between a mid point of the first pair of switches (T1, T2) and a mid point of the second pair of switches (T3, T4)
Data Source
AI summary
A power converter system includes a DC voltage source, a control system, a multilevel inverter and a balancing circuit connected to the DC voltage source, and at least two DC link series connected capacitors connected between a positive and a negative voltage supply line (V+, V−) of the DC voltage source and having a mid-point therebetween. The balancing circuit comprises a plurality of switches that are connected in series between said positive and a negative voltage supply line (V+, V−), the plurality of switches arranged in a first pair of switches and a second pair of switches. A flying capacitor is connected between a mid point of the first pair of switches and a mid point of the second pair of switches, and an inductor is connected between said DC link capacitor mid point and a mid point between the first pair of switches and second pair of switches.