DC-Link Neutral-Point Balancing for Lower Capacitor Stress
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Solution Overview
Problem
Existing power conversion systems face challenges in managing both low-frequency and high-frequency current components on DC-link capacitors, leading to increased stress and reduced efficiency and power density in parallel connected multilevel power converters.
Innovation Solution
A power conversion system with a neutral-point balancer (NPB) circuit and controller that synchronizes the carriers of DC-AC inverters and NPB circuit, reducing current stresses by switching between states to transfer charge, using high-frequency and low-frequency capacitors, and minimizing voltage oscillations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If DC-link capacitors with large capacitance in small volume are used, then power density increases, but capacitor stress from low-frequency current components remains high
Solution Approach 1:
The patent applies local quality by assigning different capacitor types to different frequency ranges. Electrolytic capacitors with high capacitance density are used specifically for low-frequency components, while film capacitors handle high-frequency components. This localized optimization allows each capacitor type to operate in its optimal performance range.
2Object-affected harmful factors
If neutral-point balancer circuit is added to reduce low-frequency current oscillations, then capacitor stress is reduced, but system complexity increases
Solution Approach 1:
The neutral-point balancer (NPB) circuit acts as an intermediary component that specifically targets and reduces low-frequency current oscillations. The NPB is controlled to switch between states in synchronization with the PWM signal, providing targeted intervention without requiring complete system redesign.
3Productivity
If capacitor bank size is reduced to increase power density, then operational efficiency improves, but ability to handle both frequency ranges simultaneously deteriorates
Solution Approach 1:
The capacitor bank is segmented into specialized sub-units for different frequency ranges. This segmentation allows each subset to be optimized for its specific frequency range, maintaining reliable handling of both high and low-frequency components while reducing the total volume compared to using a single large capacitor bank.
Data Source
AI summary
A power conversion system includes a first direct current to alternating current (DC-AC) inverter, a second DC-AC inverter, an inverter controller, a direct current (DC)-link circuit connecting the first DC-AC inverter and the second DC-AC inverter, a neutral-point balancer (NPB), an NPB circuit, and an NPB controller. The inverter controller is configured to provide a pulse-width modulation (PWM), signal to the first DC-AC inverter and the second DC-AC inverter. The NPB circuit is configured to reduce low frequency current oscillations in the DC-link circuit by switching between a first state for transferring charge from a positive voltage line to a neutral voltage line, and a second state for transferring charge from the neutral voltage line to a negative voltage line. The NPB controller is configured to control the switching of the NPB circuit and is synchronized with the PWM signal from the inverter controller.