Three-Level Inverter Control for Neutral-Point Voltage Balance
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Solution Overview
Problem
Three-level inverters in photovoltaic power generation systems face challenges in maintaining neutral-point potential balance due to adverse power grid conditions, particularly the impact of even harmonic currents on bus voltage imbalance.
Innovation Solution
A control method and system for a three-level inverter that uses a balance reference determined by the difference between the absolute values of voltages of positive and negative direct current buses and the even harmonic current in the grid-connected current to generate a current signal for balancing the voltages and suppressing even harmonic currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the three-level inverter is connected to the power grid in a grid-connected scenario, then the photovoltaic power generation system can utilize the power grid, but background harmonic voltage disturbance causes voltage imbalance between positive and negative direct current buses, resulting in neutral point deviation
Solution Approach 1:
The controller continuously monitors the neutral-point potential and grid-connected current, using feedback signals to dynamically adjust the balance reference. The balance reference is determined by combining the neutral-point potential difference with even harmonic current components, creating a closed-loop control system that automatically compensates for voltage imbalance caused by grid harmonic disturbances
Solution Approach 2:
The patent introduces a balance reference as an intermediary control parameter that mediates between the neutral-point potential balance requirement and the grid-connected current. This balance reference combines information from both the DC bus voltage difference and the even harmonic current, serving as a bridge to coordinate the conflicting requirements of maintaining neutral-point balance while operating in adverse grid conditions
2Device complexity
If conventional control methods are used without considering even harmonic current, then the control system is simple, but the even harmonic current cannot be suppressed, leading to persistent voltage imbalance and potential protective shutdown
Solution Approach 1:
The controller performs preliminary analysis of the grid-connected current to identify even harmonic components before they cause severe voltage imbalance. By detecting and compensating for even harmonic currents in advance, the system proactively prevents neutral-point potential deviation, avoiding the need for complex corrective measures and reducing the risk of protective shutdown
Solution Approach 2:
The patent changes the control parameter from simple neutral-point potential difference to a composite balance reference that includes even harmonic current components. This parameter transformation allows the controller to specifically target and suppress even harmonic effects, improving reliability without requiring substantial increases in system complexity
Data Source
AI summary
Example three-level inverters, control methods, and systems are provided. One example three-level inverter includes a first bus capacitor, a second bus capacitor, a power conversion circuit, and a controller. The first bus capacitor is connected in the middle of the current bus and the power conversion circuit. The power conversion circuit is configured to convert a direct current into a three-phase alternating current for output. The controller is configured to determine a balance reference by using a difference between absolute values of voltages of the positive and negative direct current buses and an even harmonic current in a grid-connected current, where the balance reference is used to enable the three-level inverter to generate a current signal for balancing the voltages of the positive and negative direct current buses.