Three-Level Inverter Modulation for Half-Bus Voltage Imbalance
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Solution Overview
Problem
Three-level inverters experience deviations in output voltage due to current control methods, leading to issues such as voltage and current harmonics when there is a voltage difference between the positive and negative buses.
Innovation Solution
A voltage modulation method for three-level inverters that calculates switching duty cycles based on positive and negative half-bus voltages relative to the bus midpoint, ensuring accurate output voltage by adjusting the operation of bridge arms to maintain consistency with the desired output voltage, even in the presence of bus voltage imbalances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a current control method is used to calculate duty cycle based on bus voltage and desired output voltage, then the inverter can operate with simple control logic, but voltage deviation occurs when there is a voltage difference between positive and negative buses, leading to output voltage inaccuracies and harmonics
Solution Approach 1:
The patent divides the bus voltage into two separate half-bus voltages (positive half-bus voltage and negative half-bus voltage), each controlled independently with its own duty cycle calculation. This segmentation allows the system to account for voltage imbalances between the positive and negative buses, thereby improving output voltage accuracy while maintaining relatively simple control logic for each phase
Solution Approach 2:
The patent changes the control parameter from a single bus voltage to two separate half-bus voltages (Upos and Uneg). By using different voltage parameters for positive and negative half-cycles, the system can compensate for voltage imbalances and achieve more accurate output voltage control without significantly increasing control complexity
2Device complexity
If voltage modulation is performed without considering half-bus voltage imbalances, then the control system remains simple, but harmonics are introduced in the output voltage and current
Solution Approach 1:
The patent segments the modulation process into positive half-cycle modulation and negative half-cycle modulation, each using its own half-bus voltage and duty cycle. This segmentation eliminates the harmonics caused by voltage imbalances while keeping each modulation segment relatively simple, avoiding the need for complex overall control systems
Solution Approach 2:
Instead of trying to balance the bus voltages through complex control, the patent inverts the approach by directly using the actual half-bus voltages (including their imbalances) in the duty cycle calculations. This inversion allows the system to work with the existing voltage conditions rather than attempting to correct them, simplifying the control system while eliminating harmonics
Data Source
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AI summary
Provided are a voltage modulation method and a three-level inverter, belonging to the field of inverter control technologies. The voltage modulation method is applied to a three-level inverter circuit including a neutral line relay. The voltage modulation method includes: determining a desired output voltage for each phase of the three-level inverter circuit; obtaining, in response to the neutral line relay being in an on state, a positive half-bus voltage and a negative half-bus voltage in the three-level inverter circuit; determining, based on the positive half-bus voltage and the desired output voltage, a first switching duty cycle during a positive half-cycle; determining, based on the negative half-bus voltage and the desired output voltage, a second switching duty cycle during a negative half-cycle; and driving, based on the first switching duty cycle and the second switching duty cycle, the three-level inverter circuit to output the desired output voltage. With the voltage modulation method according to the present disclosure, an actual output voltage is not affected by deviation between half-bus voltages. This enables the actual output voltage to be consistent with a desired voltage, thereby improving accuracy of the output voltage of the inverter circuit.