Three-Level Inverter Current Balancing via Pulse Delay
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Solution Overview
Problem
Existing three-level inverter systems face challenges in balancing output currents between parallel-operating inverter units, particularly due to the absence of bidirectional switches in conventional designs.
Innovation Solution
Incorporating a bidirectional switch composed of antiparallel-connected IGBTs between the neutral terminal of the DC power supply and the series connection point of the upper and lower arms, along with current detectors and ON signal delay circuits to adjust pulse signals based on output current deviations, ensuring balanced current distribution across inverter units.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional two-level inverter circuits are used without bidirectional switches, then the circuit structure is simpler, but the output current balance between parallel inverter units deteriorates
Solution Approach 1:
The inverter unit is segmented into upper and lower arms with distinct IGBT switches (Q1, Q2 for upper arm; Q3, Q4 for lower arm), allowing independent control of current flow in each arm. This segmentation enables precise control of output current to achieve balance between parallel units while maintaining modular circuit structure.
Solution Approach 2:
The control circuit uses feedback from current detectors to monitor output current and dynamically adjusts the ON/OFF timing of IGBT switches. The feedback mechanism detects current deviations and corrects them by adjusting pulse widths, ensuring balanced output current between parallel inverter units without complicating the overall circuit structure.
2Reliability
If bidirectional switches are added to achieve current balancing, then the output current balance improves, but the device complexity increases
Solution Approach 1:
The bidirectional switch configuration using antiparallel-connected IGBTs (Q3 and Q4) serves multiple functions: it enables current flow in both directions, provides neutral point connection, and facilitates active current balancing control. This multi-functionality achieves superior current balance without proportionally increasing circuit complexity.
Solution Approach 2:
The control system dynamically adjusts the switching timing and pulse widths of IGBTs based on real-time current conditions. By making the control dynamic rather than static, the system achieves adaptive current balancing that responds to varying load conditions without requiring additional hardware complexity.
3Manufacturing precision
If current balancing control is implemented through pulse width adjustment, then the current balance precision improves, but the control complexity increases
Solution Approach 1:
The control circuit pre-adjusts the pulse widths of IGBT switches based on detected current deviations before the imbalance fully develops. By taking preliminary corrective action, the system maintains current balance precision without requiring complex real-time computation or additional control components.
Solution Approach 2:
The control system changes the timing parameters (pulse widths and delays) of switch signals to achieve current balancing. By adjusting temporal parameters rather than hardware configurations, the system achieves precise current control with relatively simple control circuitry.
Data Source
AI summary
In aspects of the invention, each three-level inverter unit has an output current detector. The output from each detector is given to connection wires via a resistor, the connection wires connecting the inverter units. The voltage across the resistor is detected and the deviation, or increment, of the current value of the unit concerned from the average value is determined. The rising up edge of the ON pulses for the IGBT to be controlled is delayed, corresponding to the magnitude of the deviation. Thus, the output current is balanced between the inverter units.