Split DC-Link Multilevel Inverter Control for Capacitor Voltage Balancing
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Solution Overview
Problem
Existing multilevel inverters with split DC links face challenges in voltage balancing, leading to unequal stress on electrical components and increased total harmonic distortion, with existing solutions either increasing costs or exhibiting poor control behavior.
Innovation Solution
Calculating a modulation signal based on the actual power difference between DC link capacitors and superimposing it onto the setpoint value to generate an adapted reference signal for controlling semiconductor switches, which balances DC link capacitor voltages by compensating for power differences before they cause voltage imbalances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If additional hardware is added at the split DC link for voltage balancing, then voltage balancing performance is improved, but inverter costs and losses increase
Solution Approach 1:
The patent replaces physical hardware modifications with a control-based solution. Instead of adding balancing circuits or other hardware components to the split DC link, the invention uses a control method that processes power difference measurements and generates control signals to balance capacitor voltages, thereby substituting a hardware-based approach with a control-based approach that reduces device complexity and costs
Solution Approach 2:
The patent introduces a control unit as an intermediary that measures power differences between DC link capacitors and generates appropriate control signals. This intermediary component coordinates the balancing action without requiring direct hardware modifications to the capacitor bank, enabling voltage balancing through intelligent control rather than physical intervention
2Ease of operation
If proportional gain control is used for voltage balancing, then control implementation is simplified, but control behavior deteriorates with remaining offset error and poor transient response
Solution Approach 1:
The patent implements a feedback control mechanism where the control unit continuously measures the power difference between DC link capacitors and adjusts control signals based on this measurement. The feedback loop monitors the actual power flow and dynamically modifies switching commands to eliminate voltage imbalances, thereby improving transient response and eliminating offset errors while maintaining reasonable implementation complexity
Solution Approach 2:
The control method dynamically adjusts control parameters based on real-time power difference measurements. Rather than using fixed proportional gain, the system adapts its control action according to the current operating conditions and imbalance magnitude, enabling superior transient performance and offset elimination while keeping the control structure implementable
3Reliability
If even harmonic injection is used for voltage balancing, then voltage balancing is achieved, but output waveform quality deteriorates due to increased total harmonic distortion
Solution Approach 1:
The patent extracts and compensates for power differences at their source by measuring and controlling the power flow to each DC link capacitor separately. By addressing the root cause of voltage imbalance through direct power difference measurement and control, the method achieves voltage balancing without introducing harmonic distortion into the output waveform, effectively taking out the balancing function from the output voltage path
Data Source
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AI summary
To improve voltage balancing at the DC link capacitor voltages of a multi-level inverter with a split DC link a modulation signal (MS) with a modulation signal amplitude (AMS) as even harmonic signal of the output voltage (UAC) or output current (iAC) of the inverter (1) is calculated from an actual electric power difference (Pdiff,act) of the actual electric powers at the DC link capacitors (CDC1, CDC2) and is superimposed onto the setpoint value (SP) for setting an output voltage (uAC) or output current (iAC) of the inverter (1).