Capacitor Balancing Circuit Using Spatial Second Derivative Control
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Solution Overview
Problem
Multilevel power inverters face performance issues due to unbalanced capacitor voltages in the DC link capacitor bank, leading to harmonic generation and overvoltage across semiconductor switches, with existing solutions being cost-prohibitive or functionally inadequate.
Innovation Solution
A method and circuit that determine the voltage spatial second derivative of capacitors to decide whether to inject or extract energy, using electronic switching devices and an energy storage element, to balance capacitor voltages within specified tolerance ranges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a specialized multi-secondary winding transformer is used to enforce capacitor voltage balancing, then capacitor voltage balancing is improved, but device complexity and cost increase
Solution Approach 1:
The patent introduces an intermediary control system that uses voltage spatial second derivative calculations to determine energy injection/extraction timing. This control-mediated approach replaces the direct hardware enforcement of complex multi-secondary winding transformers, achieving capacitor balancing through intelligent control rather than complex physical structures.
Solution Approach 2:
The patent replaces the mechanical/electrical complexity of multi-secondary winding transformers with an electronic control system that calculates voltage spatial second derivatives and switches energy storage elements accordingly. This substitution of control algorithms for hardware complexity reduces device complexity while maintaining balancing effectiveness.
2Reliability
If advanced control techniques are applied to load current to manage energy flow, then capacitor voltage balancing is improved, but device complexity and implementation difficulty increase
Solution Approach 1:
The patent changes the control parameter from conventional load current management to voltage spatial second derivative-based energy flow control. By monitoring and responding to the second derivative of capacitor voltages rather than just current magnitudes, the system achieves more precise and simpler control of energy injection/extraction timing, reducing control complexity compared to advanced current management techniques.
Solution Approach 2:
The patent implements a feedback mechanism where the control system continuously monitors capacitor voltages, calculates their spatial second derivatives, and adjusts energy storage element switching based on these calculations. This closed-loop feedback with explicit mathematical criteria simplifies the control logic compared to open-loop or less-defined advanced control techniques.
3Device complexity
If conventional capacitor balancing methods are not used, then device complexity is reduced, but capacitor voltage imbalance occurs leading to harmonics and overvoltage
Solution Approach 1:
The patent applies preliminary action by proactively injecting or extracting energy from capacitors based on predicted voltage imbalance trends. By calculating voltage spatial second derivatives, the system anticipates future voltage deviations and takes corrective energy transfer actions before severe imbalance occurs, preventing harmonics and overvoltage without requiring complex post-correction circuits.
Solution Approach 2:
The patent enables the capacitor bank to self-balance through controlled energy exchange with the energy storage element. The system uses the existing operational energy flow of the power inverter to service its own balancing needs, eliminating the requirement for separate complex balancing circuits while preventing harmful voltage imbalances.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Effectively maintains all DC link capacitor bank voltages within specified tolerance ranges, preventing harmonic generation and overvoltage, while being potentially more cost-effective and functional than existing solutions.
Implementation Method 1
a bank of capacitors (the DC link) coupled to one or more DC voltage inputs is often used to provide the multiple DC voltage sources
Implementation Method 2
injecting energy into the selected one of the capacitors from an energy storage element, or extracting energy from the selected one of the capacitors into the energy storage element
Data Source
AI summary
A method of balancing voltages in a group of capacitors of a power electronic device, such as a multilevel power inverter, includes making a balancing determination regarding whether to (i) inject energy into the selected one of the capacitors from an energy storage element, or (ii) extract energy from the selected one of the capacitors into the energy storage element based on the voltage of a selected one of the capacitors, and either injecting energy into the selected one of the capacitors from the energy storage element, or extracting energy from the selected one of the capacitors into the energy storage element based on the balancing determination. Also, a voltage balancing circuit that implements the method. In one particular implementation, a spatial second derivative algorithm is used. In another particular implementation, a comparison to an average capacitor voltage is used.