Three-Level Inverter Midpoint Control With Speed-Based Gain Correction
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Solution Overview
Problem
Three-level inverters face instability due to increasing loop gain with output power, leading to unbalanced DC midpoint voltage, which results in harmonic content at low frequencies and DC drift at high frequencies, affecting power quality and switch stress.
Innovation Solution
A speed-based gain correction is introduced to the DC link midpoint controller, using a regulator transfer function and speed sensing or estimation to adjust zero-sequence voltage, maintaining constant loop gain over power variations and minimizing harmonics at low frequencies while reducing DC drift at high frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If PI regulator with constant gain is used to control DC midpoint voltage, then the control is simple, but the system becomes unstable at high output power due to increasing loop gain
Solution Approach 1:
The patent applies dynamics by making the PI regulator gains variable rather than constant. The loop gain is made dependent on the output power level through a gain scheduling mechanism, where different gain values are selected based on the operating point. This resolves the contradiction by adapting the controller parameters to match the system's changing characteristics across different power levels, maintaining stability while keeping the overall control structure relatively simple.
Solution Approach 2:
The patent changes the parameters of the PI regulator (specifically the proportional and integral gains) based on the operating conditions. By adjusting these parameters according to the output power level, the controller maintains optimal performance across the full operating range. This parameter adaptation resolves the stability issue at high power while preserving the simplicity of the PI control structure.
2Measurement precision
If high loop gain is used to improve DC midpoint voltage control accuracy, then voltage balance is improved, but harmonic content increases at low frequencies
Solution Approach 1:
The patent uses dynamic gain adjustment where the controller gain varies with the output power level. At low power levels, the gain is reduced to minimize harmonic content, while at high power levels, the gain is increased to improve voltage control accuracy. This dynamic adaptation resolves the contradiction by optimizing the gain for each operating condition rather than using a fixed high gain throughout.
Solution Approach 2:
The patent changes the PI regulator parameters based on the operating point to achieve optimal performance. By adjusting the proportional and integral gains according to the output power, the system achieves high control accuracy when needed while minimizing harmful harmonics at low power levels. This parameter scheduling resolves the contradiction between accuracy and harmonic content.
3Ease of operation
If constant PI gains are used across all operating conditions, then the control implementation is simple, but DC drift occurs at high frequencies
Solution Approach 1:
The patent implements dynamic gain scheduling where the PI regulator parameters are adjusted based on the output power level. This dynamic approach maintains DC midpoint voltage stability across different operating conditions while keeping the control implementation relatively straightforward through the use of a power-dependent gain lookup table or calculation. The dynamic parameter adjustment resolves the DC drift issue without significantly complicating the control structure.
Solution Approach 2:
The patent changes the PI regulator parameters as a function of the operating point to maintain stability across all conditions. By adapting the proportional and integral gains to the current power level, the system prevents DC drift at high frequencies while preserving ease of implementation through a systematic parameter adjustment strategy.
4Reliability
If output power dependent gain correction is applied, then system stability is improved across operating points, but device complexity increases
Solution Approach 1:
The patent applies dynamic gain scheduling to improve system stability across different operating points. While this introduces some complexity, the implementation is kept manageable through the use of a structured approach where the gain is determined as a function of the easily measurable output power, using either a lookup table or a simple calculation based on power level. This dynamic adaptation significantly improves stability while maintaining reasonable implementation complexity.
Solution Approach 2:
The patent changes the PI regulator parameters based on output power to improve stability across the full operating range. This parameter adaptation does increase complexity compared to constant gains, but the increase is justified by the significant improvement in stability and is implemented in a systematic way that keeps the added complexity manageable.
Data Source
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AI summary
A system and method (300) for controlling a DC midpoint terminal voltage of a three level inverter is provided. The method includes receiving an input power signal at a three level motor control system that includes a three level inverter, the three level inverter powering an electric motor, determining (310) in the three level motor control system, a speed value of the electric motor, and adjusting (315) a zero-sequence inverter output voltage to adjust (320) a midpoint voltage at the DC midpoint based on the determined speed value.