Inverter Overmodulation for Medium Voltage Power Cells
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Solution Overview
Problem
Existing variable frequency drive systems using series-connected inverters are inefficient in terms of size and cost, as they require multiple power cells to achieve desired output voltages, especially in medium voltage applications.
Innovation Solution
Implementing overmodulation techniques in the operation of series-connected inverters, allowing for increased output voltage without the need for higher-rated components, by operating power cells in the overmodulation region during steady-state or transient conditions, and using a control system to monitor and trigger overmodulation based on predetermined events such as motor speed or voltage changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If series-connected inverters use multiple power cells to achieve desired output voltages, then output voltage capability is improved, but device size and cost increase
Solution Approach 1:
The patent applies overmodulation technique to change the operating parameters of the inverter, allowing it to output voltages exceeding the standard DC bus voltage. By operating in the overmodulation region, the inverter can achieve higher output voltages without adding more power cells, thus resolving the contradiction between voltage capability and device complexity
Solution Approach 2:
The patent intentionally operates the inverter in an excessive modulation region where the modulation index exceeds 1.0, producing output voltages that are partially or fully excessive compared to the linear modulation limit. This allows the system to achieve desired voltage levels with fewer power cells by accepting some distortion in the output waveform
2Reliability
If series-connected inverters use more power cells to maintain redundancy, then system reliability is improved, but device cost and size increase
Solution Approach 1:
By changing the modulation parameters to operate in overmodulation region, the system can maintain reliability with fewer power cells. The control system monitors voltage levels and triggers overmodulation when needed, allowing the same voltage output to be achieved with reduced hardware while maintaining system redundancy
3Power
If standard modulation is used in inverters, then device simplicity is maintained, but output voltage capability is limited
Solution Approach 1:
The patent implements a dynamic control system that automatically transitions between linear modulation and overmodulation modes based on real-time voltage requirements. The control system dynamically adjusts the modulation index and monitors voltage levels, enabling the inverter to adapt its operation to achieve desired voltage output without manual intervention or complex external circuitry
Data Source
AI summary
A variable frequency drive including a plurality of power cells that are configured as three phases of series-connected cells to deliver power to a load. The drive is operated to increase the output voltage of each power cell using overmodulation.