Reverse-Phase Compensation Control for MMC Voltage Balance
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Solution Overview
Problem
MMC type power conversion devices face issues with capacitor voltage imbalance among phases due to increased reverse-phase voltage, leading to instability and control deterioration, particularly during system faults.
Innovation Solution
A power conversion device with a control system that includes a reverse-phase voltage compensator to stabilize system voltage by compensating for reverse-phase voltage imbalances, activated only when the imbalance is minor and sustained, using a threshold-based assessment to prevent capacitor voltage imbalances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If reverse-phase voltage compensation is performed to stabilize system voltage, then system voltage stability is improved, but capacitor voltage imbalance among phases worsens
Solution Approach 1:
The patent applies dynamics by making the reverse-phase voltage compensation function adjustable rather than fixed. The control device dynamically switches the compensation function on or off based on real-time detection of capacitor voltage balance status. When imbalance is detected, the function is deactivated; when balance is maintained, it remains active. This dynamic adaptation resolves the contradiction by allowing the system to optimize for voltage stability when safe, and prevent capacitor imbalance when necessary.
Solution Approach 2:
The patent implements feedback control by continuously monitoring capacitor voltages across all converter cells and using this information to control the reverse-phase voltage compensation function. The detection unit measures capacitor voltages, compares them against balance criteria, and feeds this information back to the control device which adjusts the compensation function accordingly. This closed-loop feedback mechanism resolves the contradiction by ensuring compensation only occurs when it won't harm capacitor voltage balance.
2Reliability
If reverse-phase voltage compensation is continuously performed, then system voltage imbalance is reduced, but capacitor voltage imbalance occurs during system faults
Solution Approach 1:
The patent applies preliminary anti-action by proactively detecting potential fault conditions through continuous monitoring of capacitor voltages before they cause harmful imbalance. The detection unit identifies when capacitor voltages are balanced, and only then permits reverse-phase voltage compensation to proceed. This preliminary check prevents the harmful effect of capacitor imbalance during system faults by stopping compensation before the fault can develop, rather than reacting after damage occurs.
Solution Approach 2:
The system dynamically adjusts the compensation function based on real-time system conditions. During normal operation with balanced capacitor voltages, the compensation function is active to maintain system voltage balance. During fault conditions where capacitor imbalance is detected, the function automatically deactivates. This dynamic behavior resolves the contradiction by adapting compensation to actual system state rather than operating in a fixed mode.
Data Source
AI summary
In a control device of a power conversion device, in order to control an AC current outputted from a power converter, a first generator generates a first voltage command value based on a positive-phase current command value and a given reverse-phase current command value. A reverse-phase voltage compensator generates the reverse-phase current command value to compensate for a reverse-phase component contained in an AC voltage of an AC power system. When a state in which imbalance is determined as being minor based on comparison of an assessment value relating to a degree of imbalance of the AC voltage of the AC power system with a threshold value continues for a certain time period or longer, the reverse-phase voltage compensator generates the reverse-phase current command value and provides the reverse-phase current command value to the first generator.