MMC Power Conversion Control for Converter Cell Voltage Balance
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Solution Overview
Problem
In Modular Multilevel Converter (MMC) systems, maintaining stable voltage across energy storage devices is crucial for continuous operation, but voltage variations due to faults in DC or AC circuits can lead to unstable feedback control and operational challenges.
Innovation Solution
The power conversion device adjusts the active power limit value when voltage variations exceed a threshold, ensuring stable operation by reducing the active power limit to prevent feedback control instability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the active power limit value is increased to improve power conversion capacity, then productivity is improved, but voltage variation across energy storage devices increases causing feedback control instability
Solution Approach 1:
The active power limit value is changed from a fixed parameter to a dynamic parameter that adapts based on voltage variation conditions. The control device monitors voltage variations across energy storage devices and adjusts the active power limit value accordingly - reducing it when voltage variation exceeds a threshold and restoring it when voltage variation is within acceptable ranges, enabling the system to maintain stability while maximizing power conversion capacity
Solution Approach 2:
The system implements a feedback mechanism where the control device continuously monitors voltage variations across energy storage devices and uses this information to adjust the active power limit value. When voltage variation exceeds a predetermined threshold, the feedback loop triggers a reduction in the active power limit value to stabilize voltage, and when voltage variation is within the threshold, the limit value is restored to its original higher level
2Reliability
If the active power limit value is reduced to maintain voltage stability, then reliability is improved, but productivity decreases
Solution Approach 1:
The active power limit value is dynamically adjusted based on real-time voltage variation conditions rather than being fixed at a conservative level. The control device increases the limit value when voltage stability is maintained and reduces it only when necessary to prevent instability, thereby maximizing productivity while ensuring reliability
Solution Approach 2:
The system changes the active power limit value parameter in response to voltage variation conditions. By monitoring voltage across energy storage devices and comparing it to a threshold, the system adjusts the active power limit value to appropriate levels - reducing it to maintain stability when voltage variation is excessive and restoring it to maximize power conversion capacity when voltage is stable
3Reliability
If feedback control is applied to maintain voltage across energy storage devices, then reliability is improved, but device complexity increases due to additional control mechanisms
Solution Approach 1:
The control device performs multiple functions using a single integrated mechanism: it monitors voltage across energy storage devices, determines whether voltage variation exceeds a threshold, adjusts the active power limit value accordingly, and restores the limit value when conditions improve. This multi-functional approach maintains voltage balance while avoiding the need for separate complex control systems for each function
Data Source
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AI summary
A power conversion device includes: an MMC-type power conversion circuit (110) configured to perform power conversion between an AC power system and a DC power system; and a central controller (120). The central controller (120) limits an active power command value and a reactive power command value to a value corresponding to an active power limit value and a value corresponding to a reactive power limit value, respectively, and controls an operation of the power conversion circuit according to the limited active power command value and the limited reactive power command value. The central controller (120) includes: an index value calculation unit (125) configured to calculate an index value that shows an extent of a variation among voltages of energy storage devices included in a plurality of converter cells; and a limiter controller (122) configured to change the active power limit value to a value smaller than the active power limit value when the index value exceeds a threshold value.