Modular Multilevel Converter DC Voltage Control During Grid Failure
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Solution Overview
Problem
In DC power transmission power conversion devices with modular multilevel converters, AC grid failures lead to unstable capacitor voltages, causing overvoltage or low voltage issues, which can result in device shutdown due to the inability to freely control AC active power on the affected AC-DC conversion terminal.
Innovation Solution
A DC power transmission power conversion device and method that detect voltage variations in DC capacitors and adjust the DC voltage command value to prevent excessive energy flow, thereby suppressing sharp voltage variations and ensuring continuous operation by controlling the DC voltage command based on predetermined values during AC grid failures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If AC grid failure occurs and DC voltage control is performed at the affected AC-DC conversion terminal, then voltage control can be maintained, but capacitor voltage becomes unstable causing overvoltage or low voltage
Solution Approach 1:
The invention introduces an intermediary control mechanism that detects capacitor voltage variations and adjusts the DC voltage command value accordingly. This intermediary control acts as a mediator between the unstable capacitor voltage and the DC voltage control system, preventing excessive energy flow to or from the capacitor while maintaining overall voltage control stability during AC grid failures.
Solution Approach 2:
The invention implements a feedback control mechanism where capacitor voltage variations are continuously monitored and fed back to adjust the DC voltage command value. When capacitor voltage deviates from the command value by exceeding a predetermined threshold, the DC voltage command is adjusted in the opposite direction, creating a closed-loop feedback system that stabilizes capacitor voltage during AC grid failures.
2Stability of the object's composition
If DC voltage command value is adjusted based on capacitor voltage variation, then sharp voltage variations are suppressed, but control complexity increases
Solution Approach 1:
The invention changes the DC voltage command value parameter dynamically based on capacitor voltage variation. When the difference between actual capacitor voltage and command voltage exceeds a predetermined threshold, the command value is adjusted by a fixed amount in the opposite direction. This parameter change approach provides a simple yet effective method to suppress sharp voltage variations without requiring complex control algorithms.
3Power
If multiple converters are multiplexed in series or parallel, then converter capacity increases and harmonics are reduced, but control difficulty increases
Solution Approach 1:
The invention segments the control of each converter into independent functional blocks: capacitor voltage detection, variation calculation, threshold comparison, and DC voltage command adjustment. Each converter's control is handled as a separate segment with identical control logic, which simplifies the overall control of multiplexed converters while maintaining high converter capacity and harmonic reduction benefits.
Data Source
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AI summary
In a power converter (1) which performs power conversion between plural-phase AC and DC, variation in voltage (Vcap) of the DC capacitor (34) in each converter cell (10) is detected, and when the variation exceeds a predetermined value, a DC voltage command value (Vdcref) as a control target value in a DC voltage control unit (21) which controls DC voltage (Vdc) between DC buses (2, 3) is corrected by being increased or decreased. Thus, when AC grid failure occurs, the capacitor voltage (Vcap) of each converter cell (10) is maintained and the operation continuity is improved.