MMC Voltage-Source Control Under Grid Voltage Imbalance
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Solution Overview
Problem
Existing power conversion devices, particularly those using modular multilevel converters (MMC), face challenges in maintaining operational continuity during system imbalances, such as those caused by accidents or voltage imbalances.
Innovation Solution
A power conversion device and control system that includes a phase-balance control unit, circulating-current control unit, correction-voltage generation unit, voltage-command calculation unit, and gate-signal generation unit. These components work together to generate arm-current command values, circulating-voltage command values, and correction-voltage command values to ensure the MMC power converter operates as a voltage source even during system imbalances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the power converter operates as a voltage source during system imbalance, then the operational continuity is improved, but overcurrent occurs due to voltage imbalance
Solution Approach 1:
The patent introduces a current-limiting impedance as an intermediary element between the voltage source and the power system. This impedance limits the overcurrent that flows during system imbalance conditions while allowing the power converter to maintain its voltage source operation, thus preventing protection shutdown while avoiding harmful overcurrent effects.
Solution Approach 2:
The patent dynamically adjusts the internal impedance of the virtual synchronous generator based on system conditions. During normal operation, the impedance is set to enable voltage source operation for improved reliability. During system imbalance, the impedance is adjusted to limit overcurrent, allowing the system to maintain operational continuity without experiencing harmful current levels.
2Reliability
If the power converter operates as a voltage source, then the operational continuity during system imbalance is improved, but the device complexity increases due to additional control units
Solution Approach 1:
The control device integrates multiple functions into unified control units that perform both voltage source control and overcurrent suppression. The virtual synchronous machine control unit and the overcurrent suppression unit work together within a single control architecture, reducing the need for separate dedicated components while achieving both improved operational continuity and overcurrent protection.
Solution Approach 2:
The patent combines the voltage source control functionality with the overcurrent suppression functionality in an integrated control system. The control device merges the operation of maintaining voltage source characteristics with the operation of limiting overcurrent during system imbalance, creating a unified control approach that improves reliability without proportionally increasing device complexity.
3Object-affected harmful factors
If the internal impedance is varied to suppress overcurrent, then the overcurrent is limited, but the output current may not meet the power output requirements
Solution Approach 1:
The patent implements dynamic impedance adjustment where the internal impedance of the virtual synchronous generator is continuously adapted based on real-time system conditions. During system imbalance, the impedance is increased to suppress overcurrent. During normal operation, the impedance is reduced to allow full power output capability. This dynamic adjustment ensures both overcurrent suppression and adequate power delivery are achieved at different operational stages.
Solution Approach 2:
The control system periodically monitors system balance conditions and adjusts the internal impedance accordingly. When system imbalance is detected, the impedance adjustment is activated to suppress overcurrent. When balance is restored, the impedance is adjusted back to enable full power output. This periodic monitoring and adjustment cycle ensures both protection and performance requirements are met.
Data Source
AI summary
A power conversion device includes: a power converter which includes a plurality of converter cells; and a control device. The control device includes: a phase-balance control unit to generate arm-current command values, based on direct-current voltages of power storage elements of the plurality converter cells; a circulating-current control unit to generate a circulating-voltage command value for controlling a circulating current, based on the arm-current command values; a correction-voltage generation unit to correct a reference-voltage command value, using a correction value for suppressing an overcurrent due to a voltage imbalance of the power system, to generate a correction-voltage command value; a voltage-command calculation unit to generate arm-voltage command values, based on the circulating-voltage command value and the correction-voltage command value; and a gate-signal generation unit to generate control signals for the plurality of switching elements of the at least one arm, according to the arm-voltage command values.