Modular Multilevel Converter Initial Charging Control
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Solution Overview
Problem
Modular multilevel converters (MMC) face challenges in initial charging of DC capacitors, as all capacitors are charged simultaneously, making it difficult to charge a specific unit cell alone, which is essential for verifying the soundness of unit cells before connecting to an AC power system, leading to potential excessive current flow and safety concerns.
Innovation Solution
A power converter apparatus with a configuration that includes a variable voltage source connected to a DC link and a variable voltage transformer between the apparatus and a three-phase power system, allowing for initial charging of a specific unit cell by bypassing charge current to the lower cascade arm IGBT of non-selected unit cells, preventing excessive voltage application and charge.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If all DC capacitors in unit cells are charged simultaneously via resistor from power system, then initial charge function is achieved, but individual verification of unit cells becomes difficult
Solution Approach 1:
The charging system is segmented to allow independent charging of individual unit cells. The control unit can selectively activate charging for specific unit cells while keeping others in standby, enabling individual verification without charging all cells simultaneously. This segmentation resolves the contradiction by providing both simultaneous charging capability and individual verification capability.
Solution Approach 2:
The charging system transitions from a static all-or-nothing charging approach to a dynamic selective charging approach. The control unit can dynamically adjust which unit cells are charged based on verification needs, switching between charging mode and verification mode as required. This dynamic control enables both simultaneous charging and individual verification.
2Reliability
If DC capacitor in unverified unit cell is charged up, then charging completeness is achieved, but safety verification of individual unit cells is compromised
Solution Approach 1:
The system performs preliminary verification of unit cells before full charging is applied. The control unit first activates selected unit cells for verification, and only after successful verification does it proceed to charge the DC capacitor in that unit cell. This preliminary action ensures reliability by verifying unit cell functionality before committing full charging power.
Solution Approach 2:
The charging system applies partial charging action initially to only the verified unit cells rather than charging all cells simultaneously. The control unit selectively applies charging voltage to unit cells that have passed verification, while keeping unverified cells in standby mode. This partial action approach maintains productivity by charging verified cells quickly while ensuring reliability through selective activation.
3Adaptability or versatility
If voltage type converter outputs voltage different from AC voltage source, then power conversion function is achieved, but excessive current flows into the converter
Solution Approach 1:
A reactor is introduced as an intermediary element between the power system and the unit cells. This reactor limits the excessive current that would otherwise flow when there is a voltage mismatch between the power converter output and the AC voltage source. The reactor acts as a buffer that allows voltage conversion while preventing harmful current surges.
Solution Approach 2:
The control unit prepares the unit cells by gradual voltage application before full power connection. It first activates unit cells at reduced voltage levels to prevent excessive current flow, then gradually increases voltage as verification progresses. This beforehand cushioning approach protects the system from current surges while maintaining voltage adaptability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables safe and individual verification of unit cell operations before linking to the power system, preventing excessive current flow and ensuring proper initial charging of DC capacitors, thereby enhancing safety and reliability.
Implementation Method 1
turn on the lower cascade arm IGBT of the bidirectional chopper circuit of the unit cell except for the unit cell selected at the time of initially charging up the DC capacitor to turn on the lower cascade arm IGBT in the unit cell except for the unit cell selected, and bypass a charge current to the DC capacitor by the lower cascade arm IGBT
Implementation Method 2
an AC voltage output from a power converter varies greatly from the voltage of AC voltage source to thereby flow an excessive current into the voltage type power converter
Implementation Method 3
The MMC is a converter using a bidirectional chopper circuit or full bridge circuit, as a unit cell, connected to a DC capacitor
Implementation Method 4
provides a variable voltage transformer between the apparatus and a three-phase power system
Data Source
AI summary
A power converter apparatus having a configuration of a plurality of unit cells, including a DC capacitor and semiconductor devices, connected in cascade, includes a variable voltage source that is connected with a DC link, and a unit having a function that initially charges up the DC capacitor in the unit cell alone selected at a time of an initial charge.