Modular Multilevel Converter Initial Charging Control
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Solution Overview
Problem
Conventional power conversion devices with modular multilevel converters face challenges in initially charging DC capacitors to the required voltage level due to charge current flowing through both phases, making it difficult to achieve voltage equality with the AC grid, leading to potential overcurrent issues.
Innovation Solution
A power conversion device with a charge control unit that includes an element driving unit to selectively turn on switching elements, allowing charge current to flow through specific phases and bypassing DC capacitors when necessary, ensuring each converter cell's DC capacitor is charged to the required voltage level efficiently.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If charge current flows via arms for two phases during initial charging, then the charging path is established, but the DC capacitors cannot be charged to the required voltage level
Solution Approach 1:
The patent segments the charging process into two distinct phases: first charging only one phase's DC capacitors to the required voltage level, then charging the other phase. This is achieved by selectively turning on switching elements in specific arms during different time periods, preventing the charge current from flowing through both phases simultaneously and ensuring each capacitor reaches the correct voltage.
Solution Approach 2:
The patent applies preliminary action by first charging the DC capacitors in one phase to the required voltage level before initiating charging in the other phase. This sequential approach ensures that when the second phase is charged, the voltage levels are properly established, preventing overcurrent issues during grid connection.
2Ease of manufacture
If DC capacitor is charged via charge resistor from AC grid, then the charging method is simple, but the MMC cannot output voltage equal to grid voltage
Solution Approach 1:
The patent employs dynamic control of switching elements to transition the system from a simple resistive charging state to an active voltage output state. By controlling the turn-on timing of switching elements in different arms, the system dynamically adjusts the charging path and eventually enables the MMC to output voltage matching the grid voltage, resolving the contradiction between simple charging and voltage matching.
3Productivity
If charge current flows through both phases simultaneously, then the charging process is fast, but overcurrent issues occur during grid connection
Solution Approach 1:
The patent applies periodic action by dividing the charging process into distinct time periods: a first time period where only one phase is charged, and a second time period where the other phase is charged. This periodic, sequential charging approach prevents simultaneous charging of both phases, eliminating overcurrent conditions while maintaining efficient charging through structured timing.
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
This configuration enables swift and reliable initial charging of DC capacitors, preventing overcurrent issues and ensuring the power conversion device can output the required AC voltage when connected to the AC grid, thereby improving the device's reliability.
Implementation Method 1
a DC capacitor 23 in each converter cell 10
Implementation Method 2
turns on one switching element 20B in each converter cell 10 as a startup element
Data Source
Figure 1
Figure 2
Figure 3
AI summary
Each phase arm (4u-4w, 5u-5w) of a power conversion device (1) includes at least one converter cell (10) connected in series. For each converter cell (10), an element driving unit (30) is provided which turns on one switching element (20B) in the converter cell (10) as a startup element. The element driving unit (30) is supplied with power from a DC capacitor (23), and when voltage of the DC capacitor (23) exceeds startup voltage Vsh, turns on the startup element. Thus, at the time of startup of the power conversion device (1), the DC capacitor (23) in each converter cell (10) is initially charged to desired voltage.