Modular Multilevel Converter Parallel Arm Voltage Balance
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Solution Overview
Problem
In power conversion devices using modular multilevel converters, connecting arms in parallel leads to unbalanced DC capacitor voltages, risking destruction of switching elements and reducing operational efficiency due to high voltage exceeding the withstand voltage of switching elements.
Innovation Solution
A power conversion device configuration with main arms and sub-arms connected in parallel, where unit converters with energy buffers and reactors are series-connected, and a control unit manages inter-parallel arm circulation currents to balance DC capacitor voltages, preventing voltage unbalance and enhancing operational stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If arms are connected in parallel to increase capacity, then the power conversion device capacity is increased, but DC capacitor voltage unbalance occurs leading to switching element destruction risk
Solution Approach 1:
The control unit continuously monitors the DC capacitor voltages in both parallel arms and dynamically adjusts the inter-parallel arm circulation current to compensate for voltage unbalance. This feedback mechanism detects the voltage difference between parallel arms and modulates the circulation current accordingly, ensuring that the DC capacitor voltages remain balanced and preventing switching element destruction while maintaining the parallel configuration for increased capacity
Solution Approach 2:
The invention changes the circulation current parameter between parallel arms to balance the DC capacitor voltages. By dynamically adjusting the magnitude and direction of the inter-parallel arm circulation current based on the detected voltage unbalance, the system maintains equal DC capacitor voltages across parallel arms, enabling safe operation at increased capacity without switching element destruction
2Power
If arms are connected in parallel to increase capacity, then the power conversion device capacity is increased, but operational efficiency decreases due to device stoppage
Solution Approach 1:
The control unit continuously monitors the DC capacitor voltages and dynamically adjusts the inter-parallel arm circulation current to prevent voltage unbalance from reaching dangerous levels. This real-time feedback control eliminates the need to stop the device for protection, maintaining continuous operation at increased capacity and preserving operational efficiency
Solution Approach 2:
The control unit performs preliminary balance control by continuously regulating the inter-parallel arm circulation current before DC capacitor voltage unbalance can cause switching element destruction. This proactive control prevents the need for emergency stoppage, ensuring continuous operation and maintaining high operational efficiency at increased capacity
Data Source
Figure 1~2
Figure 3~4
Figure 5~6
AI summary
Main arms 10UP, 10VP, 10WP, 10UN, 10VN, and 10WN and sub-arms 11UP, 11VP, 11WP, 11UN, 11VN, and 11WN are provided for each of the P-side and N-side of each phase, the main arms 10UP, 10VP, 10WP, 10UN, 10VN, and 10WN and the sub-arms 11UP, 11VP, 11WP, 11UN, 11VN, and 11WN are connected in parallel for each of the P-side and N-side of each phase of the U-phase, V-phase, and W-phase, and the power conversion control unit 13 controls an inter-parallel arms circulation current that circulates between the main arms and the sub-arms, that are connected in parallel, in the same phase.