MMC Cell Autonomous Overvoltage Protection
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Solution Overview
Problem
In Modular Multilevel Converters (MMCs), communication errors between the control device and unit converters can lead to serious failures due to difficulties in transmitting control signals, particularly when overvoltage conditions occur, as existing control configurations rely heavily on successful communication for protecting switching elements.
Innovation Solution
A power conversion device with a control circuit in each unit converter that autonomously sets the switching element to an off state when voltage exceeds a second threshold voltage, independent of signals from the control device, ensuring protection even during communication errors, and a control device that compares voltage detected values with a first threshold to transmit a control signal to all unit converters to prevent overvoltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the control device monitors voltage from all unit converters and transmits control signals, then centralized control is improved, but response time during communication errors deteriorates
Solution Approach 1:
The protection function is segmented and distributed to each unit converter individually. Each unit converter's control circuit independently monitors its own DC capacitor voltage and makes autonomous protection decisions, eliminating the time delay associated with centralized monitoring and signal transmission. This segmentation ensures that protection response time is not affected by communication delays or errors between the control device and unit converters.
2Reliability
If autonomous protection is implemented in each unit converter, then protection reliability during communication errors is improved, but device complexity increases
Solution Approach 1:
The protection functionality is implemented locally in each unit converter with a dedicated control circuit that monitors only the voltage across its own DC capacitor. This localized approach allows each unit converter to have simple, dedicated protection logic without requiring complex centralized control or inter-unit communication for protection purposes. The control circuit includes a voltage detection unit and a protection execution unit that work together to provide straightforward autonomous protection.
Data Source
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AI summary
An MMC (110) includes arms (A1 to A6) each configured with one unit converter or a plurality of cells (10) connected in series. The main circuit of the cell (10) includes a switching element and a DC capacitor. The control circuit of the cell (10) transmits a voltage detected value of the DC capacitor to a control device (120) and controls on/off of the switching element in accordance with a control signal received from the control device (120). When the voltage detected value of at least one cell (10) exceeds a first threshold voltage, the control device (120) transmits a gate block signal to the control circuit of each cell (10). The control circuit of each cell (10) autonomously fixes the switching element of its own main circuit to an off state and short-circuits the output terminals of the main circuit, irrespective of the control signal, when its own voltage detected value exceeds a second threshold voltage higher than the first threshold voltage.