Modular Converter Sub-Module DC Fault Blocking
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Solution Overview
Problem
Existing modularized multilevel converters face issues where a DC fault leads to fault currents in the AC network, requiring tripping of AC line switches for fault removal, resulting in prolonged system downtime.
Innovation Solution
A converter design with sub-modules featuring turn-off devices and diodes in antiparallel connection, along with a control method that locks the converter during DC faults, preventing AC fault currents and allowing transient DC faults to be cleared without AC line switch tripping, using series resistors to attenuate fault currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional modularized multilevel converters are used, then the converter can operate in DC networks, but when a DC fault occurs, AC line switches must be tripped to remove the fault, resulting in prolonged system downtime
Solution Approach 1:
The converter is divided into multiple sub-modules, each with independent turn-off devices and diodes. This segmentation allows selective blocking of fault currents at the sub-module level, enabling localized fault management without requiring tripping of AC line switches, thus reducing system downtime while maintaining reliability
Solution Approach 2:
Diodes are introduced as intermediary elements in antiparallel connection with turn-off devices in each sub-module. These diodes act as natural fault current blockers, preventing AC fault currents from injecting into the DC network during DC faults, thereby enabling rapid fault removal without AC line switch tripping
2Object-affected harmful factors
If AC line switches are tripped to remove DC faults, then fault current injection is prevented, but system restart time is prolonged
Solution Approach 1:
The sub-modules are pre-configured with turn-off devices and antiparallel diodes that can immediately block fault currents upon DC fault detection. This preliminary protective configuration prevents AC fault current injection without requiring AC line switch tripping, enabling rapid system restart and significantly reducing restart time
Solution Approach 2:
The converter is designed with sub-modules that have built-in fault blocking capability through turn-off devices and diodes. This preliminary action ensures that when a DC fault occurs, the system can immediately prevent fault current injection at the sub-module level, avoiding the need for AC line switch tripping and enabling quick system recovery
3Reliability
If DC breakers are installed to protect against DC faults, then fault current is blocked, but device complexity and cost increase
Solution Approach 1:
The converter is segmented into multiple sub-modules, each equipped with turn-off devices and antiparallel diodes. This segmentation distributes the fault protection function across multiple simple sub-module units, eliminating the need for complex DC breakers while maintaining reliable DC fault protection capability
Solution Approach 2:
Each sub-module is self-sufficient with its own turn-off devices and antiparallel diodes that provide inherent fault protection. The sub-modules can independently block fault currents without requiring external DC breakers, simplifying the overall converter structure and reducing device complexity while ensuring reliable DC fault protection
Data Source
Figure 1~3
Figure 4~6(b)
Figure 7~8(f)
AI summary
Disclosed are a sub-module structure formed of an energy storage element, a first turn-off device, a second turn-off device, a third turn-off device, a freewheeling diode, a series resistor, and diodes respectively in antiparallel connection with the turn-off devices, and a converter completely or partially formed of the sub-modules. Also disclosed are a relevant protection unit and a control method for the converter. The converter can be locked when a direct current (DC) fault occurs to prevent an alternating current (AC) system from injecting a fault current into a DC network, so that a transient fault of the DC network can be removed without tripping an AC line switch, thereby rapidly restarting the system. A charging resistor is comprised in the sub-module so that a charging resistor disposed at an AC side of the converter can be reduced and even may not be dispose.