Modified Half-Bridge Submodule for MMC DC Fault Suppression
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Solution Overview
Problem
Half bridge submodules in modular multi-level voltage sourced converters (MMC VSCs) cannot effectively suppress short circuit currents at DC terminals without opening AC circuit breakers or using additional DC circuit breakers.
Innovation Solution
A modified half bridge (MHB) submodule design that uses electronic switches to block fault currents by operating switches within the converter, without the need for AC or DC circuit breakers, incorporating a thyristor and diode configuration to manage current flow and voltage polarity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a conventional half bridge submodule is used, then the capital cost and losses are lower compared to full bridge submodules, but the short circuit current cannot be suppressed by electronic actions during DC faults
Solution Approach 1:
The submodule is segmented into multiple functional branches: a first branch with an energy storage device and bidirectional switch assembly, and a second branch with a bidirectional switch assembly. This segmentation allows independent control of current paths, enabling fault suppression while maintaining normal operation efficiency.
Solution Approach 2:
Bidirectional switch assemblies act as intermediaries between the energy storage device and the circuit terminals. These switch assemblies contain pairing of unidirectional electronic components that can block current in both directions, providing the necessary fault suppression capability while allowing normal power conversion operations.
2Reliability
If a mechanical circuit breaker is connected at DC terminals to suppress short circuit current, then the fault current can be blocked, but the system complexity and response time increase
Solution Approach 1:
The patent replaces mechanical circuit breakers with electronically controlled bidirectional switch assemblies containing unidirectional electronic components. This substitution enables faster electronic control of current blocking without the mechanical movement and associated delays of traditional circuit breakers.
Solution Approach 2:
The bidirectional switch assemblies are integrated directly into the submodule structure, allowing the converter itself to suppress its own fault currents through electronic control of the switch assemblies, without requiring external protection devices.
3Reliability
If AC circuit breakers are opened to stop short circuit current, then the fault current is blocked, but the AC power connection is interrupted and system availability decreases
Solution Approach 1:
The bidirectional switch assemblies serve as intermediary devices that can block fault currents at the DC side without affecting the AC side connections. This isolation allows fault suppression while maintaining AC power connection and system availability.
Solution Approach 2:
The converter system is segmented into independently controllable submodule units with integrated fault suppression capability. This allows localized fault handling at the DC side without requiring system-wide shutdown or AC connection interruption.
Data Source
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AI summary
A submodule for a modular multilevel voltage sourced converter comprises a pair of bidirectional current conducting switches assemblies, one of which is serially connected with an energy storage device such as a capacitor and other which is connected in parallel therewith, such that terminals of the submodule are provided across these parallel branches. The standalone bidirectional switch assembly comprises an antiparallel pair of controllable unidirectional-current conducting electronic components, one of which is oriented to conduct current in the same direction as a controllable unidirectional-current conducting electronic component provided in the other bidirectional switch assembly, and which furthermore is configured to conduct current while in a deactivated state such that once the currently carried current drops to zero further current conduction is blocked. Thus, in the event of a DC fault, the controllable selectively conductive switches can be operated to suppress the DC fault current without electromechanical circuit breakers.