Modular Converter Fault Management via Pre-Charged Energy Stores
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Solution Overview
Problem
High-voltage direct current transmission systems face challenges in rapidly and reliably managing insulation faults on the DC voltage side of converter assemblies, which can lead to damage and increased operating costs due to the need for prolonged isolation and high electrical losses in switching modules.
Innovation Solution
A method for fault management in modular multi-stage converters that involves actuating switching modules of the first type to charge their energy stores beyond their rated voltage, aligning the polarity with fault currents, and using zero-current control to efficiently reduce DC voltage side current, thereby minimizing hardware costs and losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the converter assembly is isolated from the AC voltage grid for fault management, then the fault can be managed safely, but the operating time is prolonged and productivity is reduced
Solution Approach 1:
The energy stores of the first type switching modules are pre-charged to a voltage in excess of their rated voltage before a fault occurs. This preliminary charging enables the converter assembly to rapidly reduce DC voltage side current during a fault without requiring prolonged isolation from the AC voltage grid, thereby maintaining both safety and productivity
Solution Approach 2:
The converter assembly dynamically switches between different operating modes: during normal operation, the first type switching modules operate at rated voltage; during fault conditions, they utilize their pre-charged energy stores to rapidly reduce current. This dynamic adaptation allows quick fault response while minimizing operational disruption
2Ease of manufacture
If switching modules are operated at rated voltage during fault management, then hardware costs are reduced, but the ability to rapidly reduce DC voltage side current is limited
Solution Approach 1:
The energy stores are pre-charged to excessive voltage during normal operation phases, preparing them in advance to deliver rapid current reduction during faults. This preliminary action enables fast response without requiring permanent hardware modifications or continuous high-voltage operation
Solution Approach 2:
The voltage parameter of the energy stores is temporarily changed to excessive voltage during fault conditions, enabling rapid current reduction. During normal operation, the voltage returns to rated levels, maintaining cost-effectiveness. This temporary parameter change resolves the contradiction between speed and cost
3Ease of operation
If the polarity of energy store voltages does not correspond to fault current polarity, then switching module operation is simpler, but fault management effectiveness is reduced
Solution Approach 1:
The control system continuously monitors the polarity of fault currents and dynamically adjusts the actuation of first type switching modules to ensure their energy store voltages correspond to the fault current polarity. This feedback mechanism maintains fault management effectiveness while keeping the control logic systematic and reliable
Solution Approach 2:
The switching modules are designed with asymmetric capabilities: first type switching modules can generate both positive and negative voltages with polarities that can be switched to match fault current direction, while second type modules have fixed polarity characteristics. This asymmetric design enables effective fault management without requiring complete symmetry in all module operations
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 approach enables reliable and efficient fault management with reduced operating costs and hardware requirements, allowing for rapid fault resolution without increasing overall costs, and includes the use of additional DC voltage side parallel paths to limit negative voltages and prevent damage to the converter assembly.
Implementation Method 1
each of which includes a plurality of semiconductor switches and at least one energy store
Data Source
AI summary
A method manages a fault on a DC voltage side of a converter assembly including a modular multistage converter with switching modules having semiconductor switches and an energy store. Some switching modules are a first type and others are a second type. During operation, a positive switching module voltage, negative switching module voltage or zero voltage are generated at terminals of switching modules of the first type, and a positive switching module voltage or zero voltage are generated at terminals of switching modules of the second type. Upon detecting a DC voltage side fault, switching modules of the first type are actuated such that the polarity of their energy store voltages corresponds to the polarity of a fault current, and energy stores of switching modules of the first type are charged to a voltage exceeding their rated voltage. A converter assembly carrying out the method is also provided.