Modular Multilevel Converter Circulating Current for Bypass Switch Detection
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Solution Overview
Problem
Modular multilevel converters face challenges in reliably detecting bypass switch failures, leading to potential arcing and system damage, especially during low power or no-load operations, due to the need for minimum current flow which increases operating costs and risks of prolonged charging times.
Innovation Solution
A method is introduced to generate or amplify internal circulating currents in the converter to quickly charge defective submodules, allowing faster detection of bypass switch failures and reducing the likelihood of arcing, by determining if the arm current is below a predetermined threshold and using neighboring submodules to monitor the state of bypass switches.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If minimum current flow is maintained during low power or no-load operations to enable bypass switch failure detection, then bypass switch failure detection capability is improved, but operating costs increase and energy losses increase
Solution Approach 1:
The patent implements periodic current injection at predetermined intervals during low power or no-load operations. Instead of maintaining continuous minimum current flow, the system periodically injects current pulses through the defective submodule to charge its capacitor. This periodic action enables bypass switch failure detection while significantly reducing average energy losses and operating costs compared to continuous current maintenance.
2Loss of energy
If arm current is reduced during low power operations, then power consumption is reduced, but bypass switch failure detection capability deteriorates
Solution Approach 1:
The patent performs preliminary current injection actions at predetermined intervals before the capacitor voltage in a defective submodule can reach dangerous levels. By proactively injecting current periodically, the system ensures the capacitor remains sufficiently charged to enable bypass switch failure detection, while maintaining overall low power consumption during low power operations. This preliminary periodic action prevents the need for continuous high current.
Solution Approach 2:
The patent dynamically changes the current parameter by injecting current pulses at predetermined intervals rather than maintaining continuous current flow. The current injection duration, amplitude, and frequency are optimized to achieve sufficient capacitor charging for bypass switch detection while minimizing energy losses. This parameter modulation resolves the contradiction between low power consumption and reliable failure detection.
3Reliability
If continuous monitoring of all submodules is implemented, then system reliability is improved, but device complexity increases
Solution Approach 1:
The patent extracts and focuses monitoring resources specifically on submodules identified as defective or at risk, rather than continuously monitoring all submodules equally. The control device identifies submodules requiring attention based on operational status and capacitor voltage levels, then directs current injection and monitoring efforts specifically to those units. This targeted approach maintains high system reliability while significantly reducing overall monitoring system complexity.
Solution Approach 2:
The patent implements a self-service monitoring mechanism where the defective submodule's capacitor voltage state serves as its own indicator. By periodically injecting current and observing the voltage response, the system allows the submodule to effectively monitor itself. The control device simply needs to track which submodules are defective and apply periodic current injection, eliminating the need for complex continuous monitoring infrastructure across all submodules.
Data Source
AI summary
A modular multilevel converter includes a plurality of submodules, each having at least two electronic switching elements, an electric energy store, two submodule connections, a bypass switch bridging the submodule, and a communication element communicating with a communication apparatus. A method for operating the modular multilevel converter includes ascertaining that the submodules have a defective submodule so that the communication element in the defective submodule does not communicate with the communication apparatus, determining whether a present arm current resulting from an operating point of the modular multilevel converter is below a predetermined threshold value, and generating or amplifying a converter-internal circular current with the defective submodule if the arm current resulting from the operating point is below the predetermined threshold value. A modular multilevel converter is also provided.