MMC Submodule Topology Circuit for DC Fault Ride-Through
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Solution Overview
Problem
Traditional modular multilevel converter (MMC) submodule topologies face issues with quick isolation and fault ride-through during short-circuit faults on the DC side, leading to system paralysis, high cost, and reduced reliability due to fault current flowing through switching devices and diodes, with existing solutions either having high device count or weak fault current blocking capabilities.
Innovation Solution
A MMC submodule topology circuit with a series connection of half-bridge submodules, thyristors, and diodes, where thyristors and diodes are used to direct fault currents through capacitors, preventing large currents from flowing through switching devices, and ensuring all capacitors are connected in series for effective fault suppression, maintaining modularity and reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a full-bridge submodule is used to realize self-blocking when the DC line fails, then the fault ride-through capability is improved, but the number of switching devices increases leading to high cost and high loss
Solution Approach 1:
The circuit is divided into multiple half-bridge submodules (first, second, third, and fourth half-bridge submodules) connected in series. Each submodule contains fewer switching devices compared to a full-bridge structure, while collectively they provide the required fault blocking capability through coordinated operation of thyristors and diodes.
Solution Approach 2:
Thyristors and diodes are introduced as intermediary components to redirect fault currents. The thyristors (first, second, third, and fourth thyristors) and diodes (first, second, third, and fourth diodes) act as mediators that guide the fault current through the capacitor branches, preventing it from flowing through the switching devices while maintaining system reliability.
2Device complexity
If the MMC submodule topology is designed with fewer switching devices to reduce cost, then the device count is reduced, but the fault current blocking capability becomes weaker
Solution Approach 1:
The thyristors are configured in advance to be able to block fault currents before the fault occurs. When a fault is detected, the thyristors can immediately redirect the fault current through the capacitor branches, providing preliminary protection to the switching devices without requiring them to have higher voltage ratings.
Solution Approach 2:
The fault current, which is normally harmful, is converted into a beneficial flow path through the thyristors and diodes that directs it through the capacitor branches. This transformation allows the fault current to be used to charge the capacitors while protecting the switching devices, turning a harmful effect into a protective mechanism.
3Reliability
If the withstand voltage of switching devices is increased to handle reverse fault currents, then the fault blocking capability is improved, but the cost and configuration difficulty increase
Solution Approach 1:
Thyristors serve as intermediary components that bear the voltage stress during fault conditions rather than the main switching devices. The thyristors are specifically designed to handle the reverse voltage and fault current, while the main switching devices (IGBTs) only need to handle normal operating voltages, thus reducing their voltage rating requirements and associated costs.
4Device complexity
If only two submodule capacitors are connected in series during reverse fault current, then the device count is kept low, but the capability to suppress short-circuit fault current becomes weak
Solution Approach 1:
The capacitor connection configuration is made dynamic through the control of thyristors. During normal operation, the capacitors are configured in their standard arrangement. During fault conditions, the thyristors dynamically reconfigure the capacitor connections to connect all four submodule capacitors in series, providing enhanced fault current suppression capability only when needed, thus maintaining modularity while improving fault performance.
Data Source
AI summary
The present disclosure relates to a submodule topology circuit for a modular multilevel converter and a method for controlling same. The submodule topology comprises an inlet port and an outlet port, at least two half-bridge submodules, a plurality of first switching devices, a plurality of thyristors and a plurality of diodes, wherein the at least two half-bridges are connected in series and are provided between the inlet port and the outlet port, and each of the half-bridge submodules is provided with an input port, a first output port and a second output port.