MMC Sub-Module Thyristor Switching for Low-Loss Safe Failure
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Solution Overview
Problem
Existing partial modules in modular multilevel converters, such as those using IGBTs and IGCTs, suffer from high electrical losses and unsafe failure states due to their switching characteristics.
Innovation Solution
The use of thyristors as switches between external connections, with a turn-off circuit and antiparallel thyristors to manage switching and failure states, along with parallel connections of switches and energy storage devices to enhance performance and reduce losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If IGBTs or IGCTs are used as switches in sub-modules, then the sub-modules can achieve controlled switching, but electrical losses are high
Solution Approach 1:
The patent changes the switching device parameter from IGBT/IGCT to thyristor, which fundamentally alters the electrical characteristics. Thyristors have lower on-state voltage drops compared to IGBTs and IGCTs, directly reducing electrical losses in the sub-module while providing inherent fail-safe conduction behavior.
Solution Approach 2:
The patent converts the potential harm of thyristor failure into a beneficial safe failure state. When a thyristor fails forward, it naturally conducts like a diode, preventing catastrophic failure and allowing the system to continue operating in a degraded mode, thus transforming a potential hazard into a protective feature.
2Loss of energy
If thyristors are used as switches, then electrical losses are reduced, but turning off the thyristor becomes more difficult
Solution Approach 1:
The patent introduces an intermediary turn-off circuit that provides the necessary reverse current to extinguish the thyristor conduction. This intermediary circuit acts as a mediator between the thyristor's natural latching behavior and the required turn-off function, enabling controlled commutation without directly modifying the thyristor device itself.
Solution Approach 2:
The turn-off circuit employs periodic or pulsed reverse current injection to achieve thyristor commutation. By applying reverse current in specific time intervals during the AC cycle, the thyristor is reliably turned off at the appropriate moments, transforming the continuous conduction challenge into a periodic control problem that can be solved with timed current pulses.
3Reliability
If antiparallel thyristors are connected, then safe failure state is achieved, but device complexity increases
Solution Approach 1:
The antiparallel thyristor configuration provides multiple functions simultaneously: it enables safe failure state through diode-like conduction, allows bidirectional current handling, and provides inherent overcurrent protection. This multi-functionality justifies the additional component by eliminating the need for separate protective devices and control circuits.
Solution Approach 2:
The antiparallel thyristor arrangement is self-protecting through its inherent electrical characteristics. When one thyristor fails forward, the antiparallel configuration automatically provides a conduction path, and the thyristors' natural diode behavior in failure states provides self-protection without requiring external protection circuits or complex control logic.
4Strength
If series connections of switching elements are used, then dielectric strength is increased, but device complexity increases
Solution Approach 1:
The patent segments the high voltage blocking requirement into multiple series-connected switching elements, each handling a portion of the total voltage. This segmentation allows the use of lower-voltage-rated, more reliable individual devices while achieving the required overall dielectric strength, and enables modular design and easier replacement of individual elements.
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 configuration significantly reduces electrical losses and ensures safe failure states by allowing thyristors to conduct like diodes in forward failure, while enabling efficient switching and increased dielectric strength and current-carrying capacity through series and parallel connections of switching elements.
Implementation Method 1
the sub-module has a turn-off circuit that can feed a turn-off current into the thyristor during active operation, which current flows from the cathode connection of the thyristor in the direction of the anode connection of the thyristor and is suitable for one of to compensate for the flow current flowing through the thyristor from the anode terminal to the cathode terminal and turn off the thyristor
Implementation Method 2
thyristors offer the advantage of a safe failure state because they conduct like diodes in the event of a forward failure
Implementation Method 3
The switch-off circuit is connected to the energy storage device on the input side and the voltage on the energy storage device is applied to it on the input side
Data Source
Figure 1~2
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Figure 5~6
AI summary
The invention relates to, inter alia, a sub-module (TM) with a first outer connection (A1) and a second outer connection (A2). At least one first switch and an electric series circuit (R), which is parallel to the switch, are connected between the two outer connections (A1, A2), said series circuit comprising a second switch (S2) and an electric energy store. According to the invention, the first switch is a thyristor (T1) or at least also comprises a thyristor (T1).