MMC Disconnecting Switch Units for DC Fault Current Isolation
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Solution Overview
Problem
MMC converter devices face challenges in quickly cutting off DC fault currents when connected through a line, leading to increased switching losses and potential damage to sub-modules.
Innovation Solution
Incorporating two disconnecting switch units with mechanical switches and diodes in opposite directions, connected in series between MMC converter devices, allowing for the formation of a reverse voltage to cut off fault currents and reduce switching losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a full bridge circuit is used in sub-modules to block fault current, then fault current blocking capability is improved, but switching loss increases by about 30% or more
Solution Approach 1:
The disconnecting switch unit is divided into two separate units connected in series on the DC line. The first disconnecting switch unit includes a mechanical switch and parallel diode, while the second disconnecting switch unit includes only a mechanical switch. This segmentation allows each unit to have optimized functions: the first unit handles reverse voltage blocking through the diode, reducing switching losses, while the second unit provides additional fault current interruption capability.
2Speed
If disconnecting switch units are added to quickly cut off DC fault current, then fault current interruption speed is improved, but device complexity increases
Solution Approach 1:
The disconnecting switch units act as intermediary devices inserted into the DC line between two MMC converter devices. These units provide a dedicated mechanism for rapid fault current interruption without requiring changes to the internal structure of the MMC converter devices themselves, thus isolating the complexity to specific components rather than the entire system.
3Reliability
If mechanical switches with parallel diodes are used in disconnecting switch units, then fault current blocking is improved, but reverse voltage formation capability must be ensured
Solution Approach 1:
The patent combines two disconnecting switch units in series on the DC line. The first unit includes a mechanical switch with a parallel diode, while the second unit includes a mechanical switch without a parallel diode. This combination allows the system to form reverse voltage across the fault location by coordinating the operation of both units, ensuring reliable fault current blocking while distributing the functional requirements across multiple components.
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
Enables quick fault current isolation and reduces switching losses by utilizing the sub-modules' voltage summation to create a reverse voltage, protecting the converter devices and minimizing energy loss.
Implementation Method 1
diodes connected in parallel to the mechanical switches, and the two diodes are provided in opposite directions to each other
Data Source
AI summary
The present invention relates to an MMC converter system which can quickly cut off fault current by installing a plurality of disconnecting switch units on a line connected between two MMC converter devices. The MMC converter system comprises: a first MMC converter device including a plurality of serially connected sub modules; a second MMC converter device including a plurality of serially connected sub modules; a first disconnecting switch unit installed on a line between the first and second MMC converter devices; and a second disconnecting switch unit connected in series to the first disconnecting switch unit on the line, wherein each of the first and second disconnecting switch units comprises a mechanical switch installed on the line so as to open and close the line and a diode connected in parallel to the mechanical switch, and the two diodes are installed in opposite directions to each other.