Voltage Source Converter Overcurrent Protection via Body Diode Commutation
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Solution Overview
Problem
Modular multilevel converters (MMC) are vulnerable to damage from overcurrent events such as DC terminal-terminal short circuits, which can cause fault currents to flow through freewheel diodes, potentially damaging semiconductor devices.
Innovation Solution
Incorporating a protection device, such as a thyristor or diode, in parallel with the MOSFET switching elements, and a controller to divert current away from the MOSFET channels and through the body diodes during overcurrent events, allowing rapid commutation to the protection device, which switches on at a higher voltage due to the high forward voltage drop across the body diodes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a protection thyristor is used with IGBT switching elements, then the submodule is protected against overcurrent events, but the thyristor must switch on at very low voltage which is difficult to achieve
Solution Approach 1:
The patent changes the voltage parameter by using body diodes with higher forward voltage drop (2-3V) compared to conventional low-voltage switching. This parameter change enables the protection device to switch on at a higher, more achievable voltage level while maintaining protection functionality
Solution Approach 2:
The body diode acts as an intermediary element between the MOSFET and the protection device. It provides a controlled voltage drop that facilitates the switching operation of the protection device, mediating the transition from low-voltage MOSFET operation to higher-voltage protection mode
2Speed
If MOSFETs are used as switching elements, then faster switching is achieved, but the body diodes have high forward voltage drop which complicates protection device switching
Solution Approach 1:
The patent converts the harmful high forward voltage drop of the body diode into a beneficial feature. This voltage drop, which initially seemed to complicate protection device switching, actually provides the necessary voltage level to enable reliable thyristor or diode protection device operation
Solution Approach 2:
The high forward voltage drop parameter of the body diode is utilized to change the operating voltage level for the protection device, transforming a potential disadvantage into an enabling characteristic for simpler protection mechanisms
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 solution effectively protects the MOSFET switching elements from damage by diverting fault currents through the body diodes and then to the protection device, ensuring rapid and efficient commutation and minimizing the risk of semiconductor device damage during overcurrent events.
Implementation Method 1
switching off the second MOSFET switching element, thereby forcing current flowing in the voltage source converter following the overcurrent event to flow through the second body diode rather than through conducting channels of the first and second MOSFET switching elements
Data Source
AI summary
The present disclosure relates to a voltage source converter (VSC) (300) comprising: a first MOSFET switching element (302) including a first body diode (306); a second MOSFET switching element (304) including a second body diode (308), the second MOSFET switching element (304) being connected in series with the first MOSFET switching element (302); a protection device (318) connected in parallel with the second MOSFET switching element (304); and a controller (312), wherein the controller (312) is configured, on detection of an overcurrent event, to: switch off the first MOSFET switching element (302); and switch off the second MOSFET switching element (304), thereby forcing current flowing in the VSC (300) following the overcurrent event to flow through the second body diode (308) rather than through conducting channels of the first and second MOSFET switching elements (302, 304).