HVDC Semiconductor Switching Circuit Stray Capacitance Protection
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Solution Overview
Problem
High voltage direct current (HVDC) power converters face challenges in managing the rate of change of current through semiconductor switching elements, which can be damaged by external stray capacitances, requiring large saturating inductors and limiting performance due to inherent limitations of main semiconductor switching elements like thyristors.
Innovation Solution
A semiconductor switching circuit with a main current branch and an auxiliary current branch in parallel, where the control unit switches on auxiliary semiconductor switching elements before or after the main element, creating an alternative current path to divert current from stray capacitances, reducing the rate of change through the main element and eliminating the need for large inductors, using voltage isolation to trigger auxiliary elements with the same control unit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If main semiconductor switching elements are used in HVDC power converters, then power conversion function is achieved, but the rate of change of current through the main element is too high causing damage from stray capacitances
Solution Approach 1:
The current path is segmented into two parallel branches: a main current branch containing the main semiconductor switching element, and an auxiliary current branch containing auxiliary semiconductor switching elements. This segmentation allows the total current to be divided, with the auxiliary branch absorbing the harmful high di/dt component from stray capacitances while the main branch handles the steady power conversion current.
Solution Approach 2:
The auxiliary current branch acts as an intermediary path that mediates the harmful effect of stray capacitances. By providing this alternative current path, the auxiliary branch protects the main semiconductor switching element from damage while still allowing the overall power conversion function to operate.
2Reliability
If large saturating inductors are added to reduce rate of change of current, then protection is improved, but device complexity and size increase
Solution Approach 1:
The patent replaces the traditional mechanical/magnetic solution (large saturating inductors) with a semiconductor-based solution. By using auxiliary semiconductor switching elements controlled by a control unit, the system achieves current surge protection through electronic control rather than passive magnetic components, reducing device complexity and size.
3Object-affected harmful factors
If auxiliary semiconductor switching elements are used to create alternative current path, then rate of change of current is reduced, but device complexity increases
Solution Approach 1:
The auxiliary semiconductor switching elements serve multiple functions: they provide an alternative current path for stray capacitance discharge, protect the main element from high di/dt, and can be controlled by the same control unit that operates the main element. This multi-functionality justifies the added complexity by providing comprehensive protection and control integration.
Data Source
Figure 1~2a
Figure 2b~3
Figure 4~5
AI summary
There is provided a semiconductor switching circuit (70), for use in a HVDC power converter, comprising: a main current branch (74) including at least one main semiconductor switching element (12) and through which current flows in a first direction when the or each main semiconductor switching element (12) is switched on; an auxiliary current branch (76) connected in parallel with the main current branch (74), the auxiliary current branch (76) including at least one auxiliary semiconductor switching element (72); and a control unit (30) operatively connected to the or each main semiconductor switching element (12) to control switching of the or each main semiconductor switching element (12), the control unit (30) operatively connected via a or a respective voltage isolation device to the or each auxiliary semiconductor switching element (72) to control switching of the or each auxiliary semiconductor switching element (72), wherein the control unit (30) is programmed to switch on the or each auxiliary semiconductor switching element (72) in the auxiliary current branch (76) before, when or after the or each main semiconductor switching element (12) is switched on to selectively create an alternative current path via the auxiliary current branch (76) whereby current flowing in the first direction through the main current branch (74) is diverted to flow through the alternative current path to reduce the rate of change of current flowing through the or each main semiconductor switching element (12) immediately after the or each said main semiconductor switching element (12) is switched on.