HVDC Semiconductor Switching Circuit Stray Capacitance Protection
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Solution Overview
Problem
High voltage direct current (HVDC) power converters face challenges due to the relatively low tolerance of semiconductor switching elements like thyristors to the rate of change of current and voltage, which can lead to damage from external stray capacitances and require large compensating components like saturating inductors.
Innovation Solution
A semiconductor switching circuit with control units that divert current and voltage through an auxiliary current path, using auxiliary semiconductor switching elements like transistors or thyristors to safely discharge stray capacitances and reduce the rate of change, thereby eliminating the need for large compensating components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If main semiconductor switching elements like thyristors are used in HVDC power converters, then AC power can be converted to DC power with cost-effective transmission, but the elements have low tolerance to rate of change of current and voltage which can lead to damage from external stray capacitances
Solution Approach 1:
An auxiliary semiconductor switching element is introduced as an intermediary component between the stray capacitances and the main semiconductor switching element. When activated, this auxiliary element provides an alternative current path that mediates the harmful effect of stray capacitance discharge, preventing direct damage to the main switching element while allowing the main element to continue its primary power conversion function.
Solution Approach 2:
The current path is segmented into multiple branches: a main current branch containing the main semiconductor switching element for primary power conversion, and an auxiliary current branch containing the auxiliary semiconductor switching element for protecting against stray capacitance effects. This segmentation allows independent optimization of each branch for its specific function.
2Reliability
If large saturating inductors (di/dt reactors) are used to compensate for low rate of change of current tolerance, then semiconductor switching elements are protected from damage, but the device complexity and size increase significantly
Solution Approach 1:
The auxiliary semiconductor switching element acts as an intermediary that provides protection against rate of change of current without requiring large saturating inductors. By controlling the auxiliary element's switching, the system achieves protection through active management rather than passive bulky components.
Solution Approach 2:
The invention replaces the mechanical/magnetic solution of large saturating inductors with an electronic control solution using auxiliary semiconductor switching elements. This substitution eliminates the need for bulky magnetic components while achieving the same protective function through electronic switching and control.
3Reliability
If auxiliary semiconductor switching elements are switched on simultaneously with main semiconductor switching elements, then the rate of change of current is reduced and stray capacitances are safely discharged, but the control complexity increases
Solution Approach 1:
The control units are configured to switch on the auxiliary semiconductor switching elements just before or simultaneously with the main semiconductor switching elements. This preliminary action ensures that the alternative current path is already available when the main elements switch, allowing stray capacitances to be safely diverted without causing damage during the switching transition.
Solution Approach 2:
The control system monitors the switching state of main semiconductor elements and automatically activates auxiliary elements in response. This feedback mechanism ensures proper coordination without requiring complex manual control, as the system self-regulates based on the operational state of the main components.
Data Source
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AI summary
In the field of HVDC power converters there is a need for an improved semiconductor switching circuit which obviates the difficulties associated with respective semiconductor switching elements exhibiting different performance characteristics and having inherent limitations in their performance. A semiconductor switching circuit (70; 80; 90) comprises a main current branch (74; 92) which includes at least one main semiconductor switching element (12) and through which current flows in a first direction (D1) when the or each main semiconductor switching element (12) is switched on. The semiconductor switching circuit (70; 80; 90) also includes an auxiliary current branch (76; 104) that is connected in parallel with the main current branch (74; 92). The auxiliary current branch (76; 104) includes at least one auxiliary semiconductor switching element (72) and the or each auxiliary semiconductor switching element (72) has a control unit (30) operatively connected therewith. The or each control unit (30) is configured to switch on the or each auxiliary semiconductor switching element (72) in the auxiliary current branch (76; 104) as the or each main semiconductor switching element (12) is switched on to selectively create an alternative current path (32; 108) via the auxiliary current branch (76; 104) whereby current flowing in the first direction (D1) through the main current branch (74; 92) is diverted instead to flow through the alternative current path (32; 108) 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.