Modular Converter Submodules for HVDC Fault Control
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Solution Overview
Problem
Current power converters, particularly U-converters, face challenges in safely and quickly controlling faults in high-voltage direct current (HVDC) networks, especially in spatially extensive and branched networks, due to high discharge currents and mechanical switch limitations, leading to inefficiencies and potential damage.
Innovation Solution
The proposed submodule design includes power semiconductor switching units in the emitter and collector connection branches, allowing for the generation of negative terminal voltages and reduced semiconductor power loss, enabling effective control of faults without additional external switches and minimizing on-state power loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional U-converters with central capacitor banks are used, then high discharge currents flow during DC voltage side short circuits, but this causes extremely high mechanical forces and arcing effects that can lead to destruction
Solution Approach 1:
The converter is divided into multiple independent submodules connected in series, each with its own capacitor bank. This segmentation distributes the total capacitor voltage across multiple units, so that during a short circuit, the discharge current from each individual capacitor is limited to a safe level rather than all capacitors discharging simultaneously through a single point.
Solution Approach 2:
The patent transitions from a two-level converter topology to a multi-level converter topology by introducing intermediate voltage levels through series-connected submodules. This dimensional change in voltage structure allows the system to limit discharge currents while maintaining the ability to control faults, effectively adding a new dimension to the voltage control capability.
2Speed
If mechanical switches are used for high-voltage direct current networks, then they can switch high fault currents, but the switching overvoltages and switch-off times are disruptive and cannot achieve quick fault control
Solution Approach 1:
The patent replaces mechanical switches with power semiconductor switching units that have no moving parts. These solid-state switches can turn on and off extremely quickly (in microseconds) without the mechanical inertia and contact bounce issues that limit mechanical switches, while also avoiding the high switching overvoltages associated with mechanical contact breaking.
3Reliability
If additional external switches are added to control faults, then fault control capability improves, but device complexity and semiconductor power loss increase
Solution Approach 1:
The power semiconductor switching units in each submodule are designed to perform multiple functions: normal power conversion operation and fault control. By programming the control system, these same switches can rapidly transition to a fault-blocking state when needed, eliminating the need for separate dedicated fault protection switches and reducing overall device complexity.
Solution Approach 2:
Each submodule is equipped with its own switching units and capacitor bank, making it self-sufficient for both normal operation and fault handling. When a fault occurs, the affected submodule can independently block the fault current using its own switches and energy storage, without requiring additional external protection devices or complex inter-submodule coordination.
4Loss of energy
If conventional submodule design is used, then modular design is achieved, but semiconductor power loss and on-state power loss are high
Solution Approach 1:
The patent optimizes the voltage and current parameters operating through the semiconductor switches by distributing the total voltage across multiple series-connected submodules. This parameter change reduces the voltage stress on each individual switch and allows operation at more efficient points on the semiconductor device characteristics, reducing conduction losses.
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 design reduces semiconductor power loss, limits the number of controllable switches, and allows uniform semiconductor equipping, enhancing fault control and operational efficiency by absorbing energy independently of current direction, thus avoiding high short-circuit currents and enabling quick recovery from faults.
Implementation Method 1
Each of these sub-units has a series circuit of two power semiconductor switching units, which are connected in parallel with an energy storage device (21, 31).
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
The invention relates to a submodule (1) for forming a converter (10). The submodule (1) in this case comprises a first subunit (2), which has: a first energy store (21); a first series circuit comprising two power semiconductor switching units (22, 23) which each have a power semiconductor which can be switched on and off and have the same forward direction and which switching units are each conductive in the direction opposite said forward direction, said first series circuit being connected in parallel with the first energy store; and a first connection terminal (X2), which is connected to the potential point between the power semiconductor switching units in the first series circuit. In addition, the submodule (1) comprises a second subunit (3), which has: a second energy store (31); a second series circuit comprising two power semiconductor switching units (32, 33), which each have a power semiconductor which can be switched on and off and have the same forward direction and which switching units are each conductive in the direction opposite said forward direction, said second series circuit being connected in parallel with the second energy store; and a second connection terminal (X1), which is connected to the potential point between the power semiconductor switching units in the second series circuit. The first subunit (2) and the second subunit (3) are additionally connected to one another via connecting means (4). The connecting means (4) in this case have an emitter connection branch (41), which connects an emitter of a first power semiconductor switching unit in the first series circuit to an emitter of a first power semiconductor switching unit in the second series circuit, a collector connection branch (42), which connects a collector of the second power semiconductor switching unit in the first series circuit to a collector of the second power semiconductor switching unit in the second series circuit, and a switching branch (43), in which a switching unit (44) is arranged and which connects the emitter connection branch to the collector connection branch. The invention is characterized by the fact that at least one power semiconductor switching unit (45, 46) is arranged in the emitter connection branch (41) or the collector connection branch (42).