Voltage Source Converter Series Limb Topology
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Solution Overview
Problem
Conventional voltage source converters for high voltage direct current (HVDC) power transmission and reactive power compensation require a large footprint and include bulky DC voltage blocking capacitors, which can lead to uncontrolled circulation currents during transient and fault conditions, and are costly and space-intensive.
Innovation Solution
A voltage source converter design featuring a series connection of converter limbs with longitudinal chain-link converters and transverse circuits containing chain-link converters, capacitive, and inductive elements, which reduces the need for large capacitors and allows for efficient operation with a smaller footprint, and a control unit that modifies voltage components to manage fault conditions and balance power exchange.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional voltage source converters use parallel converter limbs with DC voltage blocking capacitors, then the converter can block DC voltage in transverse circuits, but the footprint and cost increase significantly due to large capacitor requirements
Solution Approach 1:
The patent removes the DC voltage blocking capacitor from the transverse circuit entirely. Instead of using a capacitor to block DC voltage, the invention uses a chain-link converter configuration where the series connection of converter limbs inherently prevents DC current flow in transverse circuits, eliminating the need for large blocking capacitors and reducing footprint.
Solution Approach 2:
The patent replaces the passive electrical component (DC blocking capacitor) with an active converter structure (chain-link converter with series-connected limbs). This substitution uses the controlled switching and series connection topology to achieve DC voltage blocking without requiring large energy-storing capacitive elements.
2Reliability
If large DC voltage blocking capacitors are used in transverse circuits, then DC voltage can be blocked, but uncontrolled circulation currents occur during transient and fault conditions
Solution Approach 1:
By removing the DC blocking capacitor from the transverse circuit, the source of uncontrolled circulation currents during faults is eliminated. The chain-link converter topology provides inherent protection against such currents through its series connection structure and controlled switching, preventing the harmful circulation that occurs with large capacitors during transient conditions.
3Area of stationary object
If converter limbs are connected in series, then the footprint is reduced, but the voltage distribution and fault management become more complex
Solution Approach 1:
The patent divides the converter into multiple converter limbs connected in series, with each limb containing switching devices that can be independently controlled. This segmentation allows for simplified voltage distribution management, as each limb handles a portion of the total voltage, and the modular structure facilitates easier fault isolation and management compared to a monolithic design.
4Area of stationary object
If the number of chain-link modules is reduced, then the footprint and cost decrease, but the current rating capability may be compromised
Solution Approach 1:
The patent transitions from a parallel configuration (where current capacity scales with the number of modules) to a series configuration of converter limbs. In this series topology, the voltage is distributed across limbs while the current rating is maintained through the controlled switching and series connection, allowing reduced module count while preserving current capability through dimensional reorganization of the circuit topology.
Data Source
Figure 1
Figure 2(a)~2(b)
Figure 3(a)~3(b)
AI summary
In the field of voltage source converters for use in high voltage direct current (HVDC) power transmission and reactive power compensation, there is provided a voltage source converter (10; 70; 80; 90; 110) which comprises a plurality of converter limbs (121, 122, 123) connected in series between first and second DC terminals (16, 18) that are connectable in use to a DC network (20). Each converter limb (121, 122, 123) includes first and second series connection points (22, 24) between which extends a longitudinal chain-link converter (26) that is operable to provide a stepped variable voltage (V1arm, V2arm, V3arm). Each first series connection point (22) is electrically connected with a corresponding first AC connection terminal (42) via a first transverse branch (142). Each second series connection point (24) is electrically connected with a corresponding second AC connection terminal (46) via a second transverse branch (146). The first and second AC connection terminals (42, 46) together define an AC connection (48) that is connectable in use to a respective phase of an AC network (14). At least one transverse branch (142, 146) in each converter limb (121, 122, 123) includes a transverse circuit (44; 72; 82; 92) which has therein a transverse chain-link converter (56) that is operable to provide a stepped variable voltage (V1armT, V2 armT, V3armT).