HVDC Interconnector Switching via DC Voltage Reference Control
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Solution Overview
Problem
High voltage direct current (HVDC) power transmission networks face challenges in efficiently managing DC current thresholds to facilitate the disconnection of interconnector switches without interrupting power exchange between AC network elements, especially when AC current control is not possible or desirable.
Innovation Solution
A multi-terminal power transmission network with supplementary controllers that modify the DC voltage reference based on local DC current measurements to drive the DC current below a predetermined threshold, allowing for reliable disconnection of interconnector switches without current interruption and accommodating various power converter types and control methodologies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If regular interconnector switches are used without current interruption capability, then device complexity and cost are reduced, but the ability to disconnect the interconnection conduit safely is compromised due to DC current flowing through the switch
Solution Approach 1:
The supplementary controller preemptively modifies the DC voltage reference to drive DC current below the threshold before the interconnector switch is opened. This preliminary action ensures that when the switch opens, no significant DC current needs to be interrupted, allowing the use of simpler switch designs without compromising safety or reliability.
Solution Approach 2:
The system changes the DC voltage reference parameter dynamically based on local DC current measurements. By adjusting this voltage reference, the supplementary controller directly influences the power converter to reduce DC current flow through the interconnection conduit, enabling safe switch disconnection with regular equipment.
2Reliability
If the DC current through the interconnection conduit is reduced below threshold to enable switch disconnection, then the interconnector switch can be opened safely, but power exchange between AC network elements may be interrupted
Solution Approach 1:
The control function is segmented into two independent parts: the converter controller that maintains power exchange between AC networks, and the supplementary controller that independently manages DC current through the interconnection conduit. This segmentation allows each controller to perform its function without interfering with the other, enabling safe switch disconnection while maintaining continuous power exchange.
Solution Approach 2:
The supplementary controller acts as an intermediary that modifies the DC voltage reference to decouple the DC current flow in the interconnection conduit from the power exchange between AC networks. This intermediary control mechanism enables independent management of switch safety requirements while preserving continuous power transmission functionality.
3Measurement precision
If a supplementary controller is introduced to modify DC voltage reference for DC current control, then DC current fidelity is improved, but device complexity increases
Solution Approach 1:
The supplementary controller is designed with a universal interface that works across different power converter types and control methodologies. By establishing a standard mechanism for modifying DC voltage reference based on local current measurements, the same supplementary controller architecture can be applied universally without requiring converter-specific customization, thereby limiting the increase in overall system complexity.
4Stability of the object's composition
If the first power converter operates in voltage-frequency control mode, then AC network stability is maintained, but AC current reference cannot be changed to control DC current
Solution Approach 1:
Instead of controlling DC current by adjusting AC current reference (the conventional approach that doesn't work in voltage-frequency mode), the supplementary controller inverts the control strategy by modifying the DC voltage reference directly. This inverted approach bypasses the limitation of voltage-frequency control mode while maintaining AC network stability, as it operates independently of the AC current control channel.
Data Source
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Figure 3(a)~3(b)
AI summary
In the field of multi-terminal power transmission network, there is provided an improved multi-terminal power transmission network (10; 50) which comprises at least first and second DC terminals (12, 14) that are interconnected by a first transmission conduit (16) to permit the transfer of power between the first and second DC terminals (12, 14). The first DC terminal (12) is additionally connected, via a first interconnector switch (18), with an interconnection conduit (20) that extends in-use to a further DC terminal, and is separately connected with a first power converter (22). The first power converter (22) interconnects the first DC terminal (12) with a first AC network element (24) and includes a first converter controller (26) which is programmed to control the transfer of power between the first AC network element (24) and the first transmission and interconnection conduits (16, 20) by establishing at least a first DC voltage reference (VDC1) that the first power converter (22) is required to provide. In addition, the second DC terminal (14) is interconnected with a second AC network element (32) by a second power converter (34) which includes a second converter controller (36) that is programmed to control the transfer of power between the first transmission conduit (16) and the second AC network element (32). The first power converter (22) further includes a first supplementary controller (40) which is programmed to selectively modify, as a function of the DC current (IDC1-3) flowing in the interconnection conduit (20), the first DC voltage reference (VDC1) that the first power converter (22) is required to provide so as to drive the said DC current (IDC1-3) flowing in the interconnection conduit (20) below a predetermined threshold (IREF) and thereby facilitate disconnection of the interconnection conduit (20) from the first DC terminal (12) via opening of the first interconnector switch (18).