Common Feedback Signal for DC Grid Load Sharing
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Solution Overview
Problem
Multi-terminal DC grids face challenges in reliable power flow control and load sharing due to the lack of a common signal for load sharing between converters, leading to poor reference tracking and potential power transfer impediments during disturbances, especially when communication between converter stations is lost.
Innovation Solution
A method for controlling power flow in multi-terminal DC power transmission systems that establishes a common feedback signal based on the overall voltage level, allowing converter stations to communicate and regulate power flow using a droop control mechanism, ensuring reliable operation even during disturbances and communication failures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If droop control is used for load sharing between converters in DC grids, then communication between converter stations is avoided and local measurements are used, but reference tracking of set-points deteriorates due to voltage drops along transmission lines
Solution Approach 1:
The patent introduces a common feedback signal as an intermediary that mediates between the distributed converter stations and the central control. This signal carries voltage level information from a reference converter to all other converters, enabling them to adjust their power output based on a common reference rather than local measurements alone, thus improving reference tracking while maintaining the distributed control architecture
Solution Approach 2:
The patent implements a feedback mechanism where a common feedback signal representing the overall voltage level is established and used by all converter stations. This feedback loop allows converters to continuously adjust their operation based on the actual system state, improving reference tracking accuracy while maintaining reliability during communication failures through the use of local backup control
2Device complexity
If one station is assigned DC voltage control and other stations power control, then power flow control is simplified, but the DC voltage controlling station becomes a slack node that must have spare capacity and exposes the AC system to large power swings during disturbances
Solution Approach 1:
The patent enables each converter station to serve itself by providing autonomous control capability. Each converter can independently regulate its own power output based on the common feedback signal and local measurements, eliminating the need for one station to act as a slack node with spare capacity. This self-service approach distributes the control function across all stations, improving disturbance handling while maintaining simple control structure
Solution Approach 2:
The patent segments the voltage control function across multiple converter stations rather than concentrating it in a single slack node. Each converter station handles its own voltage regulation independently based on the common feedback signal, dividing the control responsibility and eliminating the need for any single station to have excess spare capacity, thus improving reliability during disturbances
Data Source
Figure 1~2a
Figure 2b
Figure 3~4b
AI summary
The invention relates to a method (80) for controlling power flow within a DC power flow within a DC power transmission network (1) comprising two or more interconnected converter stations (10a, 10b, 10c, 10d, 10e). The method (80) comprises the steps of: establishing a common feedback signal (U d,common,U d'common) to the converter stations, the common feedback signal (U d,common, U d',common) being based on an overall voltage level in the DC transmission, power network (1); providing, in the converter stations, a control signal (S1, S2, S3, S4) based on the common feedback signal (U d,common) and a drooped error signal (Pe, droop, U e, droop); and controlling the power flow within the DC power transmission network (1) towards set operating points ( Ud ref, PCC ref by using the control signal (S1, S2, S3, S4). The invention also relates to a control device ( 18a, 18b, 18c, 18d, 18e) and computer program products.