HVDC Bipolar Network Upgrade via Voltage Source Converters
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Solution Overview
Problem
High Voltage Direct Current (HVDC) power transmission plants with monopolar direct voltage networks experience unbalanced power transmission, leading to operational disturbances and the need for high electrode currents when changing power feeding direction, which requires costly and potentially unobtainable governmental permissions and causes environmental impacts.
Innovation Solution
Converting the direct voltage network to a bipolar configuration by adding Voltage Source Converters with switches and control means to each station, allowing current direction to be maintained in one pole conductor while reversing in the other, eliminating the need for earth electrode currents during power direction changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a monopolar direct voltage network is used for HVDC power transmission, then the device complexity is reduced, but unbalanced power transmission occurs causing operational disturbances
Solution Approach 1:
The invention divides the monopolar network into two separate pole conductors (positive and negative poles), each carrying balanced current. This segmentation allows independent control of each pole, eliminating the unbalanced operation inherent in monopolar systems while maintaining relative simplicity through modular converter designs at each station.
Solution Approach 2:
The invention introduces asymmetric switching arrangements in the converter stations, where different switching configurations are applied to positive and negative pole converters. This asymmetric control enables balanced power transmission by independently managing current flow in each pole, resolving the operational disturbances caused by symmetric monopolar configurations.
2Adaptability or versatility
If power feeding direction is changed in a monopolar network, then power transmission flexibility is improved, but high electrode currents are required causing environmental damage and requiring governmental permissions
Solution Approach 1:
Instead of reversing current direction through the same pole conductor (which causes high electrode currents), the invention inverts the active pole configuration by switching which pole (positive or negative) carries the forward current. This pole inversion approach maintains current magnitude within safe limits while achieving bidirectional power transmission without harmful electrode currents.
Solution Approach 2:
The invention introduces switching arrangements and control systems as intermediaries between the power source and pole conductors. These intermediaries enable seamless transition between power transmission directions by redirecting current through appropriate pole configurations, eliminating the need for direct current reversal that would cause harmful electrode currents.
3Reliability
If Voltage Source Converters are added to convert monopolar to bipolar configuration, then balanced power transmission is achieved, but device complexity increases
Solution Approach 1:
The invention merges the functions of monopolar operation and bipolar operation into a single integrated converter station design. By combining switching arrangements that can operate in both monopolar and bipolar modes, the system achieves balanced power transmission without requiring completely separate converter systems, thus limiting the increase in device complexity.
Solution Approach 2:
The converter stations are designed with universal switching arrangements that can perform multiple functions: monopolar operation, bipolar operation, and directional power transmission control. This multi-functionality reduces the need for specialized equipment for each mode, thereby limiting the overall device complexity increase while achieving balanced power transmission.
Data Source
Figure 1~3
Figure 4
AI summary
A plant for transmitting electric power through HVDC comprises two converter stations interconnected by a monopolar direct voltage network (7) and each having an alternating voltage side for feeding electric power from one of said alternating voltage sides to the other. Each station has a line commutated converter (3, 4). The plant is upgraded by making the direct voltage network bipolar, providing each station with a Voltage Source Converter (11, 12) and providing two switches (17-20) for both line commutated converters or both Voltage Source Converters and means for controlling said four switches.