Tripolar VSC-HVDC System With Shared Converter Valves
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Solution Overview
Problem
The existing tripolar LCC-HVDC system for converting AC lines to DC lines faces issues such as ground faults leading to DC transmission power interruption, disturbances in AC systems, and high reconstruction investment and space requirements for converter stations, making it unsuitable for urban power supply systems lacking land resources.
Innovation Solution
A tripolar VSC-HVDC system utilizing a modular multilevel converter (MMC) structure with three-phase six-bridge arm converters, where two converter valves are used between poles on the DC side, and the midpoint of each phase unit is connected to a converter transformer, allowing for constant DC current directions and periodic modulation of current orders between poles, eliminating the need for AC filters and reactive compensation devices, and enabling fault tolerance with three conductors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a tripolar LCC-HVDC system is used to convert AC lines to DC lines, then the power transmission capacity is improved, but the converter station requires more space and higher reconstruction investment
Solution Approach 1:
The patent merges the functions of multiple converter valves into a shared valve structure. The third converter valve is shared between pole 1 and pole 2, allowing the same valve to serve both poles by switching its connection. This merging reduces the total number of converter valves from six (two per pole) to five, thereby reducing converter station space and equipment quantity while maintaining tripolar power transmission capacity
Solution Approach 2:
The third converter valve is designed with multi-functionality to operate in different configurations. It can be connected to pole 1 or pole 2 depending on the operational mode, serving multiple purposes: normal tripolar operation, fault tolerance mode, and asymmetric operation. This universal valve design eliminates the need for dedicated valves for each pole, reducing overall equipment quantity and space requirements
2Power
If a tripolar LCC-HVDC system is used, then the transmission capacity is increased, but the system becomes more complex with more converter transformers, AC filters, and reactive compensation devices
Solution Approach 1:
The patent combines reactive compensation functions into the converter valve structure itself. The converter valves incorporate reactive power compensation capabilities, eliminating the need for separate AC filters and reactive compensation devices. This merging reduces equipment complexity while maintaining the ability to provide reactive power support to the AC system
Solution Approach 2:
The patent extracts and eliminates unnecessary equipment from the traditional tripolar LCC-HVDC configuration. By using VSC technology with IGBTs, the system removes the requirement for commutation voltage, AC filters, and separate reactive compensation devices, simplifying the overall system architecture while maintaining transmission capacity
3Power
If conventional tripolar LCC-HVDC is used, then power transmission is achieved, but ground faults cause DC transmission power interruption and threaten system security
Solution Approach 1:
The patent implements beforehand cushioning by designing a fault-tolerant control system that prepares for ground faults in advance. The control system continuously monitors system state and can quickly switch to asymmetric operation mode or bypass faulty lines before the fault causes complete power interruption. This preparatory measure ensures system security and maintains transmission power during ground fault conditions
Solution Approach 2:
The patent introduces dynamic operation modes that allow the system to adapt to fault conditions. The converter valves can dynamically switch between different connection configurations (symmetric tripolar, asymmetric, bipolar mode) based on real-time system state. This dynamic flexibility enables the system to maintain power transmission even when one line experiences ground faults, significantly improving reliability
Data Source
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AI summary
The present invention discloses a tripolar VSC-HVDC system and method, wherein a rectifier and an inverter consist of a three-phase six-bridge arms modular multilevel converter (MMC) respectively, and two converter valves are arranged on the DC side of the rectifier and inverter respectively, the midpoint of upper and lower converter valves of the rectifier and inverter are connected with a pole 3 DC line by a smoothing reactor, triggering of the upper and lower converter valves is controlled to change the DC voltage polarity of the pole 3 periodically, and tripolar DC transmission is realized by modulating current orders of the three poles. The technical solution provided by the invention can make full use of the three conductors for power transmission, has the characteristics of small occupied space of the converter station, low total conversion cost and high reliability, and it is particularly suitable for the implementation of conversion an AC line to a DC line for capacity increasing in large-sized urban power supply systems lacking in land resources.