Hybrid HVDC Converter Topology for Weak-Grid Commutation Stability
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Solution Overview
Problem
Existing high-voltage direct current transmission systems using line commutated converters (LCC-HVDC) are prone to commutation failure, especially when connected to weak AC systems, leading to system instability.
Innovation Solution
A hybrid DC power transmission system is proposed, comprising a rectifier side circuit with a series connection of LCC and MMC, and an inverter side circuit with three parallel MMCs and LCCs connected in series, along with a control method that includes dual-loop control for MMC and constant DC voltage and current control for LCC.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If LCC-HVDC technology is used for long-distance transmission, then transmission capacity and cost efficiency are improved, but commutation failure occurs on the inverter side when connected to weak AC systems
Solution Approach 1:
The patent combines LCC and MMC converters into a hybrid configuration where LCC handles high-voltage transmission and MMC provides stability control. The LCC and MMC are connected in series on both rectifier and inverter sides, allowing the system to leverage the high capacity of LCC while the MMC prevents commutation failure through its self-commutation capability and voltage control functions.
2Loss of energy
If transmission voltage level is increased to meet power delivery requirements, then transmission efficiency is improved, but AC system strength becomes relatively reduced leading to system instability
Solution Approach 1:
The MMC acts as an intermediary between the high-voltage LCC transmission system and the weak AC grid. It provides voltage support and stabilizes the AC system strength, enabling the LCC to operate at high voltage levels for efficient transmission without causing instability in weak AC systems.
3Ease of manufacture
If LCC is used without self-commutation function, then construction and operation costs are reduced, but the system requires strong AC system connection which reduces adaptability
Solution Approach 1:
The hybrid system achieves multi-functionality where LCC provides economical high-voltage transmission while MMC adds self-commutation capability and weak AC system compatibility. This combination allows the system to adapt to various AC system strengths without sacrificing the cost advantages of LCC technology.
Data Source
AI summary
The present invention provides a hybrid direct current transmission system and a control method thereof, which belongs to the field of power electronics. The system includes: the rectifier side circuit adopts a heterogeneous series topology structure, the high-voltage valve group adopts LCC to withstand high voltage levels, and the low-voltage valve group adopts MMC to establish and maintain the voltage stability of the grid connection point; the inverter side circuit adopts a distributed connection, wherein the high-voltage valve group adopts LCC, the low-voltage valve group adopts three parallel MMCs, and multiple inverters are distributedly connected to different AC circuits. The rectifier side is composed of LCC and MMC in series, and the inverter side is composed of three parallel MMCs and LCC in series that are distributedly connected to different AC circuits. In this way, the system can provide voltage support for the energy base, has AC and DC fault ride-through function, prevents phase change failure, and improves system stability. In addition, the distributed access method can also achieve flexible power distribution.