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China has entered a new phase of power grid development, characterized by large-scale grids, high-voltage transmission, and advanced automation. With the rapid growth of the economy, electricity demand has surged, prompting the gradual implementation of ultra-high voltage (UHV) transmission systems in China's power infrastructure. UHV transmission offers significant advantages, including large capacity, long-distance efficiency, low losses, and cost-effectiveness. It effectively addresses the limitations of existing 500kV grids, such as low transmission capacity, poor economic efficiency, and unstable operation. As a result, the construction of UHV power grids has become an essential trend in China’s energy development.
**1. Overview**
Power system stability refers to the ability of synchronous generators within the grid to maintain or regain synchronized operation after experiencing disturbances. When generators operate in sync, the power generated remains stable, and the phase angle differences between electromotive force phasors and bus voltages remain constant. If synchronization is lost, active power output fluctuates, leading to voltage swings and potential instability. If these fluctuations are not damped, the system may lose stability, resulting in cascading failures and blackouts.
**2. Stability of UHV Transmission Networks**
The UHV grid is an integral part of the broader power system, and its stability analysis shares similarities with that of traditional high-voltage grids. However, UHV transmission involves transporting large amounts of power over long distances, which introduces unique challenges. The performance of UHV lines is influenced by their transmission capacity and the characteristics of both the sending and receiving systems. Therefore, the transmission capability of UHV lines is constrained by voltage stability and power angle stability limits.
**3. Stability Principles of the UHV Grid**
Compared to conventional high-voltage lines, UHV transmission can carry significantly more power. A sudden disruption in an UHV line could cause a major power deficit, threatening the stability of the next-level 500kV grid. To ensure overall system reliability, UHV transmission often employs double-circuit lines to deliver power from generation centers to load hubs. During actual operation, the power transmitted must meet strict stability requirements, including:
1) Maintaining transient stability after a severe fault or three-phase short circuit.
2) Ensuring static stability by keeping the remaining line within safe limits and restoring normal operation quickly.
3) Preserving voltage stability by maintaining a margin within small interference limits.
4) Handling large-scale disturbances, such as a single unit tripping at the receiving end, while maintaining voltage within acceptable ranges.
**4. Economic Comparison of Transmission**
When comparing the economics of EHV and UHV transmission, factors like initial investment, operational costs, and maintenance are considered. Analysis shows that a 1100kV UHV line can transmit more than four times the power of a 500kV line. Additionally, UHV transmission incurs much lower operational and maintenance costs, with power losses about one-sixth of those in 500kV lines. These advantages make UHV a more economically viable option for long-term energy transmission.
**5. Conclusion**
Electricity plays a vital role in China’s national economy and people’s daily lives. The State Grid and other power companies play a critical role in ensuring energy security, optimizing resource allocation, and supporting sustainable development. By building a smart, strong national grid based on UHV technology, China can enhance resource efficiency, promote clean energy use, and improve the overall performance of its power industry. This strategic move is essential for the future of China’s energy landscape.
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